Passive possibilities?

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Trilloquist
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Passive possibilities?

Post by Trilloquist » Wed Feb 12, 2020 11:32 am

With a Moog DFAM as a starting point (vco, vcf, lfo, eg, vca), what sort of modest explorations could be available with passive modules? I'm still learning about the mechanics of it all, but as I understand it, passive modules dont require power. So the case itself, at least in this context, is just a box with rails?
Not that I want it to stay that way, just curious what sort of functions are provided by passive modules, if the main synth duties are covered by other semi modular gear? Is it possible to have a very limited but technically functioning rack without power and still expand on the DFAM?

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Re: Passive possibilities?

Post by Yes Powder » Wed Feb 12, 2020 12:10 pm

Off the top of my head, here's what you can get from passive modules:
Ring Modulation
Low-Pass Gates
Logic (though no N- functions since those would require voltage output when receiving nothing)
Passive filtering/EQ
Attenuators
Whatever the hell the Razmasynth Super Warp Generator is

There was someone in another thread about passive modules who mentioned once seeing a bass oscillator that was entirely powered by a gate signal, but I can't seem to find anything about it.

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Re: Passive possibilities?

Post by Pelsea » Wed Feb 12, 2020 12:20 pm

Well, clearly not voltage control. However, attenuation, mixing, and even filtering can be done with passive circuits. Some of these are quite interesting, such as the ring modulator. The gotcha is everything you do reduces the signal by 3 dB or more, so you probably have to add an amplifier at the end. This increases noise compared to the active versions of the circuits. Also, some passive operations require transformers, which are big and heavy.
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Re: Passive possibilities?

Post by Arneb » Wed Feb 12, 2020 12:42 pm

Yes Powder wrote:
Wed Feb 12, 2020 12:10 pm
Off the top of my head, here's what you can get from passive modules:
Ring Modulation
Low-Pass Gates
Logic (though no N- functions since those would require voltage output when receiving nothing)
Passive filtering/EQ
Attenuators
Whatever the hell the Razmasynth Super Warp Generator is

There was someone in another thread about passive modules who mentioned once seeing a bass oscillator that was entirely powered by a gate signal, but I can't seem to find anything about it.
That plus:
- Slew limiting
- Mixing
- Apparently there are also transformer-based passive gainers around

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Re: Passive possibilities?

Post by KSS » Wed Feb 12, 2020 8:23 pm

Pelsea wrote:
Wed Feb 12, 2020 12:20 pm
Well, clearly not voltage control.
'Vactrol' VCFs and VCAs arecan be passive.

Would be more fair to say optoisolators. Then the older lamp/LDR style 'Vactrols' remove the possbility of using the diode in the LED type as reason to cry foul at calling a vactrol passive. 8-)

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Re: Passive possibilities?

Post by theB94 » Wed Feb 12, 2020 11:41 pm

This is my little passive setup. Deciding on whether to get a vactrol lpg or a mixer for the last slot. Its a helpful and fun little box.
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Re: Passive possibilities?

Post by WLMZNHRST » Mon Mar 02, 2020 11:47 am

theB94 wrote:
Wed Feb 12, 2020 11:41 pm
This is my little passive setup. Deciding on whether to get a vactrol lpg or a mixer for the last slot. Its a helpful and fun little box.
This is really cool stuff!

I've started gathering/reading about passive DIY circuits and gadgets, and there's actually quite a lot of things you can easily build and get creative with the 'boring' basic elements. Here's the most interesting ones for me:

1) Vactrol LPG/VCA -> Always useful! Use a toggle or pot to go from filtered to VCA.
2) Attenuators -> I guess I'd almost never make dedicated ones, but just chuck em into other circuits.
3) Switched Multiple -> Much nicer than a regular passive mult, flipping switches is very fun.
4) Switched OR-combiner -> With switches to instantly mix-up multiple gate-patterns.
5) Half-wave rectifier -> Split bipolar into pos/neg, or combine pos/neg into bipolar; make your own waveshapes at audio-rate.
6) Expression Pedal -> Two 3.5mm to a stereo 6.35mm jack (maybe add an attenuator) and you can use an Expression pedal to control any CV source.
7) Solar panel -> Tried this with some guitar pedals; they give about 5V max, so you can kinda play them with your hands or any light source, very fun!
8) R2R ladder -> This weird resistor-ladder-mixer-thing, where you can plug anything anywhere and get adventurous without risk of shorting anything.

Other than those: Slew limiter should be possible, Fixed filters (bit boring I guess), Ring Mod, Jack converters, regular mixer (although other than that R2R ladder thing it's probably not worth doing passively...)

