312 Hybrid preamp | 312 to M1 and anything in between

312 Hybrid PCB by Bob Schwenkler
Head Engineer of K Records’ Dub Narcotic Studio

www.schwenkleraudio.com/hybrid312



Features and benefits of the revision 3.0 board:

• Unparallelled DIY support eliminates ALL design work, making this project accessible even to beginners. Includes step by step instructions, FULL parts lists, and complete files to have your chassis drilled and engraved (step by step instructions and front panel drill files will be available Jan/Feb
  '14). Nearly 1/2 the price of similar preamp kits!
• Can be built up as a vintage 312 type circuit, a more accurate sounding M1 type circuit, or as any of many other possible variations.
• 5 input transformer options for different flavors: Jensen JT16A, Lundahl LL1538, Cinemag CMMI-2C (threaded bushing model), OEP A262A3, and standard 312 type (eg. Cinemag 75101APC, CMMI-8PCA, Jensen JT-110k-HPC, API 2622).
• Provides repeatable and accurate gain setting with a high quality 12 position Grayhill switch.
• Also includes a fine gain adjust to precisely tailor your levels.
• 2 output options: DC servo or standard 312 type output transformer (eg. Cinemag CMOQ2L, API 2503).
• Accepts 2520, 990C, or any other discrete opamp sharing this footprint.
• Relay activated -20dB pad and polarity invert to keep signal paths short.
• Opamp input DC bias option.
• Board mounted XLR jacks and switches allow super easy mounting directly into 8″ deep chassis (similar to 7th Circle preamps).
• Fit 4 into a 1U chassis or 8+ into a 2U chassis!
• Designed for use with the Par-Metal 10 series chassis, but ought to work with any chassis that has an 8″ internal depth.
  

Order your boards at www.schwenkleraudio.com/hybrid312

Photos are of Rev. 2.3 boards. Rev. 3.1 boards have the fine gain adjust trimmer on them.

Hi bob.
Where can I download the zip file with all informations? (parts, components placements, etc...)
thanx

Hey, have you purchased the boards already? I'm going to be preparing the BOMs, front panel drill template, and schematics within the next few days, they'll be available for download on my site.

Front panel files and assembly instructions will be coming early next year.

Yes bob... and impatient to receive them!

Hi,
How much for 2 PCBs?
Where to place an order?

Hey All, didn't get thread notifications for some reason! Schematics and BOMs are now posted on my site and a bug with the Buy button is fixed. Order away!!

Anybody know of any other options other than the Cinemag CMOQ2L output for this project?

burglar said:

Anybody know of any other options other than the Cinemag CMOQ2L output for this project?

Click to expand...

Have you emailed or called Cinemag? Or even just browsed their spec sheets... I bet they've got a number of other options.

Hi!

3 questions:

1. How much does it cost approximately to buy the parts for the 312? The M-1?

2. Is there an assembly doc in the making that could help putting togheter a fitting psu?

3. How much can we possibly cram in a 2U rack without getting noise/heat issue? 10 if the power light is removed on a separate 1u case containing the psu?

I would like to put 20 channels in a 4U rack... or maybe only 18

Thank!

Hey there. There are BOMs on my site, you can check those out for parts costs where you're at to be more accurate. A full on M1 or 312 build is ~$200/ch. They can be done much cheaper without much trouble though.

No assembly docs yet, and when it's out it will be for a 1U 4 channel build. Building an external PSU is a pretty easy task, that's what I did with my 8 channel unit.

10 could be fit into a 2U probably, but not with the Jensen JT-16A, it's too tall.

I'd say go for it, that'd be a sweet setup!

Ok good!

First of all, this is my first attempt in DIY mic preamp so if I ask too much newbe questions in this thread just tell me and I can send PM instead.

From the pics of a 4 channels build on your site, I see that 5 horizontal channels would fit. So I'll design the front panel to accept 20 horizontal modules in a 4U.

I'd like to make a hybrid version (since it's possible), with a M-1 front end and a 312 back end.

