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Old 22nd April 2007, 10:06 PM   #1
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Default hum/buzz with pot all the way up

I have a valve integrated, audio innovations series 3 MK2. Uses 12AX7 and ECL86.

Lovely amp, great sound etc etc etc. I also have a chinese 6P1 amp, also ovely soudn but different flavour.


Both have a volume control that is a simple pot, 100K I think.

AFAIK, on the chinese amp this volume control is situated on the line in from the input RCAs, ie acts as a simple attenuator. Why is it when I turn the pot up all the way (ie defeated) I get hum/buzz etc etc (like full power)?

Next question, the AI300 is the same although I think that in this case the volume pot is after the preamp section and before the ECL86s. Turn it up more than halfway and you hear a significant amount of hum/buzz. Again, why is this?

I was under the impression that with the pot turned all the way up, you were basically having the amp running at full power (eg like a power amp). Reason I ask is that I've just built a passive pre and was going to use the attenuator on it to control volume rather than the one on the amp. (although even with the volume halfway up on either amp, befoer I hear buzz, I still have enough gain). A RLD should be started in the near future.

A bit rambling I know, but comments please!

Fran
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Old 22nd April 2007, 10:42 PM   #2
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Fran,

I had similar problems with my new Aikido, although the hum was not all that bad. In the end, I elected to build a simple voltage (L-pad) inside my power-amp. My main amp is an AKSA 55N, solid-state (not tube). The input impedence is around 47K. On a previous post someone suggested that 47K would be less prone to noise than the 100K end. Anyway, it worked, I purchased a bunch of high precision resistors and found the combination that gave 47K and the attenuation I wanted.

You could also look for Ground Loops. Check that your pots are fully insulated from the chassis?

Also connecting my DVD player caused a loud hum, which I removed by disconnecting the signal GND on the preamp RCA's that connect to the DVD player.

I hope this helps.

BTW, where in Ireland are you? I have family near Clonakilty, Co. Cork. It is really beautiful down there.

Charlie
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Old 23rd April 2007, 12:13 AM   #3
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all the pots I am using are just mounted directly to the chassis - so definitely the mounting part/body of the pot/attenuator (broskie stepped one in the case of the passive pre) are directly connected to the chassis. Actually now that I think about it, the darwin source selector uses 2 pots for input and output selection. These are essentialy voltage switches, switching ground on a 5VDC line to the relays on the boards. They are mounted close enough to the broskie attenuator and the body of the pots would also be directly connected to the chassis.

However, as I can see it, these pots are all meant to be connected like this to the chassis. Is there something I'm missing here?


Fran
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Old 23rd April 2007, 12:30 PM   #4
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Anyone with experience of this care to comment?


Should I look at insulating the pots across the front fascia?


Fran
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Old 24th April 2007, 02:53 AM   #5
Jeb-D. is offline Jeb-D.  United States
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I had that happen caused from grounding/power supply issues. Are you using a grounded signal source by any chance such as a PC?
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Old 24th April 2007, 09:38 AM   #6
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Sources are CDP/TT/Squeezebox/Tuner/Tape Deck.


I have asked around locally here to me to see if anyone would have a power amp I could borrow. That way I can see how much/what the preamp adds in terms of noise.

I also want to bypass the broskie stepped atten. to rule it out also. When I do that i will also isolate the pots to see if that makes any difference.

Keep the suggestions coming!



Fran
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Old 24th April 2007, 12:28 PM   #7
jkeny is offline jkeny  Ireland
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Hi Fran,
Don't know if this will help but I had the same experience recently in one channel on my Baby Huey amp - when I turned up the vol, a strong hum got louder in one channel only. I traced it down to the DC increasing on this channel as I turned up the pot. The cause of this was a wrongly adjusted CCS on the differential pair feeding the output tubes.

As I said, don't know if this helps but maybe you can check out if there is any DC on the Vol pot.

John
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Old 27th April 2007, 11:07 PM   #8
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Default OT: passive preamp

Hi all,

Finally got a chance to go and do some checking out on this. Turns out I do have some DC on the outputs. I did manage to reduce this somewhat by adding and extra ground from the PS back to the safety earth but its still there.

So I says to myself, if I add in a DC blocking cap on the outputs, lets see if that sorts out the problem. And lo and behold, it did. I just happened to have some 4.7uF 16V non-polar blackgates, so I soldered these in between signal ground and signal hot for each channel on one of the outputs, ie 1 cap per channel. Hum now completely gone.


However you shut one door and another opens! I obviously have set up some kind of filter because the broskie stepped attenuator now doesn't work! ie sound comes through when the attenuator is "off" but as soon as I switch it down to attenuate, the signal dissappears. So this must now be blocking the AC as well due to the set up between resistors in the attenuator and the caps on the outputs.

At this stage I cry HELP! Has anyone any solutions for me? To put it concisely: How can I eliminate the hum or alternatively how should I add in a blocking cap (should it be in series with the signal hot or paralleled from signal hot to ground?)

Fran

Fran
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Old 27th April 2007, 11:45 PM   #9
jkeny is offline jkeny  Ireland
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Hi Fran,
Do you get a hum with nothing connected to input of amp? If so the DC is arising in the amp & not from equipment connected to it. You need to trace back towards the input, testing for DC along the signal path.

John
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Old 27th April 2007, 11:46 PM   #10
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Hi Fran,

A little difficult off the cuff. Is it possible to post a circuit - even hand-drawn?

Regards
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