MP-3 Buffer Stage

Posted by ChasLieb (A) on November 17, 2013 at 13:40:24

I recently had my MP-3 modified by Ralph to a buffer output, removing the gain stage from this preamp. My DAC that puts out too much power, 4v in balanced operation, to avail itself of the gain stage. The power of the DAC stage meant that after the first two clicks on the volume control it was already too loud.

Ralph, suggested that he could bypass the gain stage of the MP-3 leaving only the output 6sn7 tubes as a buffer stage. My phono cart puts out a lowly .25 mv so I purchased input transformers which have a 40x gain winding allowing the phono stage voltage to come up to the voltage of the DAC

The biggest issue I had before, namely that there was too much gain- is gone. The equivalence of the signals coming into the buffer stage, means that all my music choices land between 11:00 and 3:00 on the volume knob, which is ideal as it gives me actual control over the music. Being at the high end of the volume knob also means that there are fewer resistors in the signal path, as the control is serial rather than a parallel ladder type.

I have lost the ability to trim gain on the preamp, because there is no gain to trim. I have also lost two trim pots from the circuit, FWIW. The sound is extremely clear, not to say that it wasn't already clear before, but one less gain stage tube means one less place for noise or microphonics to come into play. The phono stage is even better sounding. This may be a function of the input transformer blocking noise while providing voltage gain, or may be the result of one less gain stage to amplify noise inherent in the stage. Either way, the slight "wooshing" sound when the preamp was turned up all the way while on phono, is now gone. Also gone is the turn off "pop" that would play through even after my amps were off, and draining their caps.

What else do I lose? If I had a low power output tape deck or tuner, (which i don't) they would be unusable as there is no gain stage to turn .25 or .5v into the 2-6 volts that many amps will need.

What else do I gain? Real estate. The MP-3 performs most of the functions of the larger MA-1 except with the power supply in one chasis. This makes for a crowded interior and forecloses the substitution of volume controls due to the closely placed gain tube on the preamp. Without the gain tubes there is more room, perhaps enough to introduce a 46 position 4 pole seiden switch for a more gradual volume adjustment.

For me, this modification was just what the doctor ordered. Your mileage, as they say, may vary.

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Topic - MP-3 Buffer Stage - ChasLieb 13:40:24 11/17/13 (1)

RE: MP-3 Buffer Stage - Lew 00:09:17 11/18/13 (0)

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