RCA Ba2A Pre amp qestions & answers

[emrr]

The bottom end looks better than some, the top can be highly variable depending on test setup.  Look at my 87-A plots in my rack up thread for examples of a real one with two different input loads.  Consider that a mic will deliver a totally different load than a resistive or amp driven setup, regardless of the nominal expected Z. 

Noise looks respectable.  I'd try power rotation just to see.  Is the shield tap on the input trans grounded?  That measurement should be taken with a 150-200r across the input terminals.  Real RCA quotes -69dB (0.001 watt reference) with input terminated. 

High boost is in the 2-3 dB range, and set high enough that it usually only influences the shape of the roll off. The earliest versions used 56k/270pfd rather than 33k/330pfd, which would be a slight difference.  Consider that if r11 was 100k then c8 would be a 6 dB boost bypassing half of the voltage divider, and overall gain would fall by the same amount.  So 33-56k is a reasonable eq loss to balance against top end response leveling.  Tune shape by changing cap values, may take a handful and some test leads to see what looks best.

Q?s:

Output load?  Look at it with and without a 600-1k across the output.  Sometimes it makes no diff, sometimes a lot.

Did you try reversing the secondary of the input trans?

Did you have a look with a mic pad between the source and the input?

 

[letterbeacon]

Yes comparing my BA-2 frequency response with the plots for your 87A, the bottom end is about the same, maybe even a little better.

Yes the shield tap on the input transformer is grounded.  I'm pretty happy with the noise level, I did use carbon comp resistors throughout so I've only myself to blame!  It's just the 50Hz hum I  might try and lower somehow.

As:

Yes the amp was driving into a 10k load.  What do you suggest I do to change this?  Use a U pad with a 600r shunt resistor?

Yes I did try reversing on the secondary of the input transformer.  Weirdly, the level was much louder the other way, and the frequency response dropped off very rapidly after 1k.

No, I haven't tried it with a mic pad between the source and input.  Again, what would you suggest?  A U pad with 10k in series and a 75ohm as a shunt?

Don't get me wrong, I'm really pleased with it so far - I recorded some vocals using it with a ribbon mic last night, and it did sound a little dark, but when I dialled in some top end with an EQ it sounded great!

 

[emrr]

Can we see a clear shot of the connections you made on the input transformer?  Or tell me what pins are connected to what?  Louder one way versus the other makes me suspicious, but not sure of what.  Do you have gain measurements noted? 

output load, strap a 680r across the output terminals.  1K is also fine.  May or may not see anything other than gain change. 

standard mic U pad 620/169/620, or similar, a la Jensen.  10K would be a line pad, which usually shows even greater roll-offs in the highs. 

 

[letterbeacon]

I found this section of the BA-2 manual online:

I suppose I should be aiming for a frequency response similar to the one in the manual.  My bass response is better, the manual's plot shows it dropping off at about 200Hz, but the high end is much better.

Aha, I was just typing this and can see that you've replied.  I'll try and post a close up of the input transformer tonight, and will also try the mic U pad.

 

[emrr]

Note the stock input connection has one side of the primary grounded, and in a real one this causes a large treble boost.  Having a different input transformer, you should expect differently.  You don't want one side grounded, you want the entire primary floating.  I've never measured a BA-2 that looked like that drawing, in regards to the treble boost. 

 

[letterbeacon]

Ah I see.  On mine I have both sides floating, with one side of the secondary tied to ground.  I'll post a detailed picture of the input tx soonish, which should clear some things up.

 

[letterbeacon]

Here's a picture of the input transformer wiring which produces the frequency response I posted earlier in the thread:

The blue and brown wire comes from the + and - of the input XLR.
The grey shielded cable goes to the grid of V1.
The shield of the shielded cable goes to the shield tap on the input tx which, via the green wire, goes straight to pin 1 of the output XLR, which is connected to the chassis.
The green wire in the middle is connected to signal ground.

Unfortunately I had to send my Fireface interface to Germany yesterday as the rear input jacks have corroded (a known issue apparently), so I won't be able to do any more tests for a bit.  I'll probably borrow an interface from work this weekend to test out the mic pad you suggested earlier.

Can we see a clear shot of the connections you made on the input transformer?  Or tell me what pins are connected to what?  Louder one way versus the other makes me suspicious, but not sure of what.  Do you have gain measurements noted?

Here's a plot that I did before I sent the Fireface back.  It's with the secondary of the input transformer reversed (i.e. opposite to the way it's pictured in the photo above). 

 

[emrr]

Looks right from I can see on my phone. 

My only immediate thought is the shielded grid wire.  Shield capacitance and long wire run can kill treble.  I might replace it with unshielded for comparison.  If it's a contributor, weigh interference versus roll-off.  They used to sell low capacitance shielded for crystal mics, which is what you find on old grid caps.  Single solid conductor with a large rubber fill between it and shield. 

 

[letterbeacon]

Thanks Doug. Hmmm, I'm not using very much shielded cable in the amp, maybe about a foot in total, but I'll give swapping it for regular cable a shot.

Do you have an idea as to why the frequency response graph of the amp is so different when the secondary is reveresed?

 

[emrr]

Relationship of capacitance to ground.

 

[letterbeacon]

Another question that I've been meaning to ask is that on the schematic pin 1 of the 1620 tubes are connected to ground and are presumably connected to the metal case of the tubes.  My 1620s don't have a pin 1, is this normal?

