I finally completed my preamp to match the GM70 monoblocks after taking a break from audio for a year.
Thia scratch built 01A is a beautiful sounding preamp but very difficult to keep quiet. Circuit consists of a very simple design but with some modern power supply tricks. Instead of a choke, I used an electronic choke called a gyrator and heated the DHT filaments with Rod Colemans filament supplies very popular in the DIY market and probably licensed to some commercial manufacturers. Power supply is dead quiet using tube rectifier and Maida regulars.
I purchased several tubes(globes) just to find a couple that were not microphonic. The ST tubes however are very quiet along with the 112a tube. My next mod is to remove the resistive pot at the input and install Dave Slagles AVC at the output of the preamp. This should definately bring this preamp to new levels. At unity gain, it will also give me about 600ohms out which is great news for driving longer cables. Also, at 140Henry, the bass is still very good according to my LTSpice model. As of now, this preamp smokes my Audio Research SP6, cant wait to hear it with the AVC!
