I am having an issue when connecting my 6dj8 phono preamp to my 12au7 preamp. The audio coming from the speakers (nothing playing) sounds almost like a ground loop.
The phono pre is all DC and in a shielded box with the power coming through an umbilical.
Using the phono preamp with my 6v6 integrated amp there is no issue at all.
Things that I have tried thus far:
1) changed the cables from the phono pre to the preamp
2) Moved the phono pre to a different location in my stand
3) connected the ground from the phono pre to the ground on my preamp
4) inspected the phono preamp for a bad ground.
5) with nothing hooked to the phono preamp, I looked at the output on my scope. Nothing there (no noise output)
The preamp will work fine when I have my cd player connected. No distorted noise at all. When I connect my phono preamp and power it up, that is when I hear the hum/noise and at a pretty high level. Not even playing an album at this point, just have it connected and turned on.
Could this be an impedance matching issue with this preamp? I have attached the schematics of both.
The phono pre is all DC and in a shielded box with the power coming through an umbilical.
Using the phono preamp with my 6v6 integrated amp there is no issue at all.
Things that I have tried thus far:
1) changed the cables from the phono pre to the preamp
2) Moved the phono pre to a different location in my stand
3) connected the ground from the phono pre to the ground on my preamp
4) inspected the phono preamp for a bad ground.
5) with nothing hooked to the phono preamp, I looked at the output on my scope. Nothing there (no noise output)
The preamp will work fine when I have my cd player connected. No distorted noise at all. When I connect my phono preamp and power it up, that is when I hear the hum/noise and at a pretty high level. Not even playing an album at this point, just have it connected and turned on.
Could this be an impedance matching issue with this preamp? I have attached the schematics of both.
Attachments
Last edited:
"The preamp will work fine when I have my cd player connected."
CD play normally is not earth directly, the chassis is grounded via the screen of the interconnected cable. The screen of this cable are jointed together at output of player, so it should not be joint again at the input of the preamp. When the overall gain is low this hum due to double ground is not quite noticeable. You can try to disconnet the screen of one of the interconnect cable to see any hum.
5) with nothing hooked to the phono preamp, I looked at the output on my scope. Nothing there (no noise output)
Do you mean phono preamp input has nothing connected, included the screen of the cable? I think better to use high quality cable with one common screen from pickup to phono preamp, instead of 2 separated screen cable, esp with you overall gain is high with addition gain of preamp.
CD play normally is not earth directly, the chassis is grounded via the screen of the interconnected cable. The screen of this cable are jointed together at output of player, so it should not be joint again at the input of the preamp. When the overall gain is low this hum due to double ground is not quite noticeable. You can try to disconnet the screen of one of the interconnect cable to see any hum.
5) with nothing hooked to the phono preamp, I looked at the output on my scope. Nothing there (no noise output)
Do you mean phono preamp input has nothing connected, included the screen of the cable? I think better to use high quality cable with one common screen from pickup to phono preamp, instead of 2 separated screen cable, esp with you overall gain is high with addition gain of preamp.
I only have a 2 wire electrical system in my house as it was built back in the early 1950's, so there is no 3 prong earth ground unfortunately. All of my equipment is using 2 prong power.
For test 5, I had cables connected to the output of the phono pre and that I what I took my measurement from. There was no noise on the output.
I have tried several different cables for the output thinking that may be the cause, but thus far it has not made a difference.
For test 5, I had cables connected to the output of the phono pre and that I what I took my measurement from. There was no noise on the output.
I have tried several different cables for the output thinking that may be the cause, but thus far it has not made a difference.
Maybe adding a earth wire to your system could solve the problem:
https://electronics.stackexchange.c...at-is-the-purpose-of-a-turntables-ground-wire
The hum level maybe too low for your scope to pickup. Anyway there maybe induced hum into the tone arm for instance, you can verify by shorting the input (R1) see if the hum goes away.
This is actually the preamp that I use on my 6v6 pp amp and have no issues with the phono pre....
I guess I could go that route again until I build the Pete Millet low mu preamp (parts on the way), but for now this problem has me stumped so it's more of a curiosity thing thats bugging me to figure it out.
I guess I could go that route again until I build the Pete Millet low mu preamp (parts on the way), but for now this problem has me stumped so it's more of a curiosity thing thats bugging me to figure it out.
