[sdiy] moog module designs

[Tim Daugard]

> Well I'm guessing you're not attempting to make these modules to the
> same standards of the originals.

The things I build are for my use only, and probably will never be used
anywhere except in my studio. There is no need for anything to be
mill-spec. With the methods I've developed I can build one of my
previous designs in an hour or two from the time I start printing a copy
of the schematic and the front panel label to the time its on the rack
ready to use.

A new design takes between a week to a month - 4 to 20+ hours. Some of
that time is spent on documentaion, front panel label design,
investigating IC functions (and misfunctions - did you know that some
4027s are different from others. If you set the J and the K input high
and clock it, it might not toggle like TTL used to.)

>The price for the pots I use on
> Moogs cost more than $5 each. ;-)

The pots I use are $0.41 each in quantities of 10 or 100 I forgot which.
Unfortunately, Mouser no longer carrys them. The manufacture lists them
still on their website - 1000 peice minimum order. I'm hoping they will
come back after conversion to lead free. I bought 200 the last time.
Thats enough for a while if I don't build to many sequencer modules. THe
average is for pots per module <$2.00 per module.

Manufacturer specifies 120? cycles - some low number. I have only
replaced one so far for failure. Lots of the modules get set to a sweet
spot and left there for a long time. If they are going to fail, they
will fail in the mixer module first. I will re-evaluate them when and if
that starts happining.

> >I know that is hearesy, but my system uses a single wire patch cable
and
> >current signals instead of two wire voltage signals with shielded
plugs.
>
> How does one mult things if using current instead of voltage?
>

Inputs are nominally designed for 120 uA being a full on signal. A few
of the modules need a little more current in the range of maybe 200 uA.
Off is 0 uA, but most modules can tolerate up to 10 uA before they do
anything.

Note all currents are positive. Module inputs are not expected to source
current, only sink it. (Using the current flows from positive to ground
theory - I know it actually flows the other way, but I hate having to do
all the math with negative numbers, and the circuits work the same no
matter which theory you were thinking when you designed them.)

All module outputs are capable of providing approximately 5 mA at full.
AC outputs are actually Alternating Current outputs. A clean
non-distorting signal into one module will probabily vary between 20 and
100 uAs. Any module output can seriously overdrive the input of the next
module.

Most modules use an opamp configured as a variable inverting amplifier
to drive the output. The signals can be choked down to almost no
variation. The opamp feeds a 1K (actually 1K8) resistor connected to the
output pin. If the module has multiple outputs, each output pin has it's
own resistor.

. . . now back to your question

Multi-ing an input takes a "Y" cable - current from two outputs feed the
input and are added by the input network. Some of the input networks are
resistive summing the currents into a voltage to be sensed by an opamp.
Some of the inputs feed the current to a current limiting resistor tied
to the base of a transistor with the emitter tied directly to the
negative rail (or in a few cases to the positive rail) the transistor
does its thing with the currents.

All inputs and outputs can be tied to any power supply rail with out
problems.

Multi-ing an output is the same - use a "Y", "W", "V", "H" or any other
multi pin cable. It's similar to the Serge stacking banana plug method.

Passive filters and clippers are a special case. They have to have a DC
current path through them to prevent noise.  The ones built into a
module have a resistor to the negative rail to convert current into
voltages before the filters. Usually the filters feed a mixer/buffer.
These modules use a voltage divider input - 180K to the positive rail,
180K to the negative rail. These inputs still are at 90K ohms and can
re-reference filters outputs to a reasonable range.

In summary,
Modules
    Outputs 1K / 1mA DC coupled impediance
    Inputs  100K / 100 uA DC coupled impediance
Passive filters
    Input 100K DC || 10K AC impediance
    Output 10K to the negative rail DC \ will drive 50K to 100K with
close to the desired response (The 10K resistor on the output swamps out
input variations on the modules)  with 10-50% of the input current
consummed by the filter and the output.

> >>  know there's some loading after them, so you may well want to be
precise.
> >. . . and this is where one of the questions of the 308 vs the 071
comes
> >in. The input impediance of the 308 is 10 to 40 Megs, the 071 is much
> >greater.  I thought about hanging a 10M resistor on the 071 input to
> >reduce its impediance.
>
> The bandwidth and slew rate of the 071 is higher too.

The data sheet I found had them similiar - It may be a data sheet for a
late model LM308 even though it came from National and said it was
obsolete.

> >If someone wants to donate a 308, I could do this. Otherwise it will
> If I donated parts everytime I was asked, I'd be out of business. ;-)

That was exactly the answer I expected, and would give myself. When I
teach negotiating, one of the first things I teach is "If you don't ask
for something, you absolutely won't get it."

> I didn't like either and used Apple II's. :)
Couldn't afford one at the time. Now I have an Apple II Emulator on my
computer.


Tim Daugard
AG4GZ 30.4078N 86.6227W Alt: 3.7 M
http://home.sprintmail.com/~daugard/synth.htm

ignoring work because it's Friday, it's been a tough week, and the sea
breeze through my office window feels nice.