Hi all,
I've been spending some time tracking down those small hums which really annoy if left alone (like an itch that need scratching). I've managed to get my preamp and my power amp whisper quiet.
Yesterday I decided to try my hand at my Phono preamp. Its the preamp discussed by Theorsten in the "DC phono" thread.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=28957&highlight
I have done various things and improved things quite a bit (after making things considerably worse for a stressful while).
When I run the preamp with no input I get a bit of hum - which I attribute to the residual hum of the preamp. Then with the source deck plugged in there is a fraction more hum. The strange thing is that when I plug the turntable in the preamp becomes absolutely quiet, then over the next 30 seconds or so the hum slowly ramps up until it is back at normal levels. Its as if plugging the turntable in discharges a cap which then charges up again over the next 30secs.
It could be a slight difference in earth potential between the phono preamp and the deck causing some effect.
Anyone experienced this and can offer an explanation - if not a solution.
Thanks
Shoog
I've been spending some time tracking down those small hums which really annoy if left alone (like an itch that need scratching). I've managed to get my preamp and my power amp whisper quiet.
Yesterday I decided to try my hand at my Phono preamp. Its the preamp discussed by Theorsten in the "DC phono" thread.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=28957&highlight
I have done various things and improved things quite a bit (after making things considerably worse for a stressful while).
When I run the preamp with no input I get a bit of hum - which I attribute to the residual hum of the preamp. Then with the source deck plugged in there is a fraction more hum. The strange thing is that when I plug the turntable in the preamp becomes absolutely quiet, then over the next 30 seconds or so the hum slowly ramps up until it is back at normal levels. Its as if plugging the turntable in discharges a cap which then charges up again over the next 30secs.
It could be a slight difference in earth potential between the phono preamp and the deck causing some effect.
Anyone experienced this and can offer an explanation - if not a solution.
Thanks
Shoog
)
Where the turntable is situated can make difference too. Last winter, I noticed the hum shot up a lot as the electric heater turned on. EMF being induced from the power wires in the adjacent wall were causing this. I moved the turntable and the hum disappeared
No no no! You need to tear out all the walls and rewire your house with twisted pair.
The whole phono preamp is built within two PC power supply cases strapped together, one for the power supply and one for the circuit.
Thinking about grounding a phono preamp takes you to a whole different level than a line stage.
Yesterday I seperated the earth return for the final stage filtering caps from the first stage. That helped quite a bit. I am going to create two star grounds for the two stages, using the valve socket as the final star point. Its almost in this configuration, but needs a slight bit of tidying up.
Filament DC supply shows a nearly flat response, but I suppose there could be current pulsing which wouldn't show up on the scope. I may try lifting its potential a tad and see if that helps .
Still no explanation for the rising hum when the turntable is plugged in.
Shoog
Thinking about grounding a phono preamp takes you to a whole different level than a line stage.
Yesterday I seperated the earth return for the final stage filtering caps from the first stage. That helped quite a bit. I am going to create two star grounds for the two stages, using the valve socket as the final star point. Its almost in this configuration, but needs a slight bit of tidying up.
Filament DC supply shows a nearly flat response, but I suppose there could be current pulsing which wouldn't show up on the scope. I may try lifting its potential a tad and see if that helps .
Still no explanation for the rising hum when the turntable is plugged in.
Shoog
I have tracked down and eliminated all sources of hum except for one.
There is a low clean 50hz mains hum which I suspect is been picked up off the case. If I cup my hand around the ECC83 the hum is gone, take my hand away and it comes back.Seems as if my hand is working as a screen/drain.
Can't for the life of me work out a perminent fix for this hum - any ideas.
Shoog
There is a low clean 50hz mains hum which I suspect is been picked up off the case. If I cup my hand around the ECC83 the hum is gone, take my hand away and it comes back.Seems as if my hand is working as a screen/drain.
Can't for the life of me work out a perminent fix for this hum - any ideas.
Shoog
Is the ECC83 inside a shield? If not that is the next obvious thing to try.
Are your two chassis connected with a ground wire? Not one that is part of the circuit, just a wire from one metal case to the other. If not does connecting the two cases increase or decrease the hum?
Also how far apart are the chassis and what type of connection are you using to take power from the PS to the pre?
Are your two chassis connected with a ground wire? Not one that is part of the circuit, just a wire from one metal case to the other. If not does connecting the two cases increase or decrease the hum?
Also how far apart are the chassis and what type of connection are you using to take power from the PS to the pre?
The two chassis are bolted together. There is an earth strap between the two chassis, tied at the earth reference point for the turntable.
