RCA Ba2A Pre amp qestions & answers

[FarisElek]

Can anyone recommend me the most easily obtainable, affordable, but still great sounding transformers for this application? Input and Output. Wouldn't mind a power transformer recommendation either.

So far I've got.

Input: Jensen JT-115K-E

Output: Cienmag CMO-30/600

Power: Edcor xpwr146

Will these all works?

Also, what is the most effective way to include phantom power in this build?

Is there a power transformer that would work for two of these in one case


Ryan

 

[IanB]

Can anyone recommend me the most easily obtainable, affordable, but still great sounding transformers for this application? Input and Output. Wouldn't mind a power transformer recommendation either.

So far I've got.

Input: Jensen JT-115K-E

Output: Cienmag CMO-30/600

Power: Edcor xpwr146

Will these all works?

Also, what is the most effective way to include phantom power in this build?

Is there a power transformer that would work for two of these in one case


Ryan

I used UTC 1:10 input and Carnhill 4:1 out. I used a Hammond power tranny. 300v & 6.3v secs. I use AKG phantom box if I need it but JLM does some easy phantom/phase boards - you’ll need a power tranny to suit but JLM will tell you what you need on top of your B+ & heaters.
It’s a really simple circuit & sounds so good! You can’t go wrong eh!

 

[My3gger]

Can anyone recommend me the most easily obtainable, affordable, but still great sounding transformers for this application? Input and Output. Wouldn't mind a power transformer recommendation either.

So far I've got.

Input: Jensen JT-115K-E

Output: Cienmag CMO-30/600

Power: Edcor xpwr146

Will these all works?

Also, what is the most effective way to include phantom power in this build?

Is there a power transformer that would work for two of these in one case


Ryan

Jensen should be very good for input, Cinemag takes unbal. DC as required for BA-2 output and also have more appropriate ratio (7:1 instead of 4:1 some use). Ask Cinemag to make sure. Edcor is meant for power amp applications without phantom tap. Maybe try to find proper toroid because it is often cheaper and easier to work with.
Wish i would see more p2p projects, BA-2 is really simple for learning this technique...

 

[JW]

Can 4:1 output not drive 600 ohms properly? Because, if it can, it's more gain and the Carnhill VTB 2290 looks like an obvious choice.

 

[emrr]

Can 4:1 output not drive 600 ohms properly? Because, if it can, it's more gain and the Carnhill VTB 2290 looks like an obvious choice.

First ?  will you ever actually be driving 600 ohms?  This affects the answer enormously.

If you are, it's not a ? of whether it can drive it, it's a question of distortion and gain.  The gain loss in the tube of driving a lower ratio into 600 ohms will counter the 'increased gain' of the transformer, mostly a wash.  But distortion will be higher in the driver tube. 

The bigger question is still whether there's sufficient inductance with DC current flow to give complete low frequency response, or whether the bottom will be rolled off early. 

 

[JW]

Thanks very much Doug. I have 3 pieces of equipment that I've noticed definitely prefer to be driven by a 600 ohm 'capable' source. (Am I using the right terminology?) But anyway, DIY pultec/LA2A/1176 I certainly prefer to drive them with something like a Neve over my converters' outputs. Feels like they work better.

 

[emrr]

Thanks very much Doug. I have 3 pieces of equipment that I've noticed definitely prefer to be driven by a 600 ohm 'capable' source. (Am I using the right terminology?) But anyway, DIY pultec/LA2A/1176 I certainly prefer to drive them with something like a Neve over my converters' outputs. Feels like they work better.

Yep, all 3 of those count.  Lower the better in those cases. 

 

[JW]

As in a bigger step down (for lower impedance) if I'm reading you right? i.e. 7:1 or is it try it and see if it all comes out in the wash at 4:1? I guess the reason I ask is I'm about to put an order into Carnhill. . . . .for some other things.

 

[emrr]

Think of it backwards for the clearest picture:  If you're driving 600 ohms, the 4:1 loading on the tube is heavier, more distortion, lower level from the tube BUT less loss in the transformer.  7:1, lighter load on the tube, less distortion, higher gain, but more loss in the transformer.  How much distortion matters to you?  I can't say.  You might like either one over the other. 

 

[FarisElek]

Can I build two of these in a single box using a power transformer with double the rating? If so, how would I wire the two 240v and the two 6.3v wires to the two separate circuits? Would I wire it the same way as a single unit but split the transformer wires into two wires? Will this be very noisey?

Thank you,
Ryan

 

[jazzcrisis]

Can I build two of these in a single box using a power transformer with double the rating? If so, how would I wire the two 240v and the two 6.3v wires to the two separate circuits? Would I wire it the same way as a single unit but split the transformer wires into two wires? Will this be very noisey?

Thank you,
Ryan

That will be fine. You can split the B+ but it's better to do the heaters as you normally would.

