Question for RCA BA-31A experts

[substitute]

I just tried out PRR's variable gain mod, it sort of works.  I couldn't find a 2.5k rev log pot so I used a 2k rev log instead.  The travel on the pot is really weird, at fully ccw gain is low as I bring up the pot nothing happens until it's almost all the way up then the last few millimeters behave like a normal volume control.  In the circuit, fully ccw the pot reads about 1k across the wiper and terminal, that spot where all the volume action happens is about 500r.  How can I get the volume control to behave "normally"? 

Thanks!

for ref

Change R7 to 2K. Change R4 to 100R. Strap a 0-2K rheostat (2.5K reverse-audio pot) across terminals 46. This gives almost 20dB range of gain, both up and down from the stock gain. Keep pot-leads short. twisted, not shielded.

 

[emrr]

Probably need to be a stepped pot with resistors calculated for desired steps.  Take what you have an measure gain steps, then disconnect and measure pot resistance. Write it all down, then build a stepped pot once it makes sense. 

 

[substitute]

I was thinking something like that.  It doesn't seem like a continuos volume control is totally necessary any how.  I'm sure I'll have some questions once I start fussing about, other than that I'll let everyone know where I end up.

 

[substitute]

Ok so I set up a tone generator, wired some alligator clips to a pot, got all ready to take some measurements... And things are working great now.  I can't say what the problem was before, I used a different pot this time around but the first one measured just fine, so I don't know.  Regardless the mod seems to work well.  On a 1k test tone the volume control is very smooth and I measured about 65dbm gain on my meter with the pot all the way open.  With a crappy mic and me saying "check" everything seems fine and dandy as well.

So seems like a success.

 

[emrr]

Good to know.  I've had one sitting in the restoration pile for several years now, and haven't checked it yet. 

Are you able to look at frequency response at various settings?  It's possible full gain looks terrible, and it needs to be limited to a lower number. 

 

[substitute]

That's not something I know how to do.  If you'd like, PM me the basics, I'm game to learn something new.

 

[emrr]

Get a reference level at max gain 1k tone, then sweep tone down and make note of frequency for 1db down, 2db down, 3db down.  Then same for top end. 

Compare this to stock 40-46db gain setting, and anywhere else. Maybe check 55db, and also whatever the lowest gain is.

This is all easier with software testing, lots of free options. 

 

[rocket512]

You might consider the Jensamp by Darius Kubarth, which is an updated version, there is thread in this forum.

 

[mjrippe]

Nice!  I had not seen the Jensamp before.  He uses small capacitors with his feedback resistors to keep the frequency response consistent.  All the values are on the schematic.

 

[Nele]

I got 2 of these today for repair (A versions). Looks like they need a recap. Any suggestions what to use for replacing the old sprague electrolytics? Can I go with new Panasonics (NHG/FC) or should I use more vintage flavour ones like new sprague etc?

 

[jensenmann]

I did use Panasonic FC plus bypasscaps and I increased some of the caps when I did the racking for my BA31s. The result led to this: http://audio.kubarth.com/jensamp/
These pres are geat!

 

[Nele]

Thanks!

I had to recap the powersupply on one unit because the voltage was too low (around 17 V). The main 250uF cap was leaking.
On the second unit the input transformer was not hooked up right to the main board.

Both are working now.

I now have a small problem on one of the other units. It's working fine but it has 5db less amplification than the others. The terminals are not strapped. Any ideas? Probably R19 has drifted?

 

[substitute]

Wow, I can't believe it was almost two years ago last time I posted in this thread.  Anyhow, another pair of BA-31's crossed my work bench.  This time I made the effort to check out the frequency response.  So the set up was both units recapped, one with PRR's gain mod one without.  Test tone out of protools, monitoring the return input with the meter plugin.  So not SUPER scientific but pretty good for real world applications.  I ran a 1k tone out to both units, turned up the gain on the modded unit until the levels matched.  Swept the tone from 20 hz to 20k.  Both channels reacted the same, flat from 20k down to about 200 hz after which the response begins to drop.  After that I did the same sweep on the modded channel at various gain settings, the frequency response was the same at any position.

 

[madshamanstudios]

Wow, I can't believe it was almost two years ago last time I posted in this thread. Anyhow, another pair of BA-31's crossed my work bench. This time I made the effort to check out the frequency response. So the set up was both units recapped, one with PRR's gain mod one without. Test tone out of protools, monitoring the return input with the meter plugin. So not SUPER scientific but pretty good for real world applications. I ran a 1k tone out to both units, turned up the gain on the modded unit until the levels matched. Swept the tone from 20 hz to 20k. Both channels reacted the same, flat from 20k down to about 200 hz after which the response begins to drop. After that I did the same sweep on the modded channel at various gain settings, the frequency response was the same at any position.

Hi! I've just obtained a BA-31A with what looks like power supply and full PCB intact. Said to be linked and working in a previous studio without any modification. Seller is trusted with 100% rating and said this is a fully functioning model.

Based on schematic, can you tell me how to connect this to a power cable? Do I just follow pin out on the schematic to a regular IEC cable? Or is there a specific interface/staging that needs to happen between the pins and a modern IEC?

I apologise to all that I'm somewhat new to this. I obtained this as a learning experience. I can solder, I am reading up, and I will not take any action without confirmed knowledge. I'm very passionate about this preamp model and hoping I can do it justice. Thank you!

