Neil - Here's the problem with the power amp as Felix (Martin) discovered a few months ago. He fixed it for me (top man) but when switched on the second time heard a loud buzz, as before, and one speaker (bass driver) felt warm so I switched everything off. Anyway, here's what Martin said -
Well it is a bias problem, but it's a very subtle, thermal one... that took a while to spot!
The driver stage uses several MJE340/MJE350 transistor pairs. These are not fitted with any form of heatsinking, and on both channels this is asking one pair in particular to put up with 2.3W of dissipation in a package that can deal with about 0.6w in free air. IOW, part of the driver stage is running way over 100degrees C. When you do this to transistors, their current-gain drops, as does the turn-on voltage. This means the input stage has to work harder, but turns them on further...leading to eventual loss of offset- and bias control. Clearly the amp has put up with this for years, but one day cried enough and the failing driver transistor led to a loss of DC balance - which pushed several volts of DC into your speaker.
It's a problem that only appears once the amp is good and hot. Run up from cool, as I did when it arrived, and it behaves as nice as you like - clean scope traces, DC conditions stable, no sign of VHF instability anywhere. As it warms the DC offset drifts a little, as does the bias. Things only start to go awry when the amp is driven into a load - the issue *doesn't* show-up just sat on a bench hot but idling. And let it cool down and all is well again...
The fix is to provide heatsinking to the affected driver pair each side. Because overheating these parts has degraded their properties I will replace all 14 (7 in each channel - 55p each at Farnell) of these transistors, and fit clip-on heatsinks where required. In fact one of a pair has actually failed, but because this is inside the inner feedback loop the amp has 'worked around' it. Guess which channel!
I replaced the driver with the last one AVI had so don't want a repeat performance! That's the reason for the change.
Mr Cat - I bid on the Exposure to about £350 but was out at a gig so missed it.
May have a punt on the Primare, I've liked what I've read about it. Thanks.
