To: AH analogue@hyperreal.org,
From: "dj hohum" djhohum@gmail.com,
cc:
Subject: [AH] Juno Filter Module
Date: Wed, 18 Jul 2007 03:50:21 -0700
Hi All,
Several people asked about the details of my unfinished Juno Filter
module. I thought I'd reply to the list so that it's in the archives.
This was an old unfinished project from about five years ago when I
first thought I would build a modular. I started, stalled, and got
distracted. I only mention this because what I remembered about the
module was not exactly correct. Since it was almost finished back then
I went ahead and put it in a semi-finished state which you can see
here:
http://www.flickr.com/photos/9891418@N07/844853290/BR>
If you look at the Juno schematics you can see that the juno voice
circuit is meant to work with CMOS levels and the CMOS is powered off
of 5 volts. So for the most part, if you just duplicate the Roland
voice circuit you should be able to get the bare chip to play with
other gear so long as you are careful about the voltages.
There are some things to be careful about though. The filter CV is
inverted and has a very restricted range. I use an op-amp to invert
the signal and then scale it down to about +- 0.4 volts or so. Even at
that level turning the resonance up full the VCF as VCO tunes from a
few hz to 60khz. I have a pot on board to adjust the swing and it's
set at about +-0.25 volts or so. This allows it to tune between a few
more hz than a few hz and about 25khz or so. If you really want to be
anal you can just duplicate the entire voice circuit. Just check to
make sure that there's not excessive voltage on pin 6. I think you
still have to invert the signal though. The 560 ohm resistor is a TC
part. I didn't bother with compensation.
For the res and vca CV inputs I just duplicate the Juno voice circuit
using 2n3906 transistors. You don't need the 0.1 mf caps, I'm pretty
sure that they're there to smooth out the multiplexed CV. Just a
resistor and the transistor is all that you need.
That's really all there is too it. The input levels to the VCA needs
an attenuator but the VCF can be fed directly.
Of course there should be input and output buffers. When I get around
to it I'll put a few more on a piggy back board. In the meantime, it
sounds good. Nice and plasticky smooth! It doesn't like too much
input and will distort above about five volts. You'll know when this
is happening because it sounds like crap.
For me, the important part of the project was getting the smooth juno
sound, not making a super accurate filter. So I was quite sloppy about
the details. If your goals are different you might have a lot more
work to do. If you just want the juno filter sound in your modular,
very few external parts are necessary.
Also, I was focused on getting the filter working. I've done far less
testing with the VCA so be careful with the voltages you feed it. I
managed to latch up the chip with the gate outputs from my arp 1613,
it recovered, I thought I had destroyed the VCA. So be careful with
what voltages you feed the CV inputs. Really, a bit more work should
be done here.
If I get time this weekend I'll make some sound demos. VC resonance
with this filter is REALLY nice!
I hope this answers the questions. I don't really have a schematic
because with stuff like this I don't usually bother, I just build it
on the fly.
regardses,
Daryl
PS: The colored knobs are from futurlec. They feel as awful as they
look. If anyone knows where I can get nice rubbery knobs that fit on
knurled shafts, are large enough to cover the nut but small enough to
fit on a blacet panel, I'd appreciate knowing where.