TL071 distortion - bad amplifier design?

[AngraMelo]

I see! So you are talking about this circuit (attached)
Ill make all the changes.
What should be the voltage ratings of the 100uf caps?

[spec]

UPDATE #1  2018_12_12 (please see reply #79)

Spec, I did only the following modifications:
- Changed the 2 4.7nF for 2 47uF capacitors
- Added a BC337 between the collectors of both VAS transistors
- added a 1k resistor between 1015 collector and BC337 base
- added a 1k pot between 1815 collector and BC337 base
- added a 100uf cap across the BC337 collector and emitter
I believe that is all that is included in the schematic I posted on reply#71, am I mistaken?

No, not mistaken. That is correct but, I now think that you need to do all reply #44 mods- I am not able to determine the VAS current of the original design. Perhaps you could measure the VAS current of the original design.

Do I need to short the input to measure the current?

No, but it would be a good idea.

[Zero999]

I see! So you are talking about this circuit (attached)
Ill make all the changes.
What should be the voltage ratings of the 100uf caps?

The ones on the 15V zener diodes should be rated to at least 20V, to allow for the fact that the zener diode voltage might be higher than 15V.

The one on the BC556 can be a much lower voltage rating, say 6.3V, because the voltage across the BC556 is a couple of volts at most.

Of course higher voltage rating capacitors can be used, so 25V, 35V etc. is fine for all 100μF capacitors in the circuit.

[spec]

I see! So you are talking about this circuit (attached)
Ill make all the changes.
What should be the voltage ratings of the 100uf caps?

Yes, that is the correct circuit. The second added transistor can be most small signal PBJTs.
The 100uF cap only needs to be 4V7 or more. The 100uF capacitor only has 1V2 across it in normal operation of the amp.

Apologies for causing you extra work.

[spec]

I see! So you are talking about this circuit (attached)
Ill make all the changes.

This is an update on the subject of the original schematic VAS current (now that I have slept on it).

When I proposed the BC337 VBE multiplier for use with the original VAS, I had assumed a VAS current of 5mA to 10mA, because that sort of current is normal for a VAS and I thought I had also calculated the VAS current to be around 10mA. But that is obviously not the case because, after just doing an analysis, I find that the VAS current is in fact indeterminate, but a low value in any case. This would explain why the VBE multiplier did not work with the original VAS.

So just to confirm, I would advise to get your amplifier to the reply #44 schematic build state and I will now leave that schematic as is and not update it further so that it acts as a reference point.

[AngraMelo]

Hey Spec,
I did not have the BC556 on hand, I just ordered some. As soon as they get here Ill make the changes

[spec]

Hey Spec,
I did not have the BC556 on hand, I just ordered some. As soon as they get here Ill make the changes

Well done.

Practically any small signal PNJT will do for the top of the VAS.

Practically any small signal PNJT with a VCE of 45V or over will do for the level-shifter.

[Mark Hennessy]

As the PDF is still open on my computer...

Earlier the importance of PCB layout was mentioned. I appreciate that this is for much later, but the attached screengrab from the book illustrates just one cause of distortion caused by PCB layout - so-called "Distortion number 7: NFB takeoff point distortion" by Self.

Quoting the numbers given in the text, the distortion rose from 0.00097% to 0.0027% when making this mistake. We're only talking about a few mm of difference in terms of where the connection was made.

Obviously this won't be audible, but as I said in an earlier post, it's satisfying to avoid traps like this when the fix is free and easy, and doesn't cause any other harm.

Another example: using the ubiquitous LM3886, it's possible to make a pretty decent amp with 1kHz distortion in the 0.003% region. My lash-up - detailed here: https://www.markhennessy.co.uk/microamp/ - achieves this, despite the crude construction (I used an LM4780, which was a dual LM3886). A colleague designed a PCB to employ a pair of LM3886s, but the distortion was huge - from memory one channel was around 1% and the other was more like 0.1%. He had a hard job believing that the PCB layout could be the cause, despite the differences between channels, which was a pretty major clue, but agreed to humour me and revise the PCB. He incorporated most of my suggestions, but not all. Result: much improved distortion, but still not 0.003% and still different per channel. After some track cutting and bodge wires to implement the last of the changes I suggested we did get it very close to my version in the end.