Attachments
Last edited:
Oh dear lord, pound 4 or 5 ground rods in and earth the outlets in your listening room! I'd imagine you have knob and tube as well 🙁
Rofl.....actually I know a guy who does have tube and knob. There is also no insulation in his house....lol
Shorting the input made no difference.
I did try an old set of cables though thinking that the "fancier" cables may be the fly in the ointment. These were the type that you get when you buy a stereo component at a big box store. That didn't make any difference either.
A few other things that I can try:
Replace the 6n23p tubes with 6n1p to see if that makes a difference. I know the bias will not be correct but at least it will help me eliminate the tubes.
Examine the power supply even though I saw no ripple at all on the output of the phono pre. I am using an lr8 as the regulator for that circuit.
Examine the preamp itself. I will have to desolder some wires in order to do that since it is built into a wood cigar box (copper lined inside and grounded). I can't just flip it over and take a gander.
I did build a new preamp yesterday. A 6sn7 srpp which sounds very nice. It didn't help with the hum/buzz coming from the phono preamp, so I've eliminated the following from the equation:
1) cables - I have tried several and there was no difference.
2) the preamp - I have used two different preamps and no change
3) the turntable - shorting the input to the phono pre made no change
I did try an old set of cables though thinking that the "fancier" cables may be the fly in the ointment. These were the type that you get when you buy a stereo component at a big box store. That didn't make any difference either.
A few other things that I can try:
Replace the 6n23p tubes with 6n1p to see if that makes a difference. I know the bias will not be correct but at least it will help me eliminate the tubes.
Examine the power supply even though I saw no ripple at all on the output of the phono pre. I am using an lr8 as the regulator for that circuit.
Examine the preamp itself. I will have to desolder some wires in order to do that since it is built into a wood cigar box (copper lined inside and grounded). I can't just flip it over and take a gander.
I did build a new preamp yesterday. A 6sn7 srpp which sounds very nice. It didn't help with the hum/buzz coming from the phono preamp, so I've eliminated the following from the equation:
1) cables - I have tried several and there was no difference.
2) the preamp - I have used two different preamps and no change
3) the turntable - shorting the input to the phono pre made no change
A 50's house should not be knob and tube.
Your outlet boxes are probably grounded. A multimeter will tell you in 5 seconds. I did a friends house (50's era) a few years ago. 3 prong outlets, and pigtails - there are pigtails for this purpose, and did the whole house for <$100, including beer.
Your outlet boxes are probably grounded. A multimeter will tell you in 5 seconds. I did a friends house (50's era) a few years ago. 3 prong outlets, and pigtails - there are pigtails for this purpose, and did the whole house for <$100, including beer.
Try to see any hum when only one channel is connected. Disconnect one channel also disconnect one of the screen of the interconnect cable. Double ground termination of RCA connector through the screen of the output of the phone preamp and input of the line preamp (I'm not if you're in this situation or not) will cause local ground loop as you probable aware, apart from quality of the interconnect, maybe better or worsen humming.
With only one channel on, you can avoid hum if it is due to cross channel interference.
With only one channel on, you can avoid hum if it is due to cross channel interference.
Problem solved.....
Both stocktrader and koonw set me on the right train of though. I built the phono pre about a year ago or so. I was sitting here thinking.... ground loop.... dc heaters.... then it hit me. Did I connect the negative from the tube heater supply to the HT ground? Turns out that I did not. I heated up my soldering iron and made the connection. No more buzz.
I am guessing that since the preamp that I am using on my SET has more gain than my single stage 6sn7 on my integrated amp, the issue became more apparent, which is why I thought everything was just hunky dory before.
Thanks folks. I appreciate the help :-D
Both stocktrader and koonw set me on the right train of though. I built the phono pre about a year ago or so. I was sitting here thinking.... ground loop.... dc heaters.... then it hit me. Did I connect the negative from the tube heater supply to the HT ground? Turns out that I did not. I heated up my soldering iron and made the connection. No more buzz.
I am guessing that since the preamp that I am using on my SET has more gain than my single stage 6sn7 on my integrated amp, the issue became more apparent, which is why I thought everything was just hunky dory before.
Thanks folks. I appreciate the help :-D
lexx21, glad you found the problem. at 60 or 120 hz you will get both stages amplifying the noise x1000 or 60 db of gain because of RIAA curve.
- Status
- Not open for further replies.