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Old 28th November 2006, 02:43 PM   #1
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Default OPTs causing crosstalk?

A simple, stupid question: Can channel crosstalk be caused by a stray magnetic field from one OTP to another, if they're relatively close to each other like in a stereo amp?
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Old 28th November 2006, 09:24 PM   #2
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I suppose it would be possible if they were operating at a very high level and they were positioned just right and unshielded. However, I think it very unlikley. At least I've never experienced it or heard of such a case.

Crosstalk usually comes from low level circuits which are much more susceptible to it through improper lead dress, parts placement or poor circuit board layout. Also, from a stereo amplifier that uses a common power supply that is not sufficiently isolated or perhaps somehow faulty.

Victor
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Old 28th November 2006, 10:49 PM   #3
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It is possible, and I have heard it happen, but I don't think that it is a problem in the real world.

Below is a picture of my TubelabSE amp. I put all 3 transformers in a row right next to each other (dumb!). This amp worked fine with my 86db 7 speakers with a 7 inch woofer (limited bass response). When I took it to a friends house who had big horns that are 101 db efficient I heard HUMMMMMMMM. The hum was worse before the rectifier warmed up (no current in the OPT's). I have since rotated the power transformer 90 degrees and no more hum.

As for the crosstalk, I have performed the following test a few years back. One channel driven to full power (music) into an 8 ohm resistor. The other channel with its input shorted into headphones. There was some very faint bass sounds indicating some crosstalk. In an atempt to determine if it was electrical, I pulled the input tube for the silent channel. The sounds diminnished indicating some electrical crostalk. I then pulled the output tube, and much to my surprise the sound increased. It was still very faint into headphones, so it would never be heard through speakers. The sound was all low frequency stuff, I could make out the kick drum beat but nothing else.

This told me that SE OPT's are more succeptable to magnetic fields when there is no current flowing. I experimented with other OPT's and they all do this to some degree. These are custom Electra-Prints. The magnetic field radiated from an SE OPT goes up when it hits saturation. I have seen the effects in a neighboring OPT on an oscilloscope.
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