Class A 2A3 audio Amplifier

  
The input stage is comprised of a both halves of a 6SL7 octal dual hi-mu triode in a differential amp configuration with a 1ma constant current cathode load. I'm using field-effect (constant-current) diodes for simplicity. the diff amp approach was chosen for good power supply rejection, ease of balancing, good gain, and ease of application of *feedback*, if desired (hey, i like to keep an open mind...). It also takes care of phase-splitting right up front. Note: I've returned the constant current diode cathode to -9V instead of ground in order to avoid the non-linearities about the pinch-off region (Vpo ~= 1.5V).
Class A 2A3 audio Amplifier - schematic

The driver stage is comprised of a both halves of a 6SN7 octal dual medium-mu triode, also in a differential amp configuration, with a 4.7ma constant current cathode load. Because the normal bias gives me about 8.5V on the common cathodes I can return this constant-current diode directly to ground (Vpo ~= 3V). Output stage Push-Pull 2A3's using cathode bias. I'm currently using a quad of 750 ohm, 5W resistors bypassed by a 100uf/100V *electrolytic*. Output Transformers are stolen from a Grundig console EL84 amp. Impedance ratio is 6K:4ohm, but I'm using 8ohm speaker for 12K plate to plate load. This is about 3-4X the recommended load per RCA. Power is down considerably from what is could be. (For example, 8.5W with 4k load vs. 5.6W with 12k load.) I decided to go with this transformer for now figuring that the distortion would be minimized. Don't Know if I've passed the point of diminishing returns but there is only one set of secondary leads so I can't experiment too easily. An other *interesting* feature of this transformer ratio is that if I simply pull out one tube from each channel I am left with a Single-ended amp with a 3k load. This is right out of the book...3k load for Eb=250V. Cool.... Of course there is the matter of 60mA unbalanced DC. We'll just have to see how it works out. So far I've only briefly tried this *Single-ended* mode. Not sure if it's the naturally higher distortion, the OPT's...



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