RCA Ba2A Pre amp qestions & answers

[toniwave]

Hi everyone, I'm getting my two BA-2 back on the workbench to check for some minor hum that is probably as loud as the preamp self noise. It might be caused by a non optimal placement of a 230V-110V step down transformer. But testing seems strange:

My issue is that I'm confused on how to test the hum properly. I tried it with a 150 ohm XLR plug, but it seems to behave differently than a mic. Also, line level fed from the interface makes the hum type change (not reacting to the transformer placement at all). It feels like there are too many variables and I can't come to any conclusions. Sometimes the hum changes with moving the step down transformer, with other constellations it doesn't.

What's the proper feed to test the noise of a BA-2?

 

[dmp]

If you connect a XLR plug with 150 ohm between p2 and p3, the input transformer should be dead silent (you could also try directly connecting p2 & p3 with a wire lead as a sanity check, this only changes the impedance the grid 'sees'). You don't want the 150 ohm going to XLR p1!
With that 150 ohm connected you should hear & measure only the preamp noise. It will behave differently than with a mic connected because the mic could have some noise/hum that is masking the preamp noise.
I also measure with RMAA and a sound card connection (the level sent to the preamp needs to be reduced from line level, down ~40 dB).
This is really helpful as you can see spikes for 60 Hz (typically magnetic coupling) and/or 120 Hz (power filtering or routing). Right now you are guessing it is placement (magnetic coupling) but you can get a better idea with this measurement.

It feels like there are too many variables and I can't come to any conclusions.

I find it really helpful to start with a known good external power supply. I have a heathkit IP-17. I also have a beefy standalone DC 6.3V power supply.
I will hook these up with the 'in progress' power supply turned off and disconnected.
With the preamp working like this with good noise measurements, I move on to:
1) turn on the 'in progress' power supply. Without it connected electrically, this will show any magnetic coupling. 60 Hz noise? Try to re-orient transformers or shield the PT. Ideally for EI transformers, the cores should be at right angles and off-axis.
2) connect the 'in progress' power supply and compare. 120 Hz noise? Bad filtering, wire routing, ground layout.

Another thing to watch out for when you have the lid off and you're trying to evaluate the noise, is lights (especially on dimmer switches), monitors, or other stuff around the room that could be inducing noise in the preamp.

 

[My3gger]

Fast way for testing mains transformer induced hum is taking them out and reconnect with extended wires, then measure as suggested by Dmp. BA-2's self noise should be very low because they used triode wired pentodes at relatively low current (no partition noise, rectifier ripple is easy to filter).

 

[toniwave]

If you connect a XLR plug with 150 ohm between p2 and p3, the input transformer should be dead silent (you could also try directly connecting p2 & p3 with a wire lead as a sanity check, this only changes the impedance the grid 'sees'). You don't want the 150 ohm going to XLR p1!
With that 150 ohm connected you should hear & measure only the preamp noise. It will behave differently than with a mic connected because the mic could have some noise/hum that is masking the preamp noise.
I also measure with RMAA and a sound card connection (the level sent to the preamp needs to be reduced from line level, down ~40 dB).
This is really helpful as you can see spikes for 60 Hz (typically magnetic coupling) and/or 120 Hz (power filtering or routing). Right now you are guessing it is placement (magnetic coupling) but you can get a better idea with this measurement.



I find it really helpful to start with a known good external power supply. I have a heathkit IP-17. I also have a beefy standalone DC 6.3V power supply.
I will hook these up with the 'in progress' power supply turned off and disconnected.
With the preamp working like this with good noise measurements, I move on to:
1) turn on the 'in progress' power supply. Without it connected electrically, this will show any magnetic coupling. 60 Hz noise? Try to re-orient transformers or shield the PT. Ideally for EI transformers, the cores should be at right angles and off-axis.
2) connect the 'in progress' power supply and compare. 120 Hz noise? Bad filtering, wire routing, ground layout.

Another thing to watch out for when you have the lid off and you're trying to evaluate the noise, is lights (especially on dimmer switches), monitors, or other stuff around the room that could be inducing noise in the preamp.

I don't have a professional grade external power supply like the Heathkit, but I'll receive another external step down transformer in the next days. I'll reconnect with that and use the XLR plug and see from there. I use REW for measurement.

Thanks for the detailed response!