Electronics > Projects, Designs, and Technical Stuff
Guitar amplifier - Current-drive & Soft-clipping, combining both
TinkeringSteve:
This guy here describes (among others) two principles:
http://education.lenardaudio.com/en/13_guitar_amps_3.html
1) The current-driving of a speaker, which is supposedly closer to what old tube guitar amplifiers did vs. what usual solid state amplifiers do. I.e. using for neg. feedback, the voltage over a shunt resistor, which decreases as the speaker impedance increases, i.e. the would-be loss in output is counteracted.
2) And soft clipping (the circuit with the two output BJTs + emitter resistors).
So to better simulate the behavior of a tube amp (small wattage, like 1...5 W or so, in my case), I'd like to implement both concepts.
But in a way, they seem a bit contradictory to me, in that his example with the two transistors, it looks to me like the output gets lower when the impedance of the speaker rises - a thing that was to be avoided, as the goal of the current drive.
Yet apparently, tube amps have both.
So I guess it's a matter of scale, and balancing?
Could I implement something like that with some low Watt audio amplifier IC + those 2 (presumably bigger) BJTs at the end?
Or in a different way?
I actually tried modelling something in LTspice, with an opamp - and the 2 transisors behind it, driving the speaker, and feeding the voltage over the speaker shunt resistor way back to the inverting opamp input.
Then I fiddled with some attenuation/amplification of that feedback, but I didn't get it to work, as far as I can tell ;)
=> See attached LTspice file. (yeah, I'm just some tinkering monkey who doesn't know what he's doing, not an EE, no pretense here :D)
2N3055:
--- Quote from: TinkeringSteve on September 19, 2019, 07:22:34 pm ---This guy here describes (among others) two principles:
http://education.lenardaudio.com/en/13_guitar_amps_3.html
1) The current-driving of a speaker, which is supposedly closer to what old tube guitar amplifiers did vs. what usual solid state amplifiers do. I.e. using for neg. feedback, the voltage over a shunt resistor, which decreases as the speaker impedance increases, i.e. the would-be loss in output is counteracted.
2) And soft clipping (the circuit with the two output BJTs + emitter resistors).
So to better simulate the behavior of a tube amp (small wattage, like 1...5 W or so, in my case), I'd like to implement both concepts.
But in a way, they seem a bit contradictory to me, in that his example with the two transistors, it looks to me like the output gets lower when the impedance of the speaker rises - a thing that was to be avoided, as the goal of the current drive.
Yet apparently, tube amps have both.
So I guess it's a matter of scale, and balancing?
Could I implement something like that with some low Watt audio amplifier IC + those 2 (presumably bigger) BJTs at the end?
Or in a different way?
I actually tried modelling something in LTspice, with an opamp - and the 2 transisors behind it, driving the speaker, and feeding the voltage over the speaker shunt resistor way back to the inverting opamp input.
Then I fiddled with some attenuation/amplification of that feedback, but I didn't get it to work, as far as I can tell ;)
=> See attached LTspice file. (yeah, I'm just some tinkering monkey who doesn't know what he's doing, not an EE, no pretense here :D)
--- End quote ---
That page is so full factual errors and wrong conclusions that you should just forget about it.
Both valve and SS amp of classic design are voltage sources. Valve amps have nonlinear transformer element and higher output impedance, resulting in lower damping factor. Also, valves and bipolar transistors amplify based on different principles, and have different transfer characteristics, that influence how they distort as signal gets through the amp. That includes difference in how they go into limiting when you push them to the edge.
All that is reason for different sound. In the end, nowadays, SS amps are SUPERIOR in every characteristic. If you wish tube sound, you make preamp that has tube (or signal shaping that resembles tube) and do all tube distortion there, and end up with SS power amp..
Better find some good books on amplifiers (Douglas Self for instance) and dig in... Good luck!
TinkeringSteve:
--- Quote from: 2N3055 on September 19, 2019, 08:41:29 pm ---
That page is so full factual errors and wrong conclusions that you should just forget about it.
Both valve and SS amp of classic design are voltage sources. Valve amps have nonlinear transformer element and higher output impedance, resulting in lower damping factor. Also, valves and bipolar transistors amplify based on different principles, and have different transfer characteristics, that influence how they distort as signal gets through the amp. That includes difference in how they go into limiting when you push them to the edge.
All that is reason for different sound. In the end, nowadays, SS amps are SUPERIOR in every characteristic. If you wish tube sound, you make preamp that has tube (or signal shaping that resembles tube) and do all tube distortion there, and end up with SS power amp..
Better find some good books on amplifiers (Douglas Self for instance) and dig in... Good luck!
--- End quote ---
Thanks for your effort in writing this, but it seems clear you're not much into guitar amps.
This is a completely different arena from HIFI, very different goals, and hence, probably different ways that typical circuits work.
The reasons you gave for the difference in sound barely scratches the surface of what makes tube _guitar_ amps sound the way they do. At least that's what I gathered from this document, among others:
https://peavey.com/monitor/pvpapers/Chapter3.pdf
(the interesting stuff for here, starts ~ at page 3, transformers)
And that guy designed a bunch of famous tube amps, he knows what he is talking about.
As one can glean from this document, there are several things you would call defects, which are, nonetheless, essential in producing the typical tube guitar amp sound.
