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I've been playing again with small amplifiers. I've build a few chip amps and ALL of them sounded ok, battery powered and/or transformer powered. Depending on the pedal before it I can get some great sounds but nevertheless the feeling I get is that something is going to break inside that chip, I keep pushing it into distortion but the final result is that I'm never quite happy about the sound I can get out of it.

After searching I've came to the conclusion, you can correct me on this, that somewhere in the past that small transformer coupled guitar amps, the pignose amp for instance, disappeared into to oblivious. Nobody wants to build a discrete amplifier! What I mean is that between that 60's design and the chip amp, we still have all the textbooks class ab amplifiers, and these last amplifiers are not used anymore (I can't state any specific design made exclusive for guitar playing). Any examples?

Of course I could take a classical audio class ab amplifier (I've done that, BTW), from the Mullard book (transistor audio and radio circuits) and use it to play guitar, but that is to use a general audio amplifier and use it as a guitar amplifier. Taking in consideration that the majority of the pedals that we build and discuss in this forum are solid state, why aren't any such class-ab-design-for-a-guitar-amplifier? Of course chip amps are easy to build but the same happens with a Big Muff and it has the same number of BJTs of a typical class ab amp (4 if you are counting)!

Probably these are just thoughts of a deluded mind!

Comments and examples are welcome.

Cheers.

Quote from: tca on March 04, 2013, 10:46:48 AM
After searching I've came to the conclusion, you can correct me on this, that somewhere in the past that small transformer coupled guitar amps, the pignose amp for instance, disappeared into to oblivious. Nobody wants to build a discrete amplifier!

The transformer-based transistor amps have faded out of DIY because industry doesn't use them any more, and it's really hard to get the transfomers, I suspect. DIY audio is very much a parasite on the larger electronics and audio industries.

Quote
What I mean is that between that 60's design and the chip amp, we still have all the textbooks class ab amplifiers, and these last amplifiers are not used anymore (I can't state any specific design made exclusive for guitar playing). Any examples?

I think I have several megabytes of schematics of solid state guitar amps. Maybe the problem is that these amplifiers were never designed explicitly for guitar playing, necessarily. They were adapted from the mainstream audio amplifier practice which was current when they were designed.

Quote
Of course I could take a classical audio class ab amplifier (I've done that, BTW), from the Mullard book (transistor audio and radio circuits) and use it to play guitar, but that is to use a general audio amplifier and use it as a guitar amplifier. Taking in consideration that the majority of the pedals that we build and discuss in this forum are solid state, why aren't any such class-ab-design-for-a-guitar-amplifier? Of course chip amps are easy to build but the same happens with a Big Muff and it has the same number of BJTs of a typical class ab amp (4 if you are counting)!

Specialization for use as a guitar amp is usually done by "dumbing down" a general hifi amplifier. Tricks like softening clipping, limiting or modifying frequency response, and increasing output impedance push the classical hifi designs toward the actions of a tube amp, which we all do think of as made for guitar.

... except they largely weren't. The tube-based guitar amps of the Golden Age were themselves taken from the hifi audio practice of the time and modified to be more cheaply made, and then "corrected" to the hard, industrial use conditions of guitar amps, I think.

Hi R.G.,
 I knew you would answer.

Quote from: R.G. on March 04, 2013, 11:38:12 AM
I think I have several megabytes of schematics of solid state guitar amps. Maybe the problem is that these amplifiers were never designed explicitly for guitar playing, necessarily. They were adapted from the mainstream audio amplifier practice which was current when they were designed.

ah, so the my fundamental question remains. Is there a solid state amplifier specificity design for guitar playing? I mean, not adapted from the mainstream audio amplifier practice, made bottom up instead of top down (adapting a general audio amp)?

Thanks.

PRS had a solid state amp with transformer out.

If you look at solid state amp schematics you will see some use current feedback.

A story, a friend bought a 2x12 fender like cab.  They bought some nice 12" speakers.  I installed the speakers and to test we used a small Peavey amp IIRC about 10 watts with a 8 inch.  I disconnected the 8" and connected up the speaker cab.  It sounded good.
IMO you need a good speaker(s) for a guitar or bass amp.

I think good speakers matter more than the amp type.  I tend to like 12" speakers for guitar and 15" speakers and bigger for bass.

Let me be more specific. I have this on my breadboard

(http://www.diale.org/img/muff_amp.png)

If my calculations are correct it can deliver up to 5W with less than 100mA quiescent current, it sounds terrible for simple audio but sounds great for guitar. You may recognize the circuit driver part. Rolling down the guitar volume it sounds with much more character than any chip amp that I've build, and if you crank the volume up you'll get a good not to fuzzy overdrive sound. And that was the sound that was trying to get. I'm using a BOSS OD-3 to overdrive it and I'm still thinking what to put before it, a booster and an active tone control?!? That is what I mean my a bottom up guitar amplifier: you fix the output power, chose the power voltage and what to drive the power amp (typical tone), etc. One could the same with a chip amp, but that simple class ab topology with all its defects adds something that is hard to get with a chip amp.

Of course one could argue that it sounds better because  it has 3 stages that tunes the input signal, typical my chip amps only have a buffer and the chip itself, but you can get much more control for tone shaping in this case.

Quote from: R.G. on March 04, 2013, 11:38:12 AM
I think I have several megabytes of schematics of solid state guitar amps.

Still looking for more examples. Could you share or point to some refs?

Cheers.

Try googling "peavey schematic". That should bring up a number of them.

I think I'm still a little unclear about what you mean by "designed for guitar". Many solid state amps have been designed for guitar use. However, the basis was general audio power amp practice. Do you mean preamps, the whole input-to-output, or just the power amp?

Quote from: R.G. on March 04, 2013, 05:38:25 PM
I think I'm still a little unclear about what you mean by "designed for guitar". Many solid state amps have been designed for guitar use. However, the basis was general audio power amp practice. Do you mean preamps, the whole input-to-output, or just the power amp?

I mean the whole input-to-output, I'm thinking globally, not just taking a audio amp and making it suitable for guitar. Probably my thinking is wrong... there is no such thing as a specific guitar amp topology.

P.S. (edit)

Just got the Peavey Standard schematic. Let me look inside.

You're right - there is no such thing as a specific guitar amp topology, unless you take the very few kinds of tube amps as an example, and those are heavily hifi (such as it was at the time) derived.

On the other hand, let me pose another question. Is a great guitar sound still great went recorded and played back through hifi gear? Or is a miked guitar amp on stage still a killer sound when it's played through the main PA system?

These would seem to say that once a good guitar amp sound is created, it's preserved by high quality, high fidelity amplification and wideband, level response speakers, right?

One question that's worth thinking about is this: where is the magic of great guitar sound hidden? It's certainly not all in one place. Some of it is in the preamp circuits, some in the oddities of the power amp and ideosyncratic made-for-guitar speakers, and probably more in the player's mind and fingers.

I think speakers are underappreciated as the source of great guitar tone. They are really musical instruments in their own right, and they flavor and color everything that goes through them.

> I have this on my breadboard

If it sounds good, do it.

It lacks any real audio NFB, which is probably a good tone.

I doubt it comes close to 5 watts clean, but what's in a watt?

(It has 5-7 ohms internal resistance both ways: the 4.7 at T4, and the 680 divided by T3 hFE. So if it could really pump 18V/2.828= 6.36Vrms toward a load, this voltage is split between internal resistance and load impedance. Figure nominal 8/(6+8) or about 0.57 of 6.36. 3.63Vrms in 8 ohms is 1.6 Watts RMS at clipping.)

It is maybe barely short-proof (mostly because of those large internal losses). Short-proof is a major advantage in hi-fi and *essential* on stage.

C3 does not bypass all the way to bass. The main effect may be that input impedance falls at low frequency. (R3 is "Miller effect", divided by T1 voltage gain.) That's not a terrible thing. If you want to try full bypassing, you need to shunt the 2.5 ohms of T2 emitter. In fact to flatten the input impedance you must get much less than 2 ohms at the lowest frequency of interest (82Hz). That may be impractical. (I've had that cap physically larger than the whole amp.)

C3 = 10uFd appears to cause a bass-cut below 160Hz, which IS very customary in guitar amps.

When you add up production cost and full complete abuse-proofing, the chips win the day for any mass-market product. Current-limit, SOA limit, thermal tracking and shut-down are included for 2 cents, much less than discrete protection or warranty repair costs.

In DIY we don't care so much about assembly cost or trips back to the bench.

> If it sounds good, do it.

I will.

> It lacks any real audio NFB, which is probably a good tone.

Yes.

> I doubt it comes close to 5 watts clean, but what's in a watt?
I didn't say it was 5W clean! But you are right it gives an almost a 1W clean.

> It is maybe barely short-proof (mostly because of those large internal losses). Short-proof is a major advantage in hi-fi and *essential* on stage.
I get that.

> When you add up production cost and full complete abuse-proofing, the chips win the day for any mass-market product. Current-limit, > SOA limit, thermal tracking and shut-down are included for 2 cents, much less than discrete protection or warranty repair costs.

ah, but those chips do not sound that good, or do they?

> In DIY we don't care so much about assembly cost or trips back to the bench.

;)

Quote from: R.G. on March 04, 2013, 08:33:38 PM
These would seem to say that once a good guitar amp sound is created, it's preserved by high quality, high fidelity amplification and wideband, level response speakers, right?

Of course but the creation of the guitar sound is the problem we have in hands.

Quote from: R.G. on March 04, 2013, 08:33:38 PM
I think speakers are underappreciated as the source of great guitar tone. They are really musical instruments in their own right, and they flavor and color everything that goes through them.

The way I see it, for those DIYs that can make a 5eurs amp that sounds good, buying a 75 times priced speaker (or more)  is out of the budget, specially in Europe where guitar speakers are very expensive, but we can still do a lot with regular "hi-fi" speakers.

