I recently obtained a pair of Paul Speltz's Zero-Impedance Autoformers, and wanted to see if they made a difference with my BC22 Mk I (coupled with a BC21 preamp) driving my Magnepan 1.6QR's.
The autoformers caused my system to make a strange, loud oscilating noise intermittently. I checked all of my connetions twice over, and everything was connected properly. I also changed tubes in the BC21 just to rule out a tube going bad, but the problem persisted with new tubes.
Is it generally a bad idea to use the autoformers with the solid state BC22? What is the output impedance of the BC22? Is there something electrical going on with the design, such that the autoformers are interacting negatively with the system? Any ideas or suggestions would be greatly appreciated.
P.S. I took the Zeros out and the problem was eliminated, so I know it's related to the Zeros. Incidentally, the BC22 drives the Maggies quite well in my smallish listening room. I was just wondering if the 22 would like an 8 or 16 Ohm load better than the nominal 4 Ohms of the Maggies, hence the experiment with the Zeros.
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