Late to the party, and perhaps not of interest, I designed it for use in my home office with a 5W per channel 6J7/6V6 amp, FM tuner, and Roon endpoint using RPI 4 and Topping DX-3 pro dac. It's pretty simple and will drive almost anything within reason. Its Achille's heal is it works best with a 6N6P having good section to section match.
https://www.diyaudio.com/community/...e-stage-w-parafeed-output-transformer.378535/
I have since added a small sub woofer and class D woofer amplifier with integrated crossover.
https://www.diyaudio.com/community/...e-stage-w-parafeed-output-transformer.378535/
I have since added a small sub woofer and class D woofer amplifier with integrated crossover.
Use this as outputstage,
https://www.diyaudio.com/community/...c85-6922-6dj8-etc-mu-follower-or-srpp.388295/
And place a source selector switch and volume pot in front of it.
https://www.diyaudio.com/community/...c85-6922-6dj8-etc-mu-follower-or-srpp.388295/
And place a source selector switch and volume pot in front of it.
For my desktop I want simple. I just got back home after 10 days gone and last night I “think” I found enough parts for the simple pre.
For me, it added a bit of hum. It's supposed to help if you wind up with a ground loop problem. With all the amps and sources I've tried this preamp with I haven't had any hum problems. After building and using this preamp, I should have just omitted the switch. Its a simple circuit. It would be easy enough to go back in and lift the ground if needed after building it.
I do the same thing, for lower current applications 3 or 5A anti-parallel diodes work well in place of the bridge, but these applications are all primary fused at 1A or less. For safety the network needs to safely carry the full current. Note that safety GND is connected directly to the chassis and the ground lift network goes between the safety GND and the electronics PSU GND.
I would suggest using a step down transformer from the pro audio industry such as 4:1 ratio or 10K:600Ω. That way, you can use common audio tubes like 12AU7 or 6SN7 without feedback to have gain of 3 or 4 or 5 (depends one how the tube plate is loaded). Some even have 8:1 ratio if you're stuck with high gain tubes like 12AX7 or 6SL7. And volume control can be low value from 600Ω to 2K. Also step-down transformers tend to have wider bandwidth than step-up ones. 10K or 15K is high enough input impedance to be driven by most sources. As a bonus you get balance input and can break ground loop. And to my ears, they sound better subjectively over straight to the pot. If your source can drive 600Ω, you can even flip around and have 1:4 step-up in front of a cathode follower. Use your creativity.
Just to be clear Deaf, you install this where the house ground would normally go right to chassis?
Just had a thought. Normally the house group goes to the chassis then all the compound grounds go to chassis. If that is lifted the preamp will still work without house ground???
Safety ground / PE / house ground must be connected directly to chassis. Signal "ground" can be lifted from chassis, but, unless safety measures are taken, causes potentially dangerous conditions. For example, if lifted guitar-amp style, leakages make a voltage difference between chassis and input/output jacks, interconnect wiring, speakers and their wiring, etc. The double-diode bridge+resistor+cap keeps this leakage voltage to safe levels.
Its other function is to survive long enough to open the line fuse, which must be connected immediately at the AC wiring's line input. The absolute worst case would be a failure of that AC input line shorting to chassis before the fuse. In that worst case, the bridge+resistor+cap needs to survive long enough to open the house breaker. Makes ya worry less about all that work to file out a hole for an IEC power jack, rather than an easy round hole with grommet like the good old days.
All good fortune,
Chris
Its other function is to survive long enough to open the line fuse, which must be connected immediately at the AC wiring's line input. The absolute worst case would be a failure of that AC input line shorting to chassis before the fuse. In that worst case, the bridge+resistor+cap needs to survive long enough to open the house breaker. Makes ya worry less about all that work to file out a hole for an IEC power jack, rather than an easy round hole with grommet like the good old days.
All good fortune,
Chris
I used to be confused about this too. I hope the following helps clarify:
https://www.valvewizard.co.uk/Grounding.html
https://www.valvewizard.co.uk/Grounding.html