Quote from: tca on March 05, 2013, 03:57:36 AM
Of course but the creation of the guitar sound is the problem we have in hands.

Yes. My point was that this can be done without a power amplifier, only the preamp sections, and then this signal can be reproduced at larger volume by the much more common high fidelity audio amplifiers around us.

Quote
The way I see it, for those DIYs that can make a 5eurs amp that sounds good, buying a 75 times priced speaker (or more)  is out of the budget, specially in Europe where guitar speakers are very expensive, but we can still do a lot with regular "hi-fi" speakers.

I agree. My points in saying that were that if all you have or can afford is a hifi speaker, you will need to do more work in the low power stuff before the power amplifier/speaker setup to get the sound you are looking for, and more over, that it is *possible* to do so. There are many who would dispute that second point loudly and long.

Even with the based on hi-fi designs you find in SS amps, the pre amp is usually filtered to eliminate higher frequencies. Guitar frequencies pretty much are non-existant above about 5khz, so why bother amplifying them. The speakers chosen for most amps either 10" or 12" (25cm or 30cm) are bass drivers, and most of them bandpass themselves to below 5khz. The amps designers should take this into consideration in the design phase, but most seem to want to tack on useless bells and whistles, like clipping or like 10 band graphic equalizers that only emphasize hiss for the top 5 bands (can you say Peavey? Or Mesa?) As an aside, you might want to think about that a bit when you are trying to build a graphic eq pedal. But getting back to the subject, I have found I can get an identical (to my ears) sound out of either my SS amps or my old BF Fenders. The secret is to use the SS amp in it's clean range, and use pedals to modify the sound. That way you can avoid the ice picky harshness of SS amp distortion.

Of course, when you throw DSP's and amp modeling into the mix it's a whole different game...

Have you ever looked at the Lab Series L5 amp? It has an awesome crunch tone - think early King's X guitar sound. BB King and Allan Holdsworth were also notable users of that amp. Bob Moog/Norlin design. No output transformer. Granted it's complex, but it was designed specifically for guitar and to sound great, which I think it does.

http://www.diyguitarist.com/Schematics/L5-Schematic.jpg

Quote from: tca on March 05, 2013, 03:48:32 AM
ah, but those chips do not sound that good, or do they?

Those chips sound perfectly fine if you keep them clean and let the preamp circuiry do the clipping to make it guitar-amp-like. Letting them clip usually doesn't lead to optimal results.

Quote from: tca on March 04, 2013, 05:13:28 PM
You may recognize the circuit driver part.

fuzz face?

As Seljer says. I built a chip amp for the power part of a guitar amp, and concentrated on pre amp and tone stack for "tone" (as well as speakers). In my case this was tubes though. Sounds awesome.

Thank you for all your replies.

@Paul Marossy
I'll dive in that schematic.

@Seljer
> Those chips sound perfectly fine if you keep them clean...
That is also my feeling. That's why I thought building a class ab amplifier, then I could use the power amp to also tone the signal, besides all the things PRR said about non chip amps.

@Kesh
Actually is muff fuzz variation, if you take out the 2.7k from the second transistor emitter and the 10k collector resistor. I've breaboard that circuit just to see how it sounded and really liked the fuzz-overdrive sound that you can get out of it. I've tried to do same with the fuzz face circuit using low hfe transistors but the result was not so convincing (I've used hfe=40 BJTs). There is something special the way those transistors are coupled, the feedback loop (base 1st BJT to the emitter of 2nd BJT) makes this type of topology very responsive. BTW, if you look at the schematic of the Peavey Standard, a similar topology appears (fuzz bias, lack of better name). In the case of the Peavey AP there is boostrap-for-gain configuration on the beginning of the schematic and the vibrato has also a fuzz bias configuration. I've also played with the boostrap-for-gain (eg. Escobedo's circuit) and that configuration has some killer OD.

@bhill
About the frequency response... I'm not convinced that the upper bound for the frequency range of the guitar signal is 5KHz. Take the Fender Twin amp,  I'm pretty convinced that there is not such a cut off in that amp. Please feel free to comment on that.

Quote from: tca on March 05, 2013, 05:28:19 PM
@Seljer
> Those chips sound perfectly fine if you keep them clean...
That is also my feeling. That's why I thought building a class ab amplifier, then I could use the power amp to also tone the signal, besides all the things PRR said about non chip amps.

The first amplifier I'm aware of to employ this technique was the Thomas Organ Vox amps from the late 1960s. As long as a high feedback amplifier (which performs like the "chip amps" if protections are not activated by some signal) never clips, it puts out what it is told to. Contrary to what is politically correct in the music world, this is not necessarily bad.

Quote@bhill
About the frequency response... I'm not convinced that the upper bound for the frequency range of the guitar signal is 5KHz.

That's an approximately correct figure, give or take a bit, from the inductance and self-capacitance of the pickups. It's demonstrable. I've played my strat into a software oscilloscope and then did an FFT/spectrum on the result, and found that with volume up and tone up, it had a half-power point about 6.3kHz, depending on the note and playing. That's consistent with humbuckers having larger self inductance and self capacitance, and a lower cutoff point.

However, with all kinds of modifiers after the guitar, especially distortion pedals creating higher harmonics, what we hear today is indeed higher, with added content that was not in the original guitar signal.

QuoteTake the Fender Twin amp,  I'm pretty convinced that there is not such a cut off in that amp. Please feel free to comment on that.

It probably does have a higher cut off, as Fenders were adapted from existing hifi amps of the time, and they didn't try to cut them back.

I've noticed that the discussion of frequency response often confuses the reduction of certain frequencies with the elimination of those frequencies. The treble frequencies of a guitar pickup (and most guitar amps & speakers) are definitely reduced. That doesn't mean they're not there, or not important. Half power is a lot different than no power.

Quote from: Keppy on March 05, 2013, 06:47:39 PM
I've noticed that the discussion of frequency response often confuses the reduction of certain frequencies with the elimination of those frequencies. The treble frequencies of a guitar pickup (and most guitar amps & speakers) are definitely reduced. That doesn't mean they're not there, or not important. Half power is a lot different than no power.

Quite true.

Don't disagree, but you still have the natural response of the speakers as a limitation. I did not say that the frequencies are non-existant, they do exist in the complex waveform output from the pickups, but as harmonics, rather than fundamentals. And we add to those harmonics (or decrease) depending on how we modify the waveform before it is amplified.

But you can be very meh putting that waveform through a hi-fi system into full range speaker cabinets with 50-20khz range. It takes that complex rich waveform and just flattens everything out.

The twin, and other early Fenders, have a hi-fi heritage, but then the circuits were tweaked for instrument usage cutting some, not all, of the higher freqs. 6db/oct first order filters only decrease those freqs not eliminate. I will have to admit that I haven't figured the frequency response for the twin, prob have to sit down with the scheme in the morning and calculate things. My BF Fenders are a pair of Bandmasters, so I guess you could say I have a stereo twin with twice the number of speakers.  ;) It should be the same freqs response, but I will check before I say for sure. I do know that Leo had hearing loss in the higher frequencies so he depended on others he worked with to tell him if the design was getting too ice picky. More later...

In the meantime, we should probably get this back on track ;D

Pritchard's amps use chip -based solution for power amplification and Pritchard amps are pretty much considered to be among the best solid-state guitar amps ever made, in all kinds of aspects.


It's really all about what you put around those chips.

And if you really want to get into the circuitry of a SS amp, read teemuk's very highly recommended book about them.
http://www.ssguitar.com/index.php?topic=711.0  will get you to the book link.

Quote from: bhill on March 06, 2013, 10:27:38 AM
And if you really want to get into the circuitry of a SS amp, read teemuk's very highly recommended book about them.
http://www.ssguitar.com/index.php?topic=711.0  will get you to the book link.

Very nice!  :icon_cool:

I already have Teemu's book. Thank you for remind me of that reading.

So I've been reading (Teemu's book, Mullard's, etc) and looking for schematics of SS amps. One thing that I've noticed is that, although there is "not" (or is there?) a amplifier specific topology for guitar, there is a type of configuration that statistically pops out: 2 BJTs with a voltage feedback biasing (yes, the FF type configuration). This voltage feedback biasing appears in a simple arrangement, as in the case of a FF, but also using the class-ab to feedback the driver circuit. Two examples:
- Peavey standard, and the PA standard
- Heathkit TA-16

I've listen to the Peavey standard and it sounds great. I'm curious about that Heathkit TA-16? How does it sound? (I've goolgled that?!?)

Quote from: tca on March 11, 2013, 11:15:53 AM
although there is "not" (or is there?) a amplifier specific topology for guitar

There really isn't an amplifier topology for guitar. I believe all the ones I've ever seen are reflections of hifi practice, perhaps modified for lower cost and more ruggedness.

It's worth noting that some of the earlier amplifier setups before the diffamp input style had prominent (by hifi standards) even-order distortion, so may have been subtly more preferred by guitarists. That is definitely something the guitar ear could dig out.

Quote from: R.G. on March 11, 2013, 11:37:22 AM

Quote from: tca on March 11, 2013, 11:15:53 AM
although there is "not" (or is there?) a amplifier specific topology for guitar

There really isn't an amplifier topology for guitar.

I got that ;)

Notes on the previous schematic:
- very low input impedance
- to much gain +-50dB!
- and of course to much fuzz (obviously), that would make the amp with a very specific sound, only fuzz type sounds comes out it.

Meanwhile for those following my rumbles, here is what is on my breadboard (as the previous schematic, you may recognize the driver part):

(http://www.diale.org/img/pamp.png)

Cheers.

P.S. (edit)

I always end up using a booster to further drive the amp in to distortion (in both schematics) so I guess I should try and make a 5 transistor amplifier to get the level I want, instead of the 4 transistor classical class-ab.

