| johnR wrote on Thu, 29 January 2009 07:41 |
...it's there to keep transformer resonance under control, in combination with the resistive load that follows it in the circuit. If you take it off, the transformer is likely to ring severely at some high frequency above the audio range. This can cause audible side effects due to intermodulation etc., so removing it would be a bad idea.
If you have a signal generator and a scope, try putting a 10kHz square wave through the trans with and without the cap. You might be surprised at how bad the resonance is without it.
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Exactly right, John. That resistor and capacitor are there to damp the coil resonance of the transformer.
The famous story about Geoff Emerick detecting something wrong with a few channels in the newly installed Neve console at AIR studios is relevant here. The technicians measured a resonance on those channels at 56khz and traced it to the accidental omission of the damping resistor / capacitor on the transformers of those particular channels.
Sadly, the ultrasonic oscillation / undamped resonances which create intermodulation artifacts in the audio passband are bandied about the internets as PROOF that we can hear above 20khz, which is nonsense.
I like to explain the phenomenon thusly: We readily accept that human hearing cannot distinguish anything below 20hz.. The tremolo circuit on a fender guitar amp creates amplitude modulation at a rate between 3 and 10 hz or so.
We can "hear" the tremolo effect, right?
Yes.
Does this mean that we are "hearing" 3hz?
No. We are hearing the effect it is having on the audio passband.
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Topic: 1073 Input Transformer Cap--What does it do? (Read 1789 times)