25-02-2015, 12:39 AM
The pre-sets are for DC offset, not quiescent current.
Looking at the schematic, the effect of that pre-set failing high is much-reduced by the surrounding circuitry - the pre-sets are in series with a 470 ohm resistor, and that combination is in parallel with 680 ohms. That feeds a transistor that is a current source which loads the collector of the input transistor (not even a long-tail pair!) and also is cross-coupled into the base of said input transistor. It's actually quite hard to be completely sure how the input stage is working because of the obscure arrangements they've made between these two devices. There's even bootstrapping (via R405) to raise the input impedance...
The quiescent current is set by fixed resistors, which is A Good Thing. Unfortunately, the omission of low value emitter resistors on the output pair is Not A Good Thing, and certainly won't help reliability.
Still, at least this stuff is fixable. Not so sure about modern amplifiers with SMT devices and ICs that go obsolete a decade after entering production
Looking at the schematic, the effect of that pre-set failing high is much-reduced by the surrounding circuitry - the pre-sets are in series with a 470 ohm resistor, and that combination is in parallel with 680 ohms. That feeds a transistor that is a current source which loads the collector of the input transistor (not even a long-tail pair!) and also is cross-coupled into the base of said input transistor. It's actually quite hard to be completely sure how the input stage is working because of the obscure arrangements they've made between these two devices. There's even bootstrapping (via R405) to raise the input impedance...
The quiescent current is set by fixed resistors, which is A Good Thing. Unfortunately, the omission of low value emitter resistors on the output pair is Not A Good Thing, and certainly won't help reliability.
Still, at least this stuff is fixable. Not so sure about modern amplifiers with SMT devices and ICs that go obsolete a decade after entering production
