2A3 Hum Problems in SE (Please Help!) - diyAudio
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Old 20th March 2015, 08:37 PM   #1
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Exclamation 2A3 Hum Problems in SE (Please Help!)

Hey guys, I am FINALLY almost done with my STEM amplifier. It is a school project based on the 6SN7 and 2A3. It is a headphone amp designed for 60-600 ohms. The amp sounds fantastic other than a nasty 60hz (I think) hum that I can't seem to get rid of. I am running the B+ as a regulated 21st century Maida Regulator, and the filament supplies for the 2A3 are Pete Millett Regulated Supplies.

Info:
1. The hum gets SUPER LOUD when I DISCONNECT the RCA cables from my amp or the DAC
2. The hum persists when I remove the input tube
3. I tried using a 2.5-0-2.5 transformer to supply AC to the tubes (paralleled) but it increased the hum
4. Shorting the output eliminates hum completely
5. Bottlehead crack plugged in to the same outlet produces 0 hum, so the issue isn't my mains line
6. When I connected the - part of the filament supply to ground instead of floating, the hum was reduced significantly, but the amplifier's volume was decreased significantly, which defeats the purpose of the amplifier
7. Adding/removing grid leak resistors had 0 effect.
8. I am running the amp without a potentiometer currently, but when I added a resistor on the input to simulate a pot, nothing changed


My schematic and grounding layout have been included
Click the image to open in full size.Click the image to open in full size.Click the image to open in full size.Click the image to open in full size.Click the image to open in full size.
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Last edited by dutchgfx; 20th March 2015 at 08:53 PM. Reason: Pictures not functioning
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Old 20th March 2015, 08:49 PM   #2
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No schematics or pictures have been processed.
Please try again. Sorry.
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Old 20th March 2015, 08:51 PM   #3
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You need to remove the trailing slash from your image links.

98% of the time, this sort of hum is a grounding issue. Can you post actual construction pics.
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Old 20th March 2015, 08:57 PM   #4
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The construction is slightly different then the pictured layout, with the input jacks being grounded to a different location based on their current placement. Not sure if you can see much but ill try

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Old 20th March 2015, 09:20 PM   #5
mdamp is offline mdamp  Australia
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I see why you have HUM you need to tidy up your wiring you need to set the components out better have 1 single ground wire running from 1 side to the other connected to the mains earth so you can solder all components to ground on a common line. run a wire from 0 ohm tap from both OP Transformers to the common of your RCA inputs then at the volume pot ground the common ground to the single ground wire running from side to side that may help cut HUM!!! twist all heater wires tightly together and keep them away from any B+ power wires hope this helps

Last edited by mdamp; 20th March 2015 at 09:24 PM.
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Old 20th March 2015, 09:22 PM   #6
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Looks like you're using the chassis itself as a ground plane. No wonder you have hum. You need to do this for RF projects where you need reactance out of the DC return path, and it doesn't matter since hum frequencies are way below the signal frequencies. At AF, that doesn't matter. DC returns should be made to a star or semi-star connection to get the capacitor recharge pulses out of the DC return, otherwise, they behave like another signal. Add in another ground loop between audio components, and it just gets worse. You also need to remember that this is a SE project, and the PSRR won't be as good as for a PP output stage.

A ground loop breaker between the DC neutral and the chassis also helps.
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Old 20th March 2015, 09:33 PM   #7
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The chassis isn't actually grounded. It is labeled as such, but it is not. I am using a wood chassis.

In response to mdamp, I believe I did this. I have traced out the ground wires in cyan. As you can see, the individual sections meet at local points, which then have twisted wires the meet up and merge in to one wire. This single wire is connected to a point on the PSU. This point is then connected to a local star (Instead of the configuration pictured, I have both wires for GND in the regulator connected to the top point). This star is under the heatsink, so you can't see it well in the picture, but this star is connected to the earth pin on the IEC, the negative of the filter cap, and the center tap of the power transformer.

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Last edited by dutchgfx; 20th March 2015 at 09:38 PM.
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Old 20th March 2015, 10:33 PM   #8
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Also, the hum goes away immediately upon shutdown and it takes a few seconds before it settles in to a steady hum upon startup. I'm thinking it might be the supplies, even though I trust Pete's stuff, I feel like my grounding is fine.
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Old 20th March 2015, 10:58 PM   #9
mdamp is offline mdamp  Australia
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Quote:
Originally Posted by dutchgfx View Post
Also, the hum goes away immediately upon shutdown and it takes a few seconds before it settles in to a steady hum upon startup. I'm thinking it might be the supplies, even though I trust Pete's stuff, I feel like my grounding is fine.
I too had a problem with hum and it was a leaky power supply cap I replace them all and rewired the 6H9C/EL34 AMP I made onto a wooden board and wired in a common ground wire {see in pic} and then fired it up bit by bit meaning built the power amp side first {el34} powered it up no hum then built the preamp side and fired it up and wow no hum!!!
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Old 20th March 2015, 11:02 PM   #10
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So you think it's my power supply? Even though the capacitor is brand new, as is the regulated supply? The current format is a full wave rectifier using 2 schottky diodes into a large 220u cap, then into the regulator.
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