Help with improving amplifier bias scheme

[reddish75]

Hi,

I'm looking for any help or pointers on how the bias scheme could be improved in this circuit, as the diode strings are connected to the heatsinks it's not overly stable, varying +6-7mv on idle, the amp runs hot as it is up to 50oc at idle.

[shakalnokturn]

Is that a Luxman?

Without looking into improvements, regarding bias issues the first things to look at are the diode strings that often go intermittent and the bias trimmer itself.

[reddish75]

Thanks for the reply,

The amp is a Kenwood l01, with a funky Heatpipe for cooling. The issue is on both channels so i'm assuming that the diode strings are ok, and the trimmers are new Bourns sealed types.

Could replacing the Bias transistor be worth a try, replacing it with something that can dissipate the heat better?

[shakalnokturn]

I have one of those around, a friend's that has been waiting for repair a couple of years now. It has a burn hole in the PCB, I just can't get down to it...

I repaired one in the past with a (initially) blown output stage, a bloody mess... Being DC coupled it blew transistors out quite far back.

You say it runs hot, is the idle current adjusted correctly and stable?

If both channels have the same problem, I'd first suspect things common to both (i.e: power supply) of course it could just be bad luck and you have the same faults independently.
Try scoping both outputs simultaneously to see if the mV variations follow one another or if they are pretty much independent (if they are linked to idle current variations there could be some interaction from one channel to the other through the power supply).

The 5 legged Sony transistors used in the differential pairs in amplifiers of that era are a very common problem. Usually causing crackling sounds, or worse on DC coupled designs.