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rsbay 3rd April 2006 04:54 AM

Zen V2 with low frequency oszillation

I've changed my old Zen Amp to the Zen V2 with the Aleph current source. I took the schematic from the article with only one change - I lower the resistor value (R1) of the current source from 0.33 to 0.47 || 0.47 = 0.235 to get more current. All other values are the same.

I use my old Zen with Stacked 57th Quads (ESL) for a long time without any problems. With this new circuit I get a low frequency oszillation (a form of pumping) without an input signal. When I connect my Aleph P clone to the Zen (the preamp is switched off) the frequency changes.

I solder a 10k pot in parallel with the 1.5k resistor R15. Now I can change the amplitude and the frequency of the oszillation with the pot.

Has anyone an idea what I have to do to get a stable amplifier ? Is it worth to change R15 to a higher value ?



JCM 3rd April 2006 09:38 AM

Positive feedback there.

This reminds me the "motorboating" effect of tube amps. In that case, poor power supply regulation was at fault. But with a Class A amp, this should not be the case. Put an analog ammeter in series with the power input, see what happens. Check for HF oscillation with a 'scope.

How does it work with R15 / C6 disconnected from Q3 ?

I would disconnect that, and see how it works. Then, with no speaker load attached, connect a resistor maybe 2K or so to the base of Q3 - with no capacitor in series - and the other end of the resistor to V+ , and see what happens to the current source current. It should go down to about zero.

Make sure the capacitor C6 is correctly installed and in good condition.

It seems to me there is a positive feedback path affecting Q3. If the problem isn't with the power supply, and isn't with the C6 / R15 leg, there may be some parasitic effect there. IF capacitor C6 is bad or off-value, you can get oscillation like that.

Make sure all the ground connections are proper and firm.

Does this happen with no input, or only when a signal is present ?
Will it drive rated power into a dummy load (without the C6 / R15 connection ?

I don't think R15 is the problem, changing its value would only be a stop-gap to a problem that should not exist.

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