RCA Ba2A Pre amp qestions & answers

[My3gger]

I forgot to let you guys know what trafos Cinemag has for this amp: inputs are CMMI-10PCA and CMMI-10B, gapped outputs are CM-2092G and  CM-27101G. There are datasheets for inputs on their site.
I bought CMMI-10B because it has leads which means a less work. Next time i will try pins because shielded secondary wires really makes difference when runs are a little longer. Manufacturer told me that shield capacitance can influence frequency response, so i removed mesh shield from coaxial wire and i will put it over secondary wires to see what happens.
OT's don't have datasheets posted, here are some information: CM-2092G is gapped in the standard configuration, i didn't ask for max I, ratio is 7:1. CM-27101G can be gapped (special laminating structure with 4 "E"s in on direction and 4 in the other), it's higher quality transformer, takes up to 10mA, iirc max signal is +18dBu, also with 7:1 ratio. It was recommended to me, so i ordered one and didn't pay anything more for gapping. Maybe because i ordered more than 10 trafos all together for some other guys beside me.
So this is not that expensive after all and i really like their transformers. With all costs for shipping from US to EU they are still cheaper than Sowters and at least inputs sound better to me.

 

[MatthisD]

Thanks for posting these Cinemag details, would you happen to have the DC resistance of the windings? particularly the output primary of CM-27101G and if you know the dimensions of it.

 

[My3gger]

Thanks for posting these Cinemag details, would you happen to have the DC resistance of the windings? particularly the output primary of CM-27101G and if you know the dimensions of it.

Transformers still didn't go out of factory, we are waiting around month and a half. I will ask them if everything is ok, a friend already wrote one mail about three weeks ago and they said they will ship in a few days. This is very unusual for them, maybe it's because of this custom trafo... I think ungapped version of CM-27101 have the same core dimension as their CM-2810, i have both of them at home. When gapped version of CM-27101 arrives i will measure it, take some pics of lamination stack and let you know DCR reading.
There is also CM-9589 with 4:1 ratio which costs a little more than CM-27101. I think it takes up to 20mA with steel laminations, have to check email if anyone is interested.
Btw, compare CMMI-10PCA datasheet to Jensen JE-115KE, they look almost identical. I heard 115KE is based on old Reichenbach input transformer which is now CMMI-10PCA.

 

[MatthisD]

How long did your previous order from Cinemag take to arrive, 3 or 4 weeks?
I couldn't find it listed on the site but the datasheet is here;
http://cinemag.biz/output/PDF/CM-27101.pdf

 

[My3gger]

More like 2-3 weeks to Europe, trafos went out of factory a few days after payment for about 5-6 orders until now. I never ordered custom product or something that wasn't in stock, maybe it's this or because it was two of us talking to them this time. Anyway, they used to ship fast, communication was good and they also proposed correct ratios/models when i asked.

 

[buildafriend]

Just call dave at cinemag and ask him. He's a great communicator and he is on point.

 

[My3gger]

You are right, no problem there. I wrote an email a few hours ago and got reply from David right away. They have some lead time and also had Frankfurt. I should ask them myself before posting here...
So we have easily available trafos for BA-2, i think pretty cheap for what they are.

 

[My3gger]

Ok guys, i got CM-27101G (gapped version), here are results compared to ungapped i also have:

Ungapped CM-27101
Primary DCR: 1371R
Secondary DCR: 30R

Gapped CM-27101#G
Primary DCR: 1335R
Secondary DCR: 29,5R

My DMM is a cheap one, so this numbers are little off but consistent.
Both transformers have wire for Faraday shield, same color code and core dimensions. They sent photo of the same stacking method used for CMOQ-2x, i will try to make it smaller and attach it. Mine has 12 stacks of what looks like Iron and Nickel, so 4Fe/4Ni lams 6 times. I'm pretty sure shiny lams are Ni and other Fe. Hope you understand what i want to say, if you have any question let me know. Unfortunately i can't say how it sounds...

 

[MatthisD]

Ok guys, i got CM-27101G (gapped version), here are results compared to ungapped i also have:

Ungapped CM-27101
Primary DCR: 1371R
Secondary DCR: 30R

Gapped CM-27101#G
Primary DCR: 1335R
Secondary DCR: 29,5R

Thanks very much!

 

[My3gger]

MatthisD, why did you ask for dcr? I'm sure it reveals something, i just don't know what and why. Can you please explain me this if you can?

 

[MatthisD]

MatthisD, why did you ask for dcr? I'm sure it reveals something, i just don't know what and why. Can you please explain me this if you can?

