Invert ADSR (Modulus #4)

[Roy Tate]

From: 
        Roy Tate <roytate at ionet.net>
                                                                                       
8:18 PM

 Subject: 
        Re: Invert ADSR (Modulus #4)
     To: 
        gerryd at ntx-tech.com




Gerry Dahl wrote: 
> Hi Everyone,
> I'm looking for a way to invert the ADSR And the A(S)R on my PAIA
> Fatman. Any help on this would be great.
> Thanks.

I'll address the question "how do I invert a voltage."  I'm sorry, but I
won't be addressing the issue of using this circuit in a PAIA Fatman.

[A simple circuit]

You need an Op-Amp such as the TL071.  Use the following schematic, but
you may have to figure out the resistor values for yourself.  If you
want "unity gain" - where the input voltage equals the output voltage,
set Ra and Rb to the same value - between 1K Ohms and 100K Ohms, I would
try 100K as a starter.  Technically, this is a gain of -1 since
the output is inverted.  Power the op-amp from +/-12V or 15V (the
same power as the Fatman uses).  Normally, a 1K Ohm resistor is 
attached to pin 6 to limit the output current in case of a short to
ground later on down the signal chain (among other reasons).

Pinout:
Pin 2: Inverting Input
Pin 3: Non-Inverting Input
Pin 4: -12 or -15V
Pin 6: Voltage Out
Pin 7: +12 or +15V

Voltage In >----Ra----+-----Rb----+---1K---> Inverted Voltage
                      |           |
                      |    |\     |
    (Inverting Input) +----| \    |
                           |TL\___/
                           |71/
(Non-Inverting Input) +----| /
                      |    |/
                   Ground

Gain = -Rb / Ra

Walter Jung's IC Op-Amp cookbook has a more detailed formula on page 19
for the curious.  ISBN 0-672-22453-4.  This book covers op-amps in
general, and does not really discuss analog music synthesizer.

[The circuit you want]

There is a CV inverter module in an issue of the Modulus on-line
magazine.  Get issue #4 and look at page 6 for a schematic that inverts
and adds -10V using the 10K and 20K resistors at the top (simple
resistor divider).  You can look at the point between the 10K and 20K
resistors as another input voltage if you wish.  The Modulus CV Inverter
uses the classic "inverting summer" technique, where all of the voltages
on the left side are fed through resistors of the same value (in this
case 100K) with a 100K feedback resistor, so that we can view the op-amp
as adding the voltages together and inverting the result.  
Pin 6 of the op-amp presents us with the sum of all input voltages 
as long as the result is within the range of the power supply - 
between Pin 4 and Pin 7.  This converts a signal that goes from 0 
to 10 Volts into a signal that goes from 10 to 0 Volts.  "My" 
schematic would take 0 to 10 and convert it into 0 to -10.  You 
probably want to use the schematic from Modulus #4, since it 
requires you to know less about the Fatman's internals - like can 
the Fatman even handle negative envelopes...

Http://www.geocities.com/siliconvalley/park/2707/

If the Fatman uses 5 volt gates, you could use swap the 10K and 20K
resistors for a 5 Volt system.

[Learning More]

If you would look at the various schematics for VCOs and VCFs, you will
find that they are very similar to the Modulus #4 circuit.  Also, more
information on the inverting summer structure is available if you want
to buy the Electro-Notes complete back-issue set (see the Midwest Analog
Products link on my page).  This is an expensive purchase, but
a large percentage of the material is relevant to Analog Synth Design.

Although I'm no expert, I hope that I have helped and not hindered.  If
any other synth-diy member wishes to clarify a point or (especially)
correct some information, please do.

Regards,

Roy Tate
roytate at ionet.net
http://www.ionet.net/~roytate
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