Quote from: Keppy on March 05, 2013, 06:47:39 PM
I've noticed that the discussion of frequency response often confuses the reduction of certain frequencies with the elimination of those frequencies. The treble frequencies of a guitar pickup (and most guitar amps & speakers) are definitely reduced. That doesn't mean they're not there, or not important. Half power is a lot different than no power.

Quote from: bhill on March 05, 2013, 08:12:17 PM
Don't disagree, but you still have the natural response of the speakers as a limitation. I did not say that the frequencies are non-existant, they do exist in the complex waveform output from the pickups, but as harmonics, rather than fundamentals. And we add to those harmonics (or decrease) depending on how we modify the waveform before it is amplified.

But you can be very meh putting that waveform through a hi-fi system into full range speaker cabinets with 50-20khz range. It takes that complex rich waveform and just flattens everything out.

The twin, and other early Fenders, have a hi-fi heritage, but then the circuits were tweaked for instrument usage cutting some, not all, of the higher freqs. 6db/oct first order filters only decrease those freqs not eliminate. I will have to admit that I haven't figured the frequency response for the twin, prob have to sit down with the scheme in the morning and calculate things. My BF Fenders are a pair of Bandmasters, so I guess you could say I have a stereo twin with twice the number of speakers.  ;) It should be the same freqs response, but I will check before I say for sure. I do know that Leo had hearing loss in the higher frequencies so he depended on others he worked with to tell him if the design was getting too ice picky. More later...

since humans usually cant hear above 16khz-20khz, and since even if you could it would sound incredibly soft and weak, we can throw those frequencies out completely.

an ideal Cutoff frequency high or low pass or band pass filter would have all values within a certain range at full voltage, and immediately drop to zero voltage on all undesired values. The pass filters dont behave like this, and so attentuation occurs along frequency ranges outside the cutoff point(s). Both systems (if the former were real), have their benefits and drawbacks. If you wanted to have 500 hz at 10 volts and 1000 hz at 5 volts with a perfect filter, you would have to find a way to pass only 1000 hz+ through another network to drop it down, and so on, unless you want to lose 1000 hz altogether. With the latter method, you need to produce a circuit that just "sounds right", without actually cutting out frenquencies perfectly, which allows for a little more fudge factor, but can still be built into a complicated (or op-amp) design that can give you exactly the voltage properties per frequency you want. It just depends how much time you feel like spending making it :P

This is more or less what frequency filters and analog circuit design tutorials seem to say.

it is my personal opinion that if we didn't have harmonics in our voice boxes, we couldn't have language. Im trying to imagine a single 1000 hz tone that sounds like A in english....

If you lost harmonics in an output system, im assuming you'd just hear beeps.

Quote from: Thecomedian on March 12, 2013, 04:23:46 AM
it is my personal opinion that if we didn't have harmonics in our voice boxes, we couldn't have language. Im trying to imagine a single 1000 hz tone that sounds like A in english....

The human voice apparatus generates essentially sawtooth waveforms at the vocal cords, and the harmonics of these are shaped by resonances in the vocal tract. This is the reason that we can hear speech at different frequencies and sing. The fundamental does not carry the intelligence, nor do the harmonics, nor do the absolute positions of the resonances. What carries information in human speech is the relationship/ratio in frequency of the resonances.

Hi again, a few more notes on increasing the output impedance, or decreasing the damping factor of a SS amp using a mixed mode feedback. I've simulated both schematics with a non-ideal speaker sim and by controlling the feedback network I could get the typical non-linear response characteristic. What I also get is a 15-20dB, or bigger, reduction in the total gain of the amplifier to get this response. This of course opens another problem, that in order to get the same initial power, before the mixed feedback, the circuit needs a pre-amp and also the input of the amplifier must not clip to early... need do redesign the driver part. I do see a light in the end of the tunnel but I was thinking in a simple way of increasing the output impedance without killing "almost" all the gain! Is there a clever way of doing this without building a 60dB driver and then killing 20dB? Probably not, "there ain't no such thing as a free lunch".

I'm really enjoying Teemu's book.

Cheers.

Some words about transformer coupled SS guitar amps.
Look at my page there you can find wide range of this type amps (250mW-90W)
http://milas.spb.ru/~kmg/fetpa_en.html

Quote from: KMG on March 13, 2013, 06:42:54 AM
Some words about transformer coupled SS guitar amps.
Look at my page there you can find wide range of this type amps (250mW-90W)
http://milas.spb.ru/~kmg/fetpa_en.html

Nice amps, thanks. But I was looking for a transformer-less amp.

There is in fact a simple idea that enables to control the damping factor in a very simple way. This can be seen on the Mullard's book "Transistor audio and radio circuits" pg. 39. There are various examples of this, namely 1W, 3W (>= 20Ohm output impedance) and 10W amps (4 transistor amplifiers). The idea consists in putting the feedback not from the collector of the first transistor but from its emitter. Here is what I've cooked up:

(http://www.diale.org/img/peach.png)

The speaker qualities does shows up in this way. Resistor R3 can be varied to fit several output impedances without perturbing to much the working point.

Still have to adjust the frequency response to my taste but so far so good.

Cheers.

P.S. (edit)

R3 also controls gain. And BTW it does sound good for listen to music!

P.P.S.

I guess this is just a singleton topology that is why it works as it does and works great with a guitar, nice 2nd harmonics!

P.P.P.S.

Using this http://sound.westhost.com/project56.htm and the simulator I get roughly Zo=8Ohm, unity damping factor.

> a simple way of increasing the output impedance without killing "almost" all the gain!

You generally need significant voltage feedback at DC to set the DC operating point.

There's no God-given reason to have voltage feedback at audio.

Your NFB network tends to be a resistor from output to "-" input, then a resistor and capacitor to ground. Make that to-ground resistor zero, the cap quite large. Perhaps 68K, 470u, and <10r.

That sets the DC gain to unity, so the output DC voltage can be set near half-supply.

That sets the AC gain over 6,000, your forward path probably does not have that much gain, so the "voltage feedback" is ineffective for audio, for anything over about 35Hz.

Your plan in #34 has a "horrible" flaw (which may be a happy discovery). The output gain and NFB is very different pull-up and pull-down. Wave is lopsided.

> The speaker qualities does shows up

What, the 2dB bump at 74Hz? A classic Fender Twin shows nearer 6dB bump, because it has barely-hardly-any NFB. Some Ampegs have no NFB and 10+dB bump at bass resonance. This works best with stiff drivers in open-back cabinets. The slump from insufficient baffling is countered by the rise from low damping. (When Marshall used softer drivers that favored closed-back boxes they used more NFB than the Fender they were copying.)

tca, you use not exact emulation of speaker impedance.
This version is closer to real speaker
(http://milas.spb.ru/~kmg/files/projects/LoadBox/Load.jpg)
(http://milas.spb.ru/~kmg/files/projects/LoadBox/Cab_Load.jpg)
I use this loadbox for silent testing amps
(http://milas.spb.ru/~kmg/files/projects/LoadBox/brdtop_th.jpg)

Quote from: PRR on March 13, 2013, 06:36:41 PM
> a simple way of increasing the output impedance without killing "almost" all the gain!

You generally need significant voltage feedback at DC to set the DC operating point.

There's no God-given reason to have voltage feedback at audio.

Your NFB network tends to be a resistor from output to "-" input, then a resistor and capacitor to ground. Make that to-ground resistor zero, the cap quite large. Perhaps 68K, 470u, and <10r.

That sets the DC gain to unity, so the output DC voltage can be set near half-supply.

That sets the AC gain over 6,000, your forward path probably does not have that much gain, so the "voltage feedback" is ineffective for audio, for anything over about 35Hz.

Your plan in #34 has a "horrible" flaw (which may be a happy discovery). The output gain and NFB is very different pull-up and pull-down. Wave is lopsided.

> The speaker qualities does shows up

What, the 2dB bump at 74Hz? A classic Fender Twin shows nearer 6dB bump, because it has barely-hardly-any NFB. Some Ampegs have no NFB and 10+dB bump at bass resonance. This works best with stiff drivers in open-back cabinets. The slump from insufficient baffling is countered by the rise from low damping. (When Marshall used softer drivers that favored closed-back boxes they used more NFB than the Fender they were copying.)

I could set the DC gain to 1 and increase the gain by decreasing the value of R3 but that also makes the amp to oscillate when  the guitar is connected. By doing that the 74Hz bump can be as much as 10dB.

Quote from: KMG on March 14, 2013, 04:40:09 AM
tca, you use not exact emulation of speaker impedance.

I'll check my numbers, thanks.

Quote from: KMG on March 14, 2013, 04:40:09 AM
tca, you use not exact emulation of speaker impedance.

Just calculated both versions, blue my sim (Thiele/Small), red yours.

(http://www.diale.org/img/speaker_sim.png)

Your version seems much more harsh/drastic approach... I really don't know what is the best approximation to a real speaker.

Cheers.

I have different simulation results, solid lines - amplitude, dotted - phase.
Your resonance is about 70Hz, my - 120Hz (close to real guitar speakers).
(http://s24.postimage.org/c5oditlh1/compare.jpg)
In earlier post I show real measurements of guitar cabinet & load box with guitar amplifier having output impedance about 15 Ohm.

Quote from: KMG on March 14, 2013, 08:54:23 AM
I have different simulation results, solid lines - amplitude, dotted - phase.
Your resonance is about 70Hz, my - 120Hz (close to real guitar speakers).

ah, but I don't have a guitar speaker, just an oval bookshelf speaker!

(http://www.diale.org/img/bookshelfspeaker.png)

P.S.
Ok, just corrected a 2*pi factor in the freq! Both calcs agree now.

