Updated : sept. 6th, 2008 diode ladderVCF
En français 
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Description

Well, here is another classical and very special filter based on the EMS diode ladder filter!
This filter has a sound of its own and is the signature of the classical Pink Floyd's song "On the run".

My aim here was to design a quite faithful copy of this mythic filter while using components that minimize the burden of selecting a great number of matched transistors. This is why I used transistor arrays (CA3046, CA3096). However, because the CA3096 has become a rare part and is now overpriced, I have also designed a PCB with discrete trannies to replace it. As for all the other filters of my YUSYNTH project, I have added an input and an output buffer to adapt this filter to the input and output levels of my modular system.
I also added an input for voltage control of the resonance : everything was there in the original module but was not made available, probably because of the limited size of the patch pin matrix of the AKS/VCS3...
Because, my power supply standard is +15/-15V, I have included a power regulator IC (78L12) to convert +15V to +12V and a single transitor voltage regulator -15V to -9V (I wanted to use a 79L09 but changed my mind when I realized that this voltage regulator is quite a rare part and is difficult to source).

A short demo of this filter to exhibit its typical bitter sound, some echo was added after 20 sec.



Schematics


Basically the circuit is built around the original core of the EMS filter, though I had to change some resistor values to compensate for the fact that the active components (NPN,PNP and JFET) are modern counterparts and some of their characteristics are different from the original parts.
The major additions are : a quadruple OPA that is used to buffer the various inputs (audio, CVs) and the output. The values of the resistors of these buffers may be adjusted to audio and CV levels of any modular system. Next, I introduced a voltage control input of the RESONANCE. Eventually, I use a 78L12 and a PNP transistor to convert the power supply levels from +15V/-15V to +12V/-9V .

NOTE : the value of R47 can be changed to adjust the output level as see fit.

Printed circuit boards and component layouts

NOTE : the PCB and a component kit for this module are now made available by Bridechamber

PCB - tracks  CA3096 version



PCB - tracks  discrete version
Component layout CA3096 version




Component layout discrete version




Get the schematic as a PDF file

CA3096 version
Get the PCB as a PDF file

Discrete version
Get the PCB as a PDF file
Do not forget to solder the three straps, one is located beneath U2 !

Building details
reference
 value
 quantity
U1
 TL074 2
U2
 78L12 (TO92 package) or 7812 (TO220 package)
 1
U3
 CA3046 intersil  transistor array (Q1 to Q5)
CA3146 and CA3086 are possible substitutes
 1
U4*
 CA3096 Harris * for the transistor array version (Q7 to Q11)
 1*
Q7*,Q8*,Q11*
 BC547, * only for the discrete version
 3*
Q6,Q9*,Q10*
 BC557, * only for the discrete version 1 or 3*
Q12
 2N3819 JFET
 1
Q13
 BC327
 1
D1,D2,D3,D4,D5,D6,D7,D8,D9,
D10,D11,D12,D13,D14
 1N4148, all matched for their direct voltage drop with a maximum tolerance of 3mV
 14
R1,R2
 10 ohms
 2
R3
 22 ohms 1
R4
 150 ohms 1
R33,R35
 470 ohms 2
R10,R22,R48
 1K
 3
R24
 1.5K
 1
R23,R37
 1.8K
 2
R9
 2.2K
 1
R28*,R29*,R34
 2.7K *matched to 1%
 3
R11,R50
 4.7K
 2
R8,R12
 8.2K
 2
R36,R46,R49
 10K
 3
R26*,R30*,R31
 15K *matched to 1% 3
R42
 22K
 1
R14
 27K
 1
R21,R47**
 39K **may be changed to other values see notes
 2
R19,R43,R44
 47K
 3
R27*,R32*,R38
 68K *matched to 1% 3
R20
 82K
 1
R5,R6,R7,R15,R17,R40
 100K
 6
R13
 150K
 1
R16,R18,R39,R45
 220K
 4
R41
 680K
 1
R25
 1M
 1
C12
 10nF
 1
C13
 47nF
 1
C3,C8*,C9*,C10*,C11* 100nF *matched to 1% (not mandatory)
 5
C4,C5
 1µF/50V 2
C7
 10µF/50V
 1
C15
 16µF/50V (or use a 22µF/50V if you can't find a 16µF) 1
C1,C2
 22µF/50V
 2
C6
 47µF/50V 1
C14
 100µF/25V
 1
A1,A2
 100K 10/15 turn trimmer
 2
P1,P2
 10K lin
 2
P3,P4
 25K lin
 2
P5,P6
 50K log
 2
Jk1,Jk2,Jk3,Jk4,Jk5,Jk6
 female jack socket
 6
Do not forget to solder the two/three straps (CA3096 version/discrete version) , one is located beneath U1 !
Wiring


Front panel
Panel design


 Download the silkscreen mask as a PDF file

Download the silkscreen mask as a JPEG file


Trimming
Adjusting trimmer A1 (V/OCT tracking)
IMPORTANT NOTE : don't expect a real V/octave tracking, one may reach a good V/octave tracking only over a maximum of 2 or 3 octaves (at best) !
  1. Set the RESONANCE pot to maximum in order to obtain auto-oscillation
  2. Connect a CV/GATE controller to the V/OCT input, set it to 0V
  3. Set the FREQUENCY pot such that you obtain a C3
  4. Set the CV/GATE controller to 2V
  5. Adjust A1 such that you obtain a C5

Adjusting trimmer A2 (init frequency)
  1. Set the RESONANCE pot to min value
  2. Set the FREQUENCY pot to min value
  3. Connect a 25Hz SQUARE signal to the INPUT
  4. Adjust the A2 in order to mute the OUTPUT

IMPORTANT NOTE : the "pinch voltage" of JFETs may vary significantly even when they come from the same batch, therefore if you experience some problems with the resonance potentiometer for example if you notice that the resonance range is rather limited and the filter cannot oscillate, change the value of R41 to 220K.


References
Here are interesting links where to find schematics and infos about the EMS VCF :
One schematic very much sought after is the diode ladder VCF. It is possible to find schematics of "EMS-like" VCF using diode ladder, they work pretty well but are not the original stuff. These "ersatz" can be found  at HyperReal Music Machines  or at E-Music DIY archives .
You may also check these : Masa921 japanese site (closest to EMS VCF than the previous) and Tom Gamble's version (VCF 6c) at EFM . You may as well try the PE Minisonic2 design which is close to the original one ( Julian Bunn's Minisonic2 page  or Anders Sponton's C.A.G. )

The DIY builders' gallery
Here are the photographs of the yusynth MiniMoog VCF modules built by other synth geeks around the world.
Thank you  guys for sending me these nice photos.

Name :
Pseudo : Etaoin
Modular project : Casia modular MS01
Location : Utrecht, Netherland
Website : www.casia.org/modular 		Name :
Modular project : 
Location :
Website : 
Name :
Pseudo :
Modular project :
Location :
Website :





Name :
Pseudo :
Modular project :
Location :
Website :


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