To get an idea of the number of turns and the wire gauge that make up the primary, and to compare the various transformers available with this lamination size. I'm using a Lundahl for the moment which is a c-core dual bobbin type. The cost is equally high for this Cinemag once postage and additional taxes are included so I like to have as many details as possible before buying a couple of these.

 

[buildafriend]

I was wondering if I could beg for a little help here...

I have a friend with a UTC CR 725

I got some possibly correct specs on it through and ebay auction for one


"This sale is UTC hermaeticaly sealed audio output transformer model CR-725. This transformer was manufactured for RCA under part number B-8480798-1. The response specs are: 1/2 DB 30 to 15 KC at +20 DBM. The primary impedance is 100 ohms and the secondary can be configured for either 150 or 600 ohms. Size is 2 1/4 inches high and 1 5/8 by 1 3/16 mounting base. There are threaded studs on the bottom for mounting. This is a commercial grade transformer."

My BA-2A has a low output. I've deemed it too low for actual use. It cant even drive a 58 for a vocal. Not okay. It sounds great and its clean but using make up gain in post from a mic pre is not okay with me. I need to get it working. I'm guessing its the output transformer.

Are there any specs that I can give you guys that might help me?

 

[dmp]

Mine has about 45-50 dB I think (I omitted any tone adj)?
I wouldn't say this is "too low for actual use" (for vocals from a dynamic mic).
I would start by measuring the gain (in dB) of the whole preamp, then the gain of the input and output transformers and see if things make sense. Then go from there on making changes.

 

[dmp]

Also, it's not clear to me what the UTC CR 725 has to do with your BA-2A. Are you saying you are thinking of using the UTC 725 for the output transformer? It doesn't sound remotely like the correct output transformer for a ba-2a.

 

[lassoharp]

My BA-2A has a low output. I've deemed it too low for actual use. It cant even drive a 58 for a vocal. Not okay. It sounds great and its clean but using make up gain in post from a mic pre is not okay with me. I need to get it working. I'm guessing its the output transformer.

Then i'd say there is problem with your BA-2.  Mine is chugging away like a champ in the studio.  Really no situation I've found close to not having enough output.

A low Z 1:1 for the OT . . . . I suppose it will pass signal with unloaded secondary.  If you terminate the sec with 600r it will give the output tube a hernia.  It's the wrong tool for the job and it will likely sound not so good.

 

[My3gger]

Hi guys,

i'm changing my psu from RC filters as on schematic to fully regulated. I'm not sure what to do with R10/10k. Looks like it makes part of psu decoupling with C7, so it should probably stay in the circuit with reg. psu. Is this correct?

 

[lassoharp]

Hi guys,

i'm changing my psu from RC filters as on schematic to fully regulated. I'm not sure what to do with R10/10k. Looks like it makes part of psu decoupling with C7, so it should probably stay in the circuit with reg. psu. Is this correct?

Yes, R10 decouples and sets supply voltage for V1.  Probably wouldn't hurt to use a 22uF there.

Am curious why you are choosing a regulated supply for this circuit.

 

[My3gger]

I'm using 22u C7 (might go even higher, as on BA-11A) for a little more filtering and because of emrr's remark about old lytic's values. Both cathode caps are 68u for this second reason.
I'm changing psu because i want to have all three voltages on one board. HT is actually capacitance multiplier with zeners which comes very handy as i don't have to experiment with resistors to get right voltage. Ripple is in uV range with almost no heat from transistor at ~270V/15mA... I think this are good reasons to use it beside not needing heatsink.
Have you ever noticed any difference in sound from simple circuits like this when using reg. vs unreg.? I'm asking because never got a chance to compare on various sources, gains, etc.

 

[lassoharp]

Have you ever noticed any difference in sound from simple circuits like this when using reg. vs unreg.? I'm asking because never got a chance to compare on various sources, gains, etc.

I've never tried a regulated supply on any of the preamps I've built so can't really say.  Certainly on vari-mu limiters you can hear the increased distortion and overall poorer performance with an unregulated PS design.

If your line voltage ever dropped enough to bring the V1 plate voltage down towards 50V or so you'd likely hear the typical results for that area of operation.

 

[My3gger]

I'm making layout for BA-2 and chose RCA knob, like on Letterbeacon's build here:
http://www.groupdiy.com/index.php?topic=26675.240
There is eBay offer with similar NOS knob which has 1,5 inch diameter. It doesn't say if this includes pointer. Does anyone know if it is 1,5 inch with pointer or not? Here is eBay add, can't ask because i'm not a member:
http://www.ebay.com/itm/NOS-Original-Vintage-1-5-RCA-Viz-Bakelite-Knob-/320984836948?pt=US_Vintage_Pro_Audio_Equipment&hash=item4abc2feb54