Just found out that the oscillations that I mention in a previous post were simply bad connections to the breadboard, any bad contact makes the amp oscillate. Now it works marvelous with R6=10R although I still have some treble cut when connecting the guitar without a  buffer, it is not to drastic but sounds nicer with more highs.

Just forget to mention PRR's note:

> Your plan in #34 has a "horrible" flaw (which may be a happy discovery). The output gain and NFB is very different pull-up and pull-down. Wave is lopsided.

It is correct, the amps has an asymmetrical behavior for the positive and negative part of the signal. If overdriven it clips asymmetrical too :) .

P.S.

Some sims:

(http://www.diale.org/img/peach_graphs.png)

The frequency response was taken with the buffer.
Cheers.

So here goes some more notes. The schematic is frozen, with R6=10R, (see entry #34 (http://www.diystompboxes.com/smfforum/index.php?topic=101674.msg900623#msg900623)) that is the best sounding configuration.

Positive points:

- good compression
- sound best that any IC amp I've build
- lots of volume for clean and great when overdriven

Negative points:

- there is always a hiss at maximum gain (R6=10R) and it "disappears" with R6=1K
- there is also some fizz when playing a single note and it seems that the noise is modulated with the guitar signal, even at lower levels and because it also happens at lower volumes I think is not due to any clipping phenomenon
- when using a buffer, in this case a BOSS OD-3, the hiss/fizz reduces (but does not disappear), also if the emitter resistance (R8=4.7) is shunted  same happens hiss/fizz reduction

Taking in to account that I have the amp in the breadboard and the circuit is very sensitive to where the grounds, input and output speaker are connected and its relative position, what do you think about this? I'm going to make a metal boxed version to see if I can get any improvement on the noise. Any comments/suggestions?

Thanks.

What about a class a amp? And i DO know it is a waste of power!

(http://www.diale.org/img/smallamp.png)

Cheers.

If the hiss vanishes *abruptly* at a certain gain, the amp is oscillating supersonics which alias-down into the audio range. ("Regenerative Receiver")

> What about a class a amp?

A is one thing. Resistor-coupled is a whole different thing.

A with a choke is up to 50% efficient. ("40%" is more likely in real-world.)

A with current-source is 25% efficient. (Again, really less.)

Resistor-coupled with no DC in the load is, at best, 5.7% efficient. (Yes, noted authorities give other numbers; AFAICT they have not thought it through because the case seems trivial.) 

Your plan is non-optimum efficiency. 27W in 0.9W (undistorted Sine) out is 3.3%. OK, not that bad.

Your output impedance is just 10 ohms. Compared to nominal 8, it has damping-factor which is suited to many guitar-amp speakers, cabinets, and styles.

I do not understand your biasing. If R3 is small and R1=R2, then T1 Collector is near 1.3V? Yet T4 Base has to be real close to 18V (say 16V). I don't see how this works (I must be misreading values) and doubt it is stable for supply and part variation.

Ya know, the old "FuzzFace" plan can stabilize the 2nd-stage bias current. The drop across the 2nd emitter resistor is equal to 1st stage Vbe plus drop in 1st stage base resistor(s). In any case you ought to have a buffer between gitar and a 2-tranny amp. So you can make that base resistor small, which makes hFE tolerance great. Q1 Vbe is 0.6V for any likely current, so 2nd emitter resistor is 0.6V/1.5A= 0.4 ohms. Yes, it drifts with temperature, but ~~10% for ANY temp you are likely to play in.

Here's my real objection (pure gut-work). Resistor does over half the work. What is the sound of the resistor? Bor-ing. Nothing like a Pentode, or a BJT/FET, or even choke. IMHO, this is good for a transparent headphone-amp, maybe not for a flavor-amp.

That concept alone is not as compelling as the GREAT improvement in efficiency possible with choke-load or push-pull. We are already fighting the 0.4%-2% efficiency of the "loud"speaker; multiply that by 3% amplifier efficiency and it's pretty pathetic (0.03% overall from DC juice to air motion).

Hi PRR, thank you for your reply.

> If the hiss vanishes *abruptly* at a certain gain, the amp is oscillating supersonics which alias-down into the audio range.
> ("Regenerative Receiver")

It does... I suspected that would be the case. Back to the drawing board I guess, but that phenomenon does not appear on the sims... The amp is also noisy if nothing is connected to the input, the 100u cap on the collector of 1st transistor kills a lot of it.

> Here's my real objection (pure gut-work). Resistor does over half the work. What is the sound of the resistor? Bor-ing. Nothing like a
> Pentode, or a BJT/FET, or even choke. IMHO, this is good for a transparent headphone-amp, maybe not for a flavor-amp.

My feeling about class A amplifiers is that they are much more dull than a push-pull class ab (hard to do a straight comparison) although a high output impedance is easy to obtain in this case. Don't know why this happens? I've also tried a 18V version of a JLH type amplifier but the guitar sound was not fluid enough, kind of stuck sound. Probably lack of odd harmonics?!? Although my version of the JLH sounded great with regular music. I also tried this one

(http://www.rason.org/Projects/transaud/IMG00002.GIF)

But the same happened, dull and stuck sound with a guitar but great with regular audio. I'm missing something? What?

Thanks again.

P.S.

Ref.: http://www.rason.org/Projects/transaud/transaud.htm

Quote from: tca on March 21, 2013, 10:17:16 AM
My feeling about class A amplifiers is that they are much more dull than a push-pull class ab ...

Just realize that this feeling I get from class a amps is just to much damping (I mean low), to high output impedance. What is the typical output impedance of a guitar amplifier 3R to 10R?

Cheers.

Damping factor for guitar amps runs from >100 down to 0.1.

A basic chip-amp, or PA type amplifier, has damping in excess of 10 (may claim 1,000).

Naked no-NFB pentodes, from early Gibson one-6V6 to Ampeg VT-40, have damping near 0.1.

A broad range of classic Fenders, and some recent chip-amps adapted with series feedback, have damping factor in the general range of 1 (0.5-2).

A naked triode gives 3-5..... it is noteworthy that naked triodes have never been popular for guitar stage-amps, even though some audiophiles live them.

Damping factor is, roughly, the ratio of load Z to internal Z. Normalized to 8 ohms, the naked pentode is 80 ohms out, the solid chip-amp is <<1 ohm out.

Do NOT increase output impedance with dumb resistance, that's very wasteful. (Possible exception is bedroom amps where smaller is not cheaper, so waste power is not waste money.)

So after a lot of sims I just realize that my attempts to build a typical class ab amp started to look much like a opamp! LTP on the input, current sources, etc... I guess that is what they were invented for! The small number parts amplifier that I've found was this Red Circuit (note: you can also find it on Teemu's book)

(http://www.diale.org/img/red.png)

Ref.: http://www.redcircuits.com/Page164.htm

It is rather simple to understand: the R4 and R5 control the bias of the transistors and as the opamp requires more current to work so the transistors start to conduct more; R4 and R5 are determined such that the voltage drop, due to the  current consumption of the opamp, equals roughly .6-.7 V at idle. But my sims does not work... it seems that my simulator (TINA-TI) keeps the idle current of the opamp always constant! Still waiting for the transformer to arrive.


Meanwhile after that I've started to look for different amplifier topologies. There were two that catch my eye: the Taylor amp (check the  Scanned and cleaned up Wireless World Articles (http://www.keith-snook.info/Wireless-World-Stuff/Wireless-World-Stuff.html)) and the Ciclotron amp (the solid state version, check http://www.tubecad.com/2013/01/blog0254.htm).

Concerning the Circlotron, this version is wonderful, but it eats a few amps in order to play some real music

(http://www.tubecad.com/2013/01/05/A.%20S.%20Goldsmith%20Circlotron.png)

Does not need an output transformer and with a 2:1 input transformer a simple single ended input version is an easy build.

Cheers.

> my sims does not work... it seems that my simulator (TINA-TI) keeps the idle current of the opamp always constant!

Correct. Not the sim, but the model. Most chip opamp models do NOT simulate dynamic supply current; just a nominal idle current.

> a typical class ab amp started to look much like a opamp!

Well, you are well into a Cook's Tour (http://www.merriam-webster.com/dictionary/cook's%20tour) (or maybe Magical Mystery Tour?) of amplifier possibilities.

The reference for "Good" power amps in a modern theme is Doug Self (http://www.amazon.com/Audio-Power-Amplifier-Design-Handbook/dp/0240521625); though he doesn't teach the many-many alternate topologies.

However a Power Amp typically does NOT have to have all the features of an op-amp. It is invariably run at a medium and fixed gain. That means it does not need a wide input common-mode range; nor does it need "universal" NFB compensation. The DC gain can be zero (though in Hi-Fi there is a tendency to build for DC gain then chop subsonics).

In fact small opamps and large power-amps have converged to the same problem (with bigger parts). Sanitary NFB does generally imply simple-inverter gain structure. The LM101's topology does that, simple and neat.

> Circlotron, ...eats a few amps in order to play some real music ... Does not need an output transformer

Any Hi-Fi amplifier will need power (amps) to play amazing. Hi-Fi speakers are quite inefficient. Efficiency can be quite high in almost any sensical topology (though not if you have 6 ohm emitter resistors series with 8 ohm speaker).

The Circlotron is not an amazing invention. Look at a circuit as a string of pearls. You can move the pearls around the loop; the loop is unchanged. The Circlotron meshes the two loops of push-pull in a cat's-cradle-like way; it doesn't do anything grossly different.

Oh, there is a historic reason. McIntosh put half the load of the tube in the cathode, and patented it. This plus amazing build-quality got a good reputation. Electro-Voice rented the Circlotron patent to get about the same effect, with slightly lower build-quality, at lower price, and did well with the product. They just slid the pearls around the loop until it didn't conflict with McIntosh's "I Claim"s.

http://www.keith-snook.info/Wireless-World-Stuff/Wireless-World-Stuff.html

Several articles about audio amp designs.