It's goal is NOT perfect reproduction of the (not really pleasant) electromechanically picked up steel string sound, but to *shape* a certain sound from the raw material coming out of the pickups.
Not entirely dissimilar to the Hammond organ, whose numerous "defects" have become loved by musicians and listeners, and are now emulated in software replicas - and simpler replicas missing those "defects" sound dull / like toys :)
2N3055:
--- Quote from: TinkeringSteve on September 19, 2019, 09:47:25 pm ---
--- Quote from: 2N3055 on September 19, 2019, 08:41:29 pm ---
That page is so full factual errors and wrong conclusions that you should just forget about it.
Both valve and SS amp of classic design are voltage sources. Valve amps have nonlinear transformer element and higher output impedance, resulting in lower damping factor. Also, valves and bipolar transistors amplify based on different principles, and have different transfer characteristics, that influence how they distort as signal gets through the amp. That includes difference in how they go into limiting when you push them to the edge.
All that is reason for different sound. In the end, nowadays, SS amps are SUPERIOR in every characteristic. If you wish tube sound, you make preamp that has tube (or signal shaping that resembles tube) and do all tube distortion there, and end up with SS power amp..
Better find some good books on amplifiers (Douglas Self for instance) and dig in... Good luck!
--- End quote ---
Thanks for your effort in writing this, but it seems clear you're not much into guitar amps.
This is a completely different arena from HIFI, very different goals, and hence, probably different ways that typical circuits work.
The reasons you gave for the difference in sound barely scratches the surface of what makes tube _guitar_ amps sound the way they do. At least that's what I gathered from this document, among others:
https://peavey.com/monitor/pvpapers/Chapter3.pdf
(the interesting stuff for here, starts ~ at page 3, transformers)
And that guy designed a bunch of famous tube amps, he knows what he is talking about.
As one can glean from this document, there are several things you would call defects, which are, nonetheless, essential in producing the typical tube guitar amp sound.
It's goal is NOT perfect reproduction of the (not really pleasant) electromechanically picked up steel string sound, but to *shape* a certain sound from the raw material coming out of the pickups.
Not entirely dissimilar to the Hammond organ, whose numerous "defects" have become loved by musicians and listeners, and are now emulated in software replicas - and simpler replicas missing those "defects" sound dull / like toys :)
--- End quote ---
And that Peavey paper says nothing I didn't say. I don't car if that guy made famous amps. Technically stuff he says doesn't make sense. If he made something that sounds good it was trough experimentation without really understanding theory behind the process. Good for him and good for people that love sound of stuff he made. But he shouldn't teach theory. For example, emitter resistors (called emitter degeneration resistors) will in fact create more local negative feedback, decrease amplification factor (sensitivity of amplifier) and if there is global negative feedback, it will actually decrease output impedance of amplifier because it will linearize output stage. Less clipping will come from fact that you will get less power output at same preamp drive....
There are limiters that you install between preamp and power amp that have soft clipping. both protecting amp and making it sound better.
Speaking of Peavay, they call that tech DDT...
And I used to be musician. I know that guitar amplifier doesn't sound like a HiFi amp. I was telling you that modern way to do it is to do all the sound shaping in preamp, and basically run power amp in HiFi mode because that way you have more repeatable results and more repeatable sound.
But you, of course make amps whatever way you like. It's your stuff and your fun..
All the best on your journey..
TinkeringSteve:
--- Quote from: 2N3055 on September 19, 2019, 10:14:23 pm ---And I used to be musician. I know that guitar amplifier doesn't sound like a HiFi amp. I was telling you that modern way to do it is to do all the sound shaping in preamp, and basically run power amp in HiFi mode because that way you have more repeatable results and more repeatable sound.
--- End quote ---
OK, then I misread what you wrote.
I know that most of distortion these days happens in the preamp section.
But still, there are a lot of guitarists who, nonetheless, think that they only get the real good sound if they also crank up the amp so that the power amp begins to go into nonlinear territory.
Which could be explained by what's mentioned in the Hartley Peavey paper I linked to: Because the dynamically fluctuating (degree of) output transformer saturation, and power supply sagging, would kick in, to play a role in shaping the sound.
And this is not something (that I was aware of) that guitar preamps model.
In this light, it makes even more sens that there is now the trend for amp makers to throw one low-watt tube amp on the market after another, from 1W to 5W, ever more popular. Because if you scale everything down, you can crank up the amp, get the effects I just mentioned, without your neighbors declaring war, or your ear drums busting.
That is my hypothesis from the superficial understanding I have of these things, anyway.
About the negative feedback by the emitter transistors - yes I noticed, that was what I meant in the OP about those two goals seeming somewhat contradictory (driving with current, and soft clipping).
Then again, if tube guitar amps really were voltage sources - would it not be ok to operate them with no speaker connected?
Manufacturers usually say "warranty void if operated without speaker" - and, again, in my superficial ideas of how this works - I'm not claiming I know it all - I'm just confused - it would seem a voltage source would not care about that - very high connected resistance (i.e. nothing at all connected), there simply won't happen anything. But a current source, on the other hand, would try to drive harder when there is a very high resistance (nothing connected), and the output transformer might not like that?
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