From chordman@ix.netcom.com Tue May 7 12:17:16 1996 Received: from informatics.sunysb.edu by taz.hyperreal.com (8.6.12/8.6.5) with SMTP id MAA28808; Tue, 7 May 1996 12:17:15 -0700 Received: from pharm1.pharm.sunysb.edu by salieri.informatics.sunysb.edu with SMTP (931110.SGI/25-eef) id AA14359; Tue, 7 May 96 15:17:10 -0400 Resent-Message-Id: <9605071917.AA14359@informatics.sunysb.edu> Received: from PHARM1/SpoolDir by pharm1.pharm.sunysb.edu (Mercury 1.21); 7 May 96 15:17:10 +0500 Received: from SpoolDir by PHARM1 (Mercury 1.21); 7 May 96 15:17:04 +0500 Resent-From: "Roy Roberts" Resent-To: roy@hyperreal.com Resent-Date: Tue, 7 May 1996 15:16:59 EST Received: from SpoolDir by PHARM1 (Mercury 1.21); 5 May 96 18:28:20 +0500 Received: from taz.hyperreal.com by pharm1.pharm.sunysb.edu (Mercury 1.21); 5 May 96 18:28:16 +0500 Received: by taz.hyperreal.com (8.6.12/8.6.5) id PAA28938; Sun, 5 May 1996 15:25:34 -0700 Received: by taz.hyperreal.com (8.6.12/8.6.5) id PAA28925; Sun, 5 May 1996 15:25:32 -0700 Received: from dfw-ix9.ix.netcom.com by taz.hyperreal.com (8.6.12/8.6.5) with ESMTP id PAA28919; Sun, 5 May 1996 15:25:30 -0700 Received: from (chordman@whx-ca6-09.ix.netcom.com [204.31.115.201]) by dfw-ix9.ix.netcom.com (8.6.13/8.6.12) with SMTP id PAA00308; Sun, 5 May 1996 15:25:31 -0700 Date: Sun, 5 May 1996 15:25:31 -0700 Message-Id: <199605052225.PAA00308@dfw-ix9.ix.netcom.com> From: chordman@ix.netcom.com (Scott Gravenhorst, Synthaholic) Subject: FatMan VCF LFO and better VCO isolation To: synth-diy@horus.sara.nl Cc: analogue@hyperreal.com Sender: analogue-owner@hyperreal.com Precedence: bulk Status: OR Yesterday, I added the VCF LFO mod (method #1) which is on www.paia.com. It works very well. ALSO Here is a description of how I was able to get my FatMan's VCOs extremely close in frequency without causing soft-sync: Several months ago I had done the 'LED' modification. For those who don't know of it, it is extremely simple and by itself greatly reduces soft sync. It involves removing R38 and R39 and replacing them with (identical) LEDs. The arrow must point away from pin 1 of the 555 to which it is attached. That's it. If done correctly, the LEDs should light solidly whenever power is applied. Recently, I added an external input and I needed to be able to control each VCO level separately in order to be able to balance them against the external input. Originally, my circuit had two level controls that passed their respective signal to the 1/2 mix pot. Since this pot was always centered, it was actually redundant. I then decided to remove it from the circuit and run each VCO signal thru it's own 100K summing resistor to the VCF. A happy consequence of this was (apparently) better isolation of the VCOs, because now, I can get them much closer together without locking. I can now get them within about 1/12 of a hertz before they lock. The following "ASCII-matic" depicts the FatMan modification to further isolate the VCOs and shows where to feed your external input. The external input requires a preamp to provide a fairly large signal (8 volts p-p) I used an op-amp for that. +----------/\/\/\/-----+ | 100K | V 10K (VCO 1 level) | A -----/\/\/\/----+ | R46 | | gnd | | 100K | +---------/\/\/\/-----+-----+---> to IC17 pin 3 | | V 10K (VCO 2 level) | B ------/\/\/\/----+ | R55 | | gnd | | from output 100K | of external >------/\/\/\/-----------+ input preamp Remove R46, R55 and R57. Disconnect R56 (1/2 mix pot). Replace R46 and R55 each with a 10K pot. You can use R56 for one of these is desired. Point A comes from the positive side of C15. There is a convenient connection hole left by the removal of R46. Point B comes from the positive side of C18. There is a convenient connection point left by the removal of R55. Add in the 2 new 100K resistors as shown above. Pin 3 of IC 17 may be accessed using the convenient connection hole left by the removal of R57. -- -- Scott G., Synthaholic There is no 12 step program for synthaholics. Thank your Superior Being.