Question for RCA BA-31A experts

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O_ellinas

Member
Joined
Feb 13, 2008
Messages
9
Location
France
Hi all, long time lurker, first post here.

I just purchased a BA-31A unit off ebay that has only the UTC transformers and power supply but not the main PCB. I thought 40 euro is okay-ish just for the iron.

Now I need to build a preamp board for it so my question is: should I stick with the original RCA circuit? I have seen mixed comments about them on the net, like, they aren't exactly stellar, rather noisy and fuzzy. Or should I explore other options ?

I welcome any suggestions (preferably around discrete transistors).

Thank you
 
I like them a lot, but it is a fixed gain design. I think you'll find the iron is a lot of the sound. You could certainly work some gain variation into the existing design. Or go with a more modern design. 1200 ohm input secondary is a consideration. You could try a 990 'discrete' opamp and see if you like it there, though you'd have to remake the PSU.
 
Aha; good point about the power supply.

I think I will go the easiest route and give the 4 transistor RCA circuit a go. With some padding before and after, it should do the trick.

Thanks for your advice.
 
do it, mate. BUIlt the BA31 card. You won´t regret it. These thingies rock.

To have variable gain (kind of) I´m using mine with a simple input pad à la JLM-audio.
 
you can easily expand on the +6 gain strap in the feedback loop with a bit of additional thought. Should be able to get both less and more out of it, so why not?
 
Huh :?: I am afraid you lost me here...! :? Could you expand a little bit please? I noticed the +6db strap on the schemo but what would you do with it ?
 
it varies feedback. vary it either more or less. There will be reasonable limits, but they will be greater than 6 db.
 
I have a pair and have the original brochure/schematic. If you need a copy I'd be happy to send one to you.

Someone once told me about a mod that can boost the gain a bit. I wonder if it's the same thing...
 
[quote author="Linus"]I have a pair and have the original brochure/schematic. If you need a copy I'd be happy to send one to you.

Someone once told me about a mod that can boost the gain a bit. I wonder if it's the same thing...[/quote]

yes it is. You swith a resistor parallel to a feedback resistor which lowers the feedback. This gives you more gain.
You could try to replace the feedbackresistor with a pot to check out in which range the value can be varied without destroying the sound.
 
[quote author="jensenmann"][quote author="Linus"]I have a pair and have the original brochure/schematic. If you need a copy I'd be happy to send one to you.

Someone once told me about a mod that can boost the gain a bit. I wonder if it's the same thing...[/quote]

yes it is. You swith a resistor parallel to a feedback resistor which lowers the feedback. This gives you more gain.
You could try to replace the feedbackresistor with a pot to check out in which range the value can be varied without destroying the sound.[/quote]

I was hesitant to say, but I modeled the circuit a long time ago and found you could (in theory :oops: ) vary it quite a bit in both directions. The critical part is the compensation cap in the feedback loop, which is possibly not necessary. If you can lose it, you can get more range than with. I think you could safely get a range of 30-50 dB easily, versus the stock 40-46. 30 will probably sound very different from 50, however.
 
Chandler makes big bucks with this feature in his latest germanium series stuff. Imagine, 20 years ago a feature like this feedback thingy would have been seen as bad design :grin:
 
I have a pair and have the original brochure/schematic. If you need a copy I'd be happy to send one to you.

That would be fantastic, thanks!

I was hesitant to say, but I modeled the circuit a long time ago and found you could (in theory Embarassed ) vary it quite a bit in both directions. The critical part is the compensation cap in the feedback loop, which is possibly not necessary. If you can lose it, you can get more range than with. I think you could safely get a range of 30-50 dB easily, versus the stock 40-46. 30 will probably sound very different from 50, however.

I see. High gain/no compensation will probably raise stability issues? I foresee square wave testing in order to determine experimentally the right amount of compensation. Perhaps jumpers or DIP switches to switch compensation capacitors would do the trick but that is not very user friendly.
 
I see. High gain/no compensation will probably raise stability issues? I foresee square wave testing in order to determine experimentally the right amount of compensation. Perhaps jumpers or DIP switches to switch compensation capacitors would do the trick but that is not very user friendly.

