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Old 9th August 2011, 07:56 PM   #1
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Default Hum and noise in small amp

Hi all

Iīve built a small 6aq5/ecc81 SE amp with ez40 rectifier for my compiuter.

Since itīs assembled from whatever was around the hum wasnīt to much of a surprice and i suspected the C-R-C filter, after adding another R-C stage it was a bit less but still there.

After some listening i found the hum to be pulsating (oscilation?)
and not like a regular 50/100hz hum.

Connected it to lab supply power hum still there so i threw in a bunch of ferrit beads and 1K gridstoppers to prevent any oscilation no difference.

Pulled all tubes, still on lab power pulsating hum still there.

Iīm totally confused whatīs going on any sugestions ??
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Old 9th August 2011, 08:21 PM   #2
TheGimp is offline TheGimp  United States
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Check your transformer locations. Are the laminates perpindicualr to each other? Are they in close proximity?

schematic would be helpfull along with photos of the top and bottom.
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Old 9th August 2011, 08:33 PM   #3
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Yes theyīre close and inline since itīs a small amp.

Power xformer is shielded and grounded.

Iīll try to draw up the schematic and take a few pics.
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Old 9th August 2011, 09:23 PM   #4
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Some pics of amp and schematics

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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Old 9th August 2011, 09:42 PM   #5
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I like to rule out heater hum before I look any deeper, here's a good read on how to if you havn't already done so.

ps Lovely build

Last edited by Soonerorlater; 9th August 2011 at 09:47 PM.
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Old 9th August 2011, 10:11 PM   #6
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I tried adding a hum pot and elevating the heater no difference.

I was running it on lab power 250v regulated and 6.3v rectified and the hum still there. even with the power xformer out of the equation it has a low frequency pulsating hum.

Some kind of weird oscilation or interference betwen OPTīs ???

Iīll carry my big UPS from the computer into the shop just to rule out any chance of the mains doing something funny.

Iīve ran out of ideas.
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Old 10th August 2011, 12:22 PM   #7
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Sounds like magnetic coupling between the transformers.
Unbolt the power tx from the chassis, and with no valves in, rotate the tx with the power on and you'll find the hum will have a minima with the power tx at around 90 degrees to the others.
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Old 10th August 2011, 02:11 PM   #8
chrish is offline chrish  Australia
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For me, first thoughts for hum tend to be grounding issues. With that in mind, you appear to have a ground bus extending from your power stage towards the rear of the chassis towards the input RCA plugs. Hard to see where it attaches to because of the chassis lip. Connected to the signal input at the RCA jack?
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Old 10th August 2011, 02:56 PM   #9
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The groundbus original started at power xformer an ended at rca,12ohm/,22cap to mains grounding point at xformer.

Under dehumming i rearanged it so HT is now fed through twisted pair from power supply with some extra film caps added,grounding point connects close to rca this time.

Right now after randomly throwing in caps and ferrit beads itīs total mess.

What pussles me is that level of the hum is pulsating slowly itīs not a steady 50/100hz hum.

It must be some kind of ground issue or oscilation but i canīt put my finger on it.

If someone have a good plan iīll try it ASAP.
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Old 10th August 2011, 03:29 PM   #10
jakruby is offline jakruby  South Africa
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i built a 12ax7 phono stage that also seemed to have a similar 'oscillation' to yours. yet mine had the power supply on a separate chassis and still problems. your circuit and mine don't use any feedback so (i think) oscillation should not be an issue. have you tried more decoupling between stages and channels? and can you trace where in the circuit your hum originates or is greatest? i have yet to solve my problem, increased PSU capacitance made no difference for me.
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