Amp Trauma

I hooked up the MKIV's this AM that I just finished rebuilding. VTA boards, new larger can cap, etc.. I hooked up the Quad 57's and started to get an ever rising oscillation when I turned the amps on, so I quickly shutdown them down. I had not plugged the pre-amp into the MKIV's, so I am assuming this was the problem, in that there was no load on the inputs? I have never had this happen before, but I can't remember with my Quicksilver GLA if I ever turned it on with speakers attached, but no load on the inputs. I am not sure how to proceed at this point. Thanks for your help!!

In my experience an original Stereo 70 or Mark IV will not do this even with the reactive load presented by the Quad 57. That load is inductive at low frequencies and capacitive at high frequencies, then goes inductive again in the extreme highs. But we don't know whether the designer of the VTA boards took that load into account when designing his board. He MIGHT have increased the bandwidth of the driver and phase inverter or introduced some phase shift to where it is susceptible to oscillation, especially when the input isn't shunted by the output impedance of the preamplifier.

A well-designed amplifier should not do this. It should be unconditionally stable under conditions of normal home use, which include accidentally leaving a cable unplugged. Also there is a possibility that speaker cables could be an issue, either in themselves or adding to that of the Quad 57. Some megabuck audiophile magic cables are highly capacitive, which would only make things worse. Do your testing with zip cord, keeping it short; or a speaker cable with spaced conductors, or with individual conductors spaced apart.

My suggestion at this point would be to try a conventional dynamic loudspeaker instead, and see what happens. If both of them still do it and you have verified no errors in wiring, you may have to troubleshoot the oscillation (and if this is the case, I would not consider the VTA board to be "well-designed" for this application unless a warning is provided never to use it with electrostatic loudspeakers). I have a sneaky suspicion that it is occurring in the feedback loop, perhaps the feedback signal being altered by energy being kicked back into the output transformer from the capacitive load; but this suspicion may be due to having tweaked feedback loops on homebrew amplifiers, many years ago.



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Thanks for the info, and suggestions. The speaker cable I use is the old Mark Levinson copper cable, essential 10 guage zip cord. When I biased the amps yesterday everything seemed Ok. Since I have never had this happen before I am uncertain as to how to proceed. I have car speakers I may try to use. I just rebuilt some Rogers LS3/5A's that a big SS amp fried, so I am reluctant to use those. Thanks again.

I had the same problem with my stereo 70. Plug in a source and you will probably be OK.