RCA Ba2A Pre amp qestions & answers

emrr

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rockinrob86 said:
I added mu metal around the output transformer, and it quieted down substantially.  It was around -75db, and at that point izotope dehum takes care of the rest.  I would build it with a different pt next time, or put that one in an outside enclosure.

What I should have been expecting, but was not, was how great this thing sounds with my ribbon mics.  It should be obvious, they were made for each other and it sounds like it! 

I’m probably going to build several of these.  They’re cheap, easy to build and sound great!


Cool!  Looks good. 

Can you not move that PT to the outside?  It'd be far quieter on the outside of the chassis.

5693 tubes:  I consider mine, and any 1603/1620's scarce resources.  They really only make a difference in the first stage, if at all, so I save mine for that position and use standard 6J7 / 6SJ7 in later slots.  FWIW. 
 

rockinrob86

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EmRR said:
Cool!  Looks good. 

Can you not move that PT to the outside?  It'd be far quieter on the outside of the chassis.

5693 tubes:  I consider mine, and any 1603/1620's scarce resources.  They really only make a difference in the first stage, if at all, so I save mine for that position and use standard 6J7 / 6SJ7 in later slots.  FWIW.

I could move it.  I guess I didn't like having the connections only separated from the chassis by the little air gap, but it shouldn't be a real problem looking at it now.    I probably will eventually, but now it's nice to have it to use!

I got the 5693 for pretty cheap on ebay.  I'm considering buying a bunch of them - they work and sound great and I may start substituting 5693/6sj7 and their brethren for 12a types - they are getting expensive!
 

IanB

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rockinrob86 said:
I added mu metal around the output transformer, and it quieted down substantially.  It was around -75db, and at that point izotope dehum takes care of the rest.  I would build it with a different pt next time, or put that one in an outside enclosure.

What I should have been expecting, but was not, was how great this thing sounds with my ribbon mics.  It should be obvious, they were made for each other and it sounds like it! 

I’m probably going to build several of these.  They’re cheap, easy to build and sound great!

Awesome! Still waiting for my cases.
 

JW

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I wonder if someone can comment on having used a mic to grid transformer like the Jensen JE 115K in this circuit? https://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-115K-e1.pdf

Or similar Cinemag with not as high a step up ratio?
 

lassoharp

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I've used the Cinemags a few times for tube pre builds.  They work fine.  Largely transparent. You lose a little gain but I wouldn't consider it a problem.
 

rockinrob86

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JW said:
I wonder if someone can comment on having used a mic to grid transformer like the Jensen JE 115K in this circuit? https://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-115K-e1.pdf

Or similar Cinemag with not as high a step up ratio?

What comment do you want?  I used this one and set the front end up with the resistor like they suggested. 

It sounds great and works well.
 

Bird

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Hi
Im looking for some UTC M 8366A RCA part 205326 mic transformers for my RCA BC 3 consolette!
Anybody !!

 

dmp

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rockinrob86 said:
I added mu metal around the output transformer, and it quieted down substantially.  It was around -75db, and at that point izotope dehum takes care of the rest.  I would build it with a different pt next time, or put that one in an outside enclosure.

What I should have been expecting, but was not, was how great this thing sounds with my ribbon mics.  It should be obvious, they were made for each other and it sounds like it! 

I’m probably going to build several of these.  They’re cheap, easy to build and sound great!

I've built tube preamps with similar layouts and ended up switching the PT to a toroid. Much less hum if you have space inside. The EI PT generates a bunch of magnetic field in your enclosure. So you could move outside or just switch to a cheap Antek toroid and save some headache.
I've found using the conductive metal duct tape with mu  metal shielding works well. You want it to be continuous all around the transformer so the magnetic flux can fully travel around the interior space you are trying to isolate (i.e. Faraday cage).  Any bends should be as gentle as possible as stress reduces the permeability.


JW said:
I wonder if someone can comment on having used a mic to grid transformer like the Jensen JE 115K in this circuit?

Would sound fantastic. It is a nice input transformer. I used one in a V76 build.  They sell different versions with different levels of shielding. Since this is a relatively low gain preamp you wouldn't need the higher amounts.
 

rockinrob86

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dmp said:
I've built tube preamps with similar layouts and ended up switching the PT to a toroid. Much less hum if you have space inside. The EI PT generates a bunch of magnetic field in your enclosure. So you could move outside or just switch to a cheap Antek toroid and save some headache.
I've found using the conductive metal duct tape with mu  metal shielding works well. You want it to be continuous all around the transformer so the magnetic flux can fully travel around the interior space you are trying to isolate (i.e. Faraday cage).  Any bends should be as gentle as possible as stress reduces the permeability.

