RCA Ba2A Pre amp qestions & answers

[buildafriend]

Alright, I fixed a few funky traces from my layout, etched it, built it etc.. I seem to be getting all the right voltages. Everything looks good except for my output. My output was a acting a little strange and I think it was my output transformer selection that caused it.

I used a 0.1uF 250v WIMA film cap in place of C2 as a temporary part while I wait for the 0.47uF.

Since I still don't have my UTC A-25, would an edcor 2-1/2W 10K Ohms to 600 Ohms (4.1:1) line level matching transformer work?

I only ask because I have one available that I thought I might try to get this thing running.

URL is below.

http://www.edcorusa.com/p/160/xsm10k-600

 

[lassoharp]

Since I still don't have my UTC A-25, would an edcor 2-1/2W 10K Ohms to 600 Ohms (4.1:1) line level matching transformer work?

Electrically it should work and pass signal,  but it will not give bass response using it in this circuit.

 

[buildafriend]

This project is begging for my first ever stepped gain pot. 12 step rotary it is.

I'm assuming this circuit uses a log style pot? .. gotta learn the math behind this. .. what is shorting, what is non shorting?

everything is good to go. I just ordered a custom chassis. I'll post another pic when all is together.

 

[dustbro]

I'm assuming this circuit uses a log style pot? .. gotta learn the math behind this. ..

NewYorkDave had a great informational post about making a stepped attenuator. I'm too lazy to find it for ya, but do a search. Has all the info you need.

 

[buildafriend]

Is there any way I can get this circuit to produce a little more gain? Are all you other BA-2A cloners happy with the amount of gain it produces for you?

 

[emrr]

Not much left to do, beyond using an input transformer closer to the original ratio. Cinemag cmqee-3440a is most appropriate in their line.  50k instead of 15K, buys a few more dB. 

Bypass the eq (r11) should buy 3dB more. 

 

[lassoharp]

Are all you other BA-2A cloners happy with the amount of gain it produces for you?

Yup.  I often use SM-7 for vocals and get plenty.  Everything beyond that mic and source wise keeps vol less than 12 oclock or padded for heavy stuff.

What input iron did you use?  If modern lower ratio step up you'd lose a little relative to the original but probably not a deal breaker.

Should be getting around 50db or so.

 

[emrr]

Never any issue with gain here on an original RCA.  It might fail for spoken word with a ribbon, but probably not.  The real thing is stupidly quiet and tolerant with good tubes.  I have to pad mine 20 db on the front frequently.

 

[buildafriend]

Input Cinemeg CMMI-10C
Output Cinemag  CM-2092 (7:1, 30K-600)

I can get what I want out of it. It's very clean. I wish it had a little more gain. I think it would fail for spoken word with a ribbon, but it just sounded great through my peluso U47 copy. A phantom power circuit is all this guy really needs.

Now I REALLY WANT to start a BA-6 to pair this thing up with.

 

[buildafriend]

Not much left to do, beyond using an input transformer closer to the original ratio. Cinemag cmqee-3440a is most appropriate in their line.  50k instead of 15K, buys a few more dB. 

Bypass the eq (r11).

Connect the input transformer secondary to v1 cathode through 0.5 mfd or more, rather than ground.

Maybe I should switch to using that transformer?

 

[emrr]

It's a lot of expense for a few more dB

 

[buildafriend]

quick bass sample after adding DI
cheapie gibson bass, increased gain digitally but no compression.

https://soundcloud.com/user464938406/bass-ba-2a-completely

 

[letterbeacon]

Sounds great! Full!

 

[letterbeacon]

So I finally got round to replacing the old ADC input transformer I had in my BA-2 build with an original RCA one.  While I did this, I also re-did the heater wiring in an effort to reduce the slight hum I can hear in the amp.

I've managed to shave another 6dB off the hum, which I'm pleased about.  However, when I touch the case of the unit when it's out of the rack, the hum drops in level quite a bit which makes me suspect I've got a grounding issue somewhere in there.  I'm going to investigate further this weekend.

 

[letterbeacon]

When I have the BA-2 plugged into my RME FF400 interface fully cranked, according to my DAW I measure hum at -38dBfs.  When I touch my case the hum lowers in volume.

I have phantom power in my unit via a JLM Go Between that is fed by a ruffrecords phantom power PCB.  When I disconnect the 48v I'm feeding to the JLM PCB I measure hum at -68dBfs.  A whopping 30dB reduction in hum!

So there's obviously something wrong with the way I'm connecting the 48v, and I wondered if anyone has any insights.  Here's how I've got it set up so far:

I have a 15VA 25v-0v 25v-0v transformer connected in series to give me 50VAC.  i.e.  I have tied the 0v of one winding to the 25v of the other, giving me 50v across the other two windings on the secondaries (if that makes sense!).
The 50VAC is then connected to the phantom PCB which outputs 48VDC.
The 48VDC is connected to the 48v input on the JLM PCB
The ground output of the phantom PCB is connected to the input XLR.

Regarding the 25V and 0V winding I have tied together on the transformers secondaries -do I need to ground this?

Thank you.

 

[MagnetoSound]

Regarding the 25V and 0V winding I have tied together on the transformers secondaries -do I need to ground this?

Thank you.

No, I'm not familiar with Ian's phantom board but if you do this you will probably create a short across the bottom winding, assuming there is a full-wave bridge on the board.

Do you have a good ground at the smoothing caps on the 48v board? You may need to connect this to your star ground (if you have one) so that the ground on (mic XLR) pin 1 is not left floating.

 

[letterbeacon]

Do you have a good ground at the smoothing caps on the 48v board? You may need to connect this to your star ground (if you have one) so that the ground on (mic XLR) pin 1 is not left floating.

Mic XLR 1 is connected to the chassis as well as the ground output of the Phantom PCB so it's not floating.  Do you think this could be causing a ground loop?

 

[MagnetoSound]

Phantom ground could probably go via the ground pad on the Go-Between and up to the XLR on the screen.

 

[letterbeacon]

Thanks, I'll give it a go.  However I just found this in the Phantom PCB thread where I asked a year ago (wow, time flies) how to hook up the 0VDC:

The 0VDC should go to pin 1 of the XLR input. This same pin 1 should also be connected directly to the chassis as close to the XLR as possible. The system 0V should be connected to the chassis at the power supply.

Maybe I should take this to the Phantom PCB thread...

 

[letterbeacon]

It turns out I shouldn't have connected the shield on the pre amp side pin 1 of the XLR output.  I've disconnected it and now it's hum free.