Thanks PRR for your replies. I'll post some more ideas latter on.

Cheers.

More news! The prototype

(http://diale.org/img/red_proto.png)

and the schematic

(http://diale.org/img/red_final.png)

It has current feedback taken from an 87's Elektor article (http://users.otenet.gr/~athsam/power_amplifier_for_loudspeaker_6W_8R.htm).

Works great and loud as hell, at least for my standards, I'm just guessing my family comments... ;)

I guess is time to make a preamp...

Cheers.

P.S.

- the heaters don't need to be that big... but I'm glad I've used this ones (taken from a CRT). In case you are wondering, it DID make some smoke the first time!
- the power transformer is a 9-0-9 1A, so the amp uses +-14V

A few more notes about the previous design.

1. the input of the NE5534 should not exceed 0.6V, it breaks after that voltage (done that).
2. the BJT's do suffer from a thermal runaway/bad biasing, they start roughly at VBE=.6 and after some time, if you  push the amp a hard, VBE stays at .9V and stays there thankfully for the heat sinkers, but it will increase when heat sinks start to fail in dissipating all that heat. It will work with bigger ones. It starts as a good and low consumption class ab, and ends eating a lot of current (more than 1A at idle).
2.1 There is also the idle current of the NE5534, it drastically controls the bias of the BJT's
3. The current feedback introduces some fizz on the guitar signal.

Time to move on... not yet!

> current feedback taken from

One thing of note: if the load falls off, the voltage gain goes to "infinity" (greater than the open-loop gain of the '5534). Generally, infinite gain finds a way to oscillate. Whether 14+V swings at possibly 10KHz-1MHz are a "problem" depends on the situation. (However the output devices will probably overheat.)

Most commercial amps with current-sensing also have a voltage feedback loop, which is non-dominant with nominal load but limits the maximum gain at light load.

> bad biasing, they start roughly at VBE=0.6

Since the idle current of the chip is in no-way similar to the bias needs, IMO this scheme needs to be biased deep-B, not even try for AB. Reduce the bias resistors. The large open-loop gain will pick-up most of the slack.

Overall this is a near-perfect amplifier with some major drawbacks. Not really what you were looking for (a music-specific amp).

Find Nelson Pass website. Look at his light-bulb amps. Nifty, single-ended, MOSFET, inefficient but inexpensive. You could get much of a Watt with 15V supply and a 12V car brake-light bulb idling around 10V. Lately he has been on a current-drive kick, so there's your output impedance.

> current feedback introduces some fizz on the guitar signal.

Not so much current feedback, I suspect. The speaker is a rising impedance. With infinite output impedance, stuff above 1KHz-4KHz can get brutal. In no-NFB pentode days it was universal to hang an R-C network across the speaker. 10uFd+10r is in the ballpark. (This was easier on the tube OT's primary side where 0.1uFd+1K would do.)

> ...this scheme needs to be biased deep-B, not even try for AB

Lower the bias resistances to 15R, still works, and much cooler now!

> The speaker is a rising impedance. With infinite output impedance, stuff above 1KHz-4KHz can get brutal. In no-NFB pentode days it was universal to hang an R-C network across the speaker. 10uFd+10r is in the ballpark. (This was easier on the tube OT's primary side where 0.1uFd+1K would do.)

Very clever! Thanks. I'll try a cap in parallel with the speaker (220n ??).

> Nifty, single-ended, MOSFET, inefficient but inexpensive. You could get much of a Watt with 15V supply and a 12V car brake-light bulb idling around 10V. Lately he has been on a current-drive kick, so there's your output impedance.

I've done that also. ah, I'm sure you remember this: The Bulbamp: 1 Watt guitar class A amplifier with a light bulb (http://www.diystompboxes.com/smfforum/index.php?topic=94224.0) (it does not have a 1W!).

(http://www.diale.org/img/bulbamp.png)

It gets nicer and nicer as the voltage/amps increases, the biggest problem with these amps is the power source, big transformer, BIG caps. I'll try another run on this class a in the future. I even made a small version:

(http://www.diale.org/img/bulb-amp-m.jpg)

and a stereo version.

Just realize that the fizz is from bad biasing of the upper (PNP) transistor it simply does not conduct, I even tried to adjust the offset voltage (see datasheet) with no results. The circuit would work in class ab and not simply class b.

Just made an idiotic mistake... the output jack had a connection to ground (made because of the metal panel in which the input jacks were connected) and so the feedback did not work! It works great, no fizz, no high temperatures! Simply great!

So here is the final version of the amp:

(http://www.diale.org/img/red_final2.png)

The latter version still had some noticeable fizz... this version works better.

... time for the preamp. Because I have to limit the input of the amp to 0.6V I was thinking in doing something like the preamp project 27 from ESP (http://sound.westhost.com/project27.htm) changing the output diodes (to two and tweaking the resistor values). I'm still thinking about the tone control: a bass, middle and treble, or something like a one knob Princeton tone control...

There is in fact a very big difference in designing a preamp and a distortion box, one has to keep it simple in order to accept different stompboxes but it still needs, and it should, sound good on its own...  Any refs?


Cheers.

Here is the preamp, the tone control still needs some testing, but the clipping section works, although I find it a bit harsh when it is fully working, maybe the tone control section will tame it a little bit.

(http://www.diale.org/img/preamp.png)

Cheers.

Some notes...

I've been playing guitar with this

(http://diale.org/img/red_final.png)

One of the problems that it had was a noticeable fizz. This fizz varied in some degree and in quality as I vary the opamp, the transistors, the bias, the feedback loop (pure current, or mixed mode)... and also when changing speakers cabinets with different speakers! This fizz is very noticeable when playing between the lower B (123Hz) and lower F (175Hz) and the most accurate way of describing it is by saying that appears that the speaker has beach sand on the cone. I guess this is a pure speaker phenomenon due to the resonance frequency of the speaker or is the speaker badly tied do the cab (I think this is the correct answer, not sure yet) . Any thoughts? And BTW can this phenomenon be temperature dependent? It seems that the type of fizz changes during the day, not really sure about this either?

Cheers.

Sounds like crossover distortion.

Yep. Crossover distortion.

You could replace that circuit more or less directly with a TDA2030 or LM1875 (considering the +/- 10V power supply) and be fairly certain that you won't have crossover distortion, but will have safe area protection of the power amp and thermal shutdown if it gets too hot.

Quote from: R.G. on May 02, 2013, 11:05:39 AM
Yep. Crossover distortion.

You could replace that circuit more or less directly with a TDA2030 or LM1875 (considering the +/- 10V power supply) and be fairly certain that you won't have crossover distortion, but will have safe area protection of the power amp and thermal shutdown if it gets too hot.

I was suspecting that! I think I'll go for a LM1875, have to pay a visit to my local store.  I could also increase the power voltage but it is cheaper to buy a new chip!

But it is strange never the less... it seems a come and go phenomenon, that is why I thought about a temperature related effect (Vbe increases with temperature...).

Report soon. Thanks again for your answers.

Cheers.

P.S.
Going to make a sound sample just to future memory, that is a fizz that is worth remembering!

P.P.S
Found the SPICE/TINA-TI model: http://www.ti.com/product/lm1875#technicaldocuments

Quote from: tca on May 02, 2013, 11:35:58 AM
I was suspecting that! I think I'll go for a LM1875, have to pay a visit to my local store.  I could also increase the power voltage but it is cheaper to buy a new chip!

That particular circuit has some oddities about it. The nominal "bias" on the power devices isn't usually an issue, as the power transistors don't need bias or do anything at all until the opamp is shoving enough current out to turn them on because of the voltage drop across the resistors in the power supply leads. So the opamp actually supplies the  whole load current for a fraction of a volt away from zero, and when that gets big enough to generate a Vbe voltage across the 100R resistors, the power transistors start pumping current in. Cut-in voltage for a silicon device is about 0.45-0.5, so you're talking a current through the opamp of 0.45V/100R = 4.5ma. Not bad.

The opamp then pulls the base current for the power transistors through its power supply leads. The opamp supplies the base current after the transistor cuts in.

Can the opamp do this OK? Gotta do the math.

A TIP32 is a PNP, and will have lower gain than an NPN. We are hoping it can pull the speaker load up to most of the 10V power supply. A decent objective would be to +9V. So we are hoping the output will put 9V across an 8 ohm resistor, for about 1.125A peak. Notice that I said "resistor". A speaker is a reactive load, and may well want more than twice the current of a resistor, but we'll hope for making a resistor work first.

With a collector current of 1.25 A, what is the base current in the TIP32, and therefore the supply current in the opamp? Easy - it's collector current divided by HFE  - er, and we know HFE varies.   :icon_eek: 

OK, we can at least see how bad it could be. Time for datasheets. The TIP32 datasheet says the minimum HFE at 1A is 25. Yep, there are those "typical" graphs, but the maker won't guarantee that.  So we'll guess that HFE is as bad as 25, so the current in the base may be as big as 1.125A/25 = 0.05A. The TI datasheet for the NE5532 says that the maximum output short circuit current is 38ma. That's as much as the 5532 can put out, so it's also as much as it can suck in through its power supply lead.

Nope, the 5532 cannot be **guaranteed** to drive the output to as much voltage as the power supply voltage would otherwise let it. Typically?? Maybe. If you have a higher HFE transistor, probably. But if you're an EE, your boss will fire you when he starts getting chewed on by HIS boss for a fraction of the units failing in the field and having to be expensively replaced under warranty.

Hi R.G., thanks for your reply. Your description agrees with my idea how the circuits works.

> The nominal "bias" on the power devices isn't usually an issue.
Ok.

