John W
Vince says I am familiar with the CCT and the power supply arrangement having modified the 606 many times and it is true that the derived earth is artificially created and the 606 was not prevalent to switch-on/off plops, but the QSP is for some reason, I had tried to get the CCT to find out why and assumed that IAG had gone away from this earthing arrangement, because in the original 606 the main caps were inadequate at 6,800uf and the zero point shifted from roughly zero to a non-zero condition dependant the speaker load thus causing compression and also limiting the bass performance, the small PUMP CCT used to maintain this zero condition could only handle a very small current, about 25ma so during the 10m/s intervals the main caps could wander, one taking a charge from the other on non symmetrical audio signals, because of this action on the charge cycle the main caps would receive an equal charge and behave as two caps in parallel however on this discharge, because one cap could steal a charge from thev other,the two caps behaved as two caps in series with double the effective esr etc,this happened because the pump CCT could not supply sufficient current to equalize the charge and maintain zero volts for anything like real speaker loads and the inevitable current drain from the supply, Ross Walker was told about this and my mods. which was to increase the main caps to 10,000uf Elnas and add two reverse diodes across the caps to stop the zero point shifting by more than 0.6 volts, and keeping the pump CCT, the 909 was originally good when Stan Curtis was in charge, but when he left the main caps were downgraded to a cheaper cap which had a very poor high frequency behaviour and distortion was very much in evidence, independent testers vouched for this, and it was widely promulgated on some forum or other. ... Ross Walker stated that since the amplifier met it's specification no modification could make any difference, so the 606 for many became a mediocre amplifier working far below its real potential and sales suffered.. This amplifier has had a chequered history but is fundamentally very good. There were other things that Quad did to the power supply that defies good engineering practice and is almost beyond belief, butb that's history and I don't think people are really interested we are going back into the 80's. Vince Hawtin