Using internal power amp connectors, we get good performance for both channels so the issue above is the pre-amp:
Some Spec 1 measurementsWishing I still had my Pioneer SPEC-1 / SPEC-2 setup (for several reasons). Would have loved to see them measured.
As you can see, they use chassis as a ground return. There is only one wire going from the RCA connectors, signal live wire. So any bad connection due to corossion or so resulted in that horrible noise/hum.Here is the offending screw:
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The idle current is quite low, due to simple bias circuit with 5 diodes in series. This is the point that might have been grossly improved. Because of the low idle current, distortion is rising at low power. However, the value of distortion above 100mW is fully acceptable.
Thank you Amir.Review updated with the latest measurements.
This is the biggest issue of the power amp section. The fixed diodes do not allow for adjustment of the idle current, the idle current is low and it is the crossover distortion that degrades the parameters.
By simply changing the diodes for a Vbe multiplier, we can adjust the idle current and get much much lower distortion especially at lower power. Unfortunately, Vbe multiplier circuits were not used back in 1966. Below we can see how it works with crossover distortion reduced by a properly set idle current.
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This would be a competitive amplifier, even nowadays. Removal of crossover distortion would remove the "transistor sound" of the sixties.
Could you show results with both configurations (diodes and vbe) for the sake of comparison?
Oh, sorry I missed that!Hi, it was already shown
Thank you very much. I'd really appreciate it!THe pn's given in the jbl service manual are 12539 for the pnp and 12538 for the npn. I looked them up in an old ECG cross reference guide and they are ECG174/175 which come out as NTE175/38