A modern high-quality audio system has excellent specifications and sounds almost perfect. Almost perfect, but not quite. There is one very important attribute missing in audio systems—the attribute we call “presence”. This article discusses an alternative power amplifier design with sound that often lacks in conventional amplifiers.

Note: Due to popular demand, the LTspice files for the circuit discussed here are available at the bottom of the article.

Even the best commercially available audio systems lack real presence–while the sound can be crystal clear, you would never mistake the recorded voices for real voices, or the recorded piano for a real piano. The human ear immediately knows the difference.

As listeners, even as audiophile listeners, we don’t fuss about this lack of presence because we have come to accept that what we hear from a modern audio system is as good as it gets. Yet this just isn’t true, and it doesn’t have to be accepted.

The lack of presence occurs almost entirely as a result of distortions inherent in the fundamental design of all commercial power amplifiers. Have you noticed how much clearer headphones sound? It's due to the fact that they are driven by low-powered amplifiers.


"Block diagram of conventional class B amplifier with the two halves of a complementary output stage represented by sub-amplifiers X and Y. " From Peter Blomley's 1971 article (PDF).


In this article, I demonstrate that there is an alternative power amplifier design that suffers virtually no distortion, and provides a sound which has the presence so lacking in conventional amplifiers. This amplifier was designed over forty years ago, and yet despite its superb fidelity, it has never seen commercial production.

I will introduce the idea of “rogue frequencies” and their effect on our listening experience. I will then go on to show how this original and truly suburb amplifier successfully minimizes distortions and does not generate disturbing rogue frequencies.

This design is so effective and the output so pure that it creates an audio presence that is quite impossible to ignore.

To understand why commercial amplifiers produce a sound which is very good but which lacks presence, I will begin by discussing the sensitivity of the human ear. Then I will examine types of distortion and how these affect what we actually hear.

Finally, as a potential add-on to the original design, I will discuss distortion caused by “clipping” and how to reduce its harshness.


A Brief Anecdote about a Gramophone

Recently, I went to visit an enthusiast friend who wanted to show me a genuine 1890’s wind-up gramophone with a real thorn needle and a real “His Master’s Voice” horn that he had just bought.  


His Master's Voice, painted 1898 by Francis Baurraud.


With the purchase of the gramophone came some very old 78rpm records made directly from the original wax master—the huge horn of the recording gramophone was placed close to the orchestra and the needle cut a spiral groove, using only sound energy, in a platter covered with a thin layer of wax. There was an original record in good condition of Caruso in full glorious voice! There were also some orchestral and choir records.

These were all made before the triode was invented so no electronics were used in their manufacture. To humor my friend I agreed to listen, expecting an unpleasantly distorted sound. As I suspected, there was hiss and there were crackles, but the music! Pure and clear, you could hear each and every instrument and voice completely separately and beautifully, even with a choir and a mono recording source. Caruso really does deserve his reputation. It sounded ALIVE and PRESENT, even though it was made in 1902.

So why do modern amplifiers, with all the remarkable improvements we have achieved in electronic technology, lack this essential quality of presence?


How the Human Ear Perceives Harmonic Distortion

The human ear is extraordinarily sensitive. Under ideal conditions, the ear can hear sounds from the eardrum moving by as little as the diameter of a hydrogen atom (~10-10 m). Curiously, while the human ear can be very sensitive to some things, it isn't very sensitive to other things. For example, to notice a change in volume, the power has to be doubled (3db).   

So if you were to increase, say, the 2nd harmonic by 5%, it would be an extraordinary person who noticed. Thus, genuine pure harmonic distortion below about 10% is pretty undetectable and irrelevant for even the best hi-fi. However, harmonic distortion is easy for the engineer to measure to great levels of accuracy and down to very low levels—so it gets talked about a lot, even though it really doesn’t matter much in the long run!

Consider how you identify your mother’s voice instantly, even over the lo-fi telephone. It's done by harmonic content, and the human ear is very deeply tuned to harmonic content: "Have you got a cold, Mum!?" can be asked after just one sentence from her.

This shows us the introduction of extra harmonics is very audible indeed (as little as 0.01% is easily detectable as a different type of sound).


Intermodulation Distortion

Any two frequencies passed through a non-linear amplifier will produce the sum frequency and the difference frequency in addition to the original frequencies. The amplitude of these additional frequencies (“rogue frequencies”) is related to the amount of non-linearity. This is intermodulation distortion and it is very difficult to measure, especially at the extremely low levels that still remain significant to the human ear. These additional unwanted rogue frequencies are off-key on our standard musical scale, and even tiny amounts make the music sound “muddy”.   



Music or voice consists of hundreds of superimposed frequencies at any given moment (as described by the mathematician Joseph Fourier). As this collection of frequencies is passed through an amplifier, additional small-amplitude "rogue frequencies" are added to the original signal, and the human ear is very sensitive to this additional frequency content, and immediately identifies that the sound is not real. This is a significant reason why you never confuse, say, voices on the TV or radio with real visitors even when you are in another room.

Even very good conventional amplifiers are not mistaken for the real thing! Transducers (such as a needle on a record or a loudspeaker) are usually reasonably linear (certainly the audiophile versions) so they do not introduce many rogue frequencies although their harmonic distortion (through resonances, etc.) can be quite large. The weakness in a hi-fi system, no matter its numerically apparent superb specifications, is usually only the amplifier.


Crossover Distortion

Amplifier analysis shows that a Class B amplifier has a no-feedback distortion of about 33% and a Class A amplifier has a no-feedback distortion of about 8% and sound better than Class B. Most of the Class B distortion comes from the use of the output transistors as rectifiers to separate the plus and minus halves of the signal as well as then amplifying those halves separately thereafter (“push-pull”).  

When a power transistor is driven below a collector current of about 15mA the amplification falls dramatically. If one could prevent the current in the output power-transistors from ever going below about 15mA and into this non-linear region, it would considerably improve things. This change of amplification causes the crossover distortion characteristic of class-B amplifiers.  

Note that this crossover distortion should not be confused with the audio distortion, often also referred to as crossover distortion, which arises when audio signals are separated into frequency bands, as in loudspeaker circuits to feed the appropriate frequency range to each discrete driver unit.


