I am using a phono amp with a grounded grid first stage. It has a bad habit of picking noise up from other things plugged into the same circuit. It seems to usually be from any switching power supply on the same power supply. The same phono amp will make no noise in a different system. My questions:
1. Where is the noise getting into the amp
2. What can I do to fix it?
I have used powerline cleaners with no reults.
1. Where is the noise getting into the amp
2. What can I do to fix it?
I have used powerline cleaners with no reults.
Member
Joined 2009
Paid Member
I have used about every power supply available from fully regulated to CLC with DC heating of the tubes. All have suffered from the same thing so I do not think it is in the power supply.
Isolate the mains earth and audio earth with a pair of antiphase diodes. This can be a little tricky on a phono preamp.
Is this preamp going to be used with a moving coil cartridge?
How much will the charge current of the blocking capacitance be during tube warm-up?
If the moving coil cartridge is say 100 Ohms DCR, then at power-up, the 2700 Ohm resistor will only take a very small portion (1/27th) of the 12AX7's current during the tube's warm-up cycle.
Instead, the parallel input capacitors, in series with the 100 Ohm DCR of the cartridge, will take almost all of the tube's warm-up current.
Transient in nature, but is it enough to take out the micro-wire of the moving coil?
Hopefully, the warm up time of the 12AX7 is slow enough to keep the current low to the cartridge.
But if the power goes off for a second, then a hot-start of the preamp might burn out the coil.
The quiescent current is when the caps are charged, and the 2700 Ohm self bias takes over.
But a hot-start would present a temporary 100 Ohm self bias resistance to the 12AX7.
How much will the charge current of the blocking capacitance be during tube warm-up?
If the moving coil cartridge is say 100 Ohms DCR, then at power-up, the 2700 Ohm resistor will only take a very small portion (1/27th) of the 12AX7's current during the tube's warm-up cycle.
Instead, the parallel input capacitors, in series with the 100 Ohm DCR of the cartridge, will take almost all of the tube's warm-up current.
Transient in nature, but is it enough to take out the micro-wire of the moving coil?
Hopefully, the warm up time of the 12AX7 is slow enough to keep the current low to the cartridge.
But if the power goes off for a second, then a hot-start of the preamp might burn out the coil.
The quiescent current is when the caps are charged, and the 2700 Ohm self bias takes over.
But a hot-start would present a temporary 100 Ohm self bias resistance to the 12AX7.
Last edited:
Or,
You might connect all the switched terminating resistors directly across the moving coil.
Then use a fixed self bias resistor (and no switch selection of self bias).
Putting the termination resistors across the coil, would put more of the inrush current across the terminating resistors, instead of all the current through the coil.
Just a different idea.
You might connect all the switched terminating resistors directly across the moving coil.
Then use a fixed self bias resistor (and no switch selection of self bias).
Putting the termination resistors across the coil, would put more of the inrush current across the terminating resistors, instead of all the current through the coil.
Just a different idea.
For hum reduction, make sure to create a very local ground loop.
Moving coil ground, self bias resistor ground, switched terminating resistors grounds, and C13 ground all need to be in a local loop.
Use very short wiring for all of this local ground loop.
Only then bring out a separate wire from that local loop to the rest of the grounds.
That includes keeping the moving magnet input connector ground separate from the moving coil local ground loop.
Every wire is an inductor.
Every wire is a resistor.
Ground current through an inductor, and through a resistor is a voltage. A voltage is a signal.
If this is all on a circuit board, it is easier to say it, than to accomplish it (making a local ground loop).
Moving coil ground, self bias resistor ground, switched terminating resistors grounds, and C13 ground all need to be in a local loop.
Use very short wiring for all of this local ground loop.
Only then bring out a separate wire from that local loop to the rest of the grounds.
That includes keeping the moving magnet input connector ground separate from the moving coil local ground loop.
Every wire is an inductor.
Every wire is a resistor.
Ground current through an inductor, and through a resistor is a voltage. A voltage is a signal.
If this is all on a circuit board, it is easier to say it, than to accomplish it (making a local ground loop).
Last edited: