Ultra-simple discrete 10W audio power stage for guitar use - would this work?

I wouldn't do a discrete amplifier at 10W.. Use an amplifier module. There's many from Digikey and Mouser. Use the application notes on the datasheets.

D from BC myrealaddress(at)comic(dot)com British Columbia Canada

Hi D from BC.

I appreciate your advice, and I am aware that monolithic audio amplifier IC's or modules are available in powers up to 100W or so. However, this is a special purpose application.

I fully understand that these modules can provide absolutely wonderful THD, frequency response, efficiency (class D etc.), damping factor, et cetera. But they sound absolutely horrible if simply used in this application. People have tried it. It sounds horrible. Guitar amplifiers are generally operated partially or entirely into distortion. Even a 'clean' guitar sound is pushing various stages into the red on the transients. This is part of the sound of an electric guitar. Vacuum tubes are not necessary for this, but are well loved in this application. I've built vacuum tube amplifiers myself, and enjoy using them for this.

Yes, these distortions can be simulated in many different ways, analog or digital, and then fed to a perfectly clean power amplifier. Many people, in fact, do this. For example, I've seen bands who have no amplifiers on stage whatsoever, but prefer to run their guitars and basses through such modeling devices directly into the house PA. One problem with doing this is that your modeling setup will actually require a much more powerful amplifier in order to prevent it from ever clipping. Which, in turn, means bulkier speakers. You end up with a much heavier and more complicated setup which doesn't really provide much of an improvement in performance. That's why they still make guitar amplifiers with imperfect amplification stages.

Audiophiles don't like the 'canonical' transistor amplifier either, and I can see their point even if I don't agree with the 'magic wooden knob' school of thought. The standard design - the one that looks like a great big op-amp - doesn't handle being pushed into clipping very gracefully. If you listen to recordings with lots of dynamics, like most classical, you will probably be clipping on transients. Just like a guitar player would. The question is - how does that clipping sound? Gentle, or harsh? Does the amp take a few milliseconds to recover equilibrium afterwards?

Some people would prefer their harmonic distortion to come in more gradually, and are willing to tolerate/enjoy a little bit even at low levels. Audiophiles and guitarists are such people.

And there's nothing wrong with that, is there? We're allowed to build more than one kind of audio amplifier, at least I'd hope so. I mean, wow, there are people building single ended triode amps in this power range too.

I was just hoping people would check out the circuit and let me know what they thought of it.

Yep. I'm pretty sure that's where I want it.

It's based on a MOSFET design of John Broskie's from his Tubecad.com site. I didn't believe it when I saw it either. But SPICE seems to think it should work, and I like the simple feedback network. As Q1/ Q3 turn on, (and Q2/Q4 turn off) they pull down the positive voltage rail. This pulls UP the negative voltage rail. This change in rail voltage is coupled to the speaker through the filter caps C1/C2. As far as ripple - equal and opposite ripple components will be coupled to the speaker through C1 and C2, which should cancel. The feedback of the darlingtons tends to 'buck' the ripple as well. This whole thing simulates a lot better than it reasonably should.

And, as a design with low damping and without overall feedback, it might well be a nice thing to have overdriven by a roaring guitar.

At least I think it might sound nice. I just have to try this someday. Maybe soon.

Snip.....

patch

I went to one show where they were using wireless mics and all of a sudden somebody was ordering a hamburger and a coke. A bit of fm interference from the joint across the street. They switched out to wired mics for the next set.

That's pretty funny. I would have liked to have seen their faces.

The wireless stuff would have to be a fairly sophisticated technology like spread spectrum to avoid that type of interference. I haven't done any checking, but I would bet that it's already available.

Bob

it also preferably needs to be realtime, if there's a way to do spread-spectrum in realtime then yeah that'd work.

Echoplex. They were a tape loop echo effects machine. Most of the ones I saw used tubes. Basically, it uses two heads to record & play back the input. The playback head is on a tack, to adjust the delay. The bias to the erase head was adjustable, to set how many passes it too top erase a signal from the tape loop. They were a pain to service, because the cheap assed musicians would use them till the heads were worn out, with bad tape loops that hissed like crazy. Then they wanted everything replaced for $5, including the labor. :( They were all world class cheapskates. That was why I got out of the music business, long ago.

I've noticed that the music biz is mostly low-pay for everybody but a few superstars. All the "glamour" industries (like music, film, fashion, cuisine, even winemaking) seem to expect people to work for peanuts in exchange for being around all the "artists" and such.

My pcb layout guy is quitting and moving to LA to take courses on becoming a sound engineer. I wonder if his income will ever recover back to what we're paying him now.

John

Can he work 24/7 for years on end? If not, I doubt it. I've seen good sound engineers who had to bum rides to work, and wear ragged old clothes, because their pay was so low. All the money went to the performers and sales people. I had one TV station offer me the job of chief engineer years ago. They wanted to put me on salary, have me on call 24/7 and pay minimum wage. I laughed in their faces & walked out of the building. They had been through three engineers in the year they had been on the air, and bragged that the station was built way under budget. Their OTA signal showed just how far under they were. :(

From my father's stories it appears that Muntz used selected tubes. They were nearly impossible to repair... any tube change required an IF realignment.

...Jim Thompson

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That's right about the whole Muntzing thing. I'm already considering other parts to uglify the design. Maybe a couple clamping Zeners to prevent Vbeo breakdown, perhaps even a small ballast resistor in the collector circuit of the driver transistors to limit current.

And yeah, I don't know if I really want integrated Darlingtons. I'd rather sort through a bin of driver transistors in the hopes of finding 2 matching hFe combinations at idle current. Would be better than no matching :)

So yes, totally Muntzed.

That is actually quite easy if you know what you are doing. It can be done FHSS, DSSS or combined modes in real time. Not really any worse than 100 feet of wire. And that is with stable and constrained latency.

Status report - thing can power a 12" loudspeaker from an electric guitar but needs some preamp gain to really do this. Currently using TIP3055 and TIP42 power transistors, mounted on an old Pentium II heatsink (this CPU dissipates 25-40W). Sound is nice at these low levels, I haven't got enough gain to push it though. Gotta slap together a preamp and play it loud!

Phil - these transistors, unlike the 'real' 2N3055, maintain a minimum hFE at moderate (4A) current so as to spare the driver transistors. I also include a 47 ohm ballast resistor in the collector circuit of each driver transistor to further limit current. The mains supply is fused at 0.5A on the line side, which works out to 2.5A on the secondary (probably more like 2.25A or 2A even, with the magnetizing current and losses and all that?) I do admire and appreciate your bulletproof ethic, and if I wanted a serious design that could handle a low-impedance highly reactive load I'd definitely want to consult you. I hope this can be cranked into an 8 ohm speaker without dying, and I'll find that out soon enough. If it does, oh well, build it bigger.

To be honest, I'm amazed that this swinging supply works at all. It really seems counterintuitive. But the rails do swing, the filter caps do couple that swing to the loudspeaker.

I figure that I'll handle the Vbe issues by using a limited supply voltage (9V) for the preamp, so that it can't swing the bases enough into reverse to zener them. Probably the final preamp will have a CFP output stage and use JFETs for the lower-level stuff where voltage swing won't be as huge a deal.

Thanks for all the good advice I've received here.