Transient Intermodulation Distortion

Modern amplifier distortion is controlled by negative feedback, which reduces the distortion in proportion to the feedback. Amplification is cheaply available so the apparent non-linearity can be reduced to arbitrarily low levels by sufficient feedback.

But the feedback signal takes time to get through the amplifier and back to the input negatively to quash the distortion. So when sudden changes (transients) occur there is a period during which the naked amplifier is exposed to the world, and the non-linearity adds intermodulation rogue signals to the original, which are not entirely canceled by the feedback. This is transient intermodulation distortion. What you need is an amplifier sensibly without distortion before applying feedback The distortion of a naked class-B amplifier is so bad that most analysis seems to only consider the with-feedback distortion.

One of the reasons that modern amplifiers sound better than their older counterparts (using essentially the same class A or B circuits as always) is the increase in speed of the components. Multi-gigahertz discrete components are freely available, and even cheap power transistors have an ft of many MHz. This means that the feedback time is now very short indeed.


Clipping Distortion and How to Soften It

Transistor amplifiers driven into saturation sound horrible because the tops of the waveform are very sharply clipped off, leading to square corners and a huge explosion of unpleasant harmonics. I find myself waiting to wince when a conventional amplifier is driven hard.

On the other hand, near their clipping point, valves have quite a soft non-linear characteristic, resulting in a rounded squashed sine-wave which contain fewer spurious harmonics and sounds much better than the square-clipped sine-wave of a transistor amplifier driven hard. 

A simple circuit invented by Carl F Wheatley, Jr. (US Patent 3 786 364 / 1974) uses a single transistor and three resistors (TRP and RP1, RP2 and RP3) for each output transistor (see below).


The complete Blomley Amplifier with complementary output and clipping protection. Click to enlarge.


It measures the combination of voltage (RP3) and current (= voltage across RP2) in the output transistors and when the combination of these two voltages exceeds about 0.6V BE it turns on TRP and removes the drive to the outputs.

Note that the 0.6V is nominal and some current starts to flow when Vbe exceeds ~0.45V so this is a “soft” turn-off. It has two advantages:

  1. It makes the amplifier “clip” softly, very similar to valve designs, which makes the sound very forgiving and prevents the “cringes”.
  2. It protects the output transistors from most abuse.


Peter Blomley's New Approach to Class B Amplifier Design

In the February and March 1971 editions of Wireless World, Peter Blomley published the revolutionary and very densely concentrated article “New Approach to Class B Amplifier Design” (PDF) in two parts (patented by Plessey, No.53916.69, though this patent has long since expired).

The very clever bit of the amplifier he describes is that Blomley split the incoming signal into top and bottom halves before applying the separate signals to the output transistors. Then he was easily able to design the output transistors to work only in their linear region (above a collector current of ~15mA).

Additionally, he made the observation that with voltage signals, diodes are very non-linear, but if you use a current source, the diode is so close to the theoretical ideal that one can really call it perfect (109 difference between forward and backward current in cheap diodes).

As shown in the schematic above, he used a constant-current source (Tr6) and had a varying current sink (Tr3). The current-difference drives diodes, which are actually transistors used as diodes, (Tr4 & Tr5) to rectify the current. By using very high-frequency transistors here the transition from the “top” signal to the “bottom” signal was so fast that it was way beyond 100kHz.


"New approach to class B amplifier in which SUb-amplifiers are biased above non-linear region and fed with uni-directional signals produced by the diodes. This effectively transfers signal splitting from the sub-amplifiers to a separate part of the circuit." From Peter Blomley (PDF).


The end result of this difficult-to-understand circuitry (we are used to voltage circuits) is a Class B amplifier that has a distortion lower than 0.1% with no feedback at all. And on an oscilloscope, there is no discernible crossover distortion with no feedback.

After a little feedback is applied, there is unmeasurable intermodulation distortion, transient intermodulation distortion, and harmonic distortion. The resultant output of this amplifier is so clear that a recorded voice can easily be mistaken for a live person.  Peter Blomley’s amplifier is a Class B amplifier with much better than Class A performance.  

And yet Peter Blomley and his amplifier have gone virtually unrecognized in the audio world for more than 40 years. I suggest two reasons for this. First, his design was so original and so unexpected that few people understood it or took it seriously. Second, Blomley never put his design into commercial production because Plessey held the patent, so even fewer people were able to listen to it or review its performance.  

Most of the audio hobbyists who constructed their own Blomley amplifier modified the design and in doing so introduced distortions. I suggest you build the original design (with perhaps just the minor modifications afforded by modern components) and listen to it. This will give you a reference sound to check any further modifications with which you might like to experiment.

Unfortunately, in ignoring the Blomley design for so long, the audio world has deprived itself of a fundamentally better amplifier. We have instead put all our efforts over the last forty years in trying to mitigate what we thought were unavoidable inherent characteristics of electronic amplifiers, particularly Class B amplifiers. The boldness of Peter Blomley as a young engineer was to question how unavoidable these characteristics really were, and to then set about designing them out of his amplifier.

Today, superb high-voltage, high-speed transistors are available which makes the Blomley amplifier even better than his 1971 version.

The original amplifier design was for a 30W amplifier with a 60V power-rail, and because of the purity, this is more than adequate for normal home use. In 1971, 100V small-signal transistors were rare, but this is no longer so and an 80V power-rail can now be used, with different transistors, increasing the power to 50W. However, high sound volumes are not needed as the sound is so exceptionally clean. The huge headroom provided with most amplifiers is there so you can play them at high volume and bury the crossover-caused intermodulation distortion in the high sound-level (quite sad really).



40 years ago I fell in love with the clarity and purity of the sound from the Blomley amplifier, but it took a long time to understand the circuit and to appreciate the brilliance of Peter Blomley. Now, I have built several of these amplifiers and, provided I stick to the original Blomley design, they all sounded better than superb.

Human response, including our own, is often difficult to explain, but I have found that with a Blomley amplifier ordinary people find themselves wanting to listen to music much more than they do with a conventional top-end amplifier design. They don’t understand why, they just end up listening to more music, more often—surely the ultimate test. Listening to a Blomley amplifier is addictive. I have certainly found it so, as have many others fortunate enough to have experienced this extraordinary amplifier.

