Proper wiring to avoid ground loops

[drtechno]

The original poster, @mrtnasty

Having a look at the manual, it just states the old unbalanced connection convention for ribbons and dynamic mics like the SM7 (30, and 300) and nothing about pin 7 for balanced. Unless I'm missing something.
https://funkwerkes.com/web/wp-content/techdocs/RCA/RCA-BA-2A-BoosterAmp.pdf

 

[drtechno]

If the source is a powered device instead of a passive device, you must terminate across the front of the balanced input with a resistor sized for the load (250-600 ohms) The source wants (ie. ELM251 250 ohms, U87 600 Ohms, etc.)

 

[mrtnasty]

On page 5 (pdf page 7) of the RCA BA-2C manual found here:



It says "Whenever a balanced source is used, remove the ground lead from the input transformer primary (terminal 1), and ground terminal 7"

 

[drtechno]

On page 5 (pdf page 7) of the RCA BA-2C manual found here:



It says "Whenever a balanced source is used, remove the ground lead from the input transformer primary (terminal 1), and ground terminal 7"

But that is for an instance if it is used as an isolation amp that the source is a line out device. and in that instance, grounding pin 7 is only really needed if the 10K resistors in the C pad are not matched.

 

[mrtnasty]

But that is for an instance if it is used as an isolation amp that the source is a line out device. and in that instance, grounding pin 7 is only really needed if the 10K resistors in the C pad are not matched.

Is it wrong to ground terminal 7, or just not necessary? It definitely reduced the buzz to ground it. Maybe grounding terminal 1 would also reduce the buzz; nothing on the input transformer primary was grounded before when I was having the buzz issue. Im not using any mics that require phantom power at the moment.

 

[drtechno]

Is it wrong to ground terminal 7, or just not necessary? It definitely reduced the buzz to ground it. Maybe grounding terminal 1 would also reduce the buzz; nothing on the input transformer primary was grounded before when I was having the buzz issue.

What are you trying to connect to it?
The only time pin 7 should be grounded is never when everything is correct. If you have to connect pin 7 then there is something else not correct.

 

[ccaudle]

It says "Whenever a balanced source is used, remove the ground lead from the input transformer primary (terminal 1), and ground terminal 7"

Interesting. That wording is not found in this manual:
https://funkwerkes.com/web/wp-content/techdocs/RCA/RCA-BA-2A-BoosterAmp.pdf

 

[mrtnasty]

What are you trying to connect to it?
The only time pin 7 should be grounded is never when everything is correct. If you have to connect pin 7 then there is something else not correct.

It buzzes when I connect my RCA varacoustic ribbon microphone to it. I checked the mic yesterday, and everything seems fine, xlr pin 1 is connected to the body of the mic.

 

[mrtnasty]

Interesting. That wording is not found in this manual:
https://funkwerkes.com/web/wp-content/techdocs/RCA/RCA-BA-2A-BoosterAmp.pdf

Yea I saw that. Maybe they updated the wording when going from the BA-2A manual to BA-2C?

 

[abbey road d enfer]

Is it wrong to ground terminal 7,

Did you, as I siggested experiment with disconnecting it?
Then you'll know.

 

[ccaudle]

you must terminate across the front of the balanced input with a resistor sized for the load (250-600 ohms) The source wants (ie. ELM251 250 ohms, U87 600 Ohms, etc.)

Although it may be useful to point out for benefit of the OP that "drtechno" is the only one who recommends such a low impedance load, including among people who design audio transformers professionally. Lector cave.

Of relevance in this particular case is that the manual for the RCA Varacoustic microphone specifically states in the "Recommended Load Impedance" paragraph that "It is recommended that the microphone be worked into an unloaded input transformer."

 

[drtechno]

It buzzes when I connect my RCA varacoustic ribbon microphone to it. I checked the mic yesterday, and everything seems fine, xlr pin 1 is connected to the body of the mic.

Then which version of the mic is it? 6203 or 6204

 

[drtechno]

Although it may be useful to point out for benefit of the OP that "drtechno" is the only one who recommends such a low impedance load, including among people who design audio transformers professionally. Lector cave.

