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Old 14th November 2012, 11:12 AM   #31
tuhkam is offline tuhkam  Estonia
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If you find the root cause of your hum problem, be so kind and post it. I'm aftaid I have quite exact the same sympthoms with my EL84 SE.
When turning the volume pot louder the hum also gets louder, if the pot is maxed the hum dissapears completely.
The hum is also absent, when the I use the headphone out (it has a volume pot) on my CD player instead of line out and byass the amplifiers volume pot.
My signal ground is connected straight to power ground, safety earth connects only to chasis.
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Old 14th November 2012, 11:47 AM   #32
DF96 is offline DF96  England
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So the hum is associated with the volume pot. Either poor grounding, or capacitive coupling, or a bit of both.
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Old 14th November 2012, 01:17 PM   #33
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OK, lets see.....


now you want to hook up 1 preamp to two or one amplifiers that are DIY right?

an impedance mismatch is happening to you. at the output of the preamp to the input of the amps.

I assuming that you want the independent input controls of the amp.

this goes back to the arbitrary 100k in DIY amp designs people. 100k is arbitrary and those designs require tweaking.
and is around the lowest possible input impedance that a tube amp on the input can run.


if you were just running one amplifier, you could just increase the potentiometer on the one amp from 100k to 500K then the effective coupling impedance is 250K (this is a better loading by far) now the second amp in parallel: with its 100k substituted with a 500k would be effective loading impedence of 166K ( a little low, but reasonable). Now insert a 220K resistor between the connection of the potentiometer on the amplifier(s) and ground. you'll notice that increasing the resistance path from input grid to ground makes your problems go away.



have a nice day...
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Old 14th November 2012, 01:36 PM   #34
DF96 is offline DF96  England
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If the problem is capacitive coupling then increasing circuit impedances will make it worse.
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Old 14th November 2012, 01:54 PM   #35
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Quote:
Originally Posted by DF96 View Post
If the problem is capacitive coupling then increasing circuit impedances will make it worse.
dc coupling works, you just need to look at whats going on.

Btw, the preamp is already ac coupled.

just looking at both amps, if the 100k pot on the one amp is changed to 1M and the 470K on the other amp changed to 1M.
the the reflective Z is 250K. if you have noise in one to another its because one amp is imposing voltage on another. this can be combated by increasing the grid snubber resistor to compensate.

unless you want to give up on the dc coupling, then you can ac couple one or the other or both with a simple 1Uf 250V cap. But the need of impedance balancing is still needed.
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Old 14th November 2012, 03:23 PM   #36
DF96 is offline DF96  England
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What has DC coupling got to do with it? I have no idea what you are talking about. Please explain how raising circuit impedances will reduce hum (not noise) wnen there may be evidence that some of the hum is being caused by capacitive coupling to a high impedance point. When the slider hits max the hum disappears, because at that point the circuit node is connected directly to the low impedance output of the source.
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Old 16th November 2012, 07:40 AM   #37
Input2 is offline Input2  Canada
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It does not look like the power supply is complete, the problem is all in the power supply as it is producing ripple voltage.
Looking at the picture of the amp, wire a 4700 ohm 2 watt resistor between the plus side of the two caps.
Wire that resistor, one end to the plus side of the one cap and the other end of the resistor to the plus side of the other cap.
That should reduce it, if it does, then we can go further.

Last edited by Input2; 16th November 2012 at 07:55 AM.
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Old 16th November 2012, 10:25 PM   #38
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Quote:
Originally Posted by tuhkam View Post
If you find the root cause of your hum problem, be so kind and post it. I'm aftaid I have quite exact the same sympthoms with my EL84 SE.
.
My hum problem was an issue of a wiring mistake on my DC supply for my preamp tubes and poor construction in the amp. After fixing the heater supply in the preamp and rebuilding/new layout with the amp I have little to none hum. Thanks for asking.
Mike
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