One day I'll write all this up on my website, along with photos and measurements. It can be really quite dramatic - and is solid engineering that is very definitely not in the realm of audiophoolery. Capacitor choice can fall into that category, however. Check out DIY Audio if you want to see folk who believe that properly implemented passive components sound radically different - and if you want to know more about capacitors specifically, seek out Cyril Batemans landmark work on this.

All that said, I'll repeat my earlier recommendation - you won't find a better book on the subject than Self's. If you understand the basics of how transistor circuit work, then do yourself a favour and give it a go. Even the old PDFs that can be found on-line are great - later editions expand on the subject, but don't contradict what was originally written, and that's because it was very thoroughly researched right from the start.

[David Hess]

All that said, I'll repeat my earlier recommendation - you won't find a better book on the subject than Self's.

Bob Cordell's book might be better but they are both excellent.

[Mark Hennessy]

All that said, I'll repeat my earlier recommendation - you won't find a better book on the subject than Self's.

Bob Cordell's book might be better but they are both excellent.

I have Bob's book, and agree that it is pretty good. Personally, I think Self's is better; it goes deeper and is more rigorous. And yet manages to be more readable - to me, at least. Individual taste will obviously be a large factor.

Of course, Self has had a lot of criticism over the years because of his strong anti-subjectivist stance. Not surprising, I guess. Rational audio engineers will always have an uphill battle when threads like this exist: https://www.diyaudio.com/forums/chip-amps/40676-try-potentiometers.html 

[AngraMelo]

Hey Mark,

Keep in mind that Im a noobie at all this (electronics).
So the amplifier's resistors type would have no impact on the sound?

I cant talk at all about the impact a very specific part of the design of an amplifier, what I can do is this:
As a musician that has been for 20 years listening to a very specific recording, one can get extremely familiar with the particulars of how it sounds and be able to perceive very small nuances. How scientifically small? I dont know. It was initially a song that I really liked nowadays I use it just to test speakers and amps. But I sometimes can hear the difference from the same model of amp/speaker if they are in the same room and position.
Look, most engineers would laugh (as they have done to me) at you if you say that if you wanted to buy a guitar amp, and had all the money in the world, a tube one would be brought home.
They know it has more THD than solid state but the sound at the end is what matters. I get that it must be frustrating to listen to the enormous amount of bullshit that surrounds this subject but is the example you gave on the link measurable? Correct me if Im wrong but if you wanted to see if a pot at the beginning of the chain affected the frequency response wouldnt that be easy to measure?

[Kleinstein]

The resistor types can have some effect, but usually only very little and the loudspeaker will have a much larger effect as it's resistance is made of copper.  So at the very high end (not this circuit) things like self heating of some resistors (e.g. the gain setting ones) can have a little effect and cause distortion. Another possible effect could be noise from some resistors that seen an sizable DC bias. However a good circuit often avoids this problem by not having a DC bias at noise critical resistors. Without a bias the noise of a carbon resistor is not higher than an expensive metal foil resistor. So there is quite some audio-foolery going on.

Similar with capacitors - usually no need to absolutely have PP caps in an audio circuit. The cheaper polyster caps add some small phase shift - but this is more like equivalent to having the speaker moved a fraction of a millimeter.  It a common misunderstanding to think that dielectric absorption would cause distortion - it's a linear effect and only gives a minor effect on phase and amplitude, usually no harmonics.

It may be a little overreacting, but if one sees specs for PP caps in an amplifier, expect over-specified parts. 

[Mark Hennessy]

Hey Mark,

Keep in mind that Im a noobie at all this (electronics).
So the amplifier's resistors type would have no impact on the sound?

I cant talk at all about the impact a very specific part of the design of an amplifier, what I can do is this:
As a musician that has been for 20 years listening to a very specific recording, one can get extremely familiar with the particulars of how it sounds and be able to perceive very small nuances. How scientifically small? I dont know. It was initially a song that I really liked nowadays I use it just to test speakers and amps. But I sometimes can hear the difference from the same model of amp/speaker if they are in the same room and position.
Look, most engineers would laugh (as they have done to me) at you if you say that if you wanted to buy a guitar amp, and had all the money in the world, a tube one would be brought home.
They know it has more THD than solid state but the sound at the end is what matters. I get that it must be frustrating to listen to the enormous amount of bullshit that surrounds this subject but is the example you gave on the link measurable? Correct me if Im wrong but if you wanted to see if a pot at the beginning of the chain affected the frequency response wouldnt that be easy to measure?

