From: Joachim Verghese <jocke@... Tue Mar 12 1996
On Sun, 10 Mar 1996, I. Onsager wrote:
Weyer-Smith labs offers a modification for the ARP 4072 filter to expand frequency response. Is this Weyer-Smith proprietary information, or is it public available on the net? Anybody out there that has tried it?
I've done it to some 4075s (very similar to the 4072).
The four 4K7 (Yellow, Violet, Red) current limiting resistors which are connected between the exponential converter and the four discrete gain stages should be lowered to 2K2 (Red, Red, Red). If memory serves, these are the only 4K7 resistors in the module, so locating them shouldn't be too difficult.
Alternatively, you could add another set of 4K7s in parallel with the existing ones. These could be mounted on the solder side of the board. Beware of possible shorting to other pins, though.
-joachim
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BY: Matt Haines haines@... Thu, 7 Dec 1995
Does anyone have the pinout on the 4022 filter in the ARP Whiteface Odyssey? I'm taking the filter out to use in a Homebuilt Synth.
Thanks
I think it's the same for all the Odysseys, because the service docs talk about replacing the old ones with new ones <Gasp!> It says it's for models 2800, 2810, 2811, 2812, 2813:
Pin Connection | Pin Connection__________
1 Signal Input | 10 Out (to highpass filter)
2 RESONANCE slider (through a 22k pot) | 11 N/C
3 To wiper of VCF Balance trimpot (Pin 2) | 12 N/C
4 CV Input | 13 N/C
6 + Supply | 14 Pin 1 of VCF Balance trimpot
7 Ground | 15 N/C
8 - Supply | 16? Pin 3 of VCF Balance trimpot
I can't quite read the last pin #.
Matt Haines haines@...
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From: Joachim Verghese jocke@... Fri, 8 Dec 1995
This is actually the pinout for the later 4035 and 4075 modules. The 4023 was slightly different. The wiper of the balance trimmer is connected to -15V, and the CV input is different, if memory serves. I haven't got the schems at hand right now, but I'll look it up.
-joachim
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And, On a Different Note... ;->, Another ARP 4075 VCF Mod Article...
From: Mark Smart - http://www.marksmart.net/gearhack/arpomni/FilterMod/FilterMod.html
The 4075 is a filter module that ARP used in many different synthesizers, including the Omni 1 and Omni 2. Sometime during the design phase, a resistor value was miscalculated; as a result, the 4075's cutoff frequency will not sweep up as high as ARP really wanted it to. This may be partially responsible for the commonly held view that ARP filters do not sound as "snappy" as Moog filters.
Timothy Smith designed a modification which will allow the cutoff frequency to sweep up as high as ARP meant it to. The mod simply involves changing four resistors on the board from 4.7 to 2.2K (4K7 to 2K2). I have performed this mod on an Omni 1, and Omni 2, and a Quadra, and in all cases there was a very noticeable improvement in sound. I highly recommend it!
4075 Board Schematic The original ARP synth documents don't contain schematics of the filter modules. Fortunately, Joachim Verghese has created them; you can download the 4075 schematic here on his ARP Tech page.
4075 Board Layout Mike Novak created this very cool 4075 board layout diagram using Joachim's schematic and the filter boards from his ARP Quadra. This makes it much easier to find the four resistors in question: http://www.marksmart.net/gearhack/arpomni/FilterMod/4075Layout.JPG, or at: http://www.marksmart.net/gearhack/arpomni/FilterMod/4075Layout.JPGMark Smart Home
Last updated on 11-2-2003. Questions or comments? Email me.
I hope that some of the above mish-mash helps you out. (I believe that many of the links are still good).
HTH,
Dave Garf, Omni-head (Also uses 4075 filter)
----- Original Message ----
From: eggwheatis <eggwheatis@...>
To: vintagesynthrepair@yahoogroups.com
Sent: Tuesday, March 25, 2008 6:57:52 PM
Subject: [vintagesynthrepair] Arp Quadra info
I found this on the net about the 4075 filter "This was ARP's own
24dB/octave filter, and appeared in many of their later instruments.
It offered low noise and distortion, but a design fault (easily
rectified) limited its maximum cutoff frequency to just 12kHz or
thereabouts"
So how do you fix it? I cant find any info.
Also, my arp quadra makes a kind of low frequency thump on the attack
of the string sound..to me it seems wrong but I guess it could be
another quirk?
Any ideas about this?
Thanks, Phil.
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