Also there's apparently a passive JFET VCA circuit that takes it's power from the input signal, but it much faster than the Vactrol-based ones.
Sounds less idiot-proof though, so starting with the above list.

===

First thing I've built from the list is a (quite elaborate) passive mult/attenator thing: 2 INs, 4 OUTs with A/off/B toggle switch, and 2 separate Attenuated OUTs, that take signal from the same inputs as the mults. Quite handy all-in-one routing/attenuating possibilities.

Next up is the OR-gate combiner; It's just a multiple with some diodes, but I plan on sending some poly-rhythmic Euclidean gates to it

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Re: Passive possibilities?

Post by EPTC » Mon Mar 02, 2020 1:02 pm

theB94 wrote:
Wed Feb 12, 2020 11:41 pm
This is my little passive setup.
That's beautiful, there! Yeah, looks like it's missing a DPLPG but hadn't seen half of those things. Rectifier!

Will add this is a great passive item. After parts, comes to about $30. https://pushermanproductions.com/produc ... anel-10hp/ - Takes about five minutes to solder together (just sandwich the unsoldered pieces together with the panel and pcb)
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Re: Passive possibilities?

Post by Blairio » Tue Mar 03, 2020 2:07 pm

Just taken delivery if a Meng QI DPLPG, and totally impressed by it.

You quickly forget it is a passive module, and instead delight in the two distinct vactrol curves: one long, one short.

I have various other LPG's, but if you set aside additional control over the LPG parameters, I do not think that the DPLPG is embarrassed by any of them.

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Re: Passive possibilities?

Post by ersatzplanet » Wed Mar 04, 2020 3:24 pm

KSS wrote:
Wed Feb 12, 2020 8:23 pm
'Vactrol' VCFs and VCAs arecan be passive.

Would be more fair to say optoisolators. Then the older lamp/LDR style 'Vactrols' remove the possbility of using the diode in the LED type as reason to cry foul at calling a vactrol passive. 8-)
Though the LED in a Vactrol is an active part, the definition used here is for a passive module, that is a module that doesn't require connecting to a power supply. Since the LED in the vactrol gets it's power from the signal itself, just like a jack-light or LED patch cord, the module itself is still passive.
-James

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Re: Passive possibilities?

Post by ersatzplanet » Wed Mar 04, 2020 3:25 pm

The passive modules we make, a slide-pot module and a dual Force Sensing Resistor module.
Passive-Modules-SM.jpg
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-James

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Re: Passive possibilities?

Post by starthief » Wed Mar 04, 2020 4:09 pm

Blairio wrote:
Tue Mar 03, 2020 2:07 pm
Just taken delivery if a Meng QI DPLPG, and totally impressed by it.

You quickly forget it is a passive module, and instead delight in the two distinct vactrol curves: one long, one short.

I have various other LPG's, but if you set aside additional control over the LPG parameters, I do not think that the DPLPG is embarrassed by any of them.
Hmmm, the two vactrols in my DPLPG were really closely matched.

I think DPLPG sounds lovely. Where it falls down a bit compared to active LPGs is the minimum threshold before opening. That stops you from using low voltages gates or triggers to open it just a little bit and keep the sound dark; it also interferes with envelopes that have slow attacks. (Slow decays is less of an issue because it at least tails off with its own decay.)

Also passive LPGs play havoc with some modules, like Rampage, but that can be fixed with a buffered mult or even a passive attenuator.
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Re: Passive possibilities?

Post by Yes Powder » Wed Mar 04, 2020 4:43 pm

starthief wrote:
Wed Mar 04, 2020 4:09 pm
Hmmm, the two vactrols in my DPLPG were really closely matched.
The two on mine sound completely different. I think you just got lucky in the vactrol lottery— or unlucky if you’d prefer the variety.

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Re: Passive possibilities?

Post by dubstephead » Wed Mar 04, 2020 5:03 pm

Agree with everything above.
This thing is passive and fun... Also all Mystic Circuits 0HP modules look cool. Passive mult's are always great for Trigger / clock sources... :cloud:
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Re: Passive possibilities?

Post by Blairio » Wed Mar 04, 2020 9:09 pm

starthief wrote:
Wed Mar 04, 2020 4:09 pm
Blairio wrote:
Tue Mar 03, 2020 2:07 pm
Just taken delivery if a Meng QI DPLPG, and totally impressed by it.

You quickly forget it is a passive module, and instead delight in the two distinct vactrol curves: one long, one short.
Hmmm, the two vactrols in my DPLPG were really closely matched.
I found this content on a web shop's page. I think it came from Meng QI. "The components of the DPLPG are carefully selected in terms of musicality and sound. By using opto couplers, the two channels will most likely differ from each other. This is natural behavior caused by how opto couplers are made. Each component will have a slightly different behaviour. – In essence, it will add to the individuality of your sound!"

