Hi Everyone, Still working on a higher power version, but got side tracked to alter the original.
This is a bit of a departure for my projects and the long term upgrades to them. This headphone amp is rather nice (IMO) as it was posted but I figured it might be improved. With the indicated power supply it can have enough residual hum to be barely audible with sensitive headphones. Mine did. So hating hum as much as I do it seemed appropriate to reduce it. Using conventional techniques I could not completely eliminate it. So…having a penchant for doing “odd” things I decided to go to a non-conventional power supply. I used a 120VAC to 12VDC SMPS to power the whole thing. The 12VDC was for heaters and to power a variable voltage output SMPS that could deliver 50-400VDC at up to 200ma depending on the output voltage. I was quite happy with 50ma at 140 volts. The HV SMPS did have noise, but not what was expected. The noise was fairly wide band low frequency hiss. There was some high frequency noise, but that was easy to fix. I tried a number of things and finally settled on a choke (1.5H) input to 330uf capacitor as filtering devices. This worked perfectly. The S/N in the PS at the tube output transformer input was at -85db wideband. It is a very quiet power supply. The hum at the headphone jack is gone. So…it is a cheap solution and in a number of ways better. The HV SMPS is available from a number of sellers and there are two versions. I used the dual voltage output one as it was cheaper and easier to get. The single output one differs in that it has one less rectifier, filter and uses the boost transformer windings differently. Either will work fine. A WEB search for it should find them easily. Qunison and Smakn are the brands I found. Hook up is simple and they work well. Voltage stability is excellent. Certainly worth a try. I posted the schematic and some photos to show the project.
Good listening
Bruce
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