This is a new one to me.
I was working on a 3 channel 1930's preamp designed for ribbon microphones which has the center tap of the input transformer primaries grounded, as was the fashion.
Two channels measured correctly, with an EQ rise of a few dB up top to counter ribbon response droop. One channel was 7 dB down from the others and had a relative +44dB spike at 20kHz, with a +25dB spike an octave up, otherwise response shaped like the others. There was also a 12dB 60Hz spike. The spikes look more like a single frequency interference than a transformer resonance.
After touching up a lot of joints and checking sockets, I transposed the grid leads across a pair of the input transformers, and the good and problem results swapped channels. Eventually I spot the problem transformer does not have the primary CT connected to ground, it's been floated by someone. I have never seen this be a problem, in fact normally a solution for phantom power or unbalanced usage. Reconnecting it to ground, the problem disappeared, and response measured as the other two channels.
What mechanism might be in play here? The transformer windings all measure correctly for DC resistance and inductance. I can't think of a time I've seen any response difference after floating an input primary in similar cases.
I did not have the time to lift another channel for comparison.
The pink noise test signal was fed from a balanced output through a typical 20dB mic pad of 'U' variety.
I was working on a 3 channel 1930's preamp designed for ribbon microphones which has the center tap of the input transformer primaries grounded, as was the fashion.
Two channels measured correctly, with an EQ rise of a few dB up top to counter ribbon response droop. One channel was 7 dB down from the others and had a relative +44dB spike at 20kHz, with a +25dB spike an octave up, otherwise response shaped like the others. There was also a 12dB 60Hz spike. The spikes look more like a single frequency interference than a transformer resonance.
After touching up a lot of joints and checking sockets, I transposed the grid leads across a pair of the input transformers, and the good and problem results swapped channels. Eventually I spot the problem transformer does not have the primary CT connected to ground, it's been floated by someone. I have never seen this be a problem, in fact normally a solution for phantom power or unbalanced usage. Reconnecting it to ground, the problem disappeared, and response measured as the other two channels.
What mechanism might be in play here? The transformer windings all measure correctly for DC resistance and inductance. I can't think of a time I've seen any response difference after floating an input primary in similar cases.
I did not have the time to lift another channel for comparison.
The pink noise test signal was fed from a balanced output through a typical 20dB mic pad of 'U' variety.