Combining Green Pre and What Compressor

[Salocin]

Hi,
First post. How daunting!

I'm currently building my first piece of 'recording gear' after dabbling in effects pedals, amps and other miscellaneous electronics for the past couple of years. The project- a Green Pre and What Compressor in a single unit, as a front end for my four track. The pre will be on all the time but I want to be able to switch the compressor on and off.

The difficulty I'm having stems from the fact that the pre has a balanced input/output but the compressor does not.

The schematics I'm working from are:
http://1176neve.tripod.com/sitebuildercontent/sitebuilderpictures/gren_pre_cct4.gif
http://dt.prohosting.com/hacks/what1.gif (Although this is a stereo/dual unit I only intend to use half of it)

What I've concluded from the Green Pre schematic is that the negative portion of the balanced output is the output of the amplification stage/s (IC2a output) run through a unity gain inverting stage (IC2b). Is this correct?

If so, I'm thinking that I can insert the compressor at the output of IC2a and have the IC2b inverting section after the compressor. Possible?

Now, if I'm on the right track so far, I want to be able to switch the compressor on and off. To do this I'm planning to use a DPDT switch, with the poles on one side jumpered together, and the other side being the input/output of the compressor. I'm thinking that I could (perhaps would be better to) insert the compressor at the output of the pre's C11 capacitor and remove the compressors c3 capacitor (at its input). Would this be a good way to do this?

Thanks to anyone who can offer some advice.

 

[SSLtech]

I have never really understood the urge to put units 'all in one box'... specially if it means that later on you WON'T be able to use the compressor (in this case) for something else, if you're already using the preamp.

To me, it's rather like taking fundamental 'building-block' pieces and permanently 'bonding them together' into a form which later on LIMITS what you can do with it... but whatever.

My suggestion would be to add a simple differential input and output to the compressor. a one-op-amp differential input is easy, as is a single op-amp output inverter.

Or you can just (in the case of the green pre) use one leg of the output, and ignore the 'cold' output leg... It'll work just fine. The differential output is only there to provide a means to subtract any noise which a signal may pick up under adverse conditions over long cable runs... if the two pieces of gear are in the same case, just ignore it.

You can also do the sort of thing which you suggest of course, but what happens later on if you're so smitten by the DIY bug that you want to build an EQ? -Will you want to use it BEFORE the compressor, or AFTER? How about a De-Esser? -Basically, if you make the whole thing modular, you can arrange the order to whatever works for the particular sound which you're recording, because the order DOES make a significant sonic difference, and there is no 'right' or 'wrong' way. -The only thing that's usually for sure is that the preamp typically comes at the front!

SO by all means do that, but I'd suggest that you consider a flexible build, in order to avoid 'forcing' yourself to work in a particular way. -One thing about the SSL consoles for example is that they had significant signal-routing flexibility, you could have the filters with the EQ, the filters at the head of the channel,  the insert BEFORE the EQ but after the dynamics, the insert before the EQ (WITH the filters)  before the dynamics, the insert after the EQ after the dynamics... or any one of several OTHER combinations. -You could also take the filters or EQ or Dynamics out and use them on the module's OTHER signal path if you preferred... I quote that only as an example of how important order/sequence is.

Keith

 

[kazper]

Keith has it spot on as always!

If your set on having the units in one box, on top of Keith's recommendations you can use a couple relays to internally connect the MIC Output XLR to the Compressor Input XLR.
You would have the 4 XLR's on the back but not have to use the middle O/I's unless your varying from the normal.

As a easy optional way to do this is build them individually in one enclosure and use a small cable and plug the cable in to make them connected. Still leaves the issue of balancing out the What... but you should get the idea.

kaz

 

[PRR]

You got it right.

Except... do you NEED the balanced out?

Keith comes from a world of BIG studios. Then you do need balanced.

I often have all my stuff in a knapsack, in a town with weak radio stations. Short lines in benign rooms can be unbalanced. (70 foot unbalanced mike lines nearly work, but for many reasons I let that be balanced.)

I wish someone had built what you are building. Stereo mike-amp, safety-limiter, four RCA-pairs for recorders. Duck-taping a mikeamp to a limiter with a cable-snarl on the back is for the birds. You have to KNOW that all you will do is "capture without harm", as Keith says the creative stuff suggests flexibility.

 

[tv]

>>>> What I've concluded from the Green Pre schematic is that the negative portion of the balanced output is the output of the amplification stage/s (IC2a output) run through a unity gain inverting stage (IC2b). Is this correct?

NO. The output of IC2a is "positive".

>>>> If so, I'm thinking that I can insert the compressor at the output of IC2a and have the IC2b inverting section after the compressor. Possible?

YES and NO.
If you NEED a balanced out, you can add the "IC2b" as a "negative" out,similar to the way it's done in the "green pre". //naivelly speaking

 

[Salocin]

Thanks heaps for the input. It's giving me plenty to think about.

The balanced output is definitely a nice-to-have and not needed for this units primary purpose so I may just leave it out for the time being.
The comments on building it in a more flexible manner also got me thinking. The plan now is rather than just using a jumper on the bypass switch, run the signal through a couple of jacks which I can plug something else into (E.g. an EQ) if I want it before the compressor. Not sure yet if I'll use switched jacks or just connect then together with a short cable when not in use. This would also provide me with the ability to use the compressor by itself, if the need ever arose.

>>>> What I've concluded from the Green Pre schematic is that the negative portion of the balanced output is the output of the amplification stage/s (IC2a output) run through a unity gain inverting stage (IC2b). Is this correct?

NO. The output of IC2a is "positive".

You've confused me a little here. Is it not the IC2b stage that inverts the IC2a's positive output to provide the negative part of the balanced output?

Thanks again all. Your comments are very much appreciated.