I'd like to put the Lundahl 1538 at the input, the Hardy 990C+ and the Ed Anderson 2503 output transformer.

A quick question about the Lundahl 1538 vs 1538XL:

1. Both SPL and Focusrite use the XL version in their design, is there a benefit in using the XL instead?

ok next

I'd like the stepped input gain to drive the Lundahl/990 and the variable trim pot to feed the 2503.

So:

2.  Would it work? Is the TRIM pot before the output transformer? Sorry if the answer is obvious...

And:

3.  Does the trim pot act as an attenuation only or does it act like a "fader" to push the out level +6db or so?

Last:

4. To implement those parts (Lundahl 1538 (XL?), Hardy 990C+, EA 2503), could I built the 312 and simply swap the input xformer and opamp or the circuit parts must adapted to accept those two "non-312" parts?

Thank for your help!

On a side note, the DIY JE-990 provided by Hairball audio look like a very nice alternative to the Hardy's own 990C+ for half the price.

Any experience with this one?

This price difference will truly matter when multiplied 20 times...

Tell you what. I've got answers typed out, but why don't you start a thread in The Lab at http://groupdiy.com/index.php?board=2.0 and PM me when you have done so. That way others can benefit from the learning, and anyone who's got additional feedback can chip in too.

bobschwenkler said:

Hey, have you purchased the boards already? I'm going to be preparing the BOMs, front panel drill template, and schematics within the next few days, they'll be available for download on my site.

Front panel files and assembly instructions will be coming early next year.

Click to expand...

Anything new about above ? It appear that your ZIP contain files for old boards ...

lampas said:

bobschwenkler said:

Hey, have you purchased the boards already? I'm going to be preparing the BOMs, front panel drill template, and schematics within the next few days, they'll be available for download on my site.

Front panel files and assembly instructions will be coming early next year.

Click to expand...

Anything new about above ? It appear that your ZIP contain files for old boards ...

Click to expand...

Hm. I haven't added new files but what's there ought to be current. I'll take a look soon and report back.

These look great, Bob, and I will likely be ordering some boards soon once I figure a couple of things out.

To that end, I'm curious why you list the optional output as "DC servo"?  Looking at the schematic it's impedance balanced on the outputs unless the transformer is there.  Am I getting that right?  I thought the servo was to remove any DC that gets past the 990.

Thanks for the help,
Jim

Hey Jim,

I think I'm not totally following you. You're correct on the last bit though, the servo nulls any DC present at the output of the amp stage.

Bob,
When looking at the schematic it seems like there are two options for balanced output.  One is to use a transformer, the other is to use the "impedance balance" that's built into the circuit -- or, at least at think that is what R25 is doing there.  Is that correct?  I know that the M1 uses impedance balancing on its output if there is no output transformer.

Also, will the boards fit horizontally into a 1U Par Metal chassis if using a JT-16-A on the input?  The JT-16-A is 1.6" -- seems like a pretty tight fit, but I'd love to be able to stuff these boards into a 1U.

Lastly, I'd like to plan-out the PSU for these boards before I start work.  I've looked at the power schematic, and can't tell what is powering the relays?  At J1 I see +-24 volts coming in for the opamps, and +48 coming in for phantom, but where is the power for relays coming in?  Do you have an outboard PSU that you could recommend, or any tips for the power supply?

Sorry for the newb questions;  I've built some kits before (e.g., Hamptone), but this is more DIY than those -- I look forward to learning more!

Thanks,
Jim

Jim,

Yeah, that's correct about the output servo and impedance balancing.

I'm not sure about the 16A in a 1U. Could work but it'd be really tight. I haven't done it myself. Is the 16B shorter? I don't know if it would fit on my PCB or not... The Lundahl LL1538 is also a really fantastic option and it's much shorter than the 16A.

I spec 12V relays and use +15V to power them, still well within their tolerance. If you install the input bias circuitry the +15 is also used for that, so if it's omitted you can set the relay V+ at whatever you like.

Check out the JLM Powerstation for PSU.