> A TIP32 is a PNP, and will have lower gain than an NPN.
Yes, confirmed that experimentally!

>... a higher HFE transistor, probably.
I could try that but the TIP31/32 where just sitting here waiting to be used.

> But if you're an EE...

I'm a completely different animal: a mathematical-physicist! I can do perturbative analysis with Einstein equations but not yet with audio amplifiers.:icon_cool:

I've learn a lot so far, time for another configuration?! That chips that you mention earlier deserve some testing.

Thanks again, cheers.

P.S.
One my problems with this circuit is that I could not do a simulation with it, I usually do that with every circuit that I breadboard. PRR confirms my finding that "chip opamp models do NOT simulate dynamic supply current; just a nominal idle current. "

Quote from: tca on May 02, 2013, 04:49:23 PM
> But if you're an EE...
I'm a completely different animal: a mathematical-physicist! I can do perturbative analysis with Einstein equations but not yet with audio amplifiers.:icon_cool:

Yeah, I knew that. The "EE" stuff is just to show you how us failed mathematician/physicists live.  :icon_biggrin:

> replace that circuit more or less directly with a TDA2030 or LM1875

that is to use a general audio amplifier and use it as a guitar amplifier (http://www.diystompboxes.com/smfforum/index.php?topic=101674.msg898632#msg898632) -tca

Somehow he started on a quest for very *music amp specific* ideas and has come around to a '2030, a boring blob of Watts.

OTOH, looking back over decades of guitar amps come and gone, the only non-general plans which persist are the tube amps. There's a little simple/complex magic between the grids, plates, and transformer.

There's a few oddball transistor amps with fan-clubs; some of them seem to do one thing very well and anything else is ordinary.

This plan: as R.G. shows, it's starved at maximum output. But that's not your complaint. The sand on the cone is: up to 4mA the '5532 drives the speaker directly. '5532 is happy driving 600 ohms, NOT happy driving 8 ohms (even at small level). Gain and linearity are poor. Then up at 5mA-10mA the TIPs kick in. When they get to 5mA they add voltage gain, which goes up and up and up and UP with current. Open-loop gain may be 1/30 low for teeny signal and 30X large at maximum output. Any time you have a change-of-gain with signal level, that's distortion. The negative feedback moderates that, but who can moderate a 900:1 change of gain? I strongly suspect oscillation at high level; at low levels the '5532 gain-bandwidth is far short of spec and NFB essentially fails in the upper audio frequencies. This *can* be nice for music (Bassman Presence control) but is more often just nasty. Especially since it happens only in a narrow signal-level range around 1 milliWatt (decay of bedroom-level amp).

If you can stand the heat, hack-up a John Lindsey Hood:
http://sound.au.com/tcaas/WW4-69.gif
http://sound.au.com/tcaas/jlh1969.pdf
http://sound.au.com/tcaas/

Hi
interesting ideas.

QuoteThere's a little simple/complex magic between the grids, plates, and transformer.

So why don't more people use output transformers in solid state amps? Probably because it's easy to NOT use one. And cheap. And many people have never heard a good SS+output transformer amp. Fortunately, DIY isn't always about easy and cheap and familiar. IMO this is a large field of DIY experimentation that has been lurking in the background for about 10 years. Without the in Brian May's 'Deacy' amp and one or two others (Pignose), the benefits of OTs in solid state amps would be almost unknown.
cheers

Hi PRR, thanks for your reply.

>> replace that circuit more or less directly with a TDA2030 or LM1875
> that is to use a general audio amplifier and use it as a guitar amplifier -tca
That can not be a good thing, my statements are haunting me!

> looking back over decades of guitar amps come and gone, the only non-general plans which persist are the tube amps.
> There's a little simple/complex magic between the grids, plates, and transformer.
How far can we take those statements? That feeling thus make a SS guitar amplifier quest unfeasible, almost like a I-told-you -so statement, a highly questionable and waste of time holy grail crusade. My gut feeling is that such a good SS amp can be made with out extraterrestrial technology or other magic.

> There's a few oddball transistor amps with fan-clubs; some of them seem to do one thing very well and anything else is ordinary.
That is also my feeling.

> started on a quest for very *music amp specific* ideas and has come around to a '2030, a boring blob of Watts.
You are correct. I started with some cool discrete designs and converged to a boring blob of watts. Actually what I have on my breadboard is a TDA20* + TL* guitar amplifier! I guess these posts reveal an experimental form of the principle of least constraint (btw, my favorite way of deriving Newton's equations). Going to test some ideas with the 2030... and see how that sounds... But I/you can predict the outcome of that realization.

> If you can stand the heat, hack-up a John Lindsey Hood.

I'm glad you mention that, I did tried a 18V  small version of the JLH, at the time I didn't liked the response of it, but I SHOULD give it another try.

I have a few schematics of high voltage (typical 100V) transistor amplifier with output coupled transformer but I never found a compatible transformer to give it a go.

Cheers.

P.S.
Just for a laugh! Just blew up two LM1875, very sticky (to rail) devices.

> If you can stand the heat, hack-up a John Lindsey Hood.
I've been playing with a small 18V version of the JLH... much better now, not to much heat btw, roughly 1W rms . Didn't like the first run, but it seems a bit more knowledge helps. Actually what I'm using is a simpler version, using  the schem of fig. 7: https://www.passdiy.com/project/amplifiers/the-plh-amplifier but with BJT.

(https://www.passdiy.com/projects/images/content/plh_amplifier7.png)

I guess I should put back the first device and make something like the original

(https://www.passdiy.com/projects/images/content/plh_amplifier2.png)

Cheers.

Quote from: tca on May 06, 2013, 06:48:36 AM
Just for a laugh! Just blew up two LM1875, very sticky (to rail) devices.

Many Linn-topology amps (and this is one), including discretes, will exhibit sticking to the rail when the inputs are overdriven. It's an issue of the overdrive and feedback paths activating a positive feedback loop internally.

This is why many amplifiers have back-to-back diodes across the + and - inputs. This pretty much solves the issue of overdrive causing sticking for most cases.

For a guitar amp, it is also a good idea to look at the amount of signal that can go into the amp. If you think about it, trying to drive the input of a solid state amp with lots of feedback further than the supplies is not all that helpful. The knee of clipping is going to be harder than most musicians like, and any ephemeral advantages of "saturating the output stage" like in tube amps is not going to be there.

Thomas Organ took a practical and good-sounding approach in that they put a diode limiter before their power amps. This means that the power amp is never driven outside it's linear region, but exhibits a composite soft(ish) clipping characteristic, and never hits the oddities of the power amp being driven too hard on its input. It's well worth looking at.

Quote from: R.G. on May 07, 2013, 12:19:37 PM
For a guitar amp, it is also a good idea to look at the amount of signal that can go into the amp. If you think about it, trying to drive the input of a solid state amp with lots of feedback further than the supplies is not all that helpful. The knee of clipping is going to be harder than most musicians like, and any ephemeral advantages of "saturating the output stage" like in tube amps is not going to be there.

I understand that, but it seems very counter-intuitive not being able to saturate the output stage even if using a SS amp. I think the same effect happens, or is similar to, while playing a acoustic instrument, the player rests on the assumption that by picking the strings hard (increasing the input) some type of distortion will occur.

(edit) This could be done by overdriven the preamp of course, but can I call a one BJT booster a preamp?

Quote from: R.G. on May 07, 2013, 12:19:37 PM
Thomas Organ took a practical and good-sounding approach in that they put a diode limiter before their power amps. This means that the power amp is never driven outside it's linear region, but exhibits a composite soft(ish) clipping characteristic, and never hits the oddities of the power amp being driven too hard on its input. It's well worth looking at.

I've tested that idea on the previous NE5534 amplifier so that the abs max input would be smaller than 0.6V. I'll have a look on that ref. Thanks.

Quote from: Paul Marossy on March 05, 2013, 12:17:44 PM
Have you ever looked at the Lab Series L5 amp? It has an awesome crunch tone - think early King's X guitar sound. BB King and Allan Holdsworth were also notable users of that amp. Bob Moog/Norlin design. No output transformer. Granted it's complex, but it was designed specifically for guitar and to sound great, which I think it does.

http://www.diyguitarist.com/Schematics/L5-Schematic.jpg

I forgot to mention this Moog's distortion (http://www.google.com/patents/US4180707?dq=4,180,707&hl=en&sa=X&ei=wTmJUaS_CcKp7AaM44GYCQ&ved=0CDUQ6AEwAA). I can not see it on that schematic but these resonant filters (http://www.google.com/patents/US4117413?pg=PA2&dq=4,117,413&hl=en&sa=X&ei=CzqJUb6JK82w7Aa51YDoCA&ved=0CDgQ6AEwAA) are easy to find.

Cheers.

I tend to agree that lots of open loop gain coupled with large amounts of global negative feedback is the enemy here. You get a very "stiff" and faithful output, right up until the point where it can no longer follow the input - and then it falls over completely. OK for hifi amps, but not good for guitar.

So, intuition suggests that we either;

a) "fake" our overdriven condition, by tossing it in upstream and make sure that our amp can't rail to rail clip (like R.G. has already suggested). Perhaps we can make our amp's output a little less "stiff" by incorporating the speaker into the feedback loop, as has been done by Marshall and others, or:

b) start from scratch with a topology that isn't from a hifi cookbook, or power IC datasheet. Less loop gain, less (or no) global feedback. It would be nice to avoid the cost and sourcing hassle of an output transformer, but in many ways it makes things a bit easier - single power rail, no worries about DC offset, no need for complimentary output devices. I'm sure it's possible though to build a transformerless design using e.g. complementary output mosfets with a bit of local feedback around the driver / output stages without having to fiddle with trimmers and whatnot every five minutes.

I suppose that if it was easy, we'd all be doing it!