That's the thing about Vactrols - there seems to be a wide range of behaviour even amongst vactrols of the same spec. When buying a Doepfer A101-2 a few years back, I tried three in my local synth shop, and all three sounded distinctly different. Not bad, just different.

In my case I have been lucky with my DPLPG, as I was hoping for two different behaviours. Another use case may require Vactrols to be more closely matched. I guess it is horses for courses!

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Re: Passive possibilities?

Post by Homepage Englisch » Thu Mar 05, 2020 7:08 am

I would add passive signal router, looks interesting.

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Re: Passive possibilities?

Post by ersatzplanet » Thu Mar 05, 2020 12:48 pm

Yes Powder wrote:
Wed Mar 04, 2020 4:43 pm
starthief wrote:
Wed Mar 04, 2020 4:09 pm
Hmmm, the two vactrols in my DPLPG were really closely matched.
The two on mine sound completely different. I think you just got lucky in the vactrol lottery— or unlucky if you’d prefer the variety.
I once designed a module that took advantage of the variations in Vactrols. I called it "The Drunken Modulator" after somebody's signature I saw. It was a VERY basic circuit that took a CV input into a Vactrol and the vactrol then controlled a DC source to the output that would loosely track the input CV, adding all the Vactrol "charm" we love so much. You could have the Vactrol LEDs set so the output would follow the positive or negative side of the input CV. I even made a version that would plug into one of our FSR-4C controller modules and then you could scale the output and have a threshold triggered gate too. I got inspired after buying a bag of surplus Vactrols from Electronics Goldmine. I think it was a bag of 50 for $20 or something like that. They were not matched AT ALL. You could get pretty different responses from the same input signal.
-James

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Re: Passive possibilities?

Post by dubstephead » Fri Mar 06, 2020 3:09 pm

ersatzplanet wrote:
Thu Mar 05, 2020 12:48 pm
Yes Powder wrote:
Wed Mar 04, 2020 4:43 pm
starthief wrote:
Wed Mar 04, 2020 4:09 pm
Hmmm, the two vactrols in my DPLPG were really closely matched.
The two on mine sound completely different. I think you just got lucky in the vactrol lottery— or unlucky if you’d prefer the variety.
I once designed a module that took advantage of the variations in Vactrols. I called it "The Drunken Modulator" after somebody's signature I saw. It was a VERY basic circuit that took a CV input into a Vactrol and the vactrol then controlled a DC source to the output that would loosely track the input CV, adding all the Vactrol "charm" we love so much. You could have the Vactrol LEDs set so the output would follow the positive or negative side of the input CV. I even made a version that would plug into one of our FSR-4C controller modules and then you could scale the output and have a threshold triggered gate too. I got inspired after buying a bag of surplus Vactrols from Electronics Goldmine. I think it was a bag of 50 for $20 or something like that. They were not matched AT ALL. You could get pretty different responses from the same input signal.
Got any of these for sale!?! :cloud:

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Re: Passive possibilities?

Post by 3hands » Fri Mar 06, 2020 6:16 pm

dubstephead wrote:
Fri Mar 06, 2020 3:09 pm
ersatzplanet wrote:
Thu Mar 05, 2020 12:48 pm
Yes Powder wrote:
Wed Mar 04, 2020 4:43 pm
starthief wrote:
Wed Mar 04, 2020 4:09 pm
Hmmm, the two vactrols in my DPLPG were really closely matched.
The two on mine sound completely different. I think you just got lucky in the vactrol lottery— or unlucky if you’d prefer the variety.
I once designed a module that took advantage of the variations in Vactrols. I called it "The Drunken Modulator" after somebody's signature I saw. It was a VERY basic circuit that took a CV input into a Vactrol and the vactrol then controlled a DC source to the output that would loosely track the input CV, adding all the Vactrol "charm" we love so much. You could have the Vactrol LEDs set so the output would follow the positive or negative side of the input CV. I even made a version that would plug into one of our FSR-4C controller modules and then you could scale the output and have a threshold triggered gate too. I got inspired after buying a bag of surplus Vactrols from Electronics Goldmine. I think it was a bag of 50 for $20 or something like that. They were not matched AT ALL. You could get pretty different responses from the same input signal.
Got any of these for sale!?! :cloud:

Yes please! I would love to know if any of these are available!
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Re: Passive possibilities?

Post by JeebsFat » Sat Mar 07, 2020 4:34 am

Blanks!

sorry... :wookie:

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Re: Passive possibilities?

Post by ersatzplanet » Sat Mar 07, 2020 4:49 pm

Yes Powder wrote:
Wed Mar 04, 2020 4:