Homemade HiFi

Posted on Feb 5, 2014

An amplifier to go with an antique Meissner AM High Fidelity tuner that I had restored. But by the time the amp was finished it sounded so good that it has become my amp of choice for listening to CDs while working in the shop. The only parts purchased new were the resistors, capacitors and an aluminum chassis from Hammond. I made use of parts that had accumulated

Homemade HiFi
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from parted out chassis and Ebay auctions. I estimate that there is around $50. 00 total invested with about a week of building, rebuilding, tweaking and experimentation. The output transformer is from a Hammond church organ chassis from Ebay that also netted two 6L6 tubes, two 6SL7s and a couple of 6SN7s. I got the 5V4 from Ebay for $6. 00. The power transformer also came from a lucky Ebay win in which I got two transformers for $10. 00. The signal from one or both input jacks is combined with a resistor network consisting of two 100K resistors and a 50K gain control pot. The signal is amplified by the 6SL7 twin high mu triode connected with elements in parallel for low noise and to provide a feedback point for the global feedback. The stage has very little gain due to there being no bypass capacitor around the cathode resistor, global feedback and local feedback through the 470K resistor from the plate of the input triode of the Schmidt phase inverter. The output is passed to the grid of triode one of the Schmidt phase inverter by a. 005 uF capacitor. I found that any larger coupling capacitor here produced distortion by a phenomenon known as `blocking`. The 50K resistor to ground in the cathode of the phase inverter is large enough to provide a `constant current` source and allows the plate resistors to be the same value. Any value larger than 50K raises the cathode voltage above the heater-cathode rating of the 6SL7....

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