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Old 22nd February 2010, 02:48 PM   #1
N_Cook is offline N_Cook  England
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Default Audiolab 8000A DC thumps

Preamp problem leading to power amp problem.
First section of this preamp is the same as the first section of the 8000C preamp (murky schematic on eservice) , anyone have 85.... serial number preamp schematic ?
The setting of the preamp bias point is delayed by 4 seconds via 120K into 47uF, each channel, until sufficiently above a transistion voltage set by 4.7V zener. When it cuts in that sends a DC pulse ending up as 5V DC pulse going into the power amp. If phones are connected then a nasty double thump from the phones (each only approx 4 seconds and each produces a pulse). This occurs before the relay stand off time, but only just) but surely should not be DC pulses at all. But circuitwise it seems it must be, especially as both channels. Could icrease the timing current and adjust ratio of 120K to 330K but keep sum the same for 13V or so bias point, say 56K/390K would push this effect nearer to the mains on time and away from the relay timing point, if it gets near it then nasty relay clatter and presumably nasty DC pulses into the speakers , back emf / burnt voice coils etc. Is this a recognised problem or is it part of the design ?
When bias reaches about 5V ie enough above Q13 base voltage set by 4.7V zener then a surge seems inevitable as there is no C in the circuit for frequency response reasons presumably
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Old 22nd February 2010, 04:17 PM   #2
N_Cook is offline N_Cook  England
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As I cannot find anything wrong as far as broken wrong, I will assume this
is as designed. I cannot see why there is a low 4K7 dropper from 27V to the
4.7V zener that basically causes this 5V DC pulse, when only supplying 80uA
at most into the base of a TO92 transistor, Q13. I will try upping the
dropper and adding a C to give a time constant of 2 seconds and change the
main hold off time constant from 4 sec to 2 sec, also, but keeping 13V bias
point, both channels. A few strange noises in the first couple of seconds ,
if that and only if the phones are connected, is surely preferable to 5V DC
pulses fed , delayed, into the PA.
So both will rise approximately together and so no 5V surge is possible.
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