Music Easel Adaptation – Lowpass Gate
(used with his kind permission);
adapted by Aaron Lanterman
This is based on the lowpass gate circuit on Board 10 and Board 11 of
the Music Easel, which contains two identical LPG circuits.
You should spend some time studying the
Schematic & layouts
- Added 10/12/09: Dr. Mabuse has run into a problem with excessive
current draw cooking parts –
read about the problem and his
solution here. I personally haven’t been able to reproduce whatever
the problem is. I would be very interested to see if other people do
(or don’t) run into this problem.
- I have not been able to get the LED to light. I do not know why.
(It appears that one of my beta testers has gotten an LED to light, though.)
- I am convinced that the 50K sliders marked on the original schematics
(and this version of the board) should actually be 10K linear. The 120K
input and shaping resistors (R105, R106, R107, and R108) are off-board in
the original Easel, but included on-board in this adaptation.
- The original Easel has a 13.5 V supply, created using an op amp and a
transistor. If you have such a supply, you may hook it to the +13.5 pin and
omit R103 and R104. Otherwise, leave the +13.5 pin unconnected and use R103
and R104, which create a “soft” +13.5 V supply. In testing, this was found to
droop to between 9 V and 11 V depending on pot settings, resulting in me
being unable to open up the filter all the way using just the LOS pot.
I lowered R103 to 3K, and found that this helped counteract the droop and
I got a reasonable full-range control. You may want to experiment with
other values for R103. An alternative would be to keep
the R103/R104 ratio the same but lower the overall values, such as reducing
R103 to 1K and R104 to 9.1K. However, I have not tried this.
- The area around the vactrols is tight; be sure to install R42 and R41
before installing the vactrols. Also, the 910 pf silver mica caps are
pretty big; to get them installed I had to leave them kind of floating
above most of
the other parts.
- Q2, the buffer JFET, is a 2N4340 in the original. I picked the J201
since it happened to come with the preinstalled Eagle libraries. I used
an actual J201 in my build and it worked fine. Any JFET you have previously
successfully used as an audio buffer should work fine here.
- The need for R100, the 68K input resistor, was gleaned by studying other
parts of the original Easel schematics.
- The circuit has been tested with RC4558s. Other op amps will probably
work (many will probably work better!), but they have not been tried.
- The regular diode in the original is a 1N457.
I suspect a 1N4148s or a 1N914 will work, but I have not tested them.
- I have a tradition
of specifying 2.2 ohm resistors (should probably be 1/2 watt)
at the power inputs
to perform power supply filtering along with 10 microfarad
electrolytics. I picked 2.2 ohms since this choice shows up on some Buchla
schematics; I did not pick it through any particularly scientific means. Any
low resistance should work here.
I actually use “ferrite beads,” as suggested by Ken Stone, and not resistors
in these spots.
- Added 10/12/09:
Dr. Mabuse reports that a 0.001 uf cap (i.e. 1 nf) works
fine in place of the 910 pf cap around the LED-driving 2N1711 transistor;
this lets you save 910 pf micas for more critical audio path applications.
- Added 10/12/09: Dr. Mabuse writes: “Another sub that I decided
against but still yielded useful and interesting results was
swapping a single VTL5C3/2 for two VTL5C3s. It works both as a
filter and as a VCA but the response curve is noticeably
different (not as even and smooth) and the VCA mode
didn’t attenuate quite as much. In a pinch it think it could be used though.”
Front panel connections usually have a square and round pad together in a
white box. The round pad is the signal, and the square pad provides a
LIN – Level CV Input; amount of influnce controlled by setting of LCV pot
AIN – Audio input
AO – Audio output
LED – Hooks to the cathode (straight line part of symbol, shorter leg of
of an LED; the anode (triangle part of symbol, longer leg of actual device)
of the LED is hooked to +5 V.
SWV, SWC, and SWL – Connections for the mode switch. Use a SPDT on-off-on
switch. Connect SWC to the common connection, SWV to the lower connection,
and SWL to the upper connection. Switching to connect SWC to SWL puts the
filter in lowpass mode; switching to connect SWC to SWV puts it in VCA mode,
and switching it to the “off” position puts it in “combo” mode.
CIN – Control input. CV input
for mode control; amount of influence is controlled by the
CCV pot. If +13.5 V is input here, then the resistance of CCV corresponds to
the resistor setting on an Easel programming card, but you can put in all
sorts of varying voltages here. I
have not tried to puzzle out exactly what effect this has, i.e.
how many volts at a given pot setting is required to change modes, etc., but
I have made it switch modes. This input piles “on top of” the switch setting,
so its influence will change with switch settings.
B10P1 – Analogous to Pin 1 on Board 10; maybe useful if you are using this
to replace an original Easel board. Most users will not need this.
B10P9 – Analogous to Pin 9 on Board 10; maybe useful if you are using this
to replace an original Easel board. Most users will not need this.
LOS – Level Offset (Easel schematics and this version of the PCB say 50K,
but I recommend 10K linear)
LCV – Level CV; controls amount of influence of the LIN input (Easel
schematics and this version of the PCB say 50K, but I recommend 10K
CCV – Control CV; controls amount of influence of CCV. I specified 300K here,
but I largely pulled that number out of a hat. I would suggest a linear pot,
but I’m really not sure if a log or linear pot would be best. If +13.5 V is
put into CIN, then CCV corresponds to the resistor setting on an Easel
programming card, but you can put in all sorts of changing voltages for CIN..
These should be considered advanced projects, and should only be attempted
by people with extensive knowledge and experience in electronics,
in terms of practical construction and debugging techniques. The boards
dense and the documentation is sparse.
If you are just
getting started with Synth DIY, we recommend starting with kits
by Blacet Research or
PAiA, or boards by
from Outer Space. (There are numerous other kit and
PCB manufacturers, but those are relatively newbie-friendly.)
If you try to build one of these projects, you must assume that you will be
on your own, and be confident enough to tackle the project under those
circumstances. I am interested in learning about people’s experiences
in building the boards, and will try to answer questions over e-mail,
but I don’t have time to do any hand holding.
Any PCBs made available to the public are provided as-is, with no
guarantees or warranties whatsoever. Similarly, no guarantees or warranties
are made about the correctness or usefulness of the information on these
Any electronic project may present a risk of injury or
death, particularly when
dealing with mains voltages. It is important to follow appropriate safety
practices. The author of these
pages, Aaron Lanterman,
disclaims any liability for injury, death, or other damage caused in
using the PCBs or any of the information contained on these webpages.