Hi all,
I have a noise problem in a simple tube line stage I’m building and I’m pulling my hair out a bit. It’s a clean 60 cycle hum (not a 120Hz buzz) that is only present with linear power supplies, not audible (but probably still present) with switch switching supplies. The supplies are all isolated 12vDC and I’ve experimented with referencing to earth ground, circuit star ground, and leaving it floating. The 12v, clean and regulated, comes in and powers a 12au7’s heaters with a little filtering and then runs in parallel to an isolated DC-DC converter brick (CUI) that gives +/-24. I’m tying the -24v to common to get a +48v rail. The tube is a couple of common cathode stages with an output level pot at the end. I’ll upload a schematic later when I’m not in mobile.
The noise is independent of input, including grounding the input, and can be attenuated by the output pot. The noise disappears when I unplug the DC jack (typical barrel jack) even though the circuit can produce sound for a couple seconds after. This sent me down a simple ground loop rabbit hole for a several hours but nothing related to breaking or adding ground lines changed the noise (including coupling grounds across the DC-DC brick). Adding a ton of filtering across the heater pins attenuated it slightly which is weird because it’s not PS ripple.
When I disconnect the heater from the input power (I have it on a little header to pull easily) the noise goes away instantly. This makes me think that whatever the source is it’s being coupled in to the cathode from the heater but adding or removing or moving the ground reference of the input 0v with alligator clips once again changes nothing. I also quickly tried tying the 12v to the 48v through a resistor while the 12v was floating to see if elevating it helped. It did not and I didn’t expect it to given there was no negative voltage component.
I’m at a loss for what mechanic might cause these symptoms. Apologies for no pictures but I’m on my way to the park for a healthy distraction and I’m hoping for some ideas.
I have a noise problem in a simple tube line stage I’m building and I’m pulling my hair out a bit. It’s a clean 60 cycle hum (not a 120Hz buzz) that is only present with linear power supplies, not audible (but probably still present) with switch switching supplies. The supplies are all isolated 12vDC and I’ve experimented with referencing to earth ground, circuit star ground, and leaving it floating. The 12v, clean and regulated, comes in and powers a 12au7’s heaters with a little filtering and then runs in parallel to an isolated DC-DC converter brick (CUI) that gives +/-24. I’m tying the -24v to common to get a +48v rail. The tube is a couple of common cathode stages with an output level pot at the end. I’ll upload a schematic later when I’m not in mobile.
The noise is independent of input, including grounding the input, and can be attenuated by the output pot. The noise disappears when I unplug the DC jack (typical barrel jack) even though the circuit can produce sound for a couple seconds after. This sent me down a simple ground loop rabbit hole for a several hours but nothing related to breaking or adding ground lines changed the noise (including coupling grounds across the DC-DC brick). Adding a ton of filtering across the heater pins attenuated it slightly which is weird because it’s not PS ripple.
When I disconnect the heater from the input power (I have it on a little header to pull easily) the noise goes away instantly. This makes me think that whatever the source is it’s being coupled in to the cathode from the heater but adding or removing or moving the ground reference of the input 0v with alligator clips once again changes nothing. I also quickly tried tying the 12v to the 48v through a resistor while the 12v was floating to see if elevating it helped. It did not and I didn’t expect it to given there was no negative voltage component.
I’m at a loss for what mechanic might cause these symptoms. Apologies for no pictures but I’m on my way to the park for a healthy distraction and I’m hoping for some ideas.
Schematic attached. I’ve now isolated the jacks from chassis just to be safe despite what the schematic shows (no change). The line beneath the converter can be bridged with a header. With the header I have hum, without it I get a little additional sizzle noise but essentially the same. Originally I had a 1nF cap there per the datasheet for ripple reduction.
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You show both input and output connected to earth. Is that chassis? How is that physically, is there a distance between those earth points?
Is the 12V input grounded somewhere?
It really depends a lot on the physical layout to judge what ground current are common to what, and how they could generate an input hum signal.
For instance, if the return current from the 50V supply runs through a common ground bus or the chassis, and the input and output are grounded at different points on the chassis, you already can have a difference between the input and output grounds and that gives hum.
Jan
Is the 12V input grounded somewhere?
It really depends a lot on the physical layout to judge what ground current are common to what, and how they could generate an input hum signal.
For instance, if the return current from the 50V supply runs through a common ground bus or the chassis, and the input and output are grounded at different points on the chassis, you already can have a difference between the input and output grounds and that gives hum.
Jan
If the circuit 0V is connected to PE there should be no problems with leakage current from a 60Hz transformer, but leakage current is the only difference with linear supplies vs. Offlne switchers.
Leakage current though the heater to cathode interface can mean that the unbypassed 2.7kΩ on the first cathode can cause problems, especially if the heater insulation is failing. 5µA across 2.7k gives about 10mV of 60Hz noise - plenty enough for humm
Try 220-470µF across the 1st stage Rk, as an easy check on this.
Connecting the heaters and chassis directly to PE, and the circuit 0V to chassis via 10Ω should give the quietest setup, if you are powering the anode supply's switching supply from the heater DC
Leakage current though the heater to cathode interface can mean that the unbypassed 2.7kΩ on the first cathode can cause problems, especially if the heater insulation is failing. 5µA across 2.7k gives about 10mV of 60Hz noise - plenty enough for humm
Try 220-470µF across the 1st stage Rk, as an easy check on this.
Connecting the heaters and chassis directly to PE, and the circuit 0V to chassis via 10Ω should give the quietest setup, if you are powering the anode supply's switching supply from the heater DC
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Interesting problem.
I remember reading (Merlin B perhaps) about common mode noise riding on the heater. That’s why elevated heaters want lots of decoupling to ground.
I remember reading (Merlin B perhaps) about common mode noise riding on the heater. That’s why elevated heaters want lots of decoupling to ground.
All,
Thanks for the help. I figured it out and I was on the wrong track. My power supply is not regulated when set to 12 volts - so silly of me to overlook that assumption. It actually runs pretty hot so I added a 12v low dropout in series with the DC to DC converter to clean it up. That did the trick.
Thanks for the help. I figured it out and I was on the wrong track. My power supply is not regulated when set to 12 volts - so silly of me to overlook that assumption. It actually runs pretty hot so I added a 12v low dropout in series with the DC to DC converter to clean it up. That did the trick.