Where to get a choke for a class-A single-ended MOSFET power stage?

Thinking of a circuit like this. Simple and elegant-looking:

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Really like a very cheap version of a single-ended triode stage. Basic 'parafeed' arrangement except that no transformer is necessary. Resistors provide bias, negative feedback, and give the FET a bit of 'plate resistance' like a triode to lower output impedance.

What's great about this cheap FET circuit is that it's cheaper than the tube equivalent, much, and might sound something like. No need for expensive output transformer. A cheap FET like a IRF510, compare to a 300B, yeah. No need for heater supply! And the choke . . . the choke doesn't need to be NEARLY as large. A parafeed triode amp, to get decent bass response, might need a choke in the tens or hundreds of Henrys range. Here, the current is much greater, the voltage is much lower, impedance is much much lower, so you can get by with a choke in the millihenrys. Should be easy to find one I think.

But I'm not sure where. Any ideas where to get a 50-100 mH choke for this? Needs to take an amp or three of current without saturating. That's the hard part there I think. Lots of dual "common mode" 5-30 mH chokes for sale but I imagine those don't take much DC current at all before saturating, they're meant for AC circuits aren't they?

If I could find a choke this would be a nice cheap thing to build though. Easy enough.

Reply to
Morris Slutsky
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If it hasn't been mentioned, or you already know, I hope you visit and d/l all the free data available at:

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Mr. Pass in the KING of class-a design, offers enough free info to fill a book, and answers ?s in the forum.

When reading, you will find that a choke there is big, heavy, and wasteful. Just use a constant current design (cheaper to build too). I've built his ZEN designs, which look alot like your pix using CCS(s). There are even sources for PCBs! I promise you you'll LOVE the site.


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Did I miss something, but I didn't see what the amplifier was for. Thats funny Pass came out with that SE about the same time I drew up mine.


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I have a tee shirt from the old Georgia Champ wrestling folks that says "EVIL MEAN AND NASTY". A gift from Ole.

Well, back when the humbucking PU was made, about 6 people said they 'invented' it (not saying that you claim the same) at the same time.

Mr. Pass has too awesome of a history in amp design for anyone to doubt his actions. He also knows it's simple designs. Then again, he also YEARS ago stated that chokes are too much a bother to deal with, something you were searching for. I remember buying the white paper "Power to the People" about hi voltage power supply design, a great read (LONG LONG out of print) and the guy stated he 'designed' a quad gated MOSFET preamp. He did. I've built a few, they work great. And the same basic design was also in a book dealing with the same Mosfet called : Mosfet Circuits Handbook, by Mr. Turner.

Years and worlds apart. Both 'designed' by talented Icons in the field.

It's like when Mesa/Boogie 'designed' the 1/2 power switch, by lifting the cathode off 2 tubes..and the 'mid' switch by lifting the tone circuit ground (sometimes bypassed by a cap). Sheit, I'd been doing that too, and know folks in the UK who also did. No big deal. But no big 'design', although M/B made it most available. Carvin and Acoustic 'stole' it from M/B, sometimes as DIRECT copies..like the Acoustic. Even the schematic was almost a carbon copy..right down to the mistakes.

At least 'Earth' was smart. The Xeroxed the PCBs that Peavey made, even down to the etching 'PV' on the copper when they copied/stole PV's designs.

Sometimes the same idea hits many people at the same time. But sometimes, people like the 'Earth/Plush' people just downright steal from the designers.

Gawd, this is a fun biz. So much history, so many tales untold.



Aug 16, 2010 Hampton Beach NH.

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Wasteful is relative. Electrically, a choke-loaded single-ended amp is twice as efficient as a current-source-loaded single ended amp (because you have dissipation in the CCS that you don't with a choke), which is turn is twice as efficient as a resistor-loaded single ended amp.

Yes the Zen designs are cool. But if I wanted a badass single ended FET, a choke would be the most electrically efficient way. Cost efficient? Weight efficient? Maybe not.

It is depressing that, despite the relatively low inductance needed in a solid-state single ended circuit, the currents are higher and I need just as much iron. Huh.

