[sdiy] adding compressor to mix bus

[Erik Dower]

Hi Justin!
On Dec 23, 2011, at 2:00 AM, Justin Owen wrote:

> This sounds interesting - but I can't quite tell what you're trying to do...
> 
> What and how many sources are you trying to mix between?

It's a 6 stereo channel mixer, each channel with a 10k resistor before that first opamp (U1B) that's used for summing all of the left channels.  For simplicity sake I think about it all as a mono mixer channel since the circuitry for L/R is identical.

> What of those do you need to hear in your cue mix?

The mixer has a built-in cue system that allows me to monitor any of the 6 channels along with the main mix.  The main mix feed to the cue comes from the insert point I designated with the red slash.

> Can't tell from that schematic whether you've cut into one mono side of a mixer (i.e. source/input 1/L) or you've cut into one side of a stereo pair? Can't really tell what you're trying to do either way.

Yes, for simplicity I'm showing just the left side, however the compressor is stereo so it will be on both L and R.

> If it's a DJ mixer - are you sure it's able to output a signal big enough and of the right impedance to let the Cwejman do its thing?

It seems to be fine...Measuring a pulse wave out of a module I get 2.5v AC on my DMM, with the preamp at unity it measures 1.3v AC at the "send" point.  That being said the preamp easily lets me get 4v AC (3v AC before clipping) at the send point.

> Is the signal out of the Cwejman trying to slot back into an input designed for some other impedance?

The signal from the Cwejman is coming back right at the red slash point and it's output impedance is 1k (input is 20k).  I tried to account for this by adding a buffer with no output impedence in between the cwejman and the "return" to the mixer.

> When you say 'too much noise' - is that at the output of the Cwejman with the mixer plugged in or the output of the mixer with the Cwejman plugged back in to where you've cut your trace?

I hear it in the headphones from the monitoring section of the mixer, while monitoring the output of the Cwejman coming in where I've cut the trace.

> Sorry - that's all questions and no answers, can't see what you're trying to do in order to try and suggest something...
> 
> J

No worries!  Let me know if any other info would be useful!

Thanks!

epk

> -----Original Message-----
> From: Erik Dower [electronicpresskit at gmail.com]
> Received: 23.12.2011 06:40:59
> To: Synth DIY
> Subject: [sdiy] adding compressor to mix bus
> 
> So I'm inserting a Cwejman VC-FCS stereo compressor into the stereo bus of my mixer, but I haven't been able to quite get it right. For context sake, this is a DJ mixer that I've integrated into my suitcase style DIY performance modular synthesizer.  Unfortunately I can't just run the output of the mixer through the compressor as I need to be able to use the headphone cue system AND hear the compressed signal!
> 
> Right now it works but there is too much noise when turned up loud.  By noise I mean oscillator bleed, ground noise etc., and it's there even with nothing connected to the input.
> 
> Here's the schematic for the mixer summing stage (U1B):
> 
> http://www.dspaudio.com/mixsum.jpg
> 
> I've cut the trace after pin 7 and used that point to send to and return from the compressor.
> 
> The return from the compressor is also going through a buffer with no output impedance in order to match the expectations of the mixer.
> 
> If I remove the compressor from the insert point and just run it into the regular inputs of the mixer it's nice and clean, but once inserted again I get the noise.
> 
> Any suggestions are greatly appreciated!
> 
> Thanks!
> 
> epk
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