Broadly, there are two schools of thought in amplifier design:

1. Design an amplifier that has medium to high amounts of harmonic distortion, but play around until the circuit produces a particular spread of harmonics that is subjectively "nice".

2. Design an amplifier that has low to very low amounts of harmonic distortion, and then you don't have to worry about how the distortion sounds because it's utterly inaudible.

Perhaps unexpectedly, option 2 is much easier.

Option 1 is for guitar amplifier designers. And similar things used in the creative process - for example, a recording engineer might favour a particular vintage valve/tube microphone preamp because of what the distortion adds to female vocals. Or perhaps part or all of a mix might get "bounced" via a 1/4" tape machine to add some "analogue warmth". And I have no problem with any of that - it's all part of the performance.

However, when it comes to hi-fi, some might favour valve amps, or perhaps analogue replay media like vinyl, because of the subjective change in sound quality that these impose on the signal. OK, that's a personal choice, but my view is that I prefer to hear what the recording engineer wanted, without adding my own flavour of "niceness" to it.

In terms of how much distortion is too much for a home hi-fi, that's a difficult question to answer because not all distortion is the same. There are many different distortion mechanisms in an amplifier, and they result in different characters. But any well-designed solid-state amplifier will have less distortion than the loudspeakers, and probably less than the microphones too. So it's difficult to know what you've reached the "good enough" point. Better than 0.1% is probably a good rule of thumb - that means that the harmonics will be less than 60dB below the level of the fundamental, and if you research how the human hearing system works, you'll see why that seems like a reasonable number. Also, it's spectacularly easy to come in at well below that target. I mentioned 0.003% from an LM3886 earlier - that's 90dB of separation.

This is a good presentation from a guy from Audio Precision who (informally) demonstrates the audibility of distortion. It's very interesting.



I said earlier that there are folk who believe that properly implemented passive components sound radically different. They do not. The key part of that sentence is "properly implemented". Douglas Self has written about this in his book "Small Signal Design". Improperly implemented passive components might result in higher than ideal levels of harmonic distortion, but whether that's genuinely audible in strict AB or ABX testing (with tightly matched signal levels) is up for debate. Unfortunately, people don't just listen with their ears 

If you're new to all this, the best thing you can do is immerse yourself in books from Self and other rational authors. My biggest regret is paying far too much attention to all the BS when I was very much younger. I became a teenager in the '80s, when the subjectivist movement was gathering serious momentum and every hi-fi magazine that I enthusiastically absorbed was spouting utter nonsense about cables and suchlike. As an engineer, I'm lucky to have some out the other side of all that - many people get trapped there for ever.

[Zero999]

I've done a quick and dirty simulation of this circuit. The original version wouldn't work because the 220R resistors didn't give the transistors in the second stage enough voltage to turn on, so I increased them to 1k. I used a V_BE multiplier, similar to that proposed by spec. In real life a potential divider would need to be used to adjust the operating point.

[edit]I was wrong. It works when proper transistor models are used, rather than the generic ones. I think it's because the beta of the internal, default models was too low.[/edit]

I've used generic models for the components, so the simulatio isn't very accurate, but it gives the general idea of how this is supposed to work.

[spec]

The resistor types can have some effect, but usually only very little and the loudspeaker will have a much larger effect as it's resistance is made of copper.  So at the very high end (not this circuit) things like self heating of some resistors (e.g. the gain setting ones) can have a little effect and cause distortion. Another possible effect could be noise from some resistors that seen an sizable DC bias. However a good circuit often avoids this problem by not having a DC bias at noise critical resistors. Without a bias the noise of a carbon resistor is not higher than an expensive metal foil resistor. So there is quite some audio-foolery going on.

Similar with capacitors - usually no need to absolutely have PP caps in an audio circuit. The cheaper polyster caps add some small phase shift - but this is more like equivalent to having the speaker moved a fraction of a millimeter.  It a common misunderstanding to think that dielectric absorption would cause distortion - it's a linear effect and only gives a minor effect on phase and amplitude, usually no harmonics.

