[plumsolly]

I recently built this phone preamp: http://www225.pair.com/audio/waltzingbear/Schematics/Pacific%20Pecorders%20&%20Engineering/TX990-Schematic.pdf

The only difference in the P.S. is that the power transformer in the schematic calls for a grounded core and my toroidal transformer doesn't have a core tap.

What I am experiencing is intermittent gound-loopish noise. 120Hz and harmonics @ ~ -65dB.

What does NOT affect it:
-It is not induced emi from other units in the rack - the position of the unit in space doesn't change anything.
-It is not coming from the turntable or the first stage - the volume control between the first and second stages doesn't affect it.
-Galvanically isolating the outputs also doesn't do anything.
-Isolating from the rack rails doesn't do anything.

What DOES affect it:
-Where the preamp is plugged in - everything in the studio is run off of a single outlet - changing where the preamp is plugged in affects the likelihood of the noise appearing.
-Turning off other units in the system - specifically, the power amplifier and the console power supply.

Any ideas on where to look?

Thanks,

Ben

[JohnRoberts]

pin one, as in "pin one problem".

JR

[edit do not lift any safety grounds /edit]

[plumsolly]

pin one, as in "pin one problem".

JR

[edit do not lift any safety grounds /edit]

That’s what i thought first  - sorry, I should have said, pin one is not connected at the receiving equipment and connecting it does not change anything

[radardoug]

It could be your toroid is radiating into your active circuit. How close together are they? Otherwise look at your grounding. Also, is your supply clean?

[squarewave]

There's a lot of gain a phono preamp that will necessitate decent shielding. Inputs of high gain bits should use shielded cable and the whole thing needs to be in a metal box. And aluminum is not as good as steel especially when the continuity between different anodized pieces isn't super great.

[plumsolly]

Thanks for the reply and helping me to try and solve this.

All low-level interconnects are shielded and whatever is happening is definitely happening after the first stage where a good deal of the gain is. The box is aluminum, but it is painted, not anodized and the paint is removed and shake-proof washers are inserted in places to make sure each panel is making electrical contact with the ones it is attached to.

[plumsolly]

It could be your toroid is radiating into your active circuit. How close together are they? Otherwise look at your grounding. Also, is your supply clean?

Thanks for the reply - I don't think it can be radiation from the power transformer because it largely goes away when certain other equipment in the studio is turned off and not pulling current - if it were emi it would be pretty constant.

The grounding in the circuit is largely the same as in that schematic.

The supply is pretty clean and, again, if it were ripple from the internal PS then it would be constant. I think it has to be an interaction with the other gear - so a ground loop - but complete galvanic isolation on the output does not make it go away so...


I guess my question is: What mechanisms are there for this type of interference if it's not emi and the only galvanic connection to ground is through the safety ground and the neutral lead? Is it possible that there is a potential difference between the neutral lead and the safety ground that can cause this? Am I even thinking about this right?

Thanks again.

[Gold]

Is it a bipolar PSU? I've never had any luck using a non center tapped power transformer with bipolar supplies. It sounds like there is no ground reference and the device is floating. I've never found that to work well.

[plumsolly]

Thanks for chiming in. It is bipolar - there are dual windings on the secondary to create the center tap.

[Gold]

there are dual windings on the secondary to create the center tap.

I think that will solve your problem. Make sure to tie the center tap to 0V/chassis so the center tap isn't floating.

[plumsolly]

Thanks for thinking on this. The center tap is tied to the chassis. The only difference as far as the PS between the schematic and my implementation is the core (or core shield?) tap that is tied to the chassis with the safety ground. I don't have that on my transformer.

[Gold]

The link to the schematic doesn't work here. Is it a conventional unbalanced it phono pre for an MM cartridge? Not a transformer input for an MC cart? How are you testing it? With an actual phono cart or with a test set?

A high gain unbalanced connection is a recipe for noise. Try shorting the input and see if it still happens. If it solves the problem then the only solution with an MM cart is to keep the cables as short as possible. Or build Wayne Kirkwood's phono pre with a balanced front end. Or use an MC cart with a step up transformer and run shielded twisted pair from cart to transformer with short leads from transformer secondary to preamp.

[plumsolly]

Yeah sorry - I realized the link doesn't work - not sure why - it works if you copy and paste it.

Is it a conventional unbalanced it phono pre for an MM cartridge?

Yes.

I can say for sure the noise is not coming from the turntable - I get the noise from the preamp even when the turntable is disconnected and the volume control after the first stage has no effect on the noise I am seeing.

When the ground loop noise is not there (I have the preamp plugged into the optimal outlet, or my power amp or console power supply is off) I am getting noise figures I am fine with.

[squarewave]

The only difference as far as the PS between the schematic and my implementation is the core (or core shield?) tap that is tied to the chassis with the safety ground. I don't have that on my transformer.

Are you talking about the copper foil strap or "flux band" that goes around the outside?

[Gold]

I'd pull the op amps in the first stage and just power up the second stage. If it's the same thing I think the first stage can be eliminated. Then I'd start banging on it. I've had opamps that weren't seated well make those kinds of noises. Loose connection somewhere?

[abbey road d enfer]

Thanks for thinking on this. The center tap is tied to the chassis.

The center tap should definitely NOT be tied to the chassis. It's the regulators reference point that must be tied to chassis.
Do you have a 3-prong plug, with the earth conductor connected to chassis?
Do you use the unbalanced or balanced outputs?

[Khron]

Some (more) thoughts here: https://sound-au.com/psu-wiring.htm

[ruffrecords]

Sounds like it may be line borne interference. Do you have a filter on the mains input?

Cheers

Ian

[scott2000]

Has a goofy power strip been ruled out as well if applicable?

[plumsolly]

Man! Thank you all for thinking on this and providing insight.

The center tap should definitely NOT be tied to the chassis. It's the regulators reference point that must be tied to chassis.

It is arranged as in the attached image - where the secondary windings are joined, it is tied to the chassis. Is that correct?

Do you have a 3-prong plug, with the earth conductor connected to chassis?

Yes.

Do you use the unbalanced or balanced outputs?

Balanced. And I have completely transformer-isolated them to be sure the outputs are not the source of the ground loop.

Some (more) thoughts here: https://sound-au.com/psu-wiring.htm

Thanks - I will read that.

Sounds like it may be line borne interference. Do you have a filter on the mains input?

I do not - and that seems like a likely culprit to me as well - If it's not emi and it's not a ground loop caused by the output or the input, what else could it be?

Has a goofy power strip been ruled out as well if applicable?

I was wondering this also and was going to do some experiments today to try and get a better handle on what is happening with my mains wiring.

Thanks again,

Ben

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