Wharfedale EVP-15P amplifier fault

[flyingtele]

Hi there, I have a Wharfedale EVP-15P amplifier and it has a strange fault. If in the input level exceeds a certain point, then you get massive distortion and farting noises from the amp.
I have found it to be in the power amp and no the pre amp section as I ran my sound source direct to the power amp with the same reuslts. I would also get a loud howling noise if I moved my hand close to the rca lead, probably from some capacitance oscillation issue.
Before the amp distorts, it seems to work and sound okay, and as soon as it passes a certain level it suddenly distorts very loudly.

I have looked but cant find the schematics, and have emailed Wharfdale asking for a copy of the service manual. If anyone has a service manual or can offr some tips any help would be appreciated.

[flyingtele]

Okay I have drawn out the schematic for the first part of the input section, which is a start. Past there things get pretty complex. I only have limited test tools, limited to a couple of multimeters and an audio probe to listen to various points in the circuit etc.
Could a fault in either of the two transistors or the intro opamp cause a fault like the one I mentioned? It is a violent distortion, and is much higher in volume than the original audio signal prior to the onset of it distorting. The transistor configuration looks odd to me, I'm pretty sure I double checked when I made my notes, but I will check again tomorrow when I go back to the workshop.
Any help will be well received.

[DUPONT]

hello i'm searching for the service manual technical documentation for the amplified speaker "wharfedale EVP-15" for its maintenance . Can you send it to me at dupontolivier2@hotmail.fr

Thx .

[Grizzly616]

Did you solve the problem? Our EVP 15p produces the same farting noises since yesterday I'd love to have more info on it...

[kevvywevvywoo]

Does it distort on a scope or is it just when under load?

[Grizzly616]

sadly I don't have a scope. Also, it's not simple distortion, it's VERY loud impulses after a certain amp level. I had my friend take a look at it at his home. it was late night so he turned the knob up to 7 of 9 gave it input and the spikes or farting noises didn't occur. Maybe it would have at 8 or 9 but we'll have to test that tomorrow.

[Grizzly616]

In the end we couldn't reproduce the problem until yesterday at the end of a live show. I have a feeling that the problem is connected to heat.

[Ian Finch]

Is this a chipamp or discrete amplifier circuit? If it is evidently a fault in the amplifier and not the preamp/filter circuitry, then there may not be a lot of options for chipamp faults.

OTOH, with discrete circuitry, you may find plenty of faulty components affecting stability such as caps, semis or soldering around the feedback circuit which may be causing this. A pulsing loud noise as you describe is usually called motorboating because of the similarity to a submerged exhaust pipe noise. It is caused by overcoupling of output to input of the amplifier or even the whole circuit, by wiring, component or ageing faults. Since you may believe it is heat related, it could still be any of these.

The circuit of the opamp/Jfet switch appears to isolate the amp when signal is not present. Is there more to it at the gates? I would bypass this Jfet pair or the buffer as well if possible and see if the trouble persists. Unfortunately, I haven't seen these yet so I can't give specific assistance or locate service details.l

[Grizzly616]

Thanks for the thoughts on the matter Ian, very helpful. it seems there won't be any progress until next week, ill post the news then. in the meantime i'll try contact the original author of this thread maybe he came up with some results, just forgot to post.