La-2a Input output / transformers plethora of choices???

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bxershrts

Well-known member
Joined
Jan 12, 2015
Messages
52
Hello,

for the past 2 years I have been researching this build. and yes I have searched these forums. It appears that the bulk of the LA-2A  DIY on the net is from 2006 to 2009. I can not find anything recent!? I would like to know from confirmed recent diy la2a builds about input and output transformers as I am very concerned about what to purchase and need some help.



I hear sooo much talk about, making a transformer to kind of work. .

I want to buy transformers that perform like the originals, look like the originals(fit in the chassis) and don't need to be circuit modified. Can anyone help me out from your own personal experiences.

What I have found are these confusing problems.

1.adding a capacitor to drop the impedance to kinda make it work.
2.reverse wiring from 1:4 and 4:1
3 then the problem of the line input being to high and going into mic input
4. The transformers are to clean and no distortion or vintage grind when pushed
5. Sowter output tranny are not a circular shape and wont fit a traditional circular style chassis


The list goes on and on.

Between edcor, Jensen, Cinemag, Sowter and the esoteric UTC  and a million other ones ..is there not one that is wire for wire - similar size and shape. Impedance and ratio for the diy guy that is bolt in , no mods and works???

My concerns with purchasing the wrong transformers are : being to clean, to much gain, not being  line level, not fitting conveniently in my chassis, having to alter the tranny with ambiguous capacitor values to get something to kinda work... gains being all wrong, etc etc. Has anyone installed and purchased I/o trannys and had success with the clone being similar in function and, gain and levels to what I have used as originals in the studio?

I have built the compressor using a circuit board with circular holes for the I/o xfr
 
Why would you not buy originals?  That's the only option I see given your list.
 
I used  jensen transformers and love the sound.

http://www.jensen-transformers.com/datashts/11p1.pdf
http://www.jensen-transformers.com/datashts/10k611m.pdf

http://www.jensen-transformers.com/as/as091.pdf

I'm going to try Cinemag next.  I contacted David Geren over at Cinemag and this was his response:
-----
There are some interesting issues with the LA2A transformers.  The schematic calls out an input transformer with a pretty big step-up, being the UTC A-10.  The CMQEE-3440A ($51.39) is very close to the A-10 as to the turns ratios available.  However, two of my OEM customers are looking at coming out with similar compressors and they are finding that the turns ratio called out is too high to allow for decent compression.  (I have attached the JBL manual and a UTC catalog for your information.)  They are considering 1:4, as provided by the CMLI-10/600B ($51.10) which can handle a lot more signal level.  The 11P1 is closely related to the CMLI-15/15B ($52.34). 

The output transformer that U.R.E.I. originally used was the A24 with a turns ratio of 5:1.  To be sonically closer to the A24, use either the CM-2810 (which is what Universal Audio uses) for $41.63, or the CM-9589L with 50% high-nickel + 50% steel laminations for $76.27.  They are 4:1 and will work very well.

Happy Holidays!

All the best,
David Geren
-----------------------
 
A well known mastering room in the Los Angeles area has been using LA-2a type compressors with no input transformers for over 40 years.  One cannot "crush" and of course the limitations of the unbalanced input exist.
Am offering the suggestion of a 1:2 ratio input transformer but with bridging Z.
Candidates include:
UTC A-16 (15k:60k),
Hammond models 835, 850Q, and 560Q  (10k:40K)
 
Hi build mine with original utc (and the a10 was nos hehe). This is about 3 years ago. Last year, i build 2 unit with sowter vintage utc replica. The sowter performed well, no circuit modifications (some folks talks about 1k grid resistors to stop oscillations. I didn't put them and there way no oscillation). But comparing my utc build and the sowter one, i must say i prefer the utc one. Sowter transformer are really nice and of high quality, but the original ones sound smoother. Anyway, both are close enough to speak about accurate reproduction. Check out ebay, by the way. I saw last week a pair of a24 for about 350$
 
Hello Everyone,

Thanks all, for all your words of wisdom. It has been very helpful. I chose to go with Sowter - the 1009 and 1010 for the ptp build. Now it have to hook them up.  Has anyone purchased the 1009f and the 1010a and can offer a simple wiring guide. I have the whole chassis wired up and just have the I/o sowters to wire up.

What I can read thus far is - correct me if I have this incorrect.

Primary Side

Red - Pin 3 XLR
White - GND
Blue - PIn 2
Black -GND

Secondary


Green - Between R5 & R6
Grey - GND

the 1010

Primary

Red - C5
Yellow - GND
Black - GND

Secondary

Grey - Pin 3 XLR
White - R23 Pin 2 XLR



Can anyone provide any feedback if I have this figured out correctly?

Thanks!!!!
 
A well known mastering room in the Los Angeles area has been using LA-2a type compressors with no input transformers for over 40 years. One cannot "crush" and of course the limitations of the unbalanced input exist.
Am offering the suggestion of a 1:2 ratio input transformer but with bridging Z.
Candidates include:
UTC A-16 (15k:60k),
Hammond models 835, 850Q, and 560Q (10k:40K)
 
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