But I got a good deal on these EI transformers!!! $30 for a pair, and a pair of meters! 

I only spent 3 hours tinkering with the hum, haha.  I will probably move it to the back of the enclose, but for now it is good.  I've used it in some sessions with very pleasing results
 

rockinrob86

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So because of all of your bullying, I finally moved the Power Transformer out of the chassis and to the back.  it is now very quiet!  And it didn't take me more than 15 minutes as I was able to reuse all but one wire from before.

So note to self, keep the unshielded PT outside the chassis
 

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rock soderstrom

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rockinrob86 said:
So because of all of your bullying, I finally moved the Power Transformer out of the chassis and to the back.  it is now very quiet!  And it didn't take me more than 15 minutes as I was able to reuse all but one wire from before.

So note to self, keep the unshielded PT outside the chassis

Like!👍Did you measure your amp? Bandwith, noise or THD? 
 

rockinrob86

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Nope!

I recorded a bit of singing into an sm57, and then switched to a different preamp.  gain matched in the DAW, the noise levels were similar/acceptable, the frequency response was good. 

I compared to a peavey VMP-2.  The RCA is a bit darker/thicker, more colored, but is a very good sound. 

I don't particularly care about the specifics - it is a tool for recording, so if it works, it works! 

After using it some, I would like to build more.  have been considering building a pair in another chassis, or maybe even a quad of them.  These offer a really good value IMO and a cool color to have in the toolbox.  I really like what they do with my dynamic and ribbon mics.  Hoping to get a nice run through with the Tube LDCs soon. 
 

IanB

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Cases arrived
 

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emrr

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IanB said:
Cases arrived

About to start a 2 channel build.
I’ve got 2 Simpson meters I was going to run off a jlm buffer board strapped across the output xlr’s. Sound like a reasonable plan?
I’m using A10s for inputs and the gapped version of the 4:1 Carnhills Colin had made for LA2A builds. I’m going to put a 3 way output cap selector on a Tele switch for fun.

Cases look good!

Yes on the VU plan. 

Not sure what this input attenuator is. 

I don't think you will find need for the output attenuator either unless you plan on overdriving the whole system on purpose frequently.  I clip electric guitars with my original pretty frequently, and it never needs output control.  I haven't run it into full scale distortion on purpose though to see where it stops, I should do that. 

Are you doing that PCB version that's a modified cap coupled circuit?  If so, that WILL clip earlier, with even lower headroom.  That change to a parallel feed circuit will greatly reduce overall headroom.  It's really worth having DC through an output transformer designed for it.  You mention a gapped output, so I'm confused. 
 

IanB

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EmRR said:
Cases look good!

Yes on the VU plan. 

Not sure what this input attenuator is. 

I don't think you will find need for the output attenuator either unless you plan on overdriving the whole system on purpose frequently.  I clip electric guitars with my original pretty frequently, and it never needs output control.  I haven't run it into full scale distortion on purpose though to see where it stops, I should do that. 

Are you doing that PCB version that's a modified cap coupled circuit?  If so, that WILL clip earlier, with even lower headroom.  That change to a parallel feed circuit will greatly reduce overall headroom.  It's really worth having DC through an output transformer designed for it.  You mention a gapped output, so I'm confused.
Hey Doug.
It’s p2p. I’ll post pics as I go. I read here on this thread that noise was a common problem with the power tranny proximity, hence the separate rack box for that.
Re the output tranny, carnhill make a 4:1 gapped that can take the dc as per original schematic. I’ve used this transformer in a few other builds and it works & sounds great. Should be ok here I think.
I know I will prob have the input & output attenuators dimed much of the time but I’d rather have the option if needs arise. Inline pads are a bummer.
I’ll use nice toobs for V1 and the cheaper ones for V2 as per your suggestion.
“American Beauty” is a nod to something you actually said in a thread once. No-feedback American Beauty. Just gotta get it working. Wish me luck.
 

IanB

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It flippen works!!!
 

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IanB

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It is dead quiet (thanks to all the amazing knowledge and advice in this thread!). I’m so stoked with it! Very grown-up sounding (my first 2 valve pres so a new thing/sound for me).
Doug is right: with a dynamic & ribbon on vocal all 3 knobs are dimed but with a condenser the attenuators might be useful.
The cheap Chinese VU buffer I got is noisy as! Just ordered the JLM one which should hopefully work a treat.
So thankful for the previous 20 pages of gold that got me over the line so smoothly. Cheers boys!
REDD47 next! PSU is built and works (6x4 & 2 x OA2s = 300vdc). Turrets are done. Just gotta wire it up.
 

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