rockinrob86

Re: RCA Ba2A Pre amp qestions & answers
« Reply #380 on: June 10, 2020, 08:54:46 AM »
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!


EmRR

Re: RCA Ba2A Pre amp qestions & answers
« Reply #381 on: June 10, 2020, 01:23:03 PM »
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. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

rockinrob86

Re: RCA Ba2A Pre amp qestions & answers
« Reply #382 on: June 10, 2020, 05:26:50 PM »

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

Re: RCA Ba2A Pre amp qestions & answers
« Reply #383 on: June 11, 2020, 03:48:52 AM »
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

Re: RCA Ba2A Pre amp qestions & answers
« Reply #384 on: June 17, 2020, 06:30:08 PM »
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?

Re: RCA Ba2A Pre amp qestions & answers
« Reply #385 on: June 17, 2020, 08:03:54 PM »
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

Re: RCA Ba2A Pre amp qestions & answers
« Reply #386 on: June 17, 2020, 10:58:13 PM »
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

Re: RCA Ba2A Pre amp qestions & answers
« Reply #387 on: June 22, 2020, 02:43:23 AM »
Hi
Im looking for some UTC M 8366A RCA part 205326 mic transformers for my RCA BC 3 consolette!
Anybody !!


dmp

Re: RCA Ba2A Pre amp qestions & answers
« Reply #388 on: June 22, 2020, 10:55:05 AM »
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.


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

Re: RCA Ba2A Pre amp qestions & answers
« Reply #389 on: June 24, 2020, 11:12:27 AM »
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

Re: RCA Ba2A Pre amp qestions & answers
« Reply #390 on: June 29, 2020, 09:06:42 AM »
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
« Last Edit: June 29, 2020, 09:18:17 AM by rockinrob86 »

Re: RCA Ba2A Pre amp qestions & answers
« Reply #391 on: June 29, 2020, 10:16:04 AM »
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

Re: RCA Ba2A Pre amp qestions & answers
« Reply #392 on: July 02, 2020, 01:10:51 PM »
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. 

Re: RCA Ba2A Pre amp qestions & answers
« Reply #393 on: July 02, 2020, 01:28:02 PM »
Thanks for your feedback!I guess this is my next project.

IanB

Re: RCA Ba2A Pre amp qestions & answers
« Reply #394 on: July 12, 2020, 03:17:51 PM »
Cases arrived

EmRR

Re: RCA Ba2A Pre amp qestions & answers
« Reply #395 on: July 12, 2020, 03:40:13 PM »
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. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

IanB

Re: RCA Ba2A Pre amp qestions & answers
« Reply #396 on: July 12, 2020, 06:30:54 PM »
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.

Re: RCA Ba2A Pre amp qestions & answers
« Reply #397 on: July 12, 2020, 09:47:02 PM »

“American Beauty” is a nod to something you actually said in a thread once. No-feedback American Beauty...

Nice, I like it. 
If you want to sound crazy please tell me about THE DEEP STATE.

IanB

Re: RCA Ba2A Pre amp qestions & answers
« Reply #398 on: July 31, 2020, 10:29:31 AM »
It flippen works!!!

mjrippe

Re: RCA Ba2A Pre amp qestions & answers
« Reply #399 on: July 31, 2020, 10:41:12 AM »
Handsome work there!


 

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