It may be a little overreacting, but if one sees specs for PP caps in an amplifier, expect over-specified parts. 

I notice that you were absent when the OP asked why his amplifier wasn't working. If you are such an expert, why didn't you point out the problem, which was glaringly obvious even from a cursory inspection of the circuit.  And now that has been fixed you are, once again, on the band-wagon passing judgement from on high and, once again, in a negative and contradictory fashion.

You are making some pretty sweeping statements there, and with what purpose. What does 'there is no need' mean
And are you the arbiter of what is needed and what is not needed? You keep saying things are expensive like metal film resistors- this is not true, not in the order of things. All my low wattage resistors are metal film, and that is the complete range- they did not cost that much.

About what you need and what you don't need. Well that all depends on your own personal preferences and perception. There are no hard and fast rules, like you are implying. One person may be happy with the sound of a TV and another may only be happy with the sound of the very best high end system. Also the sound of a TV may be acceptable for listening to the news, but not serious listening to music.  And there are no morals about this, the important thing is to get a system that you like the sound of. And, in your case, if you are happy with carbon resistors and aluminum and polyester capacitors in the audio signal path  good for you, but don't keep lecturing and forcing your view on others? By the way, what is your audio equipment. Can you describe it so we know what your level is. If you like music and have never listened to a high end system you are in for a shock, of course you may have cloth ears in which case it would be wasted.

The other thing is that you need to match your system components. If you have cheap low-definition speakers, a cheap low-definition amplifier will be compatible and will produce a cheap low-definition sound. But if you have high-end speakers that same amplifier will sound awful. I have already established that the OP is a musician, has good sound perception, and has high-end speakers and other equipment.

About distortion and fidelity.  The human sensory system is not fully understood and is strange. For example second harmonic distortion can sound quite pleasant while just a sniff of third harmonic distortion will sound awful. And the distortion produced by amplifiers does sound  bad. The odd thing is that speakers have a relatively high level of distortion, even the best, but they sound OK, and even the human ear distorts.

And finally, if you are so worried about costs, the answer is to buy a ready-made amplifier from ebay for around £10UK or, failing that, buy one of the audio power amplifier chips for about £2.50.

[spec]

As a musician that has been for 20 years listening to a very specific recording, one can get extremely familiar with the particulars of how it sounds and be able to perceive very small nuances. How scientifically small? I dont know. It was initially a song that I really liked nowadays I use it just to test speakers and amps. But I sometimes can hear the difference from the same model of amp/speaker if they are in the same room and position.

This is the crux of the matter- what the system sounds like. That is the final arbiter.

Look, most engineers would laugh (as they have done to me) at you if you say that if you wanted to buy a guitar amp, and had all the money in the world, a tube one would be brought home.
They know it has more THD than solid state but the sound at the end is what matters. I get that it must be frustrating to listen to the enormous amount of bullshit that surrounds this subject but is the example you gave on the link measurable? Correct me if Im wrong but if you wanted to see if a pot at the beginning of the chain affected the frequency response wouldn't that be easy to measure?

Hmm, sounds like you have been talking to the wrong engineers, it that is what they actually were.
Any engineer worth his salt would evaluate the customer's requirement and try to understand it. Besides which, it is well known that guitar amplifiers and guitar speakers color the sound, some amplifiers in one way and another in another way. For example Fender amps produce a clean sonorous sound, while Marshal amps give a thicker bluesy sound. So guitar amplifiers are not like high end audio amplifiers which, in the main, aim to reproduce sound without any distortion. If you play a solid electric guitar through a normal audio amplifier the sound is thin, truncated, and lifeless.

There is an enormous mountain of BS about sound reproduction. but it is on both sides. On one side there is the "audiofool" brigade who like the pun in the accolade and use it on every occasion. Very often the closest they come to quality sound reproduction is a sound bar on their TV. Then there is the other end of the spectrum, the true audiofools, who talk about such things as rewiring the mains supply to their house with extra thick wire directly connected to their consumer unit, or fitting gold plated mains sockets to their house.

You mention about measuring the quality of an audio amplifier and ask if it can be measured. The straight answer is no. To illustrate this, some amplifiers, especially valve, sound fantastic, but show high levels of distortion, when measured. On the other hand, other amplifiers, nearly always transistor, have distortion figures that are so low that they literally cannot be measured, but only sound mediocre. In general, though, an amplifier with a measurably low distortion will sound better.

