Lighthouse Electric Tech Support
STA1 - Reducing 60Hz hum, stemming from AC filament supply
Under certain conditions, the STA-1 pre-amp, powered by VTPS4 PSU, can develop annoying 60 Hz hum. This occurrence can be easily eliminated, thanks to the center-taped filament transformer. The STA-1 is set-up for 12.6 V filament (AC or DC), unbalanced with one pole grounded as shown here.
By changing the filament circuitry to anti-phase (center grounded), the sine-waves of the filament current are always 180 deg. phase shifted. The out-of phase hum fields cancel each other out.
Step by step modification procedure:
1. On the STA-1 PCB, cut the traces, at each tube socket (V1, V2, V3), which connect pin #4 to ground
2. Run a separate wire from each tube socket, pin #9 (currently not connected) to PSU ground
3. Using twisted wires, connect pin #4 and pin #5 parallel (on all tube sockets) to each blue wire on the TR2 transformer as shown below.
This concludes the modification.
More suggestions for reducing hum in phono or line level preamps.
Run (twisted) filament wires close to the (metal) chassis. Keep filament wires short, not looping around, and away from the rest of the pre-amp circuits.
If wires must cross, try to keep them at right angles to avoid electromagnetic coupling.
Ground, as best as you can, the power supply negative terminals (including the grounded end of the heater supply) to the other circuit ground potentials in the same point. Ideally, this would make all paths fan out like a star, and it is called star grounding.
More on this topic here.
With good design practices, it is possible to get a completely hum-free system, even with AC on the filaments!
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