Miniwave Control Voltage Modification

The Miniwave is one very awesome module. Kudos to John Blacet and Grant Richter
for teaming up on this one. After I finished constructing mine in the MOTM format, I
found one thing I wanted to change. I did not like the fact that it takes a full 10 volts
at the "bank" or "wave" CV input to enable the control voltage to sweep the entire
selection from 0 to 15. While some voltages on the MOTM system may have a 10
volt range, some do not. One prime example is the E.G. Here is an instance where
control voltage is only 5 volts. Since the attenuator allows you to dial it back for higher
voltages, it made sense to me to increase the gain of the incoming buffer for these two
CV inputs so that if I wanted, I could get full sweep from 5 volts. I can always dial the
attenuator back to 50% for 10 volts. But, you sure cannot turn it up to 200% to make
5 volts work without the modification.

My goal was to have the internal control knob and the CV attenuator knob call up the
same bank or wave when positioned at the same point on the dial with a MOTM-800
EG delivering full voltage to the CV jack. The modification requires the addition of two

Theory behind the change: I want to change the gain of the input CV buffer without
effecting the internal bank or wave select knobs. The gain of the input buffer / summer
is determined by the ratio of R (feedback) to R (input). I want to change the gain from
unity to 2X. There are two good reasons to change the input resistor and not the
feedback resistor.

  1. The feedback resistor would need to be larger. It is more difficult to add
    series resistance as a modification than parallel resistance.
  2. Changing the feedback resistor changes the internal select knob range.
    I don’t want that.

It is easy to cut the input 100K (network) resistor in half by placing another 100K in
parallel with the existing resistor. So, thinking 2X gain is what I needed, that is what
I did. However, my MOTM-800 CV would only get me to bank or wave 14. Turns out,
I needed about 2.1 to 2.2 gain to achieve the goals I wanted. So, I changed my
resistor to 82K to get 2.2 gain. Bingo, both my incoming CV (full voltage from my EG
~ 4.9 volts) and the internal control knobs both hit bank / wave 15 at about 9 on the dial
scale. The 82K and 100K in parallel give us about 45K ohms input impedance. There
is nothing wrong with that.

So, the modification is:

Now, when you look at the PCB, you will find there are several ways to implement this change. You can go on top and hook the resistor from the cathode side of D1 to pin 2 of R2 (where your wire leaves the PCB to go to pot R2). And, do the same from the cathode side of D2 to pin 2 or R4 (where your wire leaves the PCB to go to pot R4). This might have been my choice with some heat shrink if I was doing it when I built my Miniwave. But, after-the-fact, I found it easier to tack the resistors on the bottom of the PCB.

Here is a photo of my modification:

The tape under the resistors is just to assure my resistor leads do not rub and short into
the PCB traces. It’s probably completely unnecessary, but it made me feel better.

Another way to implement this change would be to simply short RN5 pins 3 to 4 and
short PN6 pins 5 to 6. Then, a new 47K resistor could be added in the wire connection
from your pin 2 at pots R2 and R4 to their respective pins 2 at the PCB. Any method
that you use to effectively cut the input resistor on these CVs from 100K to somewhere
around 45 - 47K will accomplish the goal.

Disclaimer: I am not an engineer. I am not recommending that you modify your
Miniwave. However, I am sharing the results of my own modification for your
consideration should you find you have a similar need. I am not responsible for
damage to your Miniwave.

Larry Hendry 01/10/2002 – Revision 0

Download the MS Word document here

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