There is a coming together of the musician and engineer though. Through painstaking testing it has been found that certain techniques and components sound better than others and Douglas Self's book, describes these techniques and throughout his book Self develops, what he calls a 'blameless'' amplifier, which embodies these techniques. This book is for engineers though and not intended for musicians or the general public.

Just a word about my own experience in this field. I have been designing, building, and auditioning audio preamps, power amps, and speakers for years and, being technical, I dismissed the audiofoolery of the day. For example, I said categorically that turntables made no difference- they just went round, that amplifiers had so low distortion that they were like a piece of wire with gain and made no difference, that speaker cable made no difference. All of these were demonstrated to be wrong. I cringe remembering the time I told a guitarist that the wood of a Strat guitar made no difference. So you have to be very careful to separate armchair theory with actual listening.

And finally, any audio engineer who does not take note of the views of the listener, especially a musician, is a genuine audiofool.

[AngraMelo]

Spec,
I had BATTLES with the father of a long time ex girlfriend. He was a badass engineer that was mainly into cameras, digital image and security system. The guy was a local pop star and traveled the country with big security corporations developing new ways to see a dude in the dark dressed in black 200m away from climbimg your backyard wall and charging the least possible for it so they could compete with China.
The day I told him that most good guitar amps take tubes, he FLIPPED and had a crazy argument with me.
I told him this. If you find a way to charge the price of a solid state amp and deliver a tube sound and make a good profit, I'll sell everything I own and invest every penny in your company. I said thad we would retire in 10 years if he accomplished that. I told him that I would even pay for a prototype! No need to say that it never got out of paper and we quickly started avoiding that subject. I still remembers his face when I said That, the whole.family was there and silence was all one could hear.
But for you to have a laugh... after years playing tube amps, having intense pain from carrying that huge thing around and spending way more than reasonable, the last 5 years or so of my career was playing a small solid state. It is literally called Jazz Amp. A single 12' speaker with a kickass sound that was carried back and forth on the subway. Nowadays it is a major decoration piece in my living room.


Sent from my SM-G950F using Tapatalk

[Mechatrommer]

The day I told him that most good guitar amps take tubes, he FLIPPED and had a crazy argument with me... I still remembers his face when I said That, the whole.family was there and silence was all one could hear.

maybe he had no problem with tube or solid state. his issue i guess that you play guitar

But for you to have a laugh... after years playing tube amps, having intense pain from carrying that huge thing around and spending way more than reasonable, the last 5 years or so of my career was playing a small solid state. It is literally called Jazz Amp. A single 12' speaker with a kickass sound that was carried back and forth on the subway. Nowadays it is a major decoration piece in my living room.

so have you give a thought what you have gained throughout all those years? other than temporal joyness. your ex-g's dad gave his life to give peace to the community... cheers.

[Zero999]

Sigh.

Why does nearly every thread involving audio turn in to an argument about audiophoolery?

Who are the right and wrong engineers to ask? Just because the person you asked about something, violently disagreed with you, it doesn't mean their view is invalid. The advantages of forums such as this, over asking a friend, relative colleague etc. is, one can ask a lot of engineers and gather a consensus on a matter, not just take one person's word for it. Also remember this is a technical discussion and no one should feel offended, if they're contradicted.

There's a lot of the usual nonsense here about using passives and capacitors to avoid noise and distortion and speaker cables. There's nothing special about polypropylene, over polyester capacitors. I bet no one will be able to tell the difference, if subject to a blind test. Metal film resistors are less noisy, than carbon film, but it's wrong to say the latter shall be used everywhere in the audio amplifier, where there are plenty of parts of the circuit where it won't make any difference, i.e. the resistors used to bias the zeners diodes. It's amusing when a circuit mandates high end capacitors and metal film resistors, then uses carbon potentiometers, especially with a DC bias. In this case, the circuit has non-polarised capacitor, (220µF) carrying an AC signal, with no DC bias, which will distort the signal more than any polyester capacitor. Polarised capacitors act as poor diodes, (the electrolytic capacitor is derived from the electrolytic rectifier) and need a DC bias to avoid rectifying, thus distorting the signal. As far as resistors are concerned: I'd be more concerned about the type of resistor used for the 0R22 on the output stage, which could cause problems with oscillation if it's very inductive i,.e. wire wound, on a ferrous former.