Quote from: gritz on May 07, 2013, 01:58:37 PM
... incorporating the speaker into the feedback loop, as has been done by Marshall and others, ...

do you mean kinda like this? http://www.diystompboxes.com/smfforum/index.php?topic=81956.msg679661#msg679661 (http://www.diystompboxes.com/smfforum/index.php?topic=81956.msg679661#msg679661)

http://www.current-drive.info/9 (http://www.current-drive.info/9)

or am i barking up entirely the wrong tree, lost in the forest once more?

Quote from: artifus on May 07, 2013, 02:04:46 PM

Quote from: gritz on May 07, 2013, 01:58:37 PM
... incorporating the speaker into the feedback loop, as has been done by Marshall and others, ...

do you mean kinda like this? http://www.diystompboxes.com/smfforum/index.php?topic=81956.msg679661#msg679661 (http://www.diystompboxes.com/smfforum/index.php?topic=81956.msg679661#msg679661)

http://www.current-drive.info/9 (http://www.current-drive.info/9)

or am i barking up entirely the wrong tree, lost in the forest once more?

Yeah, that's the idea! *very basically* the typical hifi style solid state amp (discrete or on a chip) is a bit like a regulated high current power supply and it crowbars the loudspeaker into following it's will. By incorporating the speaker into the feedback loop the amp becomes more responsive to the way that the speaker's impedance changes at different frequencies - particularly at the resonant frequency of the speaker + box it sits in - it allows more of the speaker's "colour" around this point to shine through. An important part of exploiting this characteristic is the resistor and capacitor that are fitted in parallel to the speaker - as the frequency rises above the midrange, the inductance of the loudspeaker coil starts making itself felt, raising the impedance of the speaker. This can make the response very trebly as the amp tries to force more current through the speaker at high frequencies (as the speaker looks like a high resistance at high frequencies). This is tamed by the resistor / capacitor combo. The resistor is around the same value as the speaker's quoted impedance and the cap is tuned by ear to give the required response - bigger = darker, smaller = brighter.

If you have a look at some Marshall Valvestate schematics, you'll see the same principle. :)

^That and most important to protect the output stage from breaking-down and preventing the energy store in the speaker inductance to dissipate in the breakdown region of one of the output devices (zobel network), the values for C and R should be something like:

Rz=Rspeaker
Cz=Lspeaker/Rspeaker^2


Also the schem in http://www.diystompboxes.com/smfforum/index.php?topic=81956.msg679661#msg679661 sounds terrible to my ears (just breadboard that a few days ago).

Cheers.

I like your suggestion:

Quote from: gritz on May 07, 2013, 01:58:37 PM
start from scratch with a topology that isn't from a hifi cookbook, or power IC datasheet. Less loop gain, less (or no) global feedback. It would be nice to avoid the cost and sourcing hassle of an output transformer, but in many ways it makes things a bit easier - single power rail, no worries about DC offset, no need for complimentary output devices. I'm sure it's possible though to build a transformerless design using e.g. complementary output mosfets with a bit of local feedback around the driver / output stages without having to fiddle with trimmers and whatnot every five minutes.

I suppose that if it was easy, we'd all be doing it!

I'm still looking for that not-hi-fi-cookbook-or-power-ic-datasheet-topology :D And yes it is not easy, but it is a lot of fun!

Cheers.

Quote from: tca on May 07, 2013, 01:05:36 PM
I understand that, but it seems very counter-intuitive not being able to saturate the output stage even if using a SS amp. I think the same effect happens, or is similar to, while playing a acoustic instrument, the player rests on the assumption that by picking the strings hard (increasing the input) some type of distortion will occur.

I guess that it's not counter-intuitive to me.

The issue is not saturating the output stage, it's what happens to the rest of the circuit when the output stage saturates. Output stages by themselves are not sticky. They largely do what they're told as long as it's no faster than the device physics allows them to turn on and off. And it's very dependent on what's happened at the input and voltage amplifier stage(s). And it depends on whether the input common mode range or the input differential mode range is exceeded.

It's not usually an issue unless the input is overdriven a lot. It's a recognized problem with integrated opamps, and many of them have internal protection diodes for just this reason. A little careful design eliminates it while still letting the thing distort. It just doesn't latch up or stick and go up in smoke.

It's fine if the amplifier limits when driven by too much input, and it will merrily contribute distortion as expected by musicians (although perhaps a different distortion than was expected). But like all active (well, and passive, too, come to think of it) things, it has limits, and its limits are often different from other things.

> a topology that isn't from a hifi cookbook, or power IC datasheet. Less loop gain, less (or no) global feedback.

Most audio amps run Class B. It is very hard to hide the crossover distortion without ample NFB.

The JLH runs Class A. Nothing changes very fast, the way a Class B stage *switches* from top to bottom device every half-cycle.

The JLH can easily be re-rigged for little or no NFB (or for current-feedback). Though I'll wait to rack my brain until tca has one working at real power output and clean ("too" clean).

Tubes *are* an answer, for the silly reason that they appear happy when run HOT so are usually run much nearer Class A than typical transistor amps. When run into Class AB they don't typically go into crossover distortion until fairly high levels, and the effects appear over a broad range of level without the sand-on-cone tone of hard-switched transistors.

The JLH driver is heavily single-ended so even-order distortion dominates. While I don't think this is essential in the power amp (Bassman 5F6a driver and output stage is heavily odd-order), it couldn't hurt. (Tube Fenders get 1%-5% even-order in the first stage and/or a tone-driver stage. Late Ampeg V40 had a Fender-like and a dead-clean input, even I could hear more musicality in the crude Fender-inspired input.)

So here goes my little JHL guitar power amp.

(http://www.diale.org/img/jhl.png)

So far so good. It does get hot and it still needs some tweaking to make it sound more like a guitar style amp. It overdrives nicely (up to a point) and it gets loud. I'm powering it with a 18V laptop power source.

One thing that I've notice with discrete (class ab and class a) designs is that they do not complaint when using a L-Pad attenuator, something like this:

(http://www.diale.org/img/l-pad_buy.jpg)  

Chip amps don't accept all that well a L-Pad on the output, the ones I've tried start to heat up and then shuts down.

Cheers.

P.S.
I've not tested yet (just sim)  but it seems that by adding the first device (as the original JHL) similar power can be obtained with a smaller idle current.

That's not the cannonical JLH.

You have the top output (your T1) ham-strung as a constant current device.

Maybe not *constant*; but far from push-pull.

Maybe you have confused the boot-strap.

And adding the input stage can't change the power output significantly.

Wire it *just* like the original JLH. It's only a couple more parts, wired right.

And I don't think it will drive well without a preamp. Classic amplifier design is, like a concerto, in _3_ parts. Bring weak signal up to a decent level. Gain-pot. Bring decent level up to BIG level for speaker.

Don't know why chip-amps would hate an L-pad. They are fairly widely used in hi-fi remote speaker control.

> You have the top output (your T1) ham-strung as a constant current device.
> Maybe not *constant*; but far from push-pull.
> Maybe you have confused the boot-strap.

You are absolutely correct, the schematic has an error, I've confused the boot-strap. Thanks.

It should be something like this (that is what I have on the breadboard, without Q1):
(https://www.passdiy.com/projects/images/content/plh_amplifier2.png)

> Don't know why chip-amps would hate an L-pad. They are fairly widely used in hi-fi remote speaker control.
Now that you mention that, I guess the problem was that I've used the L-Pad when trying to overdrive the chip, and thus it shuts down not because of the L-Pad but due to the overdriving signal.

Just corrected the schematic.

Cheers.

Hi,
so the JLH version did sound ok... but not completely happy with it. It had some annoying fuzz/fizz/buzz when overdriven... that could be fixed, but if it is there why bother taking it out!  So I've turn to the most simple amp that I could think of, a mosfet and a light bulb at the drain (as PRR suggested). It sounded also ok, as I recall from my previous experiments. I've already seen some of the amps made by Nelson Pass (delite amp is the mosfet+bulb version) and specially the Zen amp that uses a active current source. Here is my hack on it (biased at 1A):

(http://www.diale.org/img/zen.png)

So, how does it sound? It is the most amazing amp I've build. It stable as a rock, no fuzz/fizz/buzz, it has an incredible warm sound, so amazing that I've plug it in to a hi-fi set of speakers (what a sacrilege!) and still is amazing! As R.G. point out earlier, there is no such thing as a specific guitar amplifier. It does get hot, much hotter than the JLH.

I found this topology in  "Design of VMOS Ciruits" book from the 80's (the same book that Jack mentions on his BS170 booster webpage) but Pass's patent for a very similar topology is dated from 1998 (https://docs.google.com/viewer?url=www.google.com/patents/US5710522.pdf). Go figure?!?

Cheers.

Quoteit has an incredible warm sound, so amazing that I've plug it in to a hi-fi set of speakers (what a sacrilege!) and still is amazing!

That is the sound of a percent or so of second harmonic distortion. It's very pleasant, and part of the highly-sought-after tube sound. Self biased triodes, linear as they are, have a little of it. Single-ended tube power amps have a relatively lot of it.

Sorry I didn't think of it earlier. You'd probably enjoy Rod Elliot's pages on power amps. See http://sound.westhost.com/project36.htm (http://sound.westhost.com/project36.htm) for some info on a similar design, and see the rest of his articles for general reading in a similar vein.

Quote from: R.G. on May 15, 2013, 10:43:14 AM
You'd probably enjoy Rod Elliot's pages on power amps. See http://sound.westhost.com/project36.htm (http://sound.westhost.com/project36.htm) for some info on a similar design, and see the rest of his articles for general reading in a similar vein.

Yes, thanks.

