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Old 16th April 2012, 02:22 AM   #1
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Default Amp hum with Preamp off

Hello

So I have been playing around with tube amps for a couple of years and have built 2 amps and a preamp. My question is, when my preamp is off but that amp is on I get a noticeable hum when I increase the volume on the preamp then it fades off when the volume is at max. The hum is also there when the preamp is on and has about the same volume of hum. The amps are http://diyaudioprojects.com/Schematics/DIY-Push-Pull-PP-6V6-Tube-Amplifier/ with a choke power supply, and http://diyaudioprojects.com/Tubes/KT77-Push-Pull-Tube-Amp/ The preamp is based off a schematic from http://www.funwithtubes.net/ and the last object in the signal path from the preamp is a 500k alps blue pot. When I have either amp hooked up to my computer I do have a little base line hum which is ok for now but it is no where near the amount I get from that preamp volume control. Do I have impedance mismatch? What should I look for? Thank you for helping a newbie out.

Mike
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Old 16th April 2012, 03:27 AM   #2
rman is offline rman  Canada
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Hi.

Are both power and preamp earth grounded with the signal grounds attached to earth?
If so you might have a ground loop. I think that means tiny differences in ground voltage levels are carrying a 60hz signal into your power amp. try a search on ground loops for ideas on a fix.

Cheers.
Rolf.

Last edited by rman; 16th April 2012 at 03:28 AM. Reason: Fixed what I meant to say
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Old 16th April 2012, 12:44 PM   #3
DF96 is offline DF96  England
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500k is quite a high value for a volume pot. My guess is that the input of the next stage has some stray capacitive coupling to a hum source. Could be wiring badly positioned, or the classic enormous audiophile coupling cap with huge physical size. The high source resistance from a mid-position pot then fails to 'short out' the hum current.
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Old 16th April 2012, 01:17 PM   #4
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Quote:
Originally Posted by moparmike View Post
When I have either amp hooked up to my computer I do have a little base line hum which is ok for now but it is no where near the amount I get from that preamp volume control.
This part of your problem might be fixed by using a different outlet or even an extension cord to power your amp.

I had several amps do this, they don't like to be plugged in near the computer power bar, despite all my attempts at attenuating the noise, it was never to my satisfaction.

So I took the easy way out, ran a small cord to the next outlet, now I can't tell if my amps are on unless I play something or see the tubes glowing.
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Old 17th April 2012, 02:07 AM   #5
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Ok guys thanks for all the tips. So tonight I removed all grounds from the chassis except the earth ground on one amp and preamp and tried it. No improvement so I plugged in the pre to my Bogen RP60 amp in the aux input and guess what no hum at any volume level on pre and on the Bogen well maybe a tad and max volume. So then I put in a 100k pot that I had laying around and plugged it back in the original amp and the hum is always there but isn't as loud at the peak and does go down when maxed out like the 500k pot. Again the hum is the same with the unit on or off or unplugged. I run orange drop caps for my coupling caps. It seems like my DIY projects hates each other. So what else are you guys thinking?

Mike
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Old 17th April 2012, 02:39 AM   #6
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Heater wiring maybe? Super tight twist, hug the chassis. Signal wiring at least 1" above (3D) and at right angles. Star ground referenced from the first capacitor after the rectifier.

Scott

Last edited by scott17; 17th April 2012 at 02:43 AM. Reason: add real name
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Old 17th April 2012, 05:27 AM   #7
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Quote:
Originally Posted by DF96 View Post
500k is quite a high value for a volume pot.
This is on the right track. Anywhere near the middle of the pot's range the hot input is floating pretty far from ground, and can pickup hum. This is typical of the volume controls on Fender guitars, which use higher-value volume pots than gibson. At full volume you've connected the dapming factor of some output. At minimum volume the input is grounded. But in between the input wiring can pick up hum. So you need to do two things: make it less sensitive to magnetic interference, and then reduce the 60 cycle magnetic field in the area. Use shielded cable for all wiring at the power amp inputs. Lower the input impedance of the power amp. Increase the output of the preamp and reduce the gain of the power amp. Change the volume pot for a much much lower value. Try a resistor from the pot's wiper to ground.
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Old 17th April 2012, 08:59 AM   #8
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by moparmike View Post
Ok guys thanks for all the tips. So tonight I removed all grounds from the chassis except the earth ground on one amp and preamp and tried it. No improvement so I plugged in the pre to my Bogen RP60 amp in the aux input and guess what no hum at any volume level on pre and on the Bogen well maybe a tad and max volume. So then I put in a 100k pot that I had laying around and plugged it back in the original amp and the hum is always there but isn't as loud at the peak and does go down when maxed out like the 500k pot. Again the hum is the same with the unit on or off or unplugged. I run orange drop caps for my coupling caps. It seems like my DIY projects hates each other. So what else are you guys thinking?

Mike
Sounds like the pre is OK..

What happens if you put the pot on flying leads on the power amp (Not in the preamp) and connect to the source direct to the pot?

What (if any) components are between the preamp volume pot and the output? I guess you have to be a member to see the circuit on the site you posted?

Regards
M. Gregg
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Old 17th April 2012, 10:20 AM   #9
DF96 is offline DF96  England
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Quote:
Originally Posted by cyclecamper
So you need to do two things: make it less sensitive to magnetic interference, and then reduce the 60 cycle magnetic field in the area.
No. I think you meant to say "make it less sensitive to electrostatic interference, and then reduce the 60 cycle electric field in the area." If magnetic induction was the problem then it would get better at mid-volume and worse at the two extremes.

Your advice on how to fix it is good - as it aims to prevent electric field induction. I would add: get rid of any large size coupling caps at the amp input, either use smaller ones or screen them from hum fields.
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Old 18th April 2012, 12:47 AM   #10
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So you guys have an idea of my madness I got a flickr account. I hope that that the links show up right. These drawings I edited in paint and excel so they are kinda rough but are readable. They also came off the Fun With Tubes

Preamp2 | Flickr - Photo Sharing!

003 | Flickr - Photo Sharing!

Not really proud of the last link but my construction is getting better. Yes those are Octals I used 6SL7 instead of 12ax7. I have a thing for octals and the sockets are easier to wire at least for me. All the AC is in the little boxed in area. I didn't have the alps blue pot in this pic or my signal grounds isolated from the chassis. The RCA plugs I used do have insulation keeping them separated from the chassis. Thanks for the help so far. Oh and what value of pot should I be using for that preamp? I haven't had time to do any soldering tonight so the external pot will have to wait until tomorrow.
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