It is simply untrue that amplifiers can't be measured and characterised. This is a common audiophool myth. In reality, everything which can be heard and more, can be accurately measured and characterised. Measuring equipment is well over an order of magnitude better than the human ear at picking up distortion. The problem is it's important to measure the right thing and plenty of the traditional tests used to characterise amplifiers don't. For example, pure sinewaves, which in no way resemble music or speech, are often used to obtain THD figures. The video linked to by Mark Hennessy demonstrates this perfectly and is well worth watching.

As far as the thermionic vs solid state is debate is concerned. It's true that both amplifier topologies have different characteristics, yet paradoxically can be made to sound exactly the same: please read on, I'm not trolling, its true! It's possible to make a thermionic and a solid state amplifier with imperceptible amounts of distortion, when played below the clipping threshold, at which point they'll both be audibly transparent i.e. sound the same. The difference is a thermionic amplifier will have a nicer sounding distortion, than a solid state model, when over-driven and this effect is often desirable in a guitar amplifier, where the distortion is part of the sound and hi-fidelity is not a requirement.

Over 40 years ago a double blind test was done on three amplifier designs: one thermionic and two solid state, with golden eared audiophiles and they couldn't tell the difference. Of course both of the amplifiers were designed for minimal distortion and weren't driven into clipping, but the test was for hi-fi, not a guitar amplifier.
http://www.keith-snook.info/wireless-world-magazine/Wireless-World-1978/Valves%20versus%20Transistors%20DCD.pdf

As far as hi-fi is concerned, it's fairly easy to make an amplifier an imperceptible level of distortion. In reality speakers and room acoustics are the dominant factor, not the amplifier. It's possible that a high end hi-fi system will sound the same as a TV, if they're both connected to the same speakers and played at a low enough level to avoid clipping.

This goes back to the requirements of the original poster. Is this amplifier going to be used for hi-fi, where the distortion should be minimal or a guitar, where a certain type of distortion is required? With the appropriate modifications, there's no reason why this circuit can't give a reasonable faithful reproduction of the source, but if you want a guitar amplifier, with nice distortion, then look elsewhere.

[AngraMelo]

The day I told him that most good guitar amps take tubes, he FLIPPED and had a crazy argument with me... I still remembers his face when I said That, the whole.family was there and silence was all one could hear.

maybe he had no problem with tube or solid state. his issue i guess that you play guitar

But for you to have a laugh... after years playing tube amps, having intense pain from carrying that huge thing around and spending way more than reasonable, the last 5 years or so of my career was playing a small solid state. It is literally called Jazz Amp. A single 12' speaker with a kickass sound that was carried back and forth on the subway. Nowadays it is a major decoration piece in my living room.

so have you give a thought what you have gained throughout all those years? other than temporal joyness. your ex-g's dad gave his life to give peace to the community... cheers.

The point of the story is not to show that he was a bad person, bad engineer or anything like that. As described, he was a great professional and to this day I use stuff he produces in my house and it does help to keep my family safer, for that Im eternally greatful. The main point is that low level distortion is ONE way of pursuing greatness while designing an amplifier. If money was not a problem, there is a large portion of the music business that still would not buy your amp even if it had infinit gain, infinit power and 0% distortion. For years we had people to help us with our gear and we could afford such heavy amps. The budget got thinner and tube amps were no longer an option, by buying the solid state I had to make a compromise. It does not have the sound I want but it saves me from ending up in the hospital after a small tour.
The example illustrated my point of having a very good engineer that was not concerned about what I was talking about, or trying to understand where I was coming from instead, I was made fun of for no reason. I didnt know the first thing about electronics at that time (Im still a beginner but it is completely different) and what led me to tell him about tube amps was a comment he made that tubes were no longer in use.

[AngraMelo]

Sigh.

Why does nearly every thread involving audio turn in to an argument about audiophoolery?

Who are the right and wrong engineers to ask? Just because the person you asked about something, violently disagreed with you, it doesn't mean their view is invalid. The advantages of forums such as this, over asking a friend, relative colleague etc. is, one can ask a lot of engineers and gather a consensus on a matter, not just take one person's word for it. Also remember this is a technical discussion and no one should feel offended, if they're contradicted.