 

[emrr]

I haven't noticed one that lacked pin 1. 

 

[letterbeacon]

Hmmm, very strange as both my 1620 tubes are missing them -I just assumed that they weren't supposed to have them.  Perhaps it would bring down the noise a bit more if they were screened?  I'll have a chat with the guy who sold them to me.

 

[emrr]

might or might not.  Your noise seems typical and normal from what I can see, and from your description.  Ground the outer cans yourself and see. 

 

[lassoharp]

Pin 6/ NC is the only pin that's missing on all the ones I have.  Seems strange for that particular tube.  I suppose you could run a test wire from the shell to ground and see if it helps but I doubt you'll hear any audible difference.

 

[letterbeacon]

I managed to grab some time to perform some more frequency analysis, and this is what I've come up with:

Firstly this is the frequency response with no pad whatsoever, it's just being driven by the 600r line outputs:

Now this is looking more like what I was expecting, and more like emrr's plots on his RCA pre amps.

This is the frequency response with the mic pad emrr suggested earlier in the thread - 620r series resistors and a 160r shunt resistor:

This one has much better top end than the responses I measured previously.  Curiously though it doesn't have the treble boost in it that the other plot has.  Both tests were taken with the treble boost circuitry in. It looks like the pad is taking out the treble boost for some reason.

So what does this tell me?  I guess that driving the BA-2 with the 600r line outputs is optimal, and with the mic pad in the response is still respectable.  I'm assuming that using a mic pad is the closest to simulating a mic, is that correct?

 

[lassoharp]

So what does this tell me?  I guess that driving the BA-2 with the 600r line outputs is optimal, and with the mic pad in the response is still respectable.  I'm assuming that using a mic pad is the closest to simulating a mic, is that correct?

I will second CJ's remark of "If it sounds good - go with it".  I haven't had a chance to use my BA2s on a wide selection of mics - just M-500 ribbon, SM-7 dynamic and a pair of tube mics, all of which use output transformers.  I expect there will be L to L interactions that behave differently than a pure resistive source.  I've been pleased with how all those mics respond.

I also have another tube pre that clearly starts rolling of around 2K - plot looks like your original BA-2, but doesn't sound like it's missing an excessive amount of high end.

I tend to think that with most mics the tube pre with transformer input sound like they're behaving closer to the 1st of the 2 plots you just posted - driven by 600r inputs, and with the rising response around 5K.

and it did sound a little dark, but when I dialled in some top end with an EQ it sounded great!

 

Has been pretty much standard protocol for me and that's where a Pultec style EQ comes in handy.



During the BA2 build, I sat for a while doing some A/Bing of w/w/out treble boosting circuit.  The source was line level audio going through mic to line pad.  My ear was telling me that the midrange was being altered more than just a 'straight' treble boost.  I can't recall the RDH4 page but I believe this is to be expected with this type of boosting circuit - a slight alteration of the mid response.  If I had to say which way, I would have chosen slightly dipped.

 

[letterbeacon]

I will second CJ's remark of "If it sounds good - go with it".

I've just recorded myself playing guitar and singing into it and I have to say I love it now.  It's got a really nice vintage vibe.  It does sound good, and I shall go with it!

The only thing now is the 50Hz hum.  I said it wasn't an issue before, but now I've done a bit of multitracking and also kicking up the volume in my headphones whilst tracking, the hum is definitely there and pretty distracting.  I'm going to try using shielded cable to and from the input tx to see if that will help.  I know that rotating the PT will probably help a hell of a lot, but I'd have to cut new holes in the back, and even then it's not guaranteed to help.

and it did sound a little dark, but when I dialled in some top end with an EQ it sounded great!

 

Has been pretty much standard protocol for me and that's where a Pultec style EQ comes in handy.

A p2p Pultec is definitely on my diy list now!

 

[letterbeacon]

How effective is elevating the 6.3VAC to reduce the hum?  On this page: http://www.freewebs.com/valvewizard/heater.html they mention adding a DC reference by using a voltage divider on the B+ to get between 50VDC and 90VDC and then add that to the 6.3VAC CT.

My HT is about 326VDC.  If I make R1 180k and R2 50k that should give me 70VDC.  I'll try connecting that to the CT to see where that gets me.

Does anyone have any experience of this?

 

[lassoharp]

Even though my 6.3VAC tap on the PT has a centre tap, do you think elevating the heater's voltages would reduce hum?

It's supposed to in theory.  But bigger Q:  Do you have CT connected straight to ground?  If so, try installing two 100r 2W resistors and leave the CT tied off, or better yet, install a 100-200r pot.

Before attempting I would go over the layout and make sure the filament wires aren't too near any sensitive audio wires.  I've personally never had filament hum due to layout issues and have even tried to purposefully induce it by pushing fil wires into close proximity of audio wires.

I'm going in reverse order here - rule out tubes by swapping, etc.  Also make sure that tubes are well seated into their sockets.  I just had a case of this on the recent preamp I built.  Brand new sockets that felt bitch tight, but tubes weren't all the way pushed in.  It was creating hum in output and a simple reseat cured it.

Have you done step wise troubleshooting to isolate what stage hum is coming in?  I'm assuming maybe 1st since you say you have to crank it to hear it.  You can try to check output stage by shorting V2 grid to ground (or just turning vol pot all the way down), then run another small amp of some type on the output and bring vol up and see if it's still there in the same place.

Nice build! - BTW