The ECC83 isn't shielded, and I was think that that was an obvious thing to try, though unfortunately it requires a fairly big rebuild.
I just tried an earth strap from the chassis next to the ECC83 socket to the main star ground, this has improved the hand hum situation a great deal.
I am coming the conclusion that the transformer I am using is c*&p and its radiating leakage inductance all over the case. This is bleeding right across to the valve sockets and causing most of the problem. The conclusion is that I need a new transformer and a bigger case for better isolation. As it is I think I am wasting my time trying to get it quiet.
Thanks all.
Shoog
The ECC83 isn't shielded, and I was think that that was an obvious thing to try, though unfortunately it requires a fairly big rebuild.
I just tried an earth strap from the chassis next to the ECC83 socket to the main star ground, this has improved the hand hum situation a great deal.
I am coming the conclusion that the transformer I am using is c*&p and its radiating leakage inductance all over the case. This is bleeding right across to the valve sockets and causing most of the problem. The conclusion is that I need a new transformer and a bigger case for better isolation. As it is I think I am wasting my time trying to get it quiet.
Thanks all.
Shoog
Turntable hum problems
Dear Shoog,
Although others have suggested this also, you should definitely make sure that the ground wire on the turntable is making contact with both the phono pre-amplifier ground point AND the turntable platten. On my DIY Seduction phono pre-amp if I disconnect the ground wire, I get considerable hum. I found that ground wire from my Technics turntable had lost contact with the turntable platten.
mark845
Dear Shoog,
Although others have suggested this also, you should definitely make sure that the ground wire on the turntable is making contact with both the phono pre-amplifier ground point AND the turntable platten. On my DIY Seduction phono pre-amp if I disconnect the ground wire, I get considerable hum. I found that ground wire from my Technics turntable had lost contact with the turntable platten.
mark845
I bit the bullet and decided to rebuild in a new case.
I had a suitable case so it didn't take long. There is a great deal more distance between the power transformer and the main circuit, there are also two grounded screens between as well. I introduced a high value inductor into the power supply ( I tried a smaller inductor but it introduced a resonance at about 50hz). I also placed another 65K uf of smoothing on the heater supply. I finally replaced the ECC83's base with a screened can base.
Its lashed together until I can get some decent phono plugs.
Still, the hum is all but gone at any sensible listening level. Its been well worth the frustration of debugging as I now feel confident I will have a phono preamp that I can happily live with for years. Without the residual hum it sounds very clean and forward.
Shoog
I had a suitable case so it didn't take long. There is a great deal more distance between the power transformer and the main circuit, there are also two grounded screens between as well. I introduced a high value inductor into the power supply ( I tried a smaller inductor but it introduced a resonance at about 50hz). I also placed another 65K uf of smoothing on the heater supply. I finally replaced the ECC83's base with a screened can base.
Its lashed together until I can get some decent phono plugs.
Still, the hum is all but gone at any sensible listening level. Its been well worth the frustration of debugging as I now feel confident I will have a phono preamp that I can happily live with for years. Without the residual hum it sounds very clean and forward.
Shoog
Comin' in late on this one, but you might want to try Thorsten's dirty diodes trick. Note that the grounding scheme for his phono pre design that I posted has the bridge rectifier 90 degrees INCORRECT, but if you shift it as he mentions, you'll be right on.
I've screwed up almost everything in my life, just ask my dad, but I got this thing dead quiet.
If you try this, let us know how it worked out.
Kofi
I've screwed up almost everything in my life, just ask my dad, but I got this thing dead quiet.
If you try this, let us know how it worked out.
Kofi
Thanks for that Mr Annan (what plans for your retirement ?).
I tried that this morning. Took away almost all of the remaining hum. Would you believe that I had tried that before but had got it into my head that it wouldn't work and had shorted it out. So I already had the little board to put in. I use a slightly different implementation. I have back to back diodes bypassed by a 0.68uf cap and a 100R resistor. The cap allows a perminent path to earth for radio frequencies, and the 100R is insurance should the diodes die.
I have the same arrangement in my main preamp. Works a treat there to.
Thanks
Shoog
I tried that this morning. Took away almost all of the remaining hum. Would you believe that I had tried that before but had got it into my head that it wouldn't work and had shorted it out. So I already had the little board to put in. I use a slightly different implementation. I have back to back diodes bypassed by a 0.68uf cap and a 100R resistor. The cap allows a perminent path to earth for radio frequencies, and the 100R is insurance should the diodes die.
I have the same arrangement in my main preamp. Works a treat there to.
Thanks
Shoog
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