I have built a phono amp using four 12 ax7. Circuit is grounded grid input, two Class A, and Cathode follower output. The circuit, which is properly grounded to the house and case, is prone to picking noise up from the power line created by other on line components. I recently had a ground noise on one channel. When I unplugged a two prong cord from the outlet the noise went away. How can I prevent this in the future?
Hi, do you have a proper mains input filter in your power supply, with solid grounding?
Regards, Gerrit
Regards, Gerrit
Also check the GG heater supply. Noise can be coupled from the heater to the I/P cathode. It is common to see RF chokes in the heater circuitry of "commonplace" GG amplifiers. It's a different noise that draws concern, but the general idea is valid.
Grounded Grid
I put in a grounded input filter I had lying around. I do not like the sound but if this stops the noise in the future it would be worth it. Is there a certain type? I am not familiar with them.
I put in a grounded input filter I had lying around. I do not like the sound but if this stops the noise in the future it would be worth it. Is there a certain type? I am not familiar with them.
Can you elaborate a little bit more on this point?
Is your phono preamp built with a two prong cord?
I’m puzzled you can hear the filter. Perhaps you have listened too long to music with some of the noise you got used to? And of course your interpretation will be different without the noise (due to filtering).
A few years back when I had this same issue....I swear it drove me up the poll for months. I rebuilt the phono several times before tracking the problem to the stupid desk top lamp on my workbench....it uses an electronic transformer.
Anyway what I did was pull a toroid from a decommissioned smps and wound lamp mains cord several times through it.No noise.....
Anyway what I did was pull a toroid from a decommissioned smps and wound lamp mains cord several times through it.No noise.....
If you have a grounded grid amplifier, does that mean that the phono cartridge is driving from the Cathode to Ground?
If that is not what you mean, then please post a schematic.
In any case, please post the schematic.
Because this is such a sensitive input, are you using DC to power the input tube filaments?
For such a sensitive circuit, the DC supply should be an extremely quiet linear supply, not a switcher supply.
Sometimes it is helpful to change the power plug position of the turntable (and sometimes of the line preamp or power amp that follows the phono preamp).
Many 2 wire plugs can be inserted two ways. Experiment with that.
Is the ground wire from the turntable (turntable tone arm) connected to the phono preamp chassis?
Switcher supplies on different devices that are on the same power line often do cause noise problems.
If that is not what you mean, then please post a schematic.
In any case, please post the schematic.
Because this is such a sensitive input, are you using DC to power the input tube filaments?
For such a sensitive circuit, the DC supply should be an extremely quiet linear supply, not a switcher supply.
Sometimes it is helpful to change the power plug position of the turntable (and sometimes of the line preamp or power amp that follows the phono preamp).
Many 2 wire plugs can be inserted two ways. Experiment with that.
Is the ground wire from the turntable (turntable tone arm) connected to the phono preamp chassis?
Switcher supplies on different devices that are on the same power line often do cause noise problems.
Last edited: