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Old 23rd June 2006, 08:48 AM   #11
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Join Date: Jul 2005
Location: Sussex
I now have a multimeter that i can trust.

Just noticed the bias seems to creep up from less than one millivolt up to around 8mV then i turned it off, is this normal behaviour for this amp? it suggests to me that the transistors in the bias network are getting warm fast and that this may cause thermal runaway.

Open to suggestions from everyone regarding this.

Heres a link to the schematic in another thread:

Comments on Fostex Lab 600 Amp circuit?

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Old 23rd June 2006, 01:10 PM   #12
thejohn is offline thejohn  South Africa
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Craig, I'll pull the lid off my amp this weekend and give you some feedback...
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Old 23rd June 2006, 07:47 PM   #13
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Location: Sussex
Hi John,

That would be great, like i have said my main concerns are the bias drifting (it does stabilise after about 15 minutes) and the transistors with the small clip on heatsinks heating up. If you could check your amp for the same things it would be very interesting.
Also what volatge are you running it off?
with 220v i get +/-75vDC rails as per the schematic but with my pure mains voltage ( not through a variac 240V) the rails are 85v dc which is quite a lot and is a danger to the transistors used in the circuit.
Check out the other thread i started :

amplifier bias drifting

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