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Old 4th February 2004, 02:47 AM   #1
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Default low power class A SE MOSFET amp

HI all-

Would it be possible to modify the Szekeres headphone amp to make it able to drive a 4-ohm or so speaker? Obvious problems, I see are:

Need to get rid of the output coupling cap to remove all issues involving having an electrolytic present. A direct-coupled version is available in the addendum here. This uses a dual supply (I guess I would need to increase this to +/-15 at least... maybe even as high as +/-24V.

So... how much voltage would you think I would need going to the MOSFET?

The IRF510 is rated at about 5A output. Will paralleling work when direct-coupled?

CCS will be provided by LM317 on each channel to set idle current to 1.5A.

If I changed everything above, would this make a decent low power, low cost class A amp? Anything else that should be changed?

Thanks all
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Old 4th February 2004, 06:51 AM   #2
pjacobi is offline pjacobi  Germany
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IMHO several similiar designs are floating around.

Google for '"Power Follower" MOSFET'


Peter Jacobi
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Old 5th February 2004, 05:21 PM   #3
azira is offline azira  United States
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Low-cost and class A only go together with Low power. And low power means way less than the 8 or 16-W you're talking about. The reason is that your power supply needs at even 1.5A of idle current will be 3A total (2 channels) therefore with margin a 5A transformer. 2ndly, even at 1.5A of idle, you're going to need a pretty significant heatsink to cool it, that can cost you as much as the rest of the parts.

The next issue is that this amplifier is a current buffer and not a voltage amplifier. It won't make the music any louder. For that reason, there is no need to put any higher voltages than +/- 7 volts or so. Assuming your driving it from a headphone-out type of jack, you'll be lucky if the voltage swing is 2Vp-p.

Finally... Assuming you are driving it with +/- 8 Volt rails and have a 4-ohm load. Your maximum current can be up to 2A. However, the 1.5A of class-A current will only transfer something less than 1.5 A into your load.

If you still want to build this amp, go for it and build the bipolar version. It will still work fine but I doubt you need higher voltages or to worry about the FET either.

If you're looking for something a little more, look to the Pass Zen amps, they are simple, class-A and easy to build.
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Old 5th February 2004, 05:59 PM   #4
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There's a lot of discussion about this at:

Choke Loads for Zen/Aleph Amps
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Old 5th February 2004, 08:13 PM   #5
azira is offline azira  United States
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Originally posted by Kashmire
There's a lot of discussion about this at:

Choke Loads for Zen/Aleph Amps

No... actually there really isn't..
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Old 9th February 2004, 06:14 PM   #6
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first of all, I cannot give the awnsers to you're question, but I've built the Szekeres ( http://headwize2.powerpill.org/proje...szeke1_prj.htm ) amp for my headphones, and later on I used the diagram to build 2 monoblocks for my homebuild speakers.

I built the headphones amplifier using a 18v 1A5 stabelized supply, but on the unstabelized 22v it also worked well, with little noise. Supply consisted of a 2x15v 2x1A65 trafo, windings in parallel. Capacitance 3x4700u. Source resistors 33 Ohm 5W, but lower rated devices will also work. Choice of FET is not critical, I've done some listening test afterwards with IRF620, BUZ10 and BUZ104. I noticed no difference's. Fet's MUST be mounted on a small heatsink. The sourse resistors also run hot, depending on desired power. The electrolyt output coupling caps must be of a good quality, poor caps make's it sounds muddy. I used 4700u caps. The bias-protection diodes were left out. A pot as bias resistor is nice when experimenting. The amp sounded very clear and had nice bass.

Some weeks later I tried this amp on my speakers, which was a very bad idea, it stood there for weeks, in the open air, test wire's attached to it, heating dust My Sony was unplugged, and is not used for months by now ... I decided to build 2 monoblocks (because it would not all fit in the small cases I had lying around catching ..uhm... dust). The monoblocks have 22V, 2x4700u, unstabelized, a BUZ104 MOSFET and 20 Ohm source resistance. The case was used as cooling for the FET. The amps were hard wired.

Speaker wires must be as short as possible, the act like antenna's. No additional luxury was added to keep it simple, except 2 inputs for later use with different preamps on which I'm working right now. The amp will have enough volume driving it with 2V input when experimenting.
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