312 style quad preamp

I've had these done for a while now, but realized that I never posted my end result.




I was able to mount all four channels inside a quad Purusha case by building an external power supply.



The unlabeled switch, changes the meter to read one channel at a time. I didn't have room for separate meters.



The meter holes were drilled hella crooked, but I calibrated it to my Pro Tools meters so that I could adjust it remotely without straining my eyes...



The first two channels have Profile output transformers. The last two channels use the CJ home spun output transformers. All four channels have Sowter input trannys.



I used one pico meter board to handle the metering for all four channels.







These relay boards handle all of the polarity, pad and phantom switching at the XLR connectors.







Power supply is +/-18V w/ 48V phantom made with Keith's power supply board. I picked up a NOS 3A power transformer at a local electronic store for $5, so the power supply is really heavy, but I have enough juice to easily power another Quad SSL9K box. That's why I mounted an extra 4pin connector.

Nice!

I'm about to start stuffing a similar project.

Are those the Handcrafted Tone 312 PCB's?

If so, how did you cut out space for the output Transformers?

nice job pat! :thumb:
man, that looks like a VERY tight squeeze with the relay boards and meter board in there. don't know how you did it.
is the "top secret" DOA one of fabio's 25.20 black bastards or one of your own 2520 builds which you potted? or is it really "top secret"?

@rattleyour,
i cut the output xfrmr mounting square in my handcrafted tone boards by just measuring around the perimeter of the bobbin, drew it onto the board and cut it out with a dremel. pretty easy. the square that you draw should butt up as close as possible to the trace.

i'll have to post some pics of my reliquia 312 (which i promised fabio months ago) and ed anderson 2622 input 312 (built on rob's/handcrafted tone 312 boards. still need to finish my last dual channel 312 with Jensen inputs though.

-grant

Thanks Grant, I think I might have access to a dremel, I'll check that out.

My concern is that if I mount the trasnfromers on the stand offs (as in that original blackmarket thread where we got our cards) that I won't have enough clearance to fit in a 1 rack unit case.

nice work, I dig the TOP SECRET panel.
:thumb:

Nice work, but do you think it's a good idea to use male XLRs at the output of your PSU ?

JDB.

I've made psus where the low voltage A.C. runs via a 4 pin so the
cables can be the same [ male end on preamp to avoid accidently
touching anything ] and save on too many wires for different supplies .

The major hum from the xfmr is isolated , and yeah with the weight
i wouldn't put an xfmr on standoffs , better to bind it to the case , i think

Nice work Drpat , gotta appreciatte someone who can get four pres
in a 1ru case ,

When i did that , thinking i'd save space i ended up with a way heavy rack because of the xfmrs , so i didn't save much , but Bravo .

Thanks y'all. To answer some of your questions...

Yes, these are Handcrafted Tone's PCB's, and I bought a router attachment for my Dremel tool to cut out the hole for the Profile. Works like a champ...

The "Top Secret" DOA is one of Fabio's black boxes.

It's definitely a good idea to use male XLRs at the output of an API power supply. I did it to be compatible with the Brent Averill stuff around here, since he uses the same format XLR for his API racks. He also uses male XLRs on the Neve racks that we have around here too. The P series that I used are rated at 50VAC/10A per pole, so there's plenty of headroom for my purposes.

Yeah Greg, this thing's pretty heavy, but it does save on rackspace, which is expensive real estate around here.

The 4 PIN thing is pretty universal amongst my different pres, so I have three, dual feed +/-18V PSUs, able to power up to six quad mic pres. I use one of the +/-18V feeds from this thing to power my quad 9K. I then split it off and drop it down to +/-15V via a diode network that's located inside of the 9K chassis. So most of my power supplies can be exchanged freely amongst most of my preamps. It's a no-brainer for my assistants.

[quote author="drpat"]It's definitely a good idea to use male XLRs at the output of an API power supply.[/quote]
Your call, but that makes it awfully easy to accidentally short two pins with a piece of metal.

JDB.
[and that's why most countries' electrical/safety regulations either forbid it or strongly suggest against it]

OK, I see your point now.

Like I said, I did it for compatibility reasons, and it would have been impossible to modify the BAE supplies with female XLRs, since they don't exist in that sized panel cutout.

I've been using PSUs with male XLRs for years now, and haven't had a problem yet, but hopefully, if a piece of metal does make it's way into the XLR cavity, the fuse will do it's job...

hi rattleyour,
i think i used 3/16" standoffs on mine. there was VERY little clearance to the top lid on the chassis, but it fit over the transformer without trying to "cram", "nudge", "squish" everything in if you know what i mean.
not sure if you're using the cinemag outputs as i am, but in any case the bobbins are plastic and cloth tape over the windings so no contact. i was more concerned with my PSU caps touching the top of the case!
regards,
grant

Thanks Grant, we were kind of eyeballing it last night and it'll be tight but should work out OK-- I'm using Cinemag CMOQ-2S output transformers.

LIAM

Nice work Pat! Great job on the graphics.
I like your single meter solution too. One-of-a-kind build. :thumb:

Patrick, what connection scheme do you (or Brent Averill) have with the 4-pin XLR power connector? Is it 0V/V+/V-/+48V from pins 1 to 4?

1. -18V
2. com
3. +18V
4. +48V

Male pins on the external power supply chassis connector, and female pins on the pigtail coming from the preamp.

I built a diode board to bring the 18V legs down to about 15V, and mounted it inside the preamp chassis so that I could power it with only 4 pins coming from the power supply. I also power all of my API type pres with +/-18VDC, so most of my external power supplies (except for my Neves) are standardized.

Beware that all 4pin XLRs are not created equal. Some have an offset pin1, so stick with the same manufacturer for both male and female ends, and verify that they are compatible with each other...

great Infos, thank you :thumb:

I just had a Chandler Neve Pre on my bench yesterday, which inspired me to build an external PSU, unfortunately I forgot to check the connections. They might be different than the above though...

cheers, Marten