Hi all
I´ve built a small 6aq5/ecc81 SE amp with ez40 rectifier for my compiuter.
Since it´s assembled from whatever was around the hum wasn´t 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.
I´m totally confused what´s going on any sugestions ??
I´ve built a small 6aq5/ecc81 SE amp with ez40 rectifier for my compiuter.
Since it´s assembled from whatever was around the hum wasn´t 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.
I´m totally confused what´s going on any sugestions ??
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.
http://www.freewebs.com/valvewizard/heater.html
ps Lovely build
http://www.freewebs.com/valvewizard/heater.html
ps Lovely build
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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 OPT´s ???
I´ll carry my big UPS from the computer into the shop just to rule out any chance of the mains doing something funny.
I´ve ran out of ideas.
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 OPT´s ???
I´ll carry my big UPS from the computer into the shop just to rule out any chance of the mains doing something funny.
I´ve ran out of ideas.
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?
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 it´s total mess.
What pussles me is that level of the hum is pulsating slowly it´s not a steady 50/100hz hum.
It must be some kind of ground issue or oscilation but i can´t put my finger on it.
If someone have a good plan i´ll try it ASAP.
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 it´s total mess.
What pussles me is that level of the hum is pulsating slowly it´s not a steady 50/100hz hum.
It must be some kind of ground issue or oscilation but i can´t put my finger on it.
If someone have a good plan i´ll try it ASAP.
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.
No tubes in HT and 6.3AC attached from lab supply hum there.
Today i unbolted the power xformer and turned it 90 something happened but not because i turned it, then put it om some rubber pieces little hum left no pulsation.
Doesn´t do much what way i turn it but as soon as it sits on the chassi it starts.
groundloop through transformer it´s internally shielded , ??
Mechanical vibration??
How is it transfered to the output without the the tubes present ??
I´m totally lost here
Time for disassembly and startover without internal PSU if it misbehaves on labpower and rectified fillament i´ll carry it upstairs and give it flying lessons from the window.
Today i unbolted the power xformer and turned it 90 something happened but not because i turned it, then put it om some rubber pieces little hum left no pulsation.
Doesn´t do much what way i turn it but as soon as it sits on the chassi it starts.
groundloop through transformer it´s internally shielded , ??
Mechanical vibration??
How is it transfered to the output without the the tubes present ??
I´m totally lost here
Time for disassembly and startover without internal PSU if it misbehaves on labpower and rectified fillament i´ll carry it upstairs and give it flying lessons from the window.
Something is going on with the power xformer 16.4v between inner and outer shield
So this means i have ac flowing through whatever on its way back to grounding.
My lobudget building has struck back i´ll convert to no budget and go dumpsterdiving in my storage for an old nonasia built power xformer.
Thought that little transformer looked nice shielded and everything.
So this means i have ac flowing through whatever on its way back to grounding.
My lobudget building has struck back i´ll convert to no budget and go dumpsterdiving in my storage for an old nonasia built power xformer.
Thought that little transformer looked nice shielded and everything.
You might have a transformer problem, but you definitely have a grounding layout problem. Have you connected the signal ground at the RCA to the chassis or to the end of that ground bus you have? If so you have a ground loop. You have also failed to use the technique of attaching the grounds to the bus in order of current flow. That is, your ground bus should have the grounds of each stage attached in the order of highest current to lowest current. This stops high currents in the ground bus from effecting the low current signal. Remember, your amp is amplifying the voltage difference between the signal and the signal ground. If you place the ground for your power stage 'upstream' of your small signal stage, any fluctuations in current through that ground are going to cause a voltage change as the ground bus does not have zero resistance.
Your grounding scheme has a 'Y' shape with small signal stage in the middle, and power stage at either end, with power supply ground hanging off one end of the 'Y' and the signal ground at the other. Poor transformer issue or not, that thing was always going to hum, and as you found out, no amount of ferrite beads and capacitors is going to help a ground loop! The grounds should be placed along that bus in the exact order they are represented in that schematic, signal ground, small signal tube star ground, decoupling capacitor for small signal tube, star ground for power stage, ground for output transformer, then ground of power supply filter cap. Attach one end of ground bus to chassis. Signal ground is the shield from the coax cable from the RCA. Insulate the RCA from the chassis.
I found this article really helpful getting the concepts straight in my head Star Grounding Hope this helps.
Cheers,
Chris
Your grounding scheme has a 'Y' shape with small signal stage in the middle, and power stage at either end, with power supply ground hanging off one end of the 'Y' and the signal ground at the other. Poor transformer issue or not, that thing was always going to hum, and as you found out, no amount of ferrite beads and capacitors is going to help a ground loop! The grounds should be placed along that bus in the exact order they are represented in that schematic, signal ground, small signal tube star ground, decoupling capacitor for small signal tube, star ground for power stage, ground for output transformer, then ground of power supply filter cap. Attach one end of ground bus to chassis. Signal ground is the shield from the coax cable from the RCA. Insulate the RCA from the chassis.
I found this article really helpful getting the concepts straight in my head Star Grounding Hope this helps.
Cheers,
Chris
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Toprepairman you´re probably right when i put the power xformer on some rubberbushings i quiets down and stop pulsating.
Chris the grounding isn´t tip top it looked a little better before my wild hum chase and rearranging started.
I have to make a decision between starting over on a non magnetic chassi or perhaps putting a plastic sheet on top and stargrounding the 3 xformers.
Solving the grounding or building it from scratch is about the same time
Chris the grounding isn´t tip top it looked a little better before my wild hum chase and rearranging started.
I have to make a decision between starting over on a non magnetic chassi or perhaps putting a plastic sheet on top and stargrounding the 3 xformers.
Solving the grounding or building it from scratch is about the same time
At last free from the home improvement mania. I just hate those tv shows, our tv is about to have a major breakdown and the parts for repair will me out of stock for at least a month
Transformers now elevated from chassi on a piece of wood individualy star grounded.
Groundbus rearranged.
Some hum left in channel closest to powerformer the other ok.
Many thanks to all you good people for sharing your time and knowledge in the project.
The hum left i can live with, I´ll have to stick my head right in the speaker to hear it.
Transformers now elevated from chassi on a piece of wood individualy star grounded.
Groundbus rearranged.
Some hum left in channel closest to powerformer the other ok.
Many thanks to all you good people for sharing your time and knowledge in the project.
The hum left i can live with, I´ll have to stick my head right in the speaker to hear it.
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