I've been cruising some older entries on this forum and found this old topic: http://www.diystompboxes.com/smfforum/index.php?topic=51555 Very interesting reading! One of the most interesting comments  was done by Doug H, and I quote:

Quote from: Doug_H on November 17, 2006, 08:25:52 PM
The thing I could never get around with 9v pedal circuits is the fact that tube pre's are usually running around 200-250v for their supply voltage. Then there is a tradeoff between gain and clipping headroom that you can't duplicate at 9v- unless you attenuate the guitar signal down to scale it for the circuit. So a 100mv guitar signal @ 250v scales down to a 3.6mv signal @ 9v, if you have the same gain characterstic with the JFET that you would with a tube.

Although the topic was about amp emulation with fets, the argument follows for any active device. Doug's comment confirms what I've tested in all the amps that I've done. If you attenuate the guitar signal down to scale it for the circuit (voltage power source) and keep the gain stages high enough the feel of playing with these settings is very expressive, dynamical and sounds real good. I've tested this idea with an opamp booster and common BJTs amplifiers, but there is a catch... the noise to signal ratio is very tiny and so noise is inevitable. Any thoughts on keeping the ratio  1/25 in a low voltage amplifier with a high signal to noise ratio?

Cheers.

hello mother nature?

To be honest the only passable way would be to just raise the supply voltage. The noise level doesn't really go down too much past a certain point; -90dB is usually pretty good, which would still be noticeable with the guitar signal attenuated that much. I suppose a gating circuit might help a bit, as long as the noise is less than the guitar signal.

Of course all the different noise minimizing techniques apply too. A major one is keeping the impedances low. If you buffer the guitar before attenuating you can use smaller resistors, which will cut down noise. Good shielding helps of course. Hunt down Texas Instruments' Opamps Design guide. There's a full chapter on noise.

Another suggestion, I don't know if this will even work, would be a balanced circuit. Transformers would be best. Buffer and send to transformer. Take the signal and an inverted copy from each end of the secondary. Attenuate each by the same amount, though you could use the trafo to step down too. Run each copy through identical circuits. Sum at another trafo, maybe step up if necessary. If everything is matched well, perhaps the noise will cancel. Of course, noise is random so maybe it'll be worse too. I'm just guessing here though. No idea if it will actually work. :)

> perhaps the noise will cancel

Random hiss won't cancel. (As artifus says: You can not cheat Mother Nature.)

Power supply and other noise can cancel; there's usually other ways.

Some time ago I found very interesting MOSFETS IXTP01N100D.
http://ixapps.ixys.com/DataSheet/DS98809C%28IXTP-U-Y01N100D%29.pdf
1kV of the drain voltage, 25W power dissipation. That allows to use them with standard transformers for tube amplifiers.
I already received ten of them from arrow electronics.
Now I'm waiting for the ordered transformer http://www.classictone.net/40-18045.pdf to start testing.

Also see at geofex: MOSFET Follies

And worry about the operating conditions. Tubes in general have "breakdown voltages" from plate to cathode that are well over 2x their maximum plate voltages. An output tube in a push-pull setup pulls down on its half of the primary with the center tap tied to the B+. The other half-primary, which is connected to the other output tube is pulled UP by transformer action. The tubes have to withstand nearly 2x B+ on each peak.

MOSFETs will pull the on-side down closer to ground, so the off-side will fly higher. 1000V sounds good, but is actually marginal.

MOSFETs have a characteristic somewhat like a pentode or power beam tube, OK, but are actually different beasts. An output transformer for a pair of MOSFETs is a different design than that for a couple of tubes.

And happy hunting.   :)

Quote from: R.G. on May 31, 2013, 11:21:54 AM
Also see at geofex: MOSFET Follies

And worry about the operating conditions. Tubes in general have "breakdown voltages" from plate to cathode that are well over 2x their maximum plate voltages. An output tube in a push-pull setup pulls down on its half of the primary with the center tap tied to the B+. The other half-primary, which is connected to the other output tube is pulled UP by transformer action. The tubes have to withstand nearly 2x B+ on each peak.

MOSFETs will pull the on-side down closer to ground, so the off-side will fly higher. 1000V sounds good, but is actually marginal.

MOSFETs have a characteristic somewhat like a pentode or power beam tube, OK, but are actually different beasts. An output transformer for a pair of MOSFETs is a different design than that for a couple of tubes.

And happy hunting.   :)

I know about these limitations, because I already made wide range of mosfet PA's with transformer output from 250mW to 90W.
http://milas.spb.ru/~kmg/fetpa_en.html
All of them sounds very close to tube PA's.
I look at these transistors because 2SK216 become EOL, so I need replacement for them.
http://am.renesas.com/req/support_products_list.do?layerId=555
For transformer, mentioned above PA requires about 300V supply with drive by IXTP01N100D. Twice voltage will be far enough from maximum limit.

Good.

Quote from: R.G. on May 31, 2013, 11:21:54 AM
And happy hunting.   :)

I really think that this "thing" called "tube sound" with SS devices, is a ill defined problem. There is not a unique solution, there is not a such thing as a "tube sound" stricto sensu, it is much more a physical feeling of a playful musical experience. I'm not saying that you can't get beautiful sounds by "copying" and building a 300V mosfet amp, it is not that at all. What I'm saying is that simple is best and also the best.

For instance, look at a "typical" tube amp:

1. Guitar in
2. gain stage -> tone control/tone shaping
3. repeat 2 or goto 4.
4. power amp
5. speaker

If you design a SS amp without the "tube sound restriction/bias" a lot of new sounds appear.

Cheers

Quote from: artifus on May 22, 2013, 01:17:35 PM
hello mother nature?

Mother nature works in mysterious ways! The best way to circumvent this is to have the power voltage source high enough, at least to get 20dB in the first gain stage: 20V?!.

I'm of the opinion that if something makes noises appropriate to what you're trying to do, does so repeatably and reliably, is justifiable in terms of expense and doesn't take Chuck Norris to carry it up three flights of stairs then I'm pretty pragmatic about what's inside.

Quote from: tca on May 31, 2013, 05:58:03 PM
Mother nature works in mysterious ways! The best way to circumvent this is to have the power voltage source high enough, at least to get 20dB in the first gain stage: 20V?!.

indeed she does. one of her odd ways is that most passive pick ups only put out .2 - 2 volts on average. we could raise that (say) 2v to 20v but what else do we raise with it?

i like this; lets consider this further:

QuoteFor instance, look at a "typical" tube amp:
1. Guitar in
2. gain stage -> tone control/tone shaping
3. repeat 2 or goto 4.
4. power amp
5. speaker

this could be extended to any signal 'processor'

*gosh* that sounds a bit aggressive reading it back - i didn't intend it to. genuine request for open discussion.

Quote from: tca on May 15, 2013, 10:10:10 AM
Hi,
so the JLH version did sound ok... but not completely happy with it. It had some annoying fuzz/fizz/buzz when overdriven... that could be fixed, but if it is there why bother taking it out!  So I've turn to the most simple amp that I could think of, a mosfet and a light bulb at the drain (as PRR suggested). It sounded also ok, as I recall from my previous experiments. I've already seen some of the amps made by Nelson Pass (delite amp is the mosfet+bulb version) and specially the Zen amp that uses a active current source. Here is my hack on it (biased at 1A):

(http://www.diale.org/img/zen.png)

So, how does it sound? It is the most amazing amp I've build. It stable as a rock, no fuzz/fizz/buzz, it has an incredible warm sound, so amazing that I've plug it in to a hi-fi set of speakers (what a sacrilege!) and still is amazing! As R.G. point out earlier, there is no such thing as a specific guitar amplifier. It does get hot, much hotter than the JLH.

I found this topology in  "Design of VMOS Ciruits" book from the 80's (the same book that Jack mentions on his BS170 booster webpage) but Pass's patent for a very similar topology is dated from 1998 (https://docs.google.com/viewer?url=www.google.com/patents/US5710522.pdf). Go figure?!?

Cheers.

There is something you need to add to the sim.  The source impedance "feeding" it.  Power mosfet have gate to drain and gate to source capacitance. So some of the "warm" sound can be the low pass frequency.  You might want to add a small series input resistor to the gate close to the mosfet.

Here is something to think about .  Musicman amps, some were IC and transistor even to the output stage(class b? push pull cascode with a transistor in the bottom part of the cascode)  I think a lot of the tube sound is the output transformer output tube interaction.

Also jimi had a post with a cool Kay transistor amp schematic .  SE transformer driving a push pull transformer output.  R.G. has written about the VOX solid state amps.

> There is something you need to add to the sim.  The source impedance "feeding" it.  Power mosfet have gate to drain and gate to source capacitance.
> So some of the "warm" sound can be the low pass frequency.
I've done that also, but did not show it... I've also substitute the upper CCS with a car 6.8Ohm light bulb and it works nicely. It also needs a decent preamp, which I also have. I'll post the complete schematic in a few days.

> You might want to add a small series input resistor to the gate close to the mosfet.
Done that also.

> I think a lot of the tube sound is the output transformer output tube interaction.
That is what almost everybody says. I have to build a amp with an output transformer to see what that *really* means, my biggest problem is where to find one with the right specs.

> Also jimi had a post with a cool Kay transistor amp schematic.  SE transformer driving a push pull transformer output.  
I have a lot of schematics, from late 60's and 70's, with that kind of topologies with Ge and Si devices, as always, the problem is where to find the transformer.

> R.G. has written about the VOX solid state amps.
Already read that, thanks.

Cheers.

Here goes my light bulb amp

(http://www.diale.org/img/bulb_amp_irfp240.png)

I've been playing with it for quite some time now. It does get hot (I'm using a 10cmx10cmx3.5cm heat sink, it does not take the skin off my fingers!) and has roughly 1.5A idle current. No quite certain about the preamp I've made... But the power amp part is done.


Cheers.

P.S.
It does distort nicely when overdriven.