32 channel gain for use with SSL sigma

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JessJackson

Well-known member
Joined
Dec 17, 2009
Messages
439
Location
Los Angeles
Hi guys,

I recently acquired an SSL SIGMA summing box. Tried with +20db converters, too clean no mojo, then tried with two Apollo x16 (+24db) and so much more mojo but obviously converters clip when SSL clip. whole SSL magic happens when needles are pinned.

Want to build a 1ru box on db25 interface for 32 channels of line amplification +3db or +6db  to hit before the SIGMA

looked into doing a custom board based on the SSL k line portion of an 82e01 but then thought why not just do 32 channels of THATCORP  (maybe ssl line amp has more mojo or maybe more problems)

https://www.diyrecordingequipment.com/products/balanced-input (for instance)

Any one know of anyone who's already done this or any shortcuts I might be inclined to take along the way?

Thanks for help :-]

J
 
Some converters have a trim function,  so it's possible your may be able to get more level from the converters themselves.  What converter is it?

One option is something like an INA137 can be wired for 6dB gain.  It would be unbalanced out though.

Some converters are impedance balanced out,  you could feed the signal side into a THAT1646, DRV134, etc.  This would give you differential drive and 6dB gain.

You could also just use a fully differential opamp for gain if it needs differential i/o.
 
Thanks John I'm going to look into it some more.

I thought about doing two THATCORP chips, one line input driver at 0dbu into one line output driver at +6dbu.

looking into differential op amp solutions now. cheers.
 
JessJackson said:
I thought about doing two THATCORP chips, one line input driver at 0dbu into one line output driver at +6dbu.

That would do the job as well,  but maybe a pricey solution for 32 channels..

I would also suggest looking into the front end of an instrumentation amp, see attached. Get rid of the 3rd opamp and R3 and R4.

R1 and R2 can be 50 ohm buildout resistors.  You can easily adjust differential gain,  and only need 1 quality dual opamp per channel. You would also need resistors to ground at the inputs and perhaps AC coupling caps.
 

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Thanks John, really appreciate your input. I'm revisiting this now.

I like the idea of going with the TI INA2137, In this example they show how to get +6db

Screen Shot 2021-05-20 at 2.13.28 PM.png

Would you recommend utilizing the decoupling capacitors or just use the device on its own. I was looking into powering it with a JLM Power Plant 2 :: JLM Audio Shop

Could it be possible that given the specs of the sigma, 30dbu might be out of spec for this device?
Screen Shot 2021-05-20 at 2.27.53 PM.pngScreen Shot 2021-05-20 at 2.30.01 PM.png

any further suggestions around building 16 stereo channels (32 mono) would be greatly appreciated.

Ultimately I would like to build it into a 1u rack unit with 4 dsubs to interface to the sigma with ease.
Jess
 
If the SSL has input amps, and you can access the circuit, you could just add a resistor in the feedback loop of each channel and boost the internal gain.
 
I've made something exactly for the purpose of interfacing between the converters and the Sigma. It's meant more for "colour" rather than gain - mine are configured for unity gain, but it can easily do gain boost via changing one resistor. Transformer in -> 2520 type DOA -> transformer out. It's essentially API 312 circuit minus all the extra stuff you would use for a mic preamp. Transformers are OEP A262A6Es (1:1). Relay bypass so it passes signal through when unpowered. No controls other than on and off. Two channels on one 500 series card - for the 2nd channel it uses the extra pins that Fredenstein & Midas racks connect to through Aux In and Aux Out XLRs.
 

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