In fact, it is difficult to use it for background music; people tend to stop talking and start listening to the music. Its presence is compelling.

In his article, Peter Blomley expressed the very 1970s thought that states “The performance of an amplifier of this caliber is, in my opinion, wasted in a conventional audio set-up.” I built my first Blomley, and immediately realized that I could not agree with the sentiment he had expressed. My mother, who was garrulous in the extreme, sat through the whole of the “Pirates of Penzance” without saying a word! That was the 1970s and the other components in an audio system have come a long way since then.

The choice of a Blomley amplifier is now certainly warranted and is the best way to benefit fully from the technical advances made in all the other components of an audio system.


Notes on the Circuit

In most of the amplifiers I have built I have used a quasi-complementary output transistor arrangement but nowadays matched complementary power transistors are easily available. It really doesn't seem to matter! 

Great care must be taken to physically separate the input from the output to prevent high-frequency feedback. This amplifier is quick enough to use at RF frequencies.

I have the LTspice file if anyone is interested in playing with it. [Editor's note: You can find the LTspice files below.]



  1. “New Approach to Class B Amplifier Design” by Peter Blomley in Wireless World February and March 1971
  2. “The theory of transient intermodulation distortion” by Otala, Matti and Leinonen, Eero in Acoustics, Speech and Signal Processing, IEEE Transactions on (Feb 1977) 
  3. “A Method for Measuring Transient Intermodulation Distortion (TIM)” by Eero Leinonen, Matti Otala, and John Curl in Journal of the Audio Engineering Society Volume 25 Issue 4 pp. 170-177; April 1977
  4. “Build a Low TIM Amplifier” by W Marshall Leach in Audio, Feb 1976



  Blomley Circuits_LTspice Files  

Note: The author has altered the LTspice files based on community feedback. The files featured above are the improved version, updated on 6/22/2018.




  • thy_anger 2018-05-07

    Very nice article! What about power dissipation in comparison with other topologies?

    • dermot 2018-05-09

      Because this is a completely normal (from the point of view of the output transistors) class B amplifier, the dissipation is only from working.
      I use an idle-current of 15-20 mA so the idle dissipation is ~1,2W.  Of course at full power it gets hot as expected.

  • jiggermole 2018-05-08

    I would definitely like a copy of the spice files.  I think I wanna build one.  Or similar.

  • John_Milton_666 2018-05-08

    very interesting article. If it isn’t too much of a problem, could you please email me the LT Spice file?


  • boulou292 2018-05-08

    Thanks for the read! I would love the spice files grin

  • cferrer3 2018-05-08

    Very interesting and clear article!
    As the others I also would like the Spice files to play with them.
    Do you have any “modern design” with transistors in current production?


    • dermot 2018-05-09

      This is the most unfussy amplifier I have come across.  In one application I used a few 1% resistors and eliminated the output idle-current adjustment potentiometers and all adjustment in a production circuit.
      Provided the voltage and current ratings are adequate for the location, ANY transistors will work anywhere. 
      I even tried 2N3055’s (Ft=10kHz) for the final output transistors. It didn’t quite work.  But anything with an Ft > 50kHz will work and most modern power transistors have much larger Fts.

  • rherber1 2018-05-11

    Great article Dermot.. thanks for making it available.
    I’m not into LTSpice so the flles wouldn’t help me but for those who might just want to build this amplifier is there a PCB available?

    Also a higher resolution schematic in JPG or PDF format wouldn’t go astray either.

    (Thinks: cheeky b.gger doesn’t want much does he?)

    • trys 2018-05-12

      You can open up the schematic in another tab in you browser. On a PC, right-click you mouse over the image. If using a tablet, then keep your finger pressed on the image for a second or so until the option comes up.

      • rherber1 2018-05-12

        Thanks for the tip - it does make the schematic more legible.

  • goodoldel34 2018-05-11

    Thanks Dermot ... I fully agree! ... Long time user of both vacuum tube class B amplifiers and Spice simulation (I started with the Berkeley version in the 70s), I would be very interested with your own LTSpice files ... Thanks in advance.

  • DennisJones 2018-05-11

    Very interesting and reminded me of my student days in the 60’s building the very similar Bailey and the Lindsley-Hood amplifiers.  1964 Wireless World I think.  I believe that the greatest developments in HiFi since those days have been in the field of marketing   smile

    • dermot 2018-05-18

      The Blomley amplifier is hugely different in principle to the Bailey or Linsley-Hood amplifiers (very very good though they are).  It separates the processes of rectifying the signal and amplifying into two separate places in the circuit.  All distortions without any feedback at all are below 1%.  (put a 1F (! because you can in a simulation)  capacitor to earth in each of the feedback paths.

      I agree about the marketing.  You have to LISTEN carefully and it becomes obvious…

  • rhcbiz 2018-05-11

    Im sure a lot of people would love to build this if somebody capable (i am not) and so inclined put together a circuit board and bom of current parts.

  • Billy60300 2018-05-11

    Good evening,
    I would like to know how good can be any amplifier with a single Voltage suply ? In my opinion it can only be as good as the capacitor itself: this is a weak point indeed ! anything excellent ahead of a capacitor will always be muffled by its impedance and irregularities.
    Do you have by any chance the same kind of study of this amp with a split supply, say +/-30 V ?

    • Parkera 2018-05-11

      Billy60300 - If you follow the current path of the output stage fed from a split supply, through the speaker and back THROUGH the power supply to the point you started at, you will find that the AC signal does indeed go through a capacitor.

      • dermot 2018-05-18

        Peter Blomley put some feedback in after the capacitor which appears to negate the non-linearity.  Build it (carefully) and listen. 

        I have tried to split into +- power supplies but I’m not a good enough designer.  ANYTHING I did destroyed the sound.  So I go back to the original design.

  • Superhik 2018-05-11

    Very interesting, thanks for sharing. Great article! I’d appreciate LT Spice files. Thanks.

  • gwildes1 2018-05-11

    LT spice file please.

  • pjmm 2018-05-11

    Why are the two final transistors so different in the encapsulated and electrical characteristics?

    • dermot 2018-05-18

      I’m not sure what you mean.

