RCA BA-2C Buzzing Issue

[mattrad]

Hey all, long time reader, first time poster. First of all thanks to everyone on the forum for sharing all this incredible knowledge and information so that people like me can learn and build things ourselves!

So, i'm working on a 2 channel RCA BA-2C project using one 6SN7 per channel. Trying to get the first channel going as i'm still waiting on a transformer for channel 2. Got everything hooked up, its passing signal and sounding correct but theres a somewhat serious buzz coming in, what seems to be a 120 ground loop sounding thing. It is amplified with the volume pot, and when I short the grid of the first gain stage to ground it goes away completely. Input transformer is a Harman Kardon XT3, the primary is wired + and - to pin 2 and 3 of the XLR input respectively, XLR pin 1 grounded to chassis through the XLR lug. The primary has a center tap but i'm not using it. Also, with no mic or mic cable plugged in to the input theres a louder, more prominent buzz. Not sure if that is supposed to happen or may point to a fault somewhere. Monitoring with headphones, the buzz cuts out completely when the power switch is turned off, and the mic amplifier works clean as the sound fades out. B+ voltage is slightly high without channel 2 pulling current but nothing seems way out of wack.

I've read up on grounding and tried several different ground schemes, started with preamp ground and power ground seperated and grounded to chassis at 2 points, currently running a local star ground with everything bolted to chassis at the negative end of the first filter cap. Nothing has changed the buzz in the slightest. Have also poked around messing with the lead dress with no change whatsoever. Power transformer is a Hammond, mounted off the rear of the 2U chassis on opposite end of the input section.

I've tried different cables, different outlets, different rooms, different mics, nothing seems to curb the buzz. I'm prepared to be missing something stupid and obvious. Wondering if y'all had any suggestions!

Thanks!

EDIT: I should mention that i'm using a full wave rectifier in place of the 6X5, as in example A of this diagram:

 

[mjrippe]

Have you grounded Pin 1 of the XT-3?

 

[mattrad]

Thanks for the reply. Yes, Pin 1 is grounded.

 

[mjrippe]

Ok, good. Noise with nothing connected is normal. An XLR connector with a 150 ohm resistor across pins 2+3 is handy for testing.

Are your filaments AC or DC?

 

[mattrad]

Filaments are AC, i have them twisted and elevated, tried moving em around a little bit with a chopstick while monitoring with no change at all in the buzz. Also the wires to and from the input transformer are shielded, have tried every combination of grounding/not grounding the shields, alas to no avail.

 

[rock soderstrom]

Is there really good contact between the B+ PSU ground and the heater? How is the elevation realized?

 

[mattrad]

Whoops I think I mixed up the terminology there, not elevated, i mean the heaters have a virtual center tap to ground using two 100 ohm resistors. My apologies!

 

[rock soderstrom]

Doesn't your Hammond transformer have a center tap for the heater?

 

[mattrad]

It does not. Using the Hammond 269GX.

 

[rock soderstrom]

It does not. Using the Hammond 269GX.

Ok, just an idea. Photo?

 

[mattrad]

Here ya go, not the greatest pics but will give you some idea of the layout. It was much neater before I started tearing it apart! Using the choke + capacitor method in the output section as I dont have a center gapped OT. The shield on the input XLR is currently just hanging there waiting to get reattached, don't be alarmed.

 

[radardoug]

Here ya go, not the greatest pics but will give you some idea of the layout. It was much neater before I started tearing it apart! Using the choke + capacitor method in the output section as I dont have a center gapped OT. The shield on the input XLR is currently just hanging there waiting to get reattached, don't be alarmed.

Those pots on the front panel should be wired in shielded cable for a start.

 

[mattrad]

Thanks for the suggestion. Just swapped in some shielded cable to and from the volume pot, unfortunately still no change in the buzz at all!

 

[tony hunt]

120Hz buzz with AC heaters rules out the heaters as a primary issue in my book.

I think you need to look at where you deviated from the schematic. I would qusetion myself, if the anode chokes are happy sandwiched between the PSU and it's circuitry?
I would think about that turn off fade–out giving no buzz: which parts are no longer conducting when turned off?

Later I would look at all of the wire–routing with regard to what it is running close to. I understand it is being worked on. The box looks big enough, but the wires look too short. Perhaps more use can be made of the edges of the real estate. Steel or aluminium enclosure?

 

[mattrad]

Regarding the choke location- I thought of that as well and unbolted the choke and moved it around the chassis to see if there was any change. Unfortunately it had no effect on the buzz. The enclosure is steel.

The fact that the buzz disappears immediately when the power is turned off seems to suggest that it has something to do with the power supply- so far the buzz seems to be completely unaffected by anything external. The diode rectifier... not original to the schematic, wondering if theres something i'm missing there. The rectifier IS something that would stop dead when the unit is turned off.

Ill continue to mess with the lead dress though so far its been unsuccessful. Am I correct in assuming that the fact that shorting the stage one grid to ground gets rid of the buzz, means the noise is entering the audio path somewhere between the microphone and the stage one grid? That would just leave the XLR input and the input transformer.

Thanks again!

 

[radardoug]

Have you got two grounds on the secondary of that input transformer? If shorting the first grid stops all hum, then yes, its the wiring of the transformer and XLR, or the transformer does not have good shielding.

 

[mattrad]

Only the (-) pin of the secondary is going to ground. Hmm, i've tried to move/rotate the input transformer, no change in the buzz. That would seem to rule out and issue with the transformer shielding. I've combed over the input area many times, cant see any error in the wiring. This is a tough one!

 

[Cranehazard]

Those 15k to 600 from edcor pick up a lot of noise from any power transformer around if used as an input transformer even at line level because they are not sheilded if your using it as 600 to 15k then it's much worse. Try running it unbalanced with no input transformer. I have had no luck getting an ac filament supply quiet enough for a piece of studio gear this heater noise might disappear when grounding the first stage grid because the first stage is the largest contributor.

 

[Cranehazard]

For a mic preamp you need a well sheilded input transformer or it will be too noisy for me. I have tried to use these edcor 15k 600 transformers on mics and it was extremely noisy they work excellent for output transformers if kept far from the power transformer.

 

[mattrad]

Good tip, im using the Edcor for output currently, i have a Harman Kardon XT-3 for input.

Thanks for the suggestion. Just hooked it up with no input transformer, NO BUZZ. I guess its down to something with the XT3. Strange that it would be picking up so much buzz from the PSU given that its 15 or so inches away. Maybe i'll extend the wires so I can move the XT-3 further away, see what that does.