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.


 

Related Topics

  Subject / Started by Replies Last post
9 Replies
6386 Views
Last post March 03, 2005, 11:07:04 PM
by PRR
11 Replies
3888 Views
Last post March 25, 2009, 05:19:06 PM
by gswan
0 Replies
1569 Views
Last post August 17, 2006, 10:55:40 PM
by Lowfreq
18 Replies
7755 Views
Last post May 01, 2008, 07:28:28 AM
by gary o