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.

 

[HerbertR]

Hi, I did some "simple" measurements of this SE modified GAR BA2C with my limited equipment and compared it with various preamps, like solid state preamps from AMLs, Cap'is..., tube preamps like Redd47 Style, G9...

The secondary output TX winding of the BA2C with the UTM3590 "gapped" is "solidly" terminated with a 620 ohm resistor. I don't know any source impedance of the DAW... So dear pros, don't tear me to pieces alive...

The maximum gain I could get with a 10mV 1kHz sine wave (not a very accurate measurement!) fed from the DAW is 2.72VAC. That should give a gain of about 49dB. Not bad in my opinion...


Here is a frequency response comparison between this BA2C style pre and an AML EZ573 500 with pink noise, about 40 dB of gain on both preamps. The output shows about 1 VAC on average. The high end tends to roll off above 12 kHz with about 2.5 dB up to 20 kHz. I think it's not bad for a tube preamp.

Noise is not a problem. Very quiet on both preamps... measured with a gain setting of about 45 dB on both devices, upper Graph Noisefloor GAR BA2C Style Mod, lower Graph AML EZ573


To simplify the THD, I measured 1.6% THD with the 620 ohm termination at 0 dBu / 0.775 V output / about 45 dB of gain. And about 2.5% THD at the same 45 dB gain with +4 dBu / 1.23 V output, adjusted with a 1k sine wave from the DAW and measured on the output of the Preamp. Doesn't look too bad either. But I calculated the THD only the first two Harmonics (as example 1K Sinewavefeed shows -18dB, 2K Harmonic shows -54dB, and 4K Harmonics shows about -75dB at the DAW Spectrograph, with 0.775V / 0dB Preamp-output...)
The old RCA BA2C datasheet shows less than 2% THD at 2 dBm, which - if I've calculated correctly into a 620 ohm load - should be about 1 volt...
The THD is not far from what is stated in the old BA2C datasheet.

I am not sure if the UTM inductances with "25H" are really correct. But I cannot measure the inductance properly as my cheap LCR meter is not suitable for that. It even cannot measure correctly EF Cores for Coils.... The gapped UTM 3590 looks like it is working properly in my opinion. And so does it sound.

In the next post I will explain how I made this simple modification. Regards

 

[HerbertR]

About to modify GAR BA2C

1. Remove the 1uF 400V DC block capacitor and bridge the connections with a cable...

2. Remove both 220K resistors. The Place of the removed 220K Resistor next to the large 100uF electrolytic capacitor remains empty.

3. In the other empty 220K resistor slot, solder a 10K and a ~90K resistor in series in place, as shown in my picture. The middle "Air" connection of this 10K/90K Resistors is further connected to the ground connection of the 330K shunt resistor via a good 10uF 350V electrolytic capacitor. Pay attention to the polarization like on the picture!

4. The new "gapped" output transformer is connected with + to the old "original" Out (+) connection. The - of the output transformer is connected with 240V DC to the B+ rail, NOT to the old OUT (-). !!! I used the "free" outer/top connection of the removed 220K resistor next to the large 100uF 400V ripple capacitor.

2. Replace the 47K resistor with about 10K 3W. This Value depends on the "200V AC INPUT" voltage your power supply transformer in "real" supplies. If you are powering the rectifier circuit (47pFs, 4007 diodes...) with 220V AC from the power supply transformer (when the transformer is connected to the PCB!!!) then 10K is good (you need to drop about 80 volts at 8mA). If you are only powering the rectifier with 200V AC for example then you only need a 5K resistor (here you only need to drop about 40 volts at 5mA across this resistor). At the other end of this resistor you should measure about 240V. That is the goal. Then all other voltages in the audio circuit should also be fine. That's it.


Here's a nice picture please forgive me... Colours match to a gapped UTM 3590



Btw I also rectified the Heater Voltage with an easy unregulated CRC rectifier on a small board.




Regards Herbert