79
by greg_Archive
More on the Sontec
I built a channel with the 990 op amp as the o/p driver (5534 as the i/p amp) and did some listening tests yesterday. After calibrating the offsets and noiseless switching circuit I couldn't notice a change in sound quality. I had a mono signal split to the two inputs. I monitored the o/p as a stereo signal with the eq in bypass (signal goes through the two replacement amps in bypass). There was no deviation from mono. I like listening to things this way to see if there is a difference, then trying find out what the difference is (phase shift, eq, level, and so on). In this crude listening test the two channels sounded almost identical (eq in and out). The noise floor was not improved by the new amps, but it wasn't worse either. My main complaint with the Sontec amps was that they occasionally made low level hissing and crackling noises (even new ones!).
I cut the test short when I discovered a harmonic of the power supply regulator on the right channel (very quiet, but there). This was with the eq filters at max gain. At first I thought the 990 was drawing too much current, but after I swapped the channel's op amps I realized that it was the power supply itself causing the problem. On our Sontec the bridge rectifier, a regulator, and main filter caps are about 1-2 inches from the right channel's amps and bypass circuit. The transformer is also on that side of the box. I brought the eq back to the shop and powered it up with the bench supply. No buzz. So now I am going to power it remotely.
Some other notes:
John Hardy said that his amp will work at 28 Volts but be a bit hot. He wasn't joking. It gets HOT. It's probably a good idea for me to regulate the +-28V down to 24V in the end.