40235
FILTER UPGRADE FOR THE ARP AVATAR
I
bought an old Arp Avatar guitar synthesizer from (surprise surprise!)a guitarist
. It had been lying around unused for about 15 years! Nevertheless it was in
quite reasonable condition considering, though the sliders were very stiff and
I had to replace a CA3080 OTA opamp in the VCA. There has always been a
bit of a debate about whether the Avatar was much use as a guitar
synthesizer..and often the failure of sales of the Avatar and the high R&D
costs, has always been quoted as one of the reasons for Arps fall. The great
thing about these boxes however is the fact that they contain the vco board
(BII) and filter board (C-I) found in the wonderful Arp Odyssey
synthesizer! Odysseys are now highly prized instruments and fetch
correspondingly high prices on the used market.
If you get the chance to buy an Avatar..it
could be a cheaper way to getting an almost identical sound.
Even before I got the Avatar I had
discovered Joachim Verghese’s very interesting website on technical aspects of
Arp synthesizers, called Arptech
. I was particularly intrigued by his filter design 40235 that
combines two great filters from Arp, the 4023 2-pole dynamic filter
(white odysseys) and a 4-pole Moog ladder based 4035 on one neat board. This
design was meant to be installed in the newer orange-face Odysseys (and other
Arps) to give the user a choice of filters. I decided to build a couple of
these for use in my various synth projects. My overall impression is that this
is a great little board. Anyway since getting the Avatar it was natural to
upgrade its 4-pole 4075 filter. Since I liked the 4075, rather than replace it
I wanted to add the 40235 and have a way of switching between all 3
filters on the front panel.
Here are some notes and
pictures illustrating the conversion project
The picture left shows one of two
40235 boards I have build. It’s
a
verycompact design and uses
double layer pcb’s. Its probably
better to have these made in pcb
shops, but I did these myself
using press-n-peel(not really
recommended for this job).The
empty 8-pin socket is awaiting a
CA3080 OTA.
Mounting the 3-way switch
This picture left shows the position
of the three-way slide switch,
just
above the 4075filter board.
It's a
bit of a squeeze, but mounting
it
this way avoids screw heads on
the
top panel.The switch is help
firmly
by two M3 bolts and locking
nuts.
Mounting the 40235 board
The above picture show how I mounted the 40235 board
on the back of the main C-1 filter board of the Avatar. Connection of the 40235
is via a 10 way ribbon cable and the details can be found at the Arptech link
given above. Since here we want to add the 40235 and keep the 4075
functional the wiring is slightly different than in the 40235
Arptech notes. The main idea is that the 3-way slider switch simply switches
the audio outputs of all three filters into the output pin 10 On the main
board. The problem with this setup is that one has to buffer inputs (CV and
audio) and outputs of the 3 filters. However Joachim has suggested a
rather neat way around having to add further buffer circuits. The 40235 already
has built in buffers (via NE5032 op amps) that are used in buffering
inputs/outputs of the 4023 and 4035 filters. Making use of these in the
present setup is very easy. The modifications details are (thanks to
J.Verghese!)
40235
BOARD
4075
BOARD
Pin 1 of U10 (buffered audio input)
to pin
1 of C-I board (pink wire in photo)
Pin 7 of U10 (buffered
feedback)
to pin
2 of C-I board (white wire in photo)
Pin 7 of U8 (buffered
cv)
to
pin 4 of C-I board (grey wire in photo)
With these connections, pins 1,2 and 4 of the 4075
have to be disconnected from the C-I board along with pin 10(audio out) which
has to be routed to the 3-way switch along with the out puts of the 4023 and
4035 (JP1 Pins 3 and 1 respectively) and eventually back to pin 10 of the C-I
board. The switch I used had enough poles that I could also switch
in the additional resistor to the wiper of the resonance slider (needed for
better resolution) when using the 4023filter (see the 40235
notes).
Adding front panel legend
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Using Letraset
white dry transfer lettering, I added some front panel legend next to the 3-way
switch. The switch cutout on the front panel was done using a drill and needle
files (takes a while as the panel is fairly thick steel sheet!). Then after
burnishing the legend, I masked off rectangular area around the lettering
and sprayed with matt finish clear lacquer. The
results look quite close to the original silk screening.