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Re: Input Loading for MA1 mk. III & MP3

Posted by Lew (A) on March 22, 2006 at 08:13:09

In Reply to: Input Loading for MA1 mk. III & MP3 posted by Jackb0 on March 21, 2006 at 11:34:15:

I don't know about the most recent versions of the MP3, but if yours has a single 6SN7 in the circlotron output stage, then you are probably better off with at least 1200R grid leak resistors in your amplifier, as compared to 600R ones. The output Z of an MP3 with one 6SN7 per channel in circlotron mode is a bit high to drive a classic 600-ohm load without some loss. As to how this would compare to using the standard 100Kohm input, I don't know because I never tried it with my MP1. Someone indicated to me (and I don't recall where, when, or who it was) that the servo mechanism that balances out the DC at the output of the preamp will be de-stabilized by the standard hi-Z input. Best to ask Ralph if it's OK to experiment with the 100K alternative.

One interesting and safe experiment would be to start with say a 10K resistance per phase and work your way down by half each time, e.g., 10K, then 5K, then 2.5K, then 1.2K. If at any point you notice a loss of gain or damage to hf or lf frequency response, then go back up to the next highest input resistance. If you do that, let us know the results. If my amps were not so darn heavy, I would have tried it long ago.

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