      The output stage is very unfussy as to what you use. Each transistor must be high enough voltage and current and >1MHz Ft

  • kenif 2018-05-11

    I would love to lay out a PCB for this design if someone can recommend replacement devices.

    • dermot 2018-05-18

      I don’t have a decent layout and would very much like one if you can.  I think we would all appreciate it. 

      I will look at the transistors, but the ones I can get in South Africa will be very different to the much greater range that you can get in the Northern Hemisphere. 

      Basically any modern trasnsistor with a Vce and Vcb of greater than 80v and current of 100mA for the small-signal transistors.  The two driver transistors TR9 and TR10 need to have a 1A capability.  And the splitter transistors TR4 and TR5 should be RF transistors with an Ft > 100MHz.  Final transistors (TR11 and TR12)  need to have a 10A capability.

      • kenif 2018-05-21

        So - I couldn’t resist and made a start before seeing your reply.
        Went back to Blomley’s schematic to find that I still have some of the original BCnnn transistors smile Have a schematic, working on layout, will keep you posted. I’m using through hole components so that will be easier to try different devices.

        • Darren Taylor 2018-05-21

          Thanks. I started laying out a PCB but quickly had to shift gears to a contract job. I look forward to seeing what you come up with. Although SMD is so much easier from a manufacturing standpoint and the larger SMD’s are do-able for hobbyists, I use through hole for most things I design/build if they are just for me. It certainly makes experimenting with different components for audio and even (some) RF applications much easier for a hobbyist without access to microscopes or reflow ovens.

        • dermot 2018-06-05

          The only component that is difficult to substitute is the OA47 gold-bonded germanium diode (GBGD).  Some GBGDs are too high forward voltage and result in uncontrollable idle-current.  I specified a Schottky diode but I don’t think it works.  well.  Can’t source OA47 or other GBGDs so far.  Any suggestions?

  • chief29 2018-05-11

    Great background on this Class of an audio amp.  I also, would like a copy of your LTSpice file.  Thanks again.

  • JohnBillingsley 2018-05-11

    Thank you for the very interesting article, Dermot.
    Will you please send me the spice files?

  • jwong19 2018-05-11

    Excellent post insights. I will appreciate your offer of the LTSpice.

  • jackmillerjr 2018-05-11

    Thanks for the article! I would very much appreciate a copy of the LTSpice file.

  • mgalusha 2018-05-11

    Very much enjoyed the article. Like others, I’d love a copy of the LTSpice file.

  • rstefan 2018-05-11

    I’m interested in building one of these, but I’m curious about the impedance of the load. Is this designed for a 15 ohm or greater load, or can it be used with 8 ohm speakers as well?

    • rherber1 2018-05-12

      The performance table shows it delivers 30W into 8 Ohms with a 60V regulated supply.

      • dermot 2018-05-18

        Again it’s very unfussy.  I’ve drive 4 Ohm loads quite happily.

        • rstefan 2018-05-22

          I’m trying to simulate it and it keeps clipping with anything greater than about 0.25V 1kHz sine wave at the input. I’m guessing this isn’t intended for line level audio? Or maybe something’s wrong in my simulation. Either way I’d like to build one that can handle pro line level audio.

        • dermot 2018-06-05

          The amplifier is set up for a voltage gain of 100 which is too high for normal use (usually a gain of 20?).  The net gain is set by the ration of R3 / R5 ( 4k7 / 47 )  Just alter R3 to reduce the overall gain.

  • SteveW 2018-05-11

    Thank you for a great article!  I teach Class B amplifiers at the very end of our Electronics 2 class and will include this in my student’s reading material.  I would like to have the LTSpice file if possible.  Again thanks!

    • dermot 2018-05-18

      You might find this quite a challenge - it took me quite a long time to understand it, it’s so different.

      Do you know of articles detailing the inherent distortion of Class A (8%) and class B (32%) amplifiers?  I quoted from my 50-year old memory in my university electronics course and can’t back it up.

  • apqo1 2018-05-11

    Great write-up, Dermot!  I too would like to have your Spice file.

    I’ve never built an audio amplifier, but I think this could make a rewarding first attempt.  Since you’ve successfully built these in the past, would you consider a follow-up article with a modern parts list and an enlargeable schematic?

    Also, one question:  Could you explain what you mean by, “Great care must be taken to physically separate the input from the output…”?  Are you saying they should be at opposite ends of the PCB, or separate PCBs (if this, where would you divide the circuit?), or something else?


    • dermot 2018-05-18

      Being inherently a very high frequency amplifier (works at RF if you want) the input and wires must be physically far away.  Don’t need to build onto separate boards - just be careful with routing.  My first amplifier had a single edge-connector for the board with all connections coming in there - not a good idea.

  • Costin Lixandru 2018-05-11

    Great article! I am interested in the spice file. If you know anybody making it (or at least the boards), I am interested as well. Thank you very much!

  • gimble88 2018-05-11

    Thanks for the offer. How I can I get a copy of the spice model?

  • sasaudio 2018-05-11

    Nice article, and I concur with the sensitivity of the ear. I have been in R&D, in audio, for some 40 years (50 years total), and found that alterations in the frequency response (fr), with accompanying changes in harmonic and IMD amplitude pose a real problem.

    In one of my experiments,I have incorporated a 9k resistor across a driver. Changing a 9k ohm resistor by two hundredths of an ohm very slightly varies the Fr, and thus the harmonics and IMD products. I say slightly due to the fact that I am dealing with 1 part in ~450,000. It could be less. That is an incredibly small change in fr and individual harmonics, and IMD products. Thus +/- 0,1 db from 20-20khz means nothing. By the way, Olson’s research concluded, which I concur, that the higher the harmonic number, the more sensitive the ear is, at least under some conditions.

    However, as mentioned below by another poster, the design will most likely never be totally accurate due to parts quality, in the power supply and around the active parts. This is especially true with power supply filter capacitors, the last being foremost of the caps in the signal circuitry. 

    The power supply filter capacitors and especially the last filter capacitor, has problems with ESR, DA, termination techniques etc. due to its large size. Here is a link/article, written by Walter Jung and Richard Marsh, that gives some general idea of the problems.


