I'm working on a pair of EMI RS61 line amps, encountered a strange issue. I can faintly hear whatever audio is being ran through the amps. It seems to be coming from the plate choke or output transformer. Has anyone encountered such a thing?
Tubes are NOS mil-spec. Power supply is of my own design, based on one for a similar EF37A preamp found in the EMI BTR2 tape machines. Delivers ~290VDC under load. 5R4GYA rectifier with choke input LC filtering. Large PIO caps.
Here are schematics and a response plot, which is 99% identical on both amps. 40 dB pad on the output for testing purposes. They are roughly producing the expected amount of gain (40dB), but obviously something isn't right!
Tubes are NOS mil-spec. Power supply is of my own design, based on one for a similar EF37A preamp found in the EMI BTR2 tape machines. Delivers ~290VDC under load. 5R4GYA rectifier with choke input LC filtering. Large PIO caps.
Here are schematics and a response plot, which is 99% identical on both amps. 40 dB pad on the output for testing purposes. They are roughly producing the expected amount of gain (40dB), but obviously something isn't right!