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The Not Passive Pre

Posted 16th February 2012 at 06:31 AM by hbc

Had this running for a few weeks now, and the sound coming from the Sonys altogether better, now just a bit bass heavy rather than treble shy. The speakers are mounted right next to the ceiling in the room corners, so not really a surprise.

I have simulated the crossover and equalisation circuit of the preamp section, and realised that I can add a variable resistor to adjust the amount of bass boost the circuit gives, not a horrible mod, just one pot, and a new cap, just need some time to do it.


The not passive pre has no muting circuit in at present, so power sequencing is required. I intend to make it direct coupled at the output. To do this I am planing to do a micro controller servo loop. When I made the Class A digital amplifier, mentioned earlier I used an opamp servo, and I can state that this circuit does definitely have an effect on the sound.


In the interests of minimalism the Not Passive Pre II is already being thought about. The plan is to integrate a phono section and a DAC. Initially each was to be a separate section, but do worry about all those tubes sitting there twiddling their toes whilst I listen to analogue radio, but yesterday I came up with a plan to integrate all three with some switching and probably muting... more of this later, if I can come up with an elegant way of doing it...
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