    Tube circuits also have problems as well, but smaller filter capacitors does lessen the ESR and DA problems. With that said, using specialized listening tests has confirmed that PP AB1 amps can have perfect sound, except possibly near the 20 hz level, where the bass could be slightly less. However, as the article mentions, SS virtually never passes the test in accuracy.


    Steve Sammet (retired)
    SAS Audio Labs
    ps. The pages are up for those who wish to gain some information on used components that might come up for sale (Which virtually never does). Nothing is being manufactured, retirement.

    • dermot 2018-06-05

      I would gently disagree.  Class A is when the IDLE current in the output is always sufficient to accommodate the maximum drive requirement.  Class B is when the drive current is supplied only on demand and the output transistors only have a very small idle current. Class AB is halfway between, running in class A for small signals and in class B for large signals. 

      This is a genuine class B, with an idle current as low as 15mA and a 10A+ full-load current (depending on the load impedance)

      • sasaudio 2018-06-12

        I am not sure what you are disagreeing with Dermot. I never mentioned anything about Class A, AB, or B except to say that Class AB amp can be perfect sounding, and that SS amps are not sonically accurate; this via special listening testing.

      • fuzzball57 2019-09-25

        Sorry to butt in so late. My $.02. Unless I missed something, a Class A amplifier is characterized by continuous current flow regardless of signal level. In that sense, all CE amplifier devices are class A. That applies to the output section as well.
        A class B amplifier has, ideally, no current flowing with no signal and only when the input goes +/- over a threshold does the output activate. In a perfect world, that threshold would be zero.
        A class AB amplifier works like a class B except that a small idle current is present so the output devices are always on (operating in the linear region) like a class A circuit.
        After many years (45+) experience, I have to agree with SAS that SS amps always have deficiencies.
        A final note, in the old days semiconductors were made “sloppy” (by today’s standards) and the amplifier designs, in a sense, took advantage of that (e.g., slower slew rate). SS device technology has improved considerably with smaller die sizes, faster speeds (largely due to reduced geometries), tighter optics in wafer fab, etc. One has to be careful in new designs because the faster devices can respond, um, differently in the same circuit as an older device. Others have noted this issue (my references escape me, but they are admittedly subjective).
        Still, all in all an informative article. THanks for posting.

  • starliner2000 2018-05-12

    Good article, but you should probably add that this topology is considered a separate class from A and B, and is called the class AB amp.

    • dermot 2018-06-05

      Sorry - placed my reply in the wrong comment - see above.

  • trys 2018-05-12

    A very well written article, thank you. I only woke up a couple of hours ago, and after reading it I’ve already learnt a few new things. The idea of presence is a compelling one, and I’m sure many of us now are itching to build it and have a listen. I can only imagine was an incredible guitar amplifier it would make too.

    • dermot 2018-06-05

      We used a phase splitter to operate two amplifiers in “push-pull” to get about 70W for a microphone amp. Quality was extremely well received I’m told.

  • elef 2018-05-12

    I would like to have LTSpice files.

  • jfstrama 2018-05-12

    Dermot, please send me a copy of the LTSPICE file when you get a chance. Thanks!

  • kcirtaP nnamlegoV 2018-05-12

    I am also very interested in your LTSpice files! I would highly appreciate if you could share them with us.

  • chidmf 2018-05-12

    Would also be interested in the LTSpice file. Thanks!

  • garyspang 2018-05-12

    Great article Dermot
    Please send me the LT Spice file. I may need more power for an already very clean sounding Carver 6ft ribbon speaker pair of 4 ohms impedance. Any thoughts?
    Gary KF4GGK

    • dermot 2018-06-05

      See my comment above about using two amplifiers in push-pull…

      By sourcing high-voltage transistors you could increase the power-supply to 80V easily, but building gets more challenging.

  • rherber1 2018-05-12

    Jan Didden’s website https://linearaudio.nl/power-amp-design notes includes a later Wireless World article by Mike Renardson on his Class B design - “Class B in a New Class”.
    https://linearaudio.nl/sites/linearaudio.net/files/renardson new class B.pdf

    Mike Renardson’s website can be found here https://linearaudio.nl/power-amp-design

  • Rhogst 2018-05-13

    Very good article, Dermot! I am interested in building this circuit, could you please send me the Spice files? Thank you!
    One other question… which is the best way to power this amp for a power supply? (From 110V 60Hz to ...?)

    • dermot 2018-06-05

      The power supply I used was always a 60V 10A simple supply.  Surprisingly it doesn’t seem to matter to the sound.  As I said above, with high-voltage transistors (100V are easily obtainable) you can increase the supply to 80V.  (Power is Vsq / R )

  • sasaudio 2018-05-13

    I wish I could simply bypass the inaccurate information provided, but the designer’s article has some problems that need addressing.

    1) The rise time/fall time of the amplifier is Not the only rise time we have to deal with in the audio system. We have the source, preamplifier, and speakers as well. As such, the audio system’s rise time/fall time becomes much longer than just the amplifier’s value. For simplicity, if the frequency response of the amp is -1db at 25khz, and the preamplifier’s is -1db at 25khz, the fr of the two components is -2db at 25khz, and -1db in the teens khz. As such, we have altered the rise time of the system, which is perceivable.

    Two studies demonstrated that the ear perceives rise time changes of at least 5 microseconds (us) 66.6khz, Jneutron (FermiLab, CERN, Brookhaven National Laboratory PhD EE) claims to have read a study of 2us, 166,66khz. I personally have altered the -1db from 200khz to 150khz back in the 1980s, individuals have perceived the sonic difference.

        A. Dr. Kunchur’s 5 year study involving Medical Auditory Experts, Universities, Industry, Not just EEs, have concluded at least 5us, if not less.
    http://boson.physics.sc.edu/~kunchur//Acoustics-papers.htm  (By the way, the paper has not been debunked.)

