Sonogy Cantata
This amplifier was originally made by Allego Audio Designs here in NJ before the name changed to Sonogy. This is thought to be a modified version of the very unusual Rappaport AMP-1. Perhaps the Cantata was an attempt to make the AMP-1 a more stable and robust design.
I received a pair of these amplifiers for repair. Both were configured as monoblocks, and both had obvious issues. I returned them to their original stereo configuration to see what the problem was. Right away, I noticed that the two amplifiers were not the same - different color PC boards and more importantly, different components. Guess the origial owner must not have heard the difference (and there definitely was a difference).
Both amplifiers had a noticeable hum that was intrusive on the music. Both had large 33uf electrolytic coupling caps that dried up. One of them had Wima bypass caps, and the other ERO (Roderstein), the latter of which were accidentally placed out of the circuit - they were never even used! The vbe multiplier transistors were never attached to the heatsink, so there was no thermal tracking of the bias. The power supply board was not quite the right shape for the chassis, so some dremel hacking was done at the "factory" to make it fit. The input board layout was the same for both channels, so they were interchangeable. The left channel front-end faced the rear of the amp, but the right channel's front-end faced the front of the amp and was right next to the bridge rectifier. The right channel also needed a much longer wire to go from the RCAs on the rear panel. This long wire was also close to the rectifier. The induced noise from the rectifier caued an unavoidable hum. Shielding and other hum reducing techniques were employed in an effort to quell the noise - I even built a passive DC offset circuit so I could remove the opamps as a potential source of noise. Man, I had two sleepless nights because I just could not solve the problem. I got one of the amps quiet, but not quiet enough. How did they even sell such a flawed product?
When I have something rare or weird on the bench, I do my best to model the circuit so I can understand how it works and where to look for problems. This is what the circuit looks like:
Rappaport AMP-1 Gain Board.pdf
Rappaport AMP-1 Output stage.pdf
Ultimately the problem was, IMO, that this unique design was just not a very good one, and it was not put together very well either. I wish I could have heard it without the obtrusive hum, because I think it might have sounded really good.
I love the chassis of this amp, however! It's shiny, black, has proportions not unlike that of a cinderblock and has a lot of room inside. After I returned the one that was working as well as it could, I bought the other from the owner just to gut it to house my own Crankshaft amplifer design.
This is my own TVFA Crankshaft #2 built into the chassis of the Cantata. The transformers and large filter caps are the only components left over from the Sonogy Cantata.