 

[substitute]

Hi! I've just obtained a BA-31A with what looks like power supply and full PCB intact. Said to be linked and working in a previous studio without any modification. Seller is trusted with 100% rating and said this is a fully functioning model.

Based on schematic, can you tell me how to connect this to a power cable? Do I just follow pin out on the schematic to a regular IEC cable? Or is there a specific interface/staging that needs to happen between the pins and a modern IEC?

I apologise to all that I'm somewhat new to this. I obtained this as a learning experience. I can solder, I am reading up, and I will not take any action without confirmed knowledge. I'm very passionate about this preamp model and hoping I can do it justice. Thank you!

Yup, it's just hot (black, line) to C and neutral to E.

Having done a handful of these, I don't think the variable gain mod is necessary, a pot strapped across the output works great for controlling the level.

https://groupdiy.com/threads/output-level-attenuator-for-api-312-amp-circuit.31472/
See Jeff Steigers post about half way down.

 

[madshamanstudios]

Yup, it's just hot (black, line) to C and neutral to E.

Having done a handful of these, I don't think the variable gain mod is necessary, a pot strapped across the output works great for controlling the level.

https://groupdiy.com/threads/output-level-attenuator-for-api-312-amp-circuit.31472/
See Jeff Steigers post about half way down.

Wow I'm honored to hear from you! You don't know how relieved I am that you've responded so quickly. You're keeping the RCA BA-31 bat signal alive - thank you so much! I’ll definitely look into the output attenuation! I’m a happy owner of a VP28 Platinum and big fan of Jeff.

I have two quick questions as I plan.

1. Grounding…Could you explain of how to safely ground the mic input, mic output, and the power? I’m so concerned for safety and would appreciate it immensely. Is H in P101 for the power ground? Does A in P102 feed both XLR input AND output ground wires to ground? Or does mic output not need ground since it leads to another grounded module?
2. Strapping…For the full 46db, does that simply mean connecting the transformer terminal 4 to terminal 6 with solder and wire, like a jumper? And which transformer terminals - input or output?

Again, sorry to be such a noob. I look up to experienced builders like yourself and am hoping to be diligent.

By the way, one other thing I'm excited about is using a separate phantom power box to feed a Soundelux U195 into this as well. I think I'll add a Shure A15AS in-line attenuator (-15 to -25db), as this attenuator passes phantom power. I will also use the phantom power box to run my RNDI, which will let me use the BA-31 as a direct in. How do you use your BA-31? What mics are you using? Thanks again!

 

[mjrippe]

Congrats on scoring an original "A" version! Many were "upgraded" to the B circuit in the field back in the day. From the photo it looks untouched, which means every single capacitor is original. Some folks say "if it works, leave 'em", I prefer to do the preventative maintenance and replace them all. The germanium transistors and UTC transformers are a much bigger part of the sound than dried out old caps.

 

[emrr]

I've never heard one with original caps that sounded 'good'. Usually pretty grainy/gritty. There is no desirable characteristic lost in a recap.

I don't find an output attenuator to be of any value, unless your inputs clip at +20. You're in hot distortion by then. Even if you do, then a simple -6 output switch makes more sense than the trouble of a variable control. I went to the trouble of wiring up variable output pads and I've needed them 1/10th of 1% of the time I've owned them, in 20 years. I've run a set of 8 with just a switchable input pad, but variable gain mod gives a lot more flexibility. FWIW YMMV ETC

 

[madshamanstudios]

Congrats on scoring an original "A" version! Many were "upgraded" to the B circuit in the field back in the day. From the photo it looks untouched, which means every single capacitor is original. Some folks say "if it works, leave 'em", I prefer to do the preventative maintenance and replace them all. The germanium transistors and UTC transformers are a much bigger part of the sound than dried out old caps.

Thank you! Appreciate the encouragement - I was VERY excited to find the A model. I actually was lucky enough to buy a pair, and will probably only need to keep one.

I see about 20 caps, so maybe not a huge job. Anything special I should watch out for in the recap process?

Sorry for this noob question, just trying to learn and confirm before taking actions... I see two numbers on the schematic for capacitors. For example C11 reads 35 / 50V. Would that be read 3.5mfd and 50V, and would I be looking for a radial capacitor?

 

[madshamanstudios]

I've never heard one with original caps that sounded 'good'. Usually pretty grainy/gritty. There is no desirable characteristic lost in a recap.

I don't find an output attenuator to be of any value, unless your inputs clip at +20. You're in hot distortion by then. Even if you do, then a simple -6 output switch makes more sense than the trouble of a variable control. I went to the trouble of wiring up variable output pads and I've needed them 1/10th of 1% of the time I've owned them, in 20 years. I've run a set of 8 with just a switchable input pad, but variable gain mod gives a lot more flexibility. FWIW YMMV ETC

Thanks Doug! Wow congrats on running a veritable RCA board of BA-31 preamps! And thank you for these thoughts, which really got me thinking. I plan to control condenser mic levels via input attenuation and hoping that'll work fine. Dynamic mics like my SM7B should show no problem as this preamp was design for hungry dynamic mics. So now as I consider it, the value of output attenuation is a bit confusing to me: Leaving the preamp at 40 or 46db, I assume one would already be preventing distortion at the input stage. If distortions are already prevented, why attenuate the output in circuit? Or does the BA-31A for some reason have an exceptionally high output versus modern preamps set at 46db?

By the way, I'm ichabod on gearslutz/space. I think you mentioned to me in that forum about beefing up the low end a little - any suggestions for that bigger low end? Very interested.