Quad 306 fault finding

[harrygrey382]

I'm hoping someone can give me a few hints here:
My Quad 306 cuts out at high volumes and feels really quite warm to the touch. I then need to press the reset button, and everything's fine.

It's driving Tannoy 15" monitor golds (pretty sensitive) but also cut out at high volumes with B&W 601S2s. This is turning the knob right up, it moderate levels all is fine. Sounds good, never cuts out. All wires are fine, other amps don't cut out.

This includes a 303 (runs a lot cooler, I assume this is because its AB as opposed to pure A).

I'm getting into DIY more and more at the moment - building a phono stage, new speaker cabinets, line stage etc.. I can soler and have a basic knowledge of electronics.

Is there anything obvious I should look at replacing in the 306? Or some other fault finding tactics? I'd prefer to not need to send it off to quad who will want £50 + parts.
Thanks
Harry

[harrygrey382]

no one got any pointers?

[DRC]

How hot is it when it cuts out ?
Can you hold your hand on the heatsink for 10 seconds ?
Do you have any equipment to measure peak power / power output under your operating conditions ? maybe your asking too much of it ...

There don't appear to be any bias adjustments, assuming I am looking at the right circuit Possibly the thermal trip is activating prematurely.

BTW, its looks mostly a class A/B amplifier with an additional, small class A amp so it shouldn't run too hot.

[AndrewT]

Hi,
the 306 is closer to a ClassB output stage than ClassAB.
I think it should run nearly cold at very low outputs.

If the quiescent power is causing the sinks to run warm then a small increase in temp from the normal delivery of power could cause the protection circuit to trip.
Excessive delivery of power would certainly trip out.

It is unusual to have to turn the volume control right up! What level of signal are you inputting?

50W into the 92db/W Tannoy is a bit loud, ain't it?

[harrygrey382]

yeah you can hold your fingers on the heatsink, it doesn't hurt. I read these were pure class A, guess people weren't informed.
Would measureing the output with a voltmeter tell me anything useful?

So do you guys reckon it should run pretty cool all the time? If that's the case there's definitely things wrong, it's always warm.

I was lsightly misleading before I think - the Tannoy's never get the full 50W . It cuts out at about 1 to 2 o'clock. I'd never want to give it all to them, but when you reach that volume when the music envelopes you like a duvet (but your ears DON'T ring when you turn it off) the bloody thing cuts out and throws you off.

I wouldn't mind if it was only beyond listenable levels. It's getting a mix of about 1.5V from a computer soundcard and I should think a bit less from a phono stage. Although, I'm just thinking these are at input, not after the preamp. If the input is too high would this cause cut out? If so what's the limit?

Thanks