RCA-BA-2c Build

[theholyweed]

Hey guys, Anyone here build any BA-2C style pre's?

Got UTM for output Iron, any suggestions for input TX?

Ive seen these for sale on reverb...not shielded though...

https://reverb.com/uk/item/34079409...ne-audio-transformer-50k-to-200-ohm-rca-ba-2c

 

[Murdock]

You could stay with UTM and use the UTM2572. But I've never used it so can't say anything about it's quality.
I would not use an unshielded mic input transformer.
I would be interested in your experience with the UTM output transformer once you've build it.
The stated inductance seems to be way to low for this circuit.

 

[HerbertR]

Hi, I built a BA2C style preamp a few years ago. I used GAR's BA2C PCB which has a DC capacitor block design. But after a while I found that the distortion was too much for me. I checked this layout in detail and found that it is quite easy to convert the PCB to an "original" single ended BA2C design with original values, without cutting any traces, only replacing some components and connecting on other points... in a few minutes.

I used a Sowter 9610F input transformer with 200/20K and a UTM 3590 gapped OT and NOS RCA 6J7 pentodes. Works like a charm. Full, rich, super lowend, perfectly warm-clean highend... sounds significantly different than a more "Mid" Redd47 Style Mic Pre or a cleaner G9 Studer Style preamp.

I bet the UTM 2560 Input transformer 200/47K is also an excellent choice, if not even better than my Sowter...
... regard Herbert

 

[TwentyTrees]

Herbert, that's great to hear. I'd like to do the same some day, looks like a fun build. Thanks for the info!

 

[Murdock]

Good to hear that the UTM 3590 seems to work in that circuit. But it kinda surprises me… because when I believe the Datasheet on their website the inductance seems to be way to low. 25H at 100Hz has a reactance of about 16k ohm. The output impedance of the circuit seems to be around 13-14k ohm. So that would be a matching scenario. Not good…
But that was only with simulations. Real world seems to be different.
@HerbertR are you sure the circuit is the same as the original BA-2C?

 

[HerbertR]

Hi, it follows the BA2C schematics of thr Audio path, PSU is classical linear CRCR.... Directly provided on the PCB. I ve never measured the the inductance of the OT as it is a suggested SE Transformer for this build. When I have time, I will try to measure the transformer with my sparse measuring equipment. I only can say here that it perfectly works on first sight.

 

[emrr]

I have 207H @ 120Hz for an original, and it’s really more like 50K:500.

 

[pelagos]

Hi, I built a BA2C style preamp a few years ago. I used GAR's BA2C PCB which has a DC capacitor block design. But after a while I found that the distortion was too much for me. I checked this layout in detail and found that it is quite easy to convert the PCB to an "original" single ended BA2C design with original values, without cutting any traces, only replacing some components and connecting on other points... in a few minutes.

I used a Sowter 9610F input transformer with 200/20K and a UTM 3590 gapped OT and NOS RCA 6J7 pentodes. Works like a charm. Full, rich, super lowend, perfectly warm-clean highend... sounds significantly different than a more "Mid" Redd47 Style Mic Pre or a cleaner G9 Studer Style preamp.

I bet the UTM 2560 Input transformer 200/47K is also an excellent choice, if not even better than my Sowter...
... regard Herbert

Hi HerbertR

Do you have the details of what you did o modify the GAR BA-2C?

many thanks

 

[dmp]

Hi, it follows the BA2C schematics of thr Audio path, PSU is classical linear CRCR.... Directly provided on the PCB. I ve never measured the the inductance of the OT as it is a suggested SE Transformer for this build. When I have time, I will try to measure the transformer with my sparse measuring equipment. I only can say here that it perfectly works on first sight.

You can do a frequency sweep of the preamp using RMAA or another audio software. If you try it into a typical impedance audio interface (~10k) and terminate with 600 ohm you may see the influence of the output transformer.
My sweeps of my BA2 build with RCA transformers are here for comparison.

 

[chappyclark]

Hi HerbertR

Do you have the details of what you did o modify the GAR BA-2C?

many thanks

Would be very nice to have details.