I never tried it on the bench. I kept mine all stock for the most part. I suspect the cap may be partly forced roll-off from a broadcast perspective, and not necessary in many ranges. Try without and see how stability looks. The transformers themselves already contribute a degree of compensation. I am remembering modeling showing about 68 dB open loop gain which of course may be totally wrong. If that is true, the stock 46 dB setting is already close to the minimum 20 dB feedback recommended elsewhere in some articles and papers. I believe newyorkdave posted a link to the paper in question, but I can't find it ATM. Anyway, it has to do with the harmonic artifacts created with NFB. So, in short, modification to max 50 dB would be touching the recommended range, and you could also go further with NFB and reduce to less than 40. I recall the 46 setting sounding noticeably hairier than the 40 setting. And of course, you also reduce your maximum input with increased gain.
 
Doug, many thanks. This is quite interesting. I will give it a go when time comes (right in the middle of a console recap right now) and will let you know what gives.
 
I am remembering now that these like to see a sub-1K output load, or they tend to oscillate. So play with load resistors too.
 
> I am remembering modeling showing about 68 dB open loop gain which of course may be totally wrong.

Looks odd to me.

http://www.waltzingbear.com/Schematics/RCA/BA-31A.htm

The 6190:402 ratio says closed-loop gain tends toward 1:16.4 or 24.3dB. Q3 Q4 are unity voltage gain, near enuff. The total amp, wired 600R in and out, seems to have 1:6 or 15.5dB transformer gain. Take the iron-gain as given, look at just the amp from point H to point 0.

The 402R is a feedback component. For true open-loop, short it.

Q1 works around 0.2mA so Re is near 150 ohms. Q2 works around 5mA so Re is near 6 ohms. The coupling into Q2 looks messy, but Q2's emitter is substantially grounded. So base resistance of Q2 is near 6*Hfe, say 300 ohms, far smaller than all else in this node, so 90% of Q1 AC current flows to Q2 base and is multiplied by Hfe.

Q2 Hfe is 40 typical, apparently fading to unity at 12MHz (the old data sheets are unclear). So Hfe is flat to ~~300KHz, it's an OK audio device.

Q2 load is 2K in parallel with Q3 base and the 6190 resistor. For various reasons we suspect Q3 base is much more than 2K, so pencil 1.5K here.

Voltage gain is then Hfe*1500/150, or OTOO 400, or 52dB. (Yes, with iron included this is 67.5dB, as you say.)

> minimum 20 dB feedback recommended elsewhere in some articles and papers

Oh, frag that newfangled theory. If we wanted a clean amp, we would not be using four antique transistors. Stick a 990 in there. At gain of 24 it will be clean. Even a good 5532 will be inoffensive. We love the old stuff FOR its imperfections.

Anyway, for many purposes, 20dB is not a lot better than 6dB. You wanna wander out toward 40dB NFB to get high-order residuals below what they are open-loop. And you don't get 40dB margins with essentially 2-stage gain amps.

This is a "current feedback" amp. The NFB impedance is -not- smaller than the inverting input impedance. NFB impedance impacts open-loop gain. To change gain in this design, fiddle R7. That's what RCA did with R4. That's how 90% of modern transistor amps do it. It has the helpful property that the amount of feedback stays near constant as gain is changed over a wide range. No compensation tweaks are needed when changing gain (good thing, because panel-variable compensation adds more parasitics than we can stand). The 1000pFd cap is mostly a 25KHz roll-off (broadcast bandlimiting) not compensation (though it does affect the loop margin).

With R7 shorted, amp gain is around 400, total gain between 600R taps is ~~68dB. Gain is very dependent on Hfe Q2. Distortion at high level will exceed 10% 2nd HD.

Adding R7=150 cuts gain and THD in half, but does not stabilize gain.

NFB from Q2 collector helps stabilize against Q2 variation and drift. Ideally we make R7 less than Re of Q1, say 50R. For a nominal Q2, OL gain is 400 so for 6dB NFB we want CL gain near 200. Actually the 50R will reduce OL gain to say 300, we aim for 150, then R9 could be 7,500R. This is low but not too low compared to Q2 load. 6.19K would be fine also, and suggests ~~40R at R7. Inner amp gain is near 43, total amp gain near 58. Input overload is near 10mV, which is VERY low. You really do not want this much gain.

For lower gain, raise R7. This not only changes the NFB divider ratio, it spoils open-loop gain. For R7=100R, 300R, 1K, nominal inner-amp CL gains of 61, 20, 7, the OL gain is just about 240, 133, 52. NFB margin is 4, 6.6, 7.4... pretty constant, and quite unlikely to change the compensation needed. In fact it is fairly unlikely for a 2-stage amp with just 12dB-17dB NFB to howl unless there is a transformer in the loop. (T101's parasitics may be critical.)

I would bet the "40dB 46dB" numbers are cited for 600R tappings. If you do the "logical" thing and tap the input for 150, you get 6dB more gain.

