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@Jonny69
I completed the electronics part of the build. First power-up is always nervous as, although you check everything, you never really know if you've mis-wired something and you sort of flick the switch and hope nothing goes bang. Well, nothing did, which was good news!
There are two things to set up on these: the voltage midpoint at the output transistors and the quiescent current.
The voltage midpoint needs to be set because this original design used a single rail power supply (0-27V) and a big output capacitor to block the DC getting to the speakers. On a modern amplifier you'd use +/-15V and eliminate the capacitor because the midpoint sits at 0V. On this design it needs to sit halfway up the supply voltage so that the signal can swing between 0V and and +27V at point VE or 'X' on the circuit diagram below. That voltage point is set by resistors R5 and R6. If all components were to exact spec, that should be the case, according to the original notes. The kits don't bother with this and use low tolerance resistors and trim pots so you can make adjustments. I didn't want to do it that way, so I used 1% tolerance resistors throughout and selected ones with values which were the closest to book value.
However, my VE value on the right-hand channel was still out by a volt. The reason for that is probably tolerances on the transistors. This would affect the output and the quiescent current, so I swapped out R6 for a 27k resistor and 100k trim pot in series, as per the kit version. I could then adjust the voltage back up from my measured 12.69V to the ideal 13.59V (my supply is 27.18V).
Onto the quiescent current. I disconnected the power supply rail and ran it through my multimeter on the 10A scale so I could read the current being drawn. 1.21A and the ideal when running an 8 Ohm speaker is 1.2A, so I was happy with that.
I repeated all the above with the left-hand channel. The only thing I found was the quiescent current was measuring 1.37A on that side. On the kits, again, they have a trim pot to set the quiescent current instead of using a close tolerance resistor. The main issue with trim pots in this part of the circuit, is they are actually under spec for the current going through them. So I've opted to stick with the resistors. If I've understood this correctly, if the quiescent current is too low, it will starve the output stage at high outputs. If it is too high, it is simply wasting power and it gets dumped out as heat onto the heatsinks. So I've opted to leave it, since it's only 170mA.
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