Muff's Modules & More

[os]

Since I published the details of the circuit I'd been using with Silent Way AC Encoder (http://www.expert-sleepers.co.uk/siwaacencoder.html) various people have written in with suggestions for improvements, so I thought I'd sit down and properly try them out. (A big thanks to everyone who wrote in, by the way.)

All tests were done with my Metric Halo Mobile I/O 2882 interface and an Analogue Systems RS-95 VCO.

The original circuit
This one:
I measure 8V at the output when not plugged in to anything, and get about 7 octaves of calibration.

(You'll see therefore that I have a bit of a problem with the remaining tests - 7 octaves is about as much as I can hear, and 8V is not far off the supply rails of the modular, so I can't really expect much improvement in my own setup. However, the results should be useful for those with interfaces with less output.)

An even simpler version
It was pointed out to me that one of the diodes (pick one, doesn't matter) is redundant in the circuit above. To test that theory I removed the diode from the TRS ring and left it floating. Exactly the same results as above - so yes, you can do the same thing with just one diode.

Just as simple, double the voltage
It was also mentioned that connecting the TRS shield to the TS shield is a possible source of ground loops. Moreover, the above circuit isn't taking advantage of the balanced output's full voltage swing.
So, I tried a basic half-wave rectifier (see http://www.play-hookey.com/ac_theory/ps_rectifiers.html) using the TRS's T & R as inputs, leaving the shield unconnected. So like this image, but with the capacitor as above across the TS plug:


(Side note: from here on I did all my soldering at the TS plug end, leaving the TRS shield in place to actually be a shield down the length of the cable.)

Using this circuit I measured 16V on a disconnected lead, as expected. This is clearly a no-brainer improvement over the first circuit above.

Bridge rectifier
For completeness I also tried a bridge rectifier (same link, http://www.play-hookey.com/ac_theory/ps_rectifiers.html).

The results were the same as for the half-way rectifier - 16V.

Voltage doubler
Now this is a very interesting circuit: http://www.play-hookey.com/ac_theory/ps_v_multipliers.html

I made that, and true enough, I measure 32V on a floating cable! Now that's a potentially dangerous amount of voltage to be putting into a modular, so I adjusted the 'Max Out' setting on Silent Way AC before proceeding with calibration tests.

As noted above, 32 octaves or even 12 octaves just isn't a feasible thing to hear or measure. I will say though that with this setup I measured the actual voltage on the cable when under load i.e. plugged into the VCO, and I was able to get that comfortably up to +12V.

So, for those who've been reporting only one or two octaves of calibration, do try this circuit - it's only one more capacitor than the first one, and really does double the voltage.

Let us know how it goes!

[os]

In case anyone wanted to try the new options, but wasn't sure how to translate the circuits above into cable wiring, here are some diagrams:

Half-wave rectifier:



Voltage doubler:

[jenamu6]

This is great..........I still have to try mine out.

[Arnoid]

I checked the voltage doubler circuit now i'm getting 4,60 V at max
Almost 5 octaves

First I had 2,84 V at max...

[Arnoid]

I just made a voltage tripler circuit


Now i'm getting 7V !!! at maximum...
Solid tuning over 7 octaves

[os]

[rv0]

[Klipspringer]

I made both the voltage doubler circuit and re-did the original circuit with the new suggestions.

Maximum output with the new half-wave 5.6 Volts.

Voltage doubler is, well double, 11.2 volts or so.

The original design gave me a max output of about 1.5 Volts. Seems to tracks OK now using the buffer of my MX61 into both the AsOL VCO-6 and ASol VCO-RM.

I've tried .22 uF and .1 uF caps. Seems to be good with both values.

This is with my FA-66. So it's tested and it works OK. Now I gotta buy Silent Way like I promised!

I think this needs a little box with maybe a variable cap or dip switches to select different capacitance (as that seems to be the variable for audio interface).

Sell it for maybe $39.95

[Johnisfaster]

[lombrose]

How did you guys make the tests?

[oootini]

hi guys. this all sounds totally amazing...

[lombrose]

Tested with m-audio lightbridge and presonus digimax fs, the original circuit, same as OS - 8v!!!

[PolishMilk]

I decided to have a go at this using a Focusrite Saffire audio interface. Put the voltage doubler circuit together on a breadboard using 0.1uf capacitors and 1N4148 diodes.

I'm pretty sure I put it all together right, but when it came to testing it on my modules it didn't work quite correctly. The voltage increases fine but when it comes to decreasing it gets stuck. So for example with an lfo on pitch cv it would reach the peak of the signal but never go down again.

However when I put my multimeter across the tip & sleeve of the output to measure voltage it does go down and the circuit behaves as normal, but only while the multimeter is touching. I decided to try putting a resistor [a large range of values work] between tip and sleeve of the output and hey presto it works fine. I get about 2.7v max output (even with the doubler circuit, so the interface presumably has a fairly rubbish output level) and good tracking for just over an octave.

So basically does anyone know what weirdness is going on here? Why do I need a resistor across the TS output to get the circuit to work properly? Anyone had any similar problems?

*Edit* It also does exactly the same thing using the original circuit and the half-wave rectifier

[sgnelson]

[os]

[PolishMilk]

[bsmith]

Does capacitor choice not matter as much if using the half-wave rectifier or voltage doubler or will there still be an optimal choice depending on the interface?

[os]

Capacitor choice still matters.

[PolishMilk]

Finally got my problems solved. The low output was a stupid one - there was a toggle on the software mixer for the audio interface that was dipping the volume of the output for some reason (I bought the interface new for use with Silent Way so didnt know how to use it!) With that sorted I'm getting 6v with the half-wave rectifier, 12v on the doubler and 6 octaves of perfect tracking, so SW does work with the Focusrite Saffire.

The problem with the impedance does seem to be linked to the module I was using. The cables work fine on my Arp Axxe but when plugged into the Analogue Solutions VCO-Sub the pitch rises but never falls again.

Is there a better solution to this than just strapping a resistor across the tip & sleeve? Using the Axxe as a CV master for the VCO-sub has always worked fine so there must be a workaround?

[os]