Linx power amp Ground loop? Oscillation? I'm a bit lost :-(

[steve_v]

Hi all, I just bought a Linx power 1 power amp in "good working order", but on connecting it to the preamp in my old Sansui (Great beast BTW) I get severe hum at the outputs... the amp is silent with no input so I assume this is a ground loop?
Apart from that the amp sounds great!
If I do not fully insert the RCAs (so the shield is not connected) it still hums.
I have checked the filter caps and rectifiers, but I wouldn't expect them to be the culprits anyway as the amp is silent with no inputs.
Connecting it directly to a cd player (phillips double insulated POS) there is no hum, but a quiet ticking at about 2Hz in one channel.
It also seems very hot at idle... when humming or otherwise.
When I say hot I mean I can hold the heat sinks but only just, and they are big suckers :-(
AFAIK, either the bias is miles off or it is oscillating.
I have not checked the quiescent current or tried adjusting the bias, mainly because I don't have a manual and don't know enough about amplifier design to do this without one. There is only one adjustment on each amp board, and as above I don't know what it is for :-(
I don't have a scope unfortunately but I may have access to one tomorrow.

I have had a good look at the boards and I can't SEE anything amiss... any suggestions for components to check?
I find it odd that both channels run this hot, if it was a failed component I would not expect them both to go at once!
Besides, it sounds fine...

I have no schematic or manual and I can't find any info on these amps at all, but it appears to be a fairly high quality job. Massive power supply and Van Den Hull cabling internally. Rated at 125w/200w @ 8/4 ohm.

This is a ****** of a job with no schematic and little knowledge, so any help would be very welcome :-)


Cheers.

Steve.

[Centauri]

Don't know these amps personally, but a suggestion. With the amp running and no input connected, see if there is any DC voltage at the input sockets - it may be possible the input coupling capacitors are shot, or maybe even there are none.

Cheers
Graeme

[steve_v]

Cheers, will try soon as its 4am here :-)

[Centauri]

Originally posted by steve_v
Cheers, will try soon as its 4am here :-)

Well what are you still on the net for

At least its only 2am here

[steve_v]

Nope, can't leave this one alone just yet.
Checked for dc on inputs, ~90mv dropping to 0 within a few seconds.
I take it those caps are okay?
Any ideas as to why this amp is so hot at idle? all the drivers are real hot as are the outputs.
It looks like someone has monkeyed wit this in the past... is there any reason to wind the bias right up? or is this more likely to be oscillation or something else entirely?
While I had the meter out, the DC offset is ok

[sam9]

It looks like someone has monkeyed wit this in the past... is there any reason to wind the bias right up?

There is a web page that surfaces from time to time that advocates that very thing as a "tweak". IMO a classic case of very bad advice on the web. Maybe the previous owner got pulled in by it.

BTW, the "theory" behind the advice is than cranking up the bias results in "hotter performanve", . While there may be some basis for this in the case of MOSFETs, the article does not even suggest that the victim (sorry I meant tweaker) find out first wether the ouput devices are MOSFETs ot BJTs!

[steve_v]

There is only one pot on each amp board but they look to be set at about the halfway mark, is this likely to be the bias?
What should it be connected to if it is?
Where should I measure the quiescent current?
I take it I should be measuring the voltage drop across a resistor somewhere... I really need to learn more about these things :-(

[steve_v]

As far as I can tell, the hum is not a ground loop as disconnecting the ground lead of the amp does not solve the problem.
Connecting input ground and output ground cures it, but connecting either to the case does not nor does running a ground wire between the cases of the two amps. I'm stumped.

[steve_v]

I notice that one channel the resistor between input signal ground and chassis is ~190ohm and the other is ~680ohm odd no? according to the devilishly hard to read markings (which way do you read a 6 band resistor?) they should be 10ohm. sound like it could cause either of the problems I'm having?

Cheers.

Steve.

[Centauri]

Originally posted by steve_v
Connecting input ground and output ground cures it, but connecting either to the case does not nor does running a ground wire between the cases of the two amps. I'm stumped.

Ahhh
Quite often input ground and main amp ground are isolated to prevent hum loops, but are normally referenced together with a low value resistor - maybe 10 to 100 ohms. See if you can trace the input ground back on the circuit board, looking for a resistor (or empty position on PCB !) going to main ground. If it is missing, a 10ohm 1W resistor will do the job.

If there is no connection between amp's main ground and chassis, best to add one, connected at main filter caps ground - this could be causing some instability resulting in your heat. Try these measures before considering adjusting the bias.

Cheers
Graeme