Never told this site people hire me afterwards to take the buzz and hum out of everything in broadcasting and studio sectors for the lowest noise floor. Of course you will find contradictions because the coupling from device to device is specific some times even unique.

 

[mrtnasty]

Then which version of the mic is it? 6203 or 6204

It is actually the sk-50 low impedance version

 

[mrtnasty]

Did you, as I siggested experiment with disconnecting it?
Then you'll know.

I haven't gotten around to disconnecting terminal 7 and grounding terminal 1 yet, which is how it is shown in the schematic. But just disconnecting terminal 7 there is buzz.

 

[drtechno]

For MI 6203, connect 1&6 to transformer, ground Pin 1. Its going to buzz when nothing is connected unless there is a 200 ohm resistor across it which is optionally removed on non powered source devices. It is usually landed on the input patch panel in the install.

 

[ccaudle]

xlr pin 1 is connected to the body of the mic.

And pins 2 and 3 of the XLR connect to the black and green wires from the microphone output transformer? Or black and blue?

It is actually the sk-50 low impedance version

The SK-50 should have two windings so yo can switch between reasonably low (~200 Ohms) output (black and green) and 15k imedance (black and blue, which is too high to drive a cable of significant length and also noted should be connected directly to a tube grid, not through a transformer).

grounding terminal 1 yet, which is how it is shown in the schematic

The manual does point out that grounding terminal 1 is only for unbalanced connection, not balanced.

The cable shield connects to pin 1 at the microphone end (which you already noted is connected to the microphone body) and no where else inside the microphone? Have you verified that the connection to microphone case has not become corroded over time and become high resistance?
And the cable shield connects to terminal 10 at the pre-amp?

unless there is a 200 ohm resistor across it which is optionally removed on non powered source devices

The manual for the RCA microphone does specifically note that the preamp input impedance "should exceed 2000 ohms to avoid attenuation of the low frequencies."

 

[drtechno]

It is actually the sk-50 low impedance version

SK50 in low Z mode, would be the same as MI-6203 If you wired onto the blue that would be 1250 output Z, and would have to put a 1250 ohm resistor in series and a 200 ohm across for hi-z unbalanced.

 

[drtechno]

And pins 2 and 3 of the XLR connect to the black and green wires from the microphone output transformer? Or black and blue?
The manual does point out that grounding terminal 1 is only for unbalanced connection, not balanced.

correct, and that ribbon mic is an unbalanced ribbon mic Balanced ribbon mics are 44B,74 and 77.
most broadcast mics are unbalanced. Even the sm7B that people think the presence boost and all that is going to work right with their balanced mic pre, but doesn't.

 

[mrtnasty]

And pins 2 and 3 of the XLR connect to the black and green wires from the microphone output transformer? Or black and blue?



The SK-50 should have two windings so yo can switch between reasonably low (~200 Ohms) output (black and green) and 15k imedance (black and blue, which is too high to drive a cable of significant length and also noted should be connected directly to a tube grid, not through a transformer).



The manual does point out that grounding terminal 1 is only for unbalanced connection, not balanced.

The cable shield connects to pin 1 at the microphone end (which you already noted is connected to the microphone body) and no where else inside the microphone? Have you verified that the connection to microphone case has not become corroded over time and become high resistance?
And the cable shield connects to terminal 10 at the pre-amp?



The manual for the RCA microphone does specifically note that the preamp input impedance "should exceed 2000 ohms to avoid attenuation of the low frequencies."

Ive got pins 2 and 3 of the XLR connect to the black and green wires from the microphone output transformer. I made sure that the metal internals of the microphone and the casing were connected to pin 1 of the xlr. Yes, the cable shied connects to terminal 10 of the preamp. From the RCA sk50 microphone manual:

https://coutant.org/rca6203/rcask50.pdf
It says: "It is recommended that the microphone be worked into an unloaded input transformer of 150 - 300 ohm line impedance when the microphone is connected for 200 ohm output impedance. If a transistorized preamplifier is used, its measured input impedance should exceed 2000 ohms to avoid attenuation of low frequencies..." Since the BA-2C is not transistorized, Am I not correct by using it as suggested by the first sentence?