> these like to see a sub-1K output load

That push-pull output is only push-pull when well loaded. Unloaded it becomes push-push and acts a bit oddly. It is never truly unloaded here, because of C13 and T102. There may be an unconditionally stable set of resistors, but who has time to figure? I'm sure it "wants" to be loaded with nominal 600R resistance (fader or terminated line) at the 600R tapping of T102. Since all modern loads are 10K-22K, tack 680R permanently across the output.

> the iron is a lot of the sound

If so, if there is no Germanium Fanatic in the house, I say a 5532 is quick and easy. It will run fine on the 23V dropped DC. Bias T101 pin L to half the rail. T101 H feeds 5532 input. NFB around the 5332 can be 10K and 50-5K RA pot with 470uFd to common. Put 22R in series with output and use the other half 5532 with 22R as a unity gain booster to get up to 60mA drive into the (assumed) 150R load. Use C12=100uFd to block the DC from the OT, don't use C13. Keep R17=100R to decouple PS noise, but raise C11 to several hundred uFd because 5532 works A-B.

Do note that the "Common", bypassing, and V+ and V- defy convention. (An older version musta been positive ground, and they didn't clean-sheet when NPNs became available.) If you will never build the stock circuit, you may want to alter this to make sense to conventional minds (usually negative ground).

> I noticed the +6db strap on the schemo but what would you do with it?

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.
 
Excellent; thanks PRR. Note the schematic link is labeled BA-31A, but is in fact BA-31C. 'A' version is the earliest 6 germ transistor version.

Here are response graphs I shot in Spectrafoo.

Those are graphs of restored BA-71A and BA-71B preamplifiers. The 71A is the same circuit as the 31A, etc. To further confuse things, the output caps are much larger on all three, 15 times larger on the 71B and 71T (slightly exceeding the BA-72A spec), and 5 times larger on the 71A. The unit labeled BA-71T has tantalum caps rather than standard aluminum electrolytics, and sounds quite different. More gory blow-by-blow I wrote about these down near the bottom of page 2.
 
Great post, thank you PRR.

Okay, now I have received the BA-31 yesterday, it is pristine. Looks unused except that the pcb is missing. Date code seems to say 1961.

Now my problem is: it runs on 110 volts. The schematics posted above says that 220v connection of T103 is possible. Can anyone look up the manual and tell me how to hook it up for 220v operation?
 
As I said, that schematic is for the ba-31c. the 'A' version may not connect for 220 AC. You should be able to tell by the number of power transformer taps, and whether any are paralleled. Series connection with tap 4-5 tied would be correct for 220. You may only have 4 taps. 1960-61 is correct for the 'A' version.
 
[quote author="PRR"]
I am remembering modeling showing about 68 dB open loop gain which of course may be totally wrong.
Voltage gain is then Hfe*1500/150, or OTOO 400, or 52dB. (Yes, with iron included this is 67.5dB, as you say.)[/quote]

Yes, I neglected to say I'd taken transformer gain into account.

minimum 20 dB feedback recommended elsewhere in some articles and papers
Oh, frag that newfangled theory. If we wanted a clean amp, we would not be using four antique transistors. Stick a 990 in there. At gain of 24 it will be clean. Even a good 5532 will be inoffensive. We love the old stuff FOR its imperfections.

Anyway, for many purposes, 20dB is not a lot better than 6dB. You wanna wander out toward 40dB NFB to get high-order residuals below what they are open-loop. And you don't get 40dB margins with essentially 2-stage gain amps.

Indeed. I measured 10 dB of NFB in the 'holy grail' (ha) RCA BC-2B console preamp. That is an odd one for RCA, and I personally see it as a their test bed for 9-pin tubes in broadcast preamps, being the first they issued. The octal BA-11A was still in production, and it runs 27 dB loop NFB. RCA seemed to have gone from 0 loop NFB pre-war to 24-30 dB loop NFB on almost every preamp from 1945 to 1960.

One of my other daily users has 8 dB. The Collins 6Q was made early on with 0 NFB loop, then added a loop I measured at all of 1.5 dB NFB.

The NFB impedance is -not- smaller than the inverting input impedance. NFB impedance impacts open-loop gain. To change gain in this design, fiddle R7. That's what RCA did with R4. That's how 90% of modern transistor amps do it. It has the helpful property that the amount of feedback stays near constant as gain is changed over a wide range.
I noticed the +6db strap on the schemo but what would you do with it?

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.

That addresses one of my basic questions about the different methods that might be employed.
 
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