 

[FarisElek]

That will be fine. You can split the B+ but it's better to do the heaters as you normally would.

Yes, but I only have two 6.3v. Should I split them and and deliver one of each secondary split to both sides?

Hard to describe.

 

[dmp]

Usually you put the heater windings together to all the heaters with 100 ohm resistors to ground.
I don't see any reason to split them to the two channels.
I also don't see any reason to split the high voltage. You could have separate rectifier circuits for the two channels but even that probably wouldn't be necessary. This isn't a power amp that will swing the B+

 

[FarisElek]

Finally got around to building this. Really annoying 60hz hum, nothing seems to help. But far more annoying is the steady low end roll off that starts at around 500hz. It almost sound like an old telephone.

Some of the noise I actually think has something to do with the cheap ebay anode caps I'm using. There isn't good connection or something. When I touch the anode cap wire or top of the cap it intensifies the buzz that is present.

Are there a list of voltages I should be getting near on the tube pins?


If anyone has any ideas about the roll-off, I'm all ears. I have a feeling it's transformer related. Using a custom in put of 200 to 50k, and an edcore wsm of 15k to 600. I know that output isn't desirable but I figured it would get me close. Could it be what's giving me that roll off?


Ryan

 

[rock soderstrom]

Finally got around to building this. Really annoying 60hz hum, nothing seems to help. But far more annoying is the steady low end roll off that starts at around 500hz. It almost sound like an old telephone.

Some of the noise I actually think has something to do with the cheap ebay anode caps I'm using. There isn't good connection or something. When I touch the anode cap wire or top of the cap it intensifies the buzz that is present.

Are there a list of voltages I should be getting near on the tube pins?


If anyone has any ideas about the roll-off, I'm all ears. I have a feeling it's transformer related. Using a custom in put of 200 to 50k, and an edcore wsm of 15k to 600. I know that output isn't desirable but I figured it would get me close. Could it be what's giving me that roll off?


Ryan

AFAIK, the BA2A needs a gapped output transformer. There is DC current in the primary of your ungapped Edcor now. This will result in a bass roll off.

Btw, the BA2A works with grid caps, so it is quite normal that this area is very sensitiv.

Edit: I need urgently an Edcor WSM 15k to 600R at the moment.

 

[FarisElek]

AFAIK, the BA2A needs a gapped output transformer. There is DC current in the primary of your ungapped Edcor now. This will result in a bass roll off.

Btw, the BA2A works with grid caps, so it is quite normal that this area is very sensitiv.

Edit: I need urgently an Edcor WSM 15k to 600R at the moment.

Agh makes sense. Oh well. I'd sell you my transformer but I'm in Texas and sending a receiving to Germany has taken a minimum of a month for me. Also, I definitely could use this for another project coming up.


Ok, very newb question for later in this build. But I've been reading a lot about load resistors and I had some questions. On the original schematic is says Load 250 ohm on the output. Is that literally a 250ohm resistor between pin 2 and 3 of the xlr output? and there is something similar on the input that says 30ohm. Is there a 30 ohm resistor there between pin 2 and 3 of input xlr?


Thank you,
Ryan

 

[rock soderstrom]

Agh makes sense. Oh well. I'd sell you my transformer but I'm in Texas and sending a receiving to Germany has taken a minimum of a month for me. Also, I definitely could use this for another project coming up.


Ok, very newb question for later in this build. But I've been reading a lot about load resistors and I had some questions. On the original schematic is says Load 250 ohm on the output. Is that literally a 250ohm resistor between pin 2 and 3 of the xlr output? and there is something similar on the input that says 30ohm. Is there a 30 ohm resistor there between pin 2 and 3 of input xlr?


Thank you,
Ryan

These values are AC impedances of the sources and subsequent devices for which the amplifier was designed.

Today, it is more common to assume 200R sources, such as dynamic microphones. Most devices your amp will play into today have up to 10k input impedance. Worst case scenario are still vintage devices like LA2a and 1176 with 600R. Therefore the output impedances are still very low.

So there are no resistors permanently installed. This is done only in the test case to simulate the impandances of the sources or subsequent devices.

PS: you are right, shipping iron over the great lake makes no sense atm.

 

[BluegrassDan]

Edit: I need urgently an Edcor WSM 15k to 600R at the moment.

How many do you want?

I bet shipping them to Berlin from the USA will cost more than the transformers themselves.

 

[rock soderstrom]

How many do you want?

I bet shipping them to Berlin from the USA will cost more than the transformers themselves.

Yes, that's right. In fact, it would only make sense to buy several transformers together at once.

Is no longer necessary, Don Audio, the European distributor of Edcor, can deliver again since last Friday. I got my 15k/600R yesterday!

Thanks for your offer though!

 

[PermO]

Congratulations !

That's good news

My transformers from the US are still in transit... two months now (USPS), I think they have crossed the pond by now... I sure hope so...