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 Forum index » DIY Hardware and Software
Serge Universal Audio Processor
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bearblock



Joined: Sep 27, 2006
Posts: 90
Location: uk
Audio files: 1

PostPosted: Mon Jun 10, 2013 2:56 am    Post subject: Serge Universal Audio Processor
Subject description: Pan / Crossfade / Dual VCA
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This sounds like an incredibly useful module, and I was wondering if anyone in this community has ever tried to clone it, or maybe had the same idea independently?

As I understand it, this circuit has three switchable modes:

1) Dual VCA. Two independent VCAs. Each one has audio in & out and CV in jacks, plus two pots for initial gain and modulation depth. Ideally the mod depth is bipolar (i.e. zero at 12 o'clock)

2) VC Pan. The incoming audio signals are summed and sent to the inputs of both amps. The incoming CVs are also summed and sent to both amps, but the CV going to the second amp is inverted.

3) VC Crossfade. The outputs of both amps are summed and sent to both outputs. As before, the incoming CVs are summed and sent to both amps, but the CV going to the second amp is inverted.

I guess this could work after a fashion with pretty much any VCA core, but as I have a couple of THAT 2181's lying around I thought I'd try using them. Not sure if the exponential gain control is what I want here though? Does that mean the gain control is volts / decibels? And is this the same as 'equal power' like on the Serge? #back2school
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clee



Joined: Dec 08, 2009
Posts: 42
Location: Chicago

PostPosted: Wed Jun 12, 2013 11:30 am    Post subject: Reply with quote  Mark this post and the followings unread

I built an equal power panner using an Ap note from THAT. It uses the 2180 chip. Don't know what the differences are between the 2180 and 2181. My build is here,

http://www.clsound.com/VC_Panner.html

The circuit may not be the most efficient way to get equal power pans, but I searched for a while and this was pretty much the only thing I found. Linear VCAs give you OK panning, but there is a little bit of an uneven feel when doing VC movement across the pan.

2 chips give you one channel of pan/xfade. To do a Serge UAP you would need to add a VCA for each input in front of the panner.

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bearblock



Joined: Sep 27, 2006
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PostPosted: Thu Jun 13, 2013 3:44 am    Post subject: Reply with quote  Mark this post and the followings unread

Awesome, thanks! Very Happy I'll have to have a proper look at the documentation when I get back from work.

I would like the module I make to also have the third mode which decouples the VCAs and lets you use them independently. My system has two lowpass gates and a ring mod, but no dedicated VCAs. Hopefully I can find a switch with enough poles!

The 2180 is pre-trimmed, and the 2181 needs an external trimpot on pin 4. More work to implement but better potential THD.

I didn't realize the UAP has an extra VCA in front of each channel - can probably do without this to be honest. Panel space is at a premium. I nearly just bought a Doepfer 134-1, but like I said above I really need the twin VCA mode.
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clee



Joined: Dec 08, 2009
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Location: Chicago

PostPosted: Thu Jun 13, 2013 6:03 am    Post subject: Reply with quote  Mark this post and the followings unread

bearblock wrote:

I didn't realize the UAP has an extra VCA in front of each channel - can probably do without this to be honest. Panel space is at a premium. I nearly just bought a Doepfer 134-1, but like I said above I really need the twin VCA mode.


I've never seen the inside of a UAP. Serge was a very clever designer, so I don't know if he came up with a way to do it with just 2 VCAs. Because the response in non-linear I don't think you can just scale the CV range to get overall gain control.

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bearblock



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PostPosted: Sat Aug 03, 2013 1:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

It looks like the behaviour I'm after would need a 6P3T switch, which don't exist AFAIK, so I'm looking at using 3 x DPDT analog switch ICs. I can't find any in DIP packages though, does anyone know of one?
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clee



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PostPosted: Sat Aug 03, 2013 4:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

Look at the Maxim DG series. Not shure which one will fit what you need. The nice thing is that unlike the CMOS switches they use bipolar power and can take full AC signals.
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bearblock



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PostPosted: Sun Aug 04, 2013 8:23 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Smile DG303 looks perfect.
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