Ashdown EVO II 500

[Mark Black]

Hi again -

Anybody have (and care to share) a schematic for the power section of an Ashdown EVO II 500 bass amp?

Or an idea where to look for the cause of my remaining problem:

The amp came in with shorted output & driver transistors, and showed signs of having been worked on before. I replaced all output & driver transistors and checked all the other components in the output section.

The amp now fires up & passes signal but has an anomaly on the negative peaks at onset of clipping which I guess I would characterize as "latch-up". Instead of gracefully coming out of clip and following the sine wave signal back toward zero volts the waveform continues at the negative rail voltage briefly then snaps to continue the sine wave forming an extended 90 degree "corner" as seen on the scope. The positive peaks seem to act normally and as I would expect at onset of clipping.

It's just another thing I haven't seen before so I would welcome ideas or commentary from someone who has...

Thanks!

Mark

[Steve Conner]

Hi Mark

That behaviour does happen to a certain extent in all solid-state power amps. But the "sticking" usually only lasts a very short time, so it's only noticeable with a high frequency test tone like 10kHz.

It's caused by charge storage effects in the transistors, so I'd guess someone replaced a transistor in the negative side with a slower part instead of the original. Either that, or a resistor somewhere that would normally help discharge a transistor base got disconnected during the repair job, or burnt up when the output transistors blew. Or maybe Ashdown's output stage is asymmetrical and that's just how it works. (are all the output transistors the same type, or are there two different types? what type are they for that matter?) It's hard to be more specific without seeing the schem.

If it uses MOSFETs, what I said kind of still applies, but you should check the zener diode that is usually between gate and source. And read "gate" instead of "base" :P

[Mark Black]

Thanks for the reply Steve.

This thing is indeed a symmetric push-pull using bipolar transistors instead of the MOSFET's I am used to seeing in Ashdowns.

I think I will give them another try for a schematic - they ignored my first request...

Thanks again!

[Enzo]

Try calling on the phone.