On Tuesday 08 April 2008, Tim wrote: [...] >The lastest thing to expend my time on has been restoring a 1960s or >1970s audio mixer (haven't quited figured it out), with all discrete >components (you could keep it going until it mechanially falls apart), >multi-tapped coil passive equalisation, P&G faders, and transformer >coupling everywhere. While playing, I discovered something I really >didn't expect - if you ground one side or the other of a floating >transformer output, the voltage across the output terminals changes. > >Crude ASCII-art transformer cct: > > signal source ----- || ------| > 3 || E | > 3 || E VU meter on output > 3 || E | > 3 || E | > ground ----------- || ------| > >You get three different meter readings depending on whether you don't >ground either output terminal, or which side of the meter you do ground. >There's about a 2dB change. No, it's not grounded anywhere else, nor is >anything other than a passive VU meter connected to the transformer, and >I can't find any primary/secondary leakage. The sound source is a >symmetrical sine wave. > >There's always something out there to surprise you. In this case, I believe I would insert a transformer between the signal source and your input transformer. I'd suspect something in the generator was responding to the grounding of its output, particularly if the generators output is an electronically balanced output without a transformer to isolate it. Grounding one side will often cause a rise in the opposite sides output because of the extreme levels of feedback in such circuitry. If you still see that effect then, there is something bogus about that input tranny. And since they are often wound with 40+ gauge wire, I would not be willing to bet they are really good as little as 5 years later, due as much to the solders amalgamation with the copper of so fine a wire, the failure rate under those conditions is legendary. -- Cheers, Gene "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) If this is a service economy, why is the service so bad?
