Post by Brickhead on Jul 21, 2020 9:30:34 GMT
This is probably of little interest to anyone but 888, but one is reason enough for me to post 
(Whether this belongs under Hardware or General is not clear to me, so I picked the less crowded alternative. Move if needed.)
Hum is the well-known curse of amplifier DIY'ers and guitarists alike. Here are some tricks I've picked up or revisited lately, feel free to correct me or add your own.
Hum from input shields (typically 2x AC frequency):
Short the shields at chassis entry (if > 1 channel), connect one lead only directly to ground star.
Hum from safety ground loop (typically 2x AC frequency):
Do NOT disconnect safety ground.
Use a ground-lift circuit between signal ground and safety ground - 3 elements in parallel:
- A 10R 2W resistor, to resist hum currents but still keep the potentials fairly equal.
- A rectifier bridge wired to form two anti-parallel diode pairs in series, to handle fault situations when the potentials differ by more than 1.4-2 volts. (Short the +/- pins and use the AC pins.) Choose one with sufficient short-term current capability to trip the breaker.
- A 10-100nF ceramic capacitor to short RF noise.
Hum from dirty B+ (typically 2x AC frequency):
Use regulated B+. Most solid-state regulators cannot handle large voltage drops, but a mosfet current-source (or a triode) in front of a low-dropout regulator would protect it from the brunt of the voltage (and power dissipation, as a bonus).
Use RC filtering. Increase the B+ voltage to compensate for the voltage drop.
Use an amplifier topology with good PSRR. One neat trick is to use a differential stage of sorts and feed a suitable fraction of the AC component from B+ into the inverting input.
Magnetically induced hum (typically 1x AC frequency):
Twist all AC lead pairs tightly together, to have the two currents cancel each other's magnetic fields as much as possible.
Orient transformers so that their strongest field doesn't project towards small signal circuitry.
Keep transformers and AC leads away from small signals. Distance is king.
Less effective: Shield with expensive mu-metal.
Even less effective: Shield with mild steel.
(Whether this belongs under Hardware or General is not clear to me, so I picked the less crowded alternative. Move if needed.)
Hum is the well-known curse of amplifier DIY'ers and guitarists alike. Here are some tricks I've picked up or revisited lately, feel free to correct me or add your own.
Hum from input shields (typically 2x AC frequency):
Short the shields at chassis entry (if > 1 channel), connect one lead only directly to ground star.
Hum from safety ground loop (typically 2x AC frequency):
Do NOT disconnect safety ground.
Use a ground-lift circuit between signal ground and safety ground - 3 elements in parallel:
- A 10R 2W resistor, to resist hum currents but still keep the potentials fairly equal.
- A rectifier bridge wired to form two anti-parallel diode pairs in series, to handle fault situations when the potentials differ by more than 1.4-2 volts. (Short the +/- pins and use the AC pins.) Choose one with sufficient short-term current capability to trip the breaker.
- A 10-100nF ceramic capacitor to short RF noise.
Hum from dirty B+ (typically 2x AC frequency):
Use regulated B+. Most solid-state regulators cannot handle large voltage drops, but a mosfet current-source (or a triode) in front of a low-dropout regulator would protect it from the brunt of the voltage (and power dissipation, as a bonus).
Use RC filtering. Increase the B+ voltage to compensate for the voltage drop.
Use an amplifier topology with good PSRR. One neat trick is to use a differential stage of sorts and feed a suitable fraction of the AC component from B+ into the inverting input.
Magnetically induced hum (typically 1x AC frequency):
Twist all AC lead pairs tightly together, to have the two currents cancel each other's magnetic fields as much as possible.
Orient transformers so that their strongest field doesn't project towards small signal circuitry.
Keep transformers and AC leads away from small signals. Distance is king.
Less effective: Shield with expensive mu-metal.
Even less effective: Shield with mild steel.