There's a lot of the usual nonsense here about using passives and capacitors to avoid noise and distortion and speaker cables. There's nothing special about polypropylene, over polyester capacitors. I bet no one will be able to tell the difference, if subject to a blind test. Metal film resistors are less noisy, than carbon film, but it's wrong to say the latter shall be used everywhere in the audio amplifier, where there are plenty of parts of the circuit where it won't make any difference, i.e. the resistors used to bias the zeners diodes. It's amusing when a circuit mandates high end capacitors and metal film resistors, then uses carbon potentiometers, especially with a DC bias. In this case, the circuit has non-polarised capacitor, (220µF) carrying an AC signal, with no DC bias, which will distort the signal more than any polyester capacitor. Polarised capacitors act as poor diodes, (the electrolytic capacitor is derived from the electrolytic rectifier) and need a DC bias to avoid rectifying, thus distorting the signal. As far as resistors are concerned: I'd be more concerned about the type of resistor used for the 0R22 on the output stage, which could cause problems with oscillation if it's very inductive i,.e. wire wound, on a ferrous former.

It is simply untrue that amplifiers can't be measured and characterised. This is a common audiophool myth. In reality, everything which can be heard and more, can be accurately measured and characterised. Measuring equipment is well over an order of magnitude better than the human ear at picking up distortion. The problem is it's important to measure the right thing and plenty of the traditional tests used to characterise amplifiers don't. For example, pure sinewaves, which in no way resemble music or speech, are often used to obtain THD figures. The video linked to by Mark Hennessy demonstrates this perfectly and is well worth watching.

As far as the thermionic vs solid state is debate is concerned. It's true that both amplifier topologies have different characteristics, yet paradoxically can be made to sound exactly the same: please read on, I'm not trolling, its true! It's possible to make a thermionic and a solid state amplifier with imperceptible amounts of distortion, when played below the clipping threshold, at which point they'll both be audibly transparent i.e. sound the same. The difference is a thermionic amplifier will have a nicer sounding distortion, than a solid state model, when over-driven and this effect is often desirable in a guitar amplifier, where the distortion is part of the sound and hi-fidelity is not a requirement.

Over 40 years ago a double blind test was done on three amplifier designs: one thermionic and two solid state, with golden eared audiophiles and they couldn't tell the difference. Of course both of the amplifiers were designed for minimal distortion and weren't driven into clipping, but the test was for hi-fi, not a guitar amplifier.
[url]http://www.keith-snook.info/wireless-world-magazine/Wireless-World-1978/Valves%20versus%20Transistors%20DCD.pdf[/url]

As far as hi-fi is concerned, it's fairly easy to make an amplifier an imperceptible level of distortion. In reality speakers and room acoustics are the dominant factor, not the amplifier. It's possible that a high end hi-fi system will sound the same as a TV, if they're both connected to the same speakers and played at a low enough level to avoid clipping.

This goes back to the requirements of the original poster. Is this amplifier going to be used for hi-fi, where the distortion should be minimal or a guitar, where a certain type of distortion is required? With the appropriate modifications, there's no reason why this circuit can't give a reasonable faithful reproduction of the source, but if you want a guitar amplifier, with nice distortion, then look elsewhere.

The engineer´s point was clear, he thought that tubes in a amp made no sense. Even after I told him that the distortion they add is why they are still used. I dont know if he view was invalid or not. I simply stated a fact: if guitar players like some distortion in their sound, they will most likely look for an amp with tubes if they can afford it. The forum is a great place to learn! No wonder I cant keep myself from reading the posts!
I have nothing but gratitude for you guys!

If the whole point of using a tube amp is so you can have the characteristic sound of their distortion, how can you say that it is possible to make a solid state sound just like a tube amp?
They are made with low wattage on purpose so they will start to distort even at moderate volume. What we are looking for is exactly that light distortion it adds to the sound. If you say your solid state amp sounds just like a tube amp except when distortion starts to happen, isnt that the same as saying that your car does exact the same thing as an airplane except flying?

[Zero999]

The engineer´s point was clear, he thought that tubes in a amp made no sense. Even after I told him that the distortion they add is why they are still used. I dont know if he view was invalid or not. I simply stated a fact: if guitar players like some distortion in their sound, they will most likely look for an amp with tubes if they can afford it. The forum is a great place to learn! No wonder I cant keep myself from reading the posts!
I have nothing but gratitude for you guys!

