UA 610 Microphone Preamplifier Help thread

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muffy1975

Well-known member
Joined
Mar 21, 2006
Messages
330
Location
UK
HI,

following John Hinson's drawing, I got to let you know this thing sounds really nice. Using a humble OEP mic transformer and a gapped output transformer I'm getting  nicely distorted Doors / Neil Young  sounds. The PSU is a modification of the G9 PSU .

The High shelf EQ is perfect, however the +6 dB bass boost does not work. Is there something wrong with the drawing around the LF switch?  I can get the bass cut to engage , however the bass boost just stays flat.

Many thanks to John Hinson for the drawing, which I gather was derived from an early 610 console.

Michael

 

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  • 610-A UA 610 Preamp Schematic.pdf
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Glad you got it sorted Michael. 
Even though it's correct as drawn, I think the schematic could have been a little clearer, sorry for that.
Enjoy ☺
 
For clarity, it is worth saying you do need a gapped transformer because the primary carries the plate current of the output stage.

Cheers

Ian
 
Hey Michael,
I think B+ was around the 275V range on the original desk but, as you discovered, the operating points don't really change much from 250 to 300V so you're good to go. 
Your description is exactly how I would describe the tone too, the 12AX7 stages supplying the hair that you hear 👍.  The main difference between this and a current UA box will be that your output stage now won't crap out early since it can swing more volts.

Yep, I think gain is a bit north of 60dB with your transformer set, would have been a bit more with a UTC O1 input like the original but 60dB is more than respectable I think

Anyway, it's really great to know someone built this and likes it, thanks 😊
 
John’s annual post.  ;) Glad to see you staying on schedule, man.

You’ve probably answered this at some point, but what off the shelf OPT comes closest to the original, in your opinion (assuming you have one on that)?


thanks,
BT
 
rackmonkey said:
John’s annual post.  ;) Glad to see you staying on schedule, man.

You’ve probably answered this at some point, but what off the shelf OPT comes closest to the original, in your opinion (assuming you have one on that)?

😀

Stateside, taking a peek at Cinemag's site, looks like the  CMO-30/600 would work.  Just let them know there'd be about 7mA single-ended current through the primary. 
Over here in the UK, I would just ask Sowter what they have similar in 30K/600  7:1 ratio. 
Lundahl will also have something but would no doubt be a bit cleaner sounding, which may not be what you want? 

I shoved a Cinemag  CM-9661A  50% nickel in a proto many years ago.  It isn't gapped but I had no problems with it.  Best check with David over @ Cinemag though if you went that route.

Cheers mate 👍
 
Thanks, John.

Looks like the Sowter 9980 designed for the REDD47 would fit the bill if Sowter will gap it appropriately. Only off-the shelf 7:1 I saw on their site.

Cinemag also has the CM-27101, which is close. It doesn't mention it in the datasheet, but they do a gapped version. I have that one but haven't experimented with it.

 
ruffrecords said:
what about a UTC A26?

Forgot about that one, yes 30K to multiple line. 

Lundahl ll1630/5mA looks OK too,  data sheet here: 
https://www.lundahltransformers.com/wp-content/uploads/datasheets/1630.pdf
 

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