        B. https://www.physiology.org/doi/10.1152/jn.2000.83.6.3548 (Oohashi study)
    ” Department of KANSEI Brain Science, ATR Human Information Processing Research Laboratories, Kyoto 619-0288;
    Department of Network Science, Chiba Institute of Technology, Narashino 275-0016;”

    ” Department of Brain Pathophysiology, Kyoto University School of Medicine, Kyoto 606-8507;
    Biomedical Imaging Research Center, Fukui Medical University, Fukui 910-1193;
    1 Department of KANSEI Brain Science, ATR Human Information Processing Research Laboratories, Kyoto 619-0288;” etc, etc, etc, etc,

    2) Next, to keep things simple, we have the problems of ESR and DA.
        A. ESR is a series of resistances with intervals of capacitance between each resistor (theoretically infinite), thus the ESR can Not be linear.
        B. The DA voltage of a capacitor varies over minutes, not microseconds. As such, it can vary the bias of an active device, depending upon the impedances involved, and insulating material used. Electrolytics are normally in the vicinity of 4% or so DA.

    3) Next, the fr influences not only the overall frequency response, but the amplitude relationship between the fundamental and harmonics. The ear is extremely sensitive to such, the higher the harmonic the more sensitive the ear.

    4) One could not perceive a sonic difference between capacitors, obvious either the audio system is quite inferior. By the way, Walter Jung’s article clearly demonstrates how inaccurate large electrolytic capacitors, used in the power supply, are. The last one or two filter caps are clearly in the signal path. The output electrolytic capacitor is so so, but not exceptional.

    5) Did I miss any mention of layout to prevent channel separation problems? It is important as in a typical home setup, the sonics will not mimic the lab measurements.

    I don’t mean to be so ruff, and one can build the amp, but please don’t expect totally accurate performance, because it won’t.


    Steve Sammet
    SAS Audio Labs (retired)

    • dermot 2018-06-05

      I agree with all your comments Steve.  For example I was very unhappy with the output capacitor but we had designed a board for work at the time so easily tried it with audio.  NONE OF THE ABOVE MATTERED when listening!  We did build each channel on a separate board but used a common power supply which is a no-no.  It didn’t matter!  In the early 1970s pickups and loudspeakers were not brilliant.  It didn’t matter!
      The sound was pure and two of us mistook a record for real life simultaneously.

      • sasaudio 2018-06-12

        Glad to hear of your findings Dermot. The difference between our findings might lie in that I am in a lab situation, with excellent acoustical surroundings etc. In testing my speakers,
        a 1 part in 400,000 change in resistor value across the full range driver (extremely small change in frequency response) was quite perceivable.

        Cheers and no harm meant.

        steve sammet (retired)
        SAS Audio Labs

        • dermot 2018-06-13

          I’m not a professional audio designer so my experience is very very limited compared to you.  I defected to computers in 1972 and dabbled in audio thereafter. 
          I can only comment on my observations of ordinary people listening to the Blomley. 
          The music demanded total attention and talking around the sound was extremely difficult even for musical philistines.  I used to put a (very good) tape-casette recorder in the signal path if I wanted background music. 
          I know the difference that a proper environment makes for hearing detail and often wished I had an anechoic room, antisocial though it might be, to be able to compare sound quality.  Even in a noisy environment I could clearly hear the difference in cheap and decent pre-amp cables.
          Thanks for your input.

  • Analog_Tim 2018-05-14

    Most interesting article |Dermot. Could you please send me the LT Spice files?

  • rcjcarr 2018-05-14

    Thanks for the article; very informative. If you could send me the spice file that would be appreciated. Looks like an interesting circuit.

    • etippelt 2018-05-14

      Yes please for a copy of the spice file…

  • mszoke01 2018-05-14

    Great article!  It is inspiring me to build the circuit and test it.  Let us know when you upload the LTSpice files.  Also, a recommended BOM including some suggested alternate components would be good.  Some of the Transistors listed in the circuit diagram are obsolete and are tricky to source.

  • Chrysostum 2018-05-15

    Could you please email me the schematic and parts list?
    Thank you,
    great article.

  • Chrysostum 2018-05-15

    LT Spice Files will be fine, thanks.
    All the best

  • genrose 2018-05-15

    i would like the lt spice files too.. how would i get it?... by the way..i would attempt to design it using a bipolar supply too!

  • sparkoid 2018-05-16

    Thanks for the interesting read. Could I please get a copy of the Spice file also. Very interested to build this amp.

  • igninja 2018-05-17

    Dear Dermot, very interesting article! Please send me LTSpice file. I will design PCB for it and build one.

  • andythompson 2018-05-17

    Great article. Can you send me the LTSPICE circuit file?

  • pleriche 2018-05-17

    An interesting article, and I’ve long thought intermodulation distortion, not harmonic distortion is what causes a muddy and indistinct sound. But I don’t think you can have one without the other as they’re both produced by any non-linearity.The classic example is the extreme non-linearity of a diode used to demodulate an AM signal - all it’s doing is creating sum and difference intermodulation products between the carrier and sidebands, which, of course, include the audio signal. So if an amplifier truly has negligible harmonic distortion it can’t produce intermodulation distortion.

    Also, I’m not quite convinced that the time the feedback signal takes to get back to the input can be a factor with transistors having an Ft orders of magnitude beyond the audio. I confess that my grasp of poles and zeros in my graduate control systems course many years ago was less than complete, but any significant delay would surely cause instability.

    But I’ll stop short of dismissing it all as snake oil. Lab tests of THD using a continuous sine wave from a signal generator are unlikely to reflect real world performance. Transients may cause the the power supply to dip due, for example, to capacitor ESR. And when did you ever see a THD figure for those horrible analoggy things that covert your finely honed electrical energy into sound? In fact, capacitors, too, are non-linear, which is why the dream of supercapacitors replacing batteries has failed to materialise. (At high electric field strengths the energy stored by a dielectric no longer increases with the square of the voltage.) So I suspect the debate will go on.

    • dermot 2018-06-05

      The Blomley amplifier has less than 0.01percent harmonic distortion WITHOUT ANY FEEDBACK APPLIED.  And the amplifier appears not to change the distortions in the signal from a signal generator.  Just listen to it.

  • Emilio Pérez 2018-05-18

    Very interesting, I’d also appreciate LT Spice files. Thanks in advance.

  • oldenburg 2018-05-19

    Nice Article!
    I created the LTspice simulation from the picture in the article. Ithink there is an error in R22 and R23 with the value of 1m. LTspice will interpret this as 1 milliohm.
    For soft clipping maybe 1 meg would be better? Then “1meg” should be used.
    Using 0 milliohm as default ESR in caps and power-supplies lead to overoptimistic simulations. Insert some realistic number here.