Reply to
Morris Slutsky

Yes, but more hum and magnetic interference.

I'd bet you don't know how to design CCS(s) that interact with the circuit. Didn't Mr. Pass teach you anything? Maybe it was over your head. Was for me till I built a few. Once you see what happens with SE designs, and how current is used, you get the picture. I'm not talking single device CCS devices, which are little better then resistors, I'm talking controlled transistor/Mosfet devices that read the amp and adjust to the need. A choke just sits there doing the best it can. Fighting what the PS feeds it.

See what I said above. A choke is a one trick pony, often done well, but a interactive CCS reads the circuit and does what is needed to keep the current flowing full tilt.

Yeah, more parts. But better operation.


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I was able to make an LTSpice simulation that gives about 37% maximum efficiency with a 6 VDC power supply, 1.94 VAC input, and 3.13 VAC output into an 8 ohm load, at 200 Hz. It appears to work from 20 Hz to 20 kHz. But the biasing is very critical, and probably will be unstable with power supply, temperature and device variations. I used an STD30NF06L MOSFET which has a low turn-on voltage. R3 in the schematic is not needed, but the V2 signal source should be low impedance.

I have some 100 mH chokes rated at 10 amps that I found at a Hamfest in a pile of freebies. But they are probably 30 lb each.

I agree that this is not really a very practical design, and much better alternatives exist.


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Reply to
Paul E. Schoen

Probably more parts would be better in every measurable way! You don't build a single-ended class A amplifier for any objective reason anyway. It's something you do for subjective, not objective reasons. Like, say, building a guitar amplifier that uses 2 EL34 tubes with 300 V of plate supply and a 5K load. Not ideal in any design sense. I do like the way it sounds, though.

I understand that in general all coils will pick up interference. But seriously, a choke that runs into an 8 ohm speaker?

Maybe it was over my head, maybe it wasn't, you decide I guess. Sure I read his website already. And I guess I see his Aleph-type current sources as not really single ended amplifiers. In fact, I know someone with a Pass power amp and it sounds very very good. I could almost see switching from a choke design to an Aleph type thing. But that's really, really push-pull from my point of view. Nothing wrong with that, I could have more than 1 transistor per channel - it'd be so hilariously cool to do it with 1, though, you can understand that.

Efficiency is something that single ended designs often suck at. Push- pull stages - ideal class B is 78.5% efficient, ideal class A push- pull is 50% efficient, class AB lies in between. Ideal single ended class A with an ideal choke or transformer is also 50% efficient. With an ideal CCS load, 25%, and with a resistor load 12.5%.

Pass has a few designs up on his DIY site that lie between these situations. He has, for example, resistor-loaded single ended amps (with horribly low efficiency). He also suggests use of incandescent lamps as loads, which have a strong temperature coefficient and could perhaps even be seen as something BETWEEN a resistor and a CCS load. He has CCS loaded single ended class A, the "Zen" amplifier.

And the Aleph is also sort of an in between thing, between CCS load and push-pull class A.

What the Aleph current source does, as far as I can tell, is sense the current into the load and adjust itself accordingly. It's responding to the bottom FET that actually is providing the transconductance which drives the load. The amount of current sensing/adjustment seems to be variable - from pure CCS on one side, to fully compensating load current swings on the other - at that point, what you've got is a pretty clever way to do push-pull without an explicit phase inverter stage. A lot like an SRPP or some push-pull pentode stages that don't require a phase inverter - works so long as you stay in class A. And so you can push it up to the same efficiency level as a choke loaded stage if you want to.

I will consider such designs. I suppose chokes being expensive and heavy and all. But there's something kind of elegant and brute force about a single MOSFET just sitting there heating away.

Assuming I can keep a stable bias on the damned thing and get a decent choke in the first place, which might be pretty hard actually.

Reply to
Morris Slutsky



Not practical but fun perhaps. Probably I really need a fancier circuit that separates DC bias from signal feedback. I'll look into the CH-1 and it's relatives (the CH-2 might actually be better for this).

Thanks for all your advice.

Reply to
Morris Slutsky

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