Hi,
I have a Fender-style preamp with one 12ax7.
It gives me hum which seems to be related to the power supply. It sounds like this:
(normalized).
It's not unbearably loud, I would say it's liveable, but it still is really annoying.
The thing is when I disconnect it from the mains, I get a few seconds of absolutely beautifully hum-free (while there is a charge in the capacitors) guitar sounds.
I believe the preamp circuit doesn't have ground loops (for every ground point in the circuit I have a separate track going to one single point which is connected to the power supply's 0V). The power supply I use is https://www.aliexpress.us/item/1005005909458258.html (seems to be pretty solid).
I have tried a huge 8 Henry choke on the 300V output of the power supply. I have tried balancing the output signal with the transformer (hoping the balanced result will cancel this hum), but nothing seems to help, the hum is still there. I have even dangerously tried to temporarily disconnect the mains earth connection (just to quickly check) and the noise gets much worse when I do that.
I would prepare myself for living with it if not for these beautiful few seconds of beautiful quietness when I disconnect it from the mains...
Could someone help identifying the problem (and finding the solution)?
Thanks.
I have a Fender-style preamp with one 12ax7.
It gives me hum which seems to be related to the power supply. It sounds like this:
It's not unbearably loud, I would say it's liveable, but it still is really annoying.
The thing is when I disconnect it from the mains, I get a few seconds of absolutely beautifully hum-free (while there is a charge in the capacitors) guitar sounds.
I believe the preamp circuit doesn't have ground loops (for every ground point in the circuit I have a separate track going to one single point which is connected to the power supply's 0V). The power supply I use is https://www.aliexpress.us/item/1005005909458258.html (seems to be pretty solid).
I have tried a huge 8 Henry choke on the 300V output of the power supply. I have tried balancing the output signal with the transformer (hoping the balanced result will cancel this hum), but nothing seems to help, the hum is still there. I have even dangerously tried to temporarily disconnect the mains earth connection (just to quickly check) and the noise gets much worse when I do that.
I would prepare myself for living with it if not for these beautiful few seconds of beautiful quietness when I disconnect it from the mains...
Could someone help identifying the problem (and finding the solution)?
Thanks.
It's a clean channel of Dumble ODS from here:
Without JFET circuit on the input, and without PAB switch. a part that starts with RV1-7 33k resistor and ends with C12 0.05f capacitor (of course followed by the Master POT, with the difference that I have a 500K pot instead of 1M).
From the power perspective, from this schematics, I use a 20u capacitor that's on B+4, then I have the same 2.2k resistor to B+5 and a couple of 10u capacitors on B+5. The power supply's 300V is connected to the B+4 point (everything before is of course omitted), 12ax7 is fed from the B+5 point (everything after B+5 is also omitted).
I haven't connected heater and HV grounds. I have just tried and it doesn't help. I also remember reading that they shouldn't be connected.
Without JFET circuit on the input, and without PAB switch. a part that starts with RV1-7 33k resistor and ends with C12 0.05f capacitor (of course followed by the Master POT, with the difference that I have a 500K pot instead of 1M).
From the power perspective, from this schematics, I use a 20u capacitor that's on B+4, then I have the same 2.2k resistor to B+5 and a couple of 10u capacitors on B+5. The power supply's 300V is connected to the B+4 point (everything before is of course omitted), 12ax7 is fed from the B+5 point (everything after B+5 is also omitted).
I haven't connected heater and HV grounds. I have just tried and it doesn't help. I also remember reading that they shouldn't be connected.
Heaters shall never be floating. They need to be somehow referenced to ground, or elevated.
The signal is coming out from C12, right?
I assume you do not have an oscilloscope. Can you use a sound analyser app with your sound card, like REW?
https://www.diyaudio.com/community/threads/how-to-distortion-measurements-with-rew.338511/
It can show the the hum frequency components.
https://www.diyaudio.com/community/threads/how-to-distortion-measurements-with-rew.338511/
It can show the the hum frequency components.
The preamp is mono. I currently use it with the transformer on the output (10k:10k transformer). Primary outer legs are connected to the preamp output/ground, secondary outer legs are connected to left/right channels of the stereo input and center tap is connected to the sound card input ground. Then in the digital domain I multiply the value of the right channel by -1 and sum it with the left channel. But I get the same hum even if I connect just one single channel without transformer.
Should I just connect the probe to the output of C12 and hit auto? What am I looking for?
Should I just connect the probe to the output of C12 and hit auto? What am I looking for?
@jcalvarez thanks for your help. It seems like I've solved the problem. It was the power supply's fault. I've got another one (https://www.aliexpress.us/item/3256806060916473.html) which is a bit smaller and seems less solid, but the hum is gone with it! This time I really seem to have my PCB circuit right 
. The downside of the new power supply is a bit of mechanical high pitched sound (the one that is typical for the switching power supplies), but I suppose it will be negligible once I have it boxed.
. The downside of the new power supply is a bit of mechanical high pitched sound (the one that is typical for the switching power supplies), but I suppose it will be negligible once I have it boxed.