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anonymous 2 February 2003

For anyone interested in weird and wonderful circuits, I have the schematics for the Joemeek VCQ1 - micpre / processor.

It features the "meequalizer" and a photo-electric compressor.

Let me know you're interested and I'll send it over.

Mats

mr.karlsson@bredband.net

Comments

Kev Sun, 02/02/2003 - 10:43

Is that the VC1Q or the VC1Qcs (current sense)

I have the second and would like a schematic.

.... but I am most interested in the schematic for the VC1Q with the TX input. I had a few emails with Ted Fletcher and he said they dropped the TX in favour of the current sense input as it was lower noise ... and I'm sure there was another reason ???

Still it would be interesting to see the before and after schematics.

anonymous Tue, 02/04/2003 - 17:39

Hey Mats!

What's the op-amp in the mic pre? Haven't seen that kind of topology before; is that a linear gain pot and if so, does it have a good 'feel'?

About the opto-compressor element...do you know what make it is? Is it two separate devices (like in my cheap little one; two orange LED's shining on an LDR), or is it a manufactured device? A company called Silonex make these types of devices manufactured for audio use, but I have yet to come across a manufacturer that uses them...thoughts, anyone? And whilst on that subject...what opto-devices do focusrite use in their platinum range?

Silonex Inc.

Bjorn

anonymous Tue, 02/04/2003 - 18:49

Bjorn,
On the subject of optos, there are some fairly extensive prior threads. A lot of manufacturers use the Vactrol brand. You can get then from Newark in the US and there is a European partner....guys?

Overview:
http://optoelectronics.perkinelmer.com/producttemplates/Category.asp?levelId=14370&s=2&ss=1

PDF with lots of good info/applications:
http://pki1.mondosearch.com/cgi-bin/MsmGo.exe?grab_id=68845178&EXTRA_ARG=&CFGNAME=MssFind%2Ecfg&host_id=5&page_id=387&query=vactrol

Gerry

anonymous Thu, 02/06/2003 - 14:49

Originally posted by Bjorn Zetterlund:
What's the op-amp in the mic pre? Haven't seen that kind of topology before; is that a linear gain pot and if so, does it have a good 'feel'?

The mic pre is made up from two opamps, the first as a non-inverting, the other as an inverting amp. For a linear dB law, a non-inverting opamp needs a reverse log pot whereas an inverting one needs a regular log pot.

Using a linear gain pot in this configuration balances things very nicely. The dB gain vs pot position curve is slightly "S" shaped but you really can't tell. It feels very right to me.
The cool thing is that the 1:st stage does most of the work during the first half of the pot range, and in the second half, the 2:nd opamp kicks in.

I have not used the JoeMeek myself but I have built several preamps using this configuration. I first saw it some 22 years ago in an article in "Studio Sound" by Steve Dove. If you read Glen Ballou's book on recording technology you'll find the article in revised form there.

No idea about the compressor bit - sorry!

Mats

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