If the whole point of using a tube amp is so you can have the characteristic sound of their distortion, how can you say that it is possible to make a solid state sound just like a tube amp?
They are made with low wattage on purpose so they will start to distort even at moderate volume. What we are looking for is exactly that light distortion it adds to the sound. If you say your solid state amp sounds just like a tube amp except when distortion starts to happen, isnt that the same as saying that your car does exact the same thing as an airplane except flying?

Yes, I do understand that the whole point of a tube amplifier is the distortion. Perhaps the person who you spoke to doesn't get this or believes the extra cost doesn't justifies it. In any case he's entitled to his point of view.

To answer your question. You're talking about a guitar amplifier, which will be driven into distortion, where as the paper I linked to is about hi-fi, which has totally different requirements to a guitar amplifier. A high amplifier should never be driven into distortion. The results of the experiment are that for hi-fi, both solid state and thermionic amplifiers give equally good sound quality, so there's no point in selecting one over the other, based on how they sound. The solid state designs are just cheaper and more reliable but people may like the way thermionic tubes look and the idea of having something old. For a guitar amplifier, a tube amp is a totally different beast to a solid state amp and there are good reasons to choose it, for the way it sounds.

[AngraMelo]

The engineer´s point was clear, he thought that tubes in a amp made no sense. Even after I told him that the distortion they add is why they are still used. I dont know if he view was invalid or not. I simply stated a fact: if guitar players like some distortion in their sound, they will most likely look for an amp with tubes if they can afford it. The forum is a great place to learn! No wonder I cant keep myself from reading the posts!
I have nothing but gratitude for you guys!

If the whole point of using a tube amp is so you can have the characteristic sound of their distortion, how can you say that it is possible to make a solid state sound just like a tube amp?
They are made with low wattage on purpose so they will start to distort even at moderate volume. What we are looking for is exactly that light distortion it adds to the sound. If you say your solid state amp sounds just like a tube amp except when distortion starts to happen, isnt that the same as saying that your car does exact the same thing as an airplane except flying?

Yes, I do understand that the whole point of a tube amplifier is the distortion. Perhaps the person who you spoke to doesn't get this or believes the extra cost doesn't justifies it. In any case he's entitled to his point of view.

To answer your question. You're talking about a guitar amplifier, which will be driven into distortion, where as the paper I linked to is about hi-fi, which has totally different requirements to a guitar amplifier. A high amplifier should never be driven into distortion. The results of the experiment are that for hi-fi, both solid state and thermionic amplifiers give equally good sound quality, so there's no point in selecting one over the other, based on how they sound. The solid state designs are just cheaper and more reliable but people may like the way thermionic tubes look and the idea of having something old. For a guitar amplifier, a tube amp is a totally different beast to a solid state amp and there are good reasons to choose it, for the way it sounds.

The article is great! Really appreciated it. I agree with that guitar amps and hi-fi are totally different things. I expressed myself badly on the last comment

[Mechatrommer]

The budget got thinner and tube amps were no longer an option, by buying the solid state I had to make a compromise. It does not have the sound I want but it saves me from ending up in the hospital after a small tour.

so i guess you dont need this thread. your heavily distorted amp originally should give you the better (or whitier) sound you required, no? i agree with earlier commenter that good engineer should listen to their clients. so maybe another thread on how to purposely introduce distortion out of solid state to get tube's quality sound?

[spec]

The budget got thinner and tube amps were no longer an option, by buying the solid state I had to make a compromise. It does not have the sound I want but it saves me from ending up in the hospital after a small tour.

so i guess you dont need this thread. your heavily distorted amp originally should give you the better (or whitier) sound you required, no? i agree with earlier commenter that good engineer should listen to their clients. so maybe another thread on how to purposely introduce distortion out of solid state to get tube's quality sound?

Not sure if you are leg pulling or not, but making transistor amps sound like valve amps and also producing distortion, has been standard practice for years in the music industry.

Incidentally, just in case, there is a vast range of guitar and bass speakers that have a tailored frequency response and generate a lot of distortion, of all kinds, to add character to the sound.

https://www.jensentone.com/