    Power supply caps ESR plus the source impedance of the power supply generates a load dependent voltage that will find its way the the loudspeakers. This is seldom discussed. Try inserting this in your simulation. In real life you could load the amp with a power resistor and tap the signal in the power supply, maybe to another amp and listen to, or measure, the junk from the power supply.

    The audiophile area is a minefield of facts, opinions, and emotions. It is often hard to sort thru what you hear and read. Much of the discussions of amps etc are trying to separate facts from emotions and with marketing efforts included, to make life easy. Professional musicians one should think have the best taste in selecting sound reproducing equipment but the reality is different.
    My opinion is that musicians listens to music and audiophiles listen to what is wrong with the music.

    I think making a little PCB for this amp would be a snap. Blomley had some real balls when he said the amp was too good for the common people.

    You can measure all kinds of things in audio signals, but no instrumentation exists that I know could find out if a music piece was played badly, which the ear could find instantly. Sound reproduction equipment are measured and quality graded with instruments that are available, regardless of relevance, with criteria carried over from the vacuum-tube era.
    Strangely enough the old tube stuff sounded good compared with early transistor amps. They measured horribly compared to solid state amps but still sounded better. The challenges with tube amps have now been easier to solve. Power supplies can be made much more robust and quiet. DC-powering filaments is no problem either to help getting rid of hum.
    Making a good tube amp will be expensive, output transformers and the tubes are not cheap. The reward could be good. Tubes have a much simpler transfer function than bipolar transistors and simpler to explain. 
    Either way you will have fun.



  • oldenburg 2018-05-20

    I ran some simulation with LTspice and found out some things.
    The 5KHz simulated signal source is rife with harmonics. Surprise! Stronger odds than evens.
    The soft clipper works OK with anything from 1Meg to 640K @  R22,23. Even order harmonics are generated to match the odd ones when approaching saturation.
    Removing the R6 feedback resistor lowers harmonics 20dB, and matches the input source.
    Power supply rejection is -55dB to the output.
    I designed a 10x10cm PCB a Sunday afternoon if anyone is interested in stuffing it. I found some interesting 100V 15A 4MHz Darlingtons for the output. I added an extra set of these for more current capability. Ballast resistors may be needed for even current sharing, perhaps, but part tolerances and using the same heatsink may help the match.
    Other fatter complementary output transistors are available in Darlington config with 120V 25A 50MHz for a few bucks more.
    This circuit lack any kind of protection, like fuses, inductor in the output. Maybe not needed?
    The input is not symmetric like later amps, and overloads asymmetrically. Upper TR7 base drive is 17 times lower than the TR8 bottom half.
    Lots of stuff to look at.

    • mszoke01 2018-05-21

      Interesting simulation results.  I have also made my own LTspice model based upon the picture in the article.  Note that there are also a few type-o errors which could cause some confusion (e.g. R9 should be 6.8k, R3 should be 4.7k). I tried 1m, 1k and 1Meg for R22 and R23.  I checked the picture in this article against the original 1971 circuit diagram and note the two added transistors TRP1 and TRP2.  Think I modelled the circuit correctly but there seems to be an error in my model cannot get stable output signal.  I would like to compare LTSpice models.  There is a LTSpice Yahoo Group. Is anyone interested in collaborating there on this LTSpice model?

      • oldenburg 2018-05-21

        “...Note that there are also a few type-o errors which could cause some confusion (e.g. R9 should be 6.8k, R3 should be 4.7k)...”
        Not a typo per se, but an alternative way of expressing a decimal point, this is commonly used as a dot may disappear in small print.

        The “..RP…” labeled parts belong to the soft clipping circuit not included in the 1971 Peter Blomley schematic.

        My simulation runs without any problems, look for errors.
        This is not not a “model” but a simulation schematic. “Models” are the individual part used in common lingo.

        Yes. join the Yahoo group. I have an account there. They have loads of models you cannot find elsewhere.

        The sim will run much faster if you skip the initial DC operating point. The startup includes a spike that will pummel you eardrums. Back in the day relays where used for outputs. Also temperature compensation diodes etc. I sorta lost interest in Transistor amps as the tube amps sounded better. This could be a fun little circuit to play with.

        Another transistor amp I heard in the early 70’s was the Spectra Sonics 701, which sounded really good and also had an excellent reputation in pro circles. It has some similarities with this and is a more classical OP amp differential design. It required the use of one input audio transformer per channel, but it could easily be hooked up in a bridged config, bi-amping and such could be easily done. PNP power transistors were not so good back then and that amp used a quasi-complementary output, with six TO-220 Darlingtons, which you still can get.
        Those amps where on plug in cards and gobs of them could be stuffed in a chassis.

        • mszoke01 2018-05-22

          Thanks for the tips and lingo corrections.  I checked my simulation schematic and there were no schematic errors.  As you suggested I deleted the startup input spike (i.e. changed “.tran 0 1 0 startup” to “.tran 0 1 0” and deleted “.include relay.lib” and it worked!  The circuit amplified the input sine wave +- 3mV to +-300mV.  Thinking I can now use this model to figure out each resistor power ratings needed.  Any tips on choosing best resistors or tips on what small signal diode can be used to replace the Germanium diode (i.e. 1N34A)?

    • Darren Taylor 2018-05-22

      If you shoot me the files, I’ll throw a couple together and measure the performance with my (admittedly) pedestrian (HP/Agilent/Rigol/Instrument and Measurement mics) gear. I started laying out a PCB but had to shift gears to a more time sensitive project.

    • dermot 2018-06-05

      I found that the “soft clipping” didn’t work with modern high beta transistors.  Worked like a charm in real life in the 1970s with beta-20 power transistors!  So later versions of spice model left out these transistors. With sufficient beta in the transistors, the feedback overcomes any form of clipping. 
      R22, 23 were supposed to be 1 milliohm.  Supposed to short out the drive to the power-tripples.when either the current or voltage combination got too big.  Too crude.

      Careful of even Darlington transistors on the same die.  Seems to introduce some non-linearity compared to discrete devices???

  • kenif 2018-05-24

    Schematic and PCB layout completed and would appreciate feedback.
    The idea was to come up with a mono development board using mainly through-hole devices. There are SM cap pads on the back in parallel with the electrolytics. For the output pair I’ve chosen OnSemi DH44V10 and DH45V10 (I assume these are inherited from Motorola). These are pretty close to the originals, but there are several other options. The rest of the transistors are Diodes (née Zetex, old favourites and still a wide range of bipolar). 
    It’s a long thin board at 2 x 7” (50 x 178 mm). RCA phono input on the left; power and speaker out on the right are by terminal blocks. The output pair mount on a small TO-220 heatsink, which should be enough for development. BOM to follow in a couple of days, plus a page to discuss the design decisions here: http://www.thundernerd.com/amp/index.html
    Here is the schematic: http://www.thundernerd.com/amp/Amp-3.pdf
    And here is the layout: http://www.thundernerd.com/amp/Amp-A.PNG
    If this works out I will do a stereo version with large heatsinks - this version is for playtime. Comments, complaints, and suggestions for improvement are welcome: .(JavaScript must be enabled to view this email address).

    • Darren Taylor 2018-05-25

      Much thanks. As mentioned previously, I started laying out the PCB but was pulled away for some contract work. It’s amazing what you give-up when someone offers you a nice contract gig! I will have time this weekend to work on my layout and check out yours, too. I might pass on finishing my own design in favor of yours after I have a chance to look at it. That would be nice as I would likely be able to build it up this weekend, also. Now that would be a productive long weekend!

      • dermot 2018-06-05

        How can I contact you to leave Spice files (which have been sent to the Editor already) or where can I upload them to?

  • a1c3m49 2018-05-25

    I would like the LTSpice files too.

  • johannesrexx 2018-05-25

    May I have a copy of the SPICE files please?

    • boyet0173 2018-05-25

      Hi all,  have someone working pcb layout?  Interested to build.  Thanks,

  • sreaves 2018-05-26


    I would like to have those LTS files.

  • Rob_mx 2018-05-28

    Nice article. Could I get the LTS files?

  • kapplasse 2018-05-29

    Very interesting as a DIY-Project.
    I hoping for the same musical revelation as when I built the Kaneda preamp with the 2SK30AGR transistors.
    A couple of questions though:
    -The pots, R20/R21, they are 500 Ohms right? What is the meaning of Rtap=1k and tap=0.5? Is .61 and .39 some
    kind of percentage figures?
    - I was thinking of using BDX33C/34C as Power transistors, are those OK to use?


    • dermot 2018-06-05

      Sorry - in the simulation at the time I used a coupled resistor pair to simulate a pot.  And the .39 was the wiper percentage to adjust the idle current to 20mA by trial and error.  It’s supposed to be a 500ohm preset pot.

  • Bob.Campchero 2018-05-29

    Thank you for writing the article.  This will be my first DIY attempt.  Please send the LTSpice file, and any information on parts to replace the obsolete items, and the potentiometer.

  • kenif 2018-05-31

    The boards have been ordered, extras available.
    Will keep you up to date as the build progresses.
    Here’s the story so far:

  • stakata 2018-06-01

    I’m a bit late to the party, but could you send me the LTSpice as well?  I’d like to explore the simulation.

  • haiqu 2018-06-02

    Spice file please.

    • dermot 2018-06-06

      Spice files are now attached - see the end of the article. 

      If your simulation needs extra files I’ll add them.  Let me know.

      • oldenburg 2018-06-11

        The schematic file has several errors:
        Diode D3 is missing,
        C5 off by an order of magnitude,
        Transistors are all the ‘wrong”  (B) beta group.

        Feedback resistor R6 seems to increase harmonics 20dB and lower gain ~ 1dB.

        • oldenburg 2018-06-11

          ...and the transistor types on the output are unspecified and incorrectly connected.
          Sim runs OK after fixing these.

        • dermot 2018-06-21

          I hang my head in shame…  The lower output circuitry is a complete mess.  Apologies.

          D3 is indeed missing

          C5 is correct as Peter Blomley specified and it gives ~100kHz 3db point.

          R6 appears to virtually remove the distortion of the output capacitor and as Blomley says in his article, “R6 generates another (feedback) loop effectively placing a damping resistance across the [apparent] output inductance.”

          beta seems to be very unimportant for functioning in this circuit, but I haven’t checked in detail.  Apparently, compared to then available 1970 transistors, all modern transistors have sufficient gain for the purpose.

        • oldenburg 2018-06-22

          The simulation self-oscillates with 330pF with both B and C group, but runs stable with 33pF. Up to 120pF works, but some gain peaking at 2 MHz.
          -3db @ 1MHz with 33pF.
          Changing to B group has a drastic impact on gain bandwidth.

        • dermot 2018-06-25

          The simulation I’ve just posted above doesn’t seem to oscillate with 330pF but it does seem to hinge a lot in the choice of transistors.  I chose LTSpice-available transistors without reference to more than the voltage and current capability just to get the simulation working. 
          Remember that the circuit was designed for 1970’s transistors - a power transistor with a beta of 30 was more than adequate!

        • oldenburg 2018-06-26

          The link to simulation file looks like it has not been changed.
          Stability greatly depends also on the choice of transistor models. 
          Feedback resistor 22k affects the current sharing between upper and lower half of output. I played with beta groups to get a better current balance.
          I’m not sure its real or not, but some MHz switching artifacts exists at the input transistor base current.

  • suikerbuik 2019-02-14

    Very nice articel.
    Ik am also over 40 years developing Blomley amplifiers and enjoy listening to them.
    I am willing to share the scheme and spice data. Please let me know.

    Great to hear an explanation for the great sound.

    • madsl 2019-05-05

      Hi. I am looking into making my own Blomley amplifier. Could you share the schematic you are using?

  • court4audio 2019-03-31

    Thanks for the great article. I am always seeking ways to make better sounding solid state audio power amplifiers. The better designed tube amps tend to sound superior. Have you compared your amp to the best tube amps?

  • eloco 2019-05-14

    A very interesting article especially the parts about the missing presence. I have been looking at germanium transistors’ and rectifier diodes recently and have read some articles that suggest a far more life like sound from a good design amp that uses germanium especially when it comes to the human voice. Have you ever thought of building this amplifier using germanium semiconductors? It is something I would like to try myself but at the moment I lack the knowledge.