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
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
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.
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.
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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.
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.
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
Mike
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.
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
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.cyclecamper said:
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.
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.
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.
One more message just to try this photo thing. Setup when I just fired up the preamp.
An externally hosted image should be here but it was not working when we last tested it.
Well,
The number one question has got to be, why have you got a volume control on the input (Is this some kind of balance control?) and output of the preamp?
You only need one on the input and drive straight from the last tube to the power amp..
2nd question is why have you got a potential divider on the input to the preamp?
I would expect a balance / volume on the input and direct drive out of the last tube?
Just a thought...if the two pots on the input were volume individual left and right you would have balance and volume..or put bal/volume on the input..
Regards
M. Gregg
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Here is some info..
Have a look at the volume and balance circuits..
Potentiometers (Beginners' Guide to Pots)
Or just adapt what you are using now..
Regards
M. Gregg
Have a look at the volume and balance circuits..
Potentiometers (Beginners' Guide to Pots)
Or just adapt what you are using now..
Regards
M. Gregg
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Yes that is a balance control right after the input and before the tubes. I would rather not have to adjust 2 volume pots at the same time. I listen to a lot of music in stereo so not having to adjust the balance several times to get it perfect is preferred. I have seen the dual pots with a split shaft that would work nice because I could adjust left and right and still turn the whole thing up and down or I hope that’s how it would work. I did read the website and have seen it before but the question that remains is how does a person decide what value of pot is appropriate for the unit? Tonight I should have some time and just for trial I will remove the balance circuit and put the volume pot in its place. I will let you know what I find out.
Mike
Mike
Ok,
This is what I would do..
remove the 500K pot from the output and fit a 100K resistor so the ouput cap is Grounded via the resistor..Take the output to the phono straight from the cap after the last tube.
Fit a 100K Log pot as the volume on the front end of the preamp with your balance control..
If you dont have a 100K log pot use a resistor to set the resistance taper to log as shown in the link.
As to what size pot do I use it depends on the application, however you won't go wrong to stick with 100K as volume others may use a different value. The idea is to not load the previous stage but still keep the connecting lead close to gnd with no signal. There is more such as the effect of the resistance in line with the tube grid. Etc. (Miller Capacitance)..
Regards
M. Gregg
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Link for Miller effect,
You have to remember that the top section of the volume control is in series with the grid..The lower the volume the more resistance is in series..
The Valve Wizard
Regards
M. Gregg
You have to remember that the top section of the volume control is in series with the grid..The lower the volume the more resistance is in series..
The Valve Wizard
Regards
M. Gregg
So after a evening of soldering I did make the modifications M. Gregg recommended. The hum is still there. Even with the preamp unplugged. When I did turn it on the hum disappeared for a second and then came back at louder than ever. Plugged in my laptop and the amp doesn't hum and music sounds great. I even used a cheesy RCA to 3.5mm adapter in to the RCA cables that I have been using. I did make the flying leads pot and the amp stops humming at 450 ohms. One question I do have, isn't there a lot of stray capacitance to ground if I put one pot in line with another which would cause me to lose some high frequencies? What are you guys thinking that I should try next? Thank you for the help too.
Update. Tonight I plugged in a cd player to the preamp and plugged it in to the aux port of my Bogen RP60 amp and guess what it works with out the hum. But when I do crank up the volume on the Bogen amp I do get a higher pitch hum like 120 hz but that can be fixed later. Probity due to the heater supply. This whole deal has me baffled and what I should do with the preamp and why it won't work with my amps.
Mike
Mike
Sounds like you have more than one fault,
This isn't magic even if it seems like it.. it can only be grounding or psu problems,
If it hums when unplugged then it could be that the chassis is now not at Gnd..you can prove this by disconnecting the supply to the mains Tx and plug back in..
The thing to do is work on the other hum that you have and some times it will effect the situation and give clues to further problems. Just don't give up.
The phono sockets should be isolated from the chassis and a star ground should be in place.. If the preamp works with another amp then you need to ask what is different with the grounding when linked to the different power amps. (Taking into account the difference on the input side of the power amps)
Never remove a safety mains chassis earth to get rid of hum...
Regards
M. Gregg
This isn't magic even if it seems like it.. it can only be grounding or psu problems,
If it hums when unplugged then it could be that the chassis is now not at Gnd..you can prove this by disconnecting the supply to the mains Tx and plug back in..
The thing to do is work on the other hum that you have and some times it will effect the situation and give clues to further problems. Just don't give up.
The phono sockets should be isolated from the chassis and a star ground should be in place.. If the preamp works with another amp then you need to ask what is different with the grounding when linked to the different power amps. (Taking into account the difference on the input side of the power amps)
Never remove a safety mains chassis earth to get rid of hum...
Regards
M. Gregg
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