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dano12 12th February 2009 08:40 PM

ISO "Simplest" possible amplifier
I'm in a minimalist phase. Can you fine folks suggest ideas for the "simplest" possible amp, with simple defined as:

- lowest parts count
- transistor based
- semi to marginally decent performance
- wall wart power
- 2 to 8 watts



mjf 12th February 2009 10:31 PM

2 Attachment(s)
perhaps a three transistor amp. like that old fairchild amp........

darian 12th February 2009 11:24 PM

The profet!!!

dano12 21st February 2009 06:28 PM

Thanks guys, I'll give those a look.

In the meantime, I came across this schematic:

I've built it up, but no sound. The notes say to bias at 1/2 of Vcc at the drain of Q1, but I can get nowhere near that. I can adjust the trimmer to get 1/2 Vcc at the gate...

Any ideas on what I've done wrong?

Rikard Nilsson 21st February 2009 07:01 PM

The trimpot supplies the gate with any voltage between ground and full power supply voltage. So if you canīt find the DC operating point there and can verify that the gate voltage really moves Iīd say there is only two more components that can be faulty, the FET or the 50 ohm load resistor.

The resistor can be measured very simply, and if itīs ok it has to be a blown FET. They go S/C, so the resistor should become hot if this is the case.


pacificblue 21st February 2009 07:30 PM


Originally posted by dano12
Any ideas on what I've done wrong?
Turn C1 around. It is a bad solution to use it with its positive side to the left and charge it on its negative side on the right through R1 with +9..+28 V.

The input high-pass f3 is 1 / (2*pi*R1*C1). With R1 set to maximum (10k) you get a nice 1,6 Hz high-pass. With R1 set near minimum (e. g. 1 Ohm) the f3 point will be 16000 Hz. Not much left to hear, if everything below 16 kHz is filtered out.

You were probably trying to build this amplifier?

dano12 21st February 2009 09:59 PM

Thanks for the ideas. I checked the resistor and it is good.

I guess I should have socketed the FET (argh)

I'll try reversing the capacitor and putting in a new FET.

Thanks again.

SQLGuy 21st February 2009 10:19 PM

What's your Vcc?

This FET is rated 20V max Vgs. If you're using the 28V Vcc, and you start with the pot turned all the way up, you'll blow the gate first thing.

woody 21st February 2009 11:17 PM

I just wanted to point out that that metal power resistor is
suposed to be mounted on a big heatsink.

psychosteve 21st February 2009 11:22 PM

if thats a pic of your amp id also say that even for testing, the fet heat sink is way, WAY, to small.
bigger ones are easy to find in old pc's and and the power supply's of old pc's. (make sure they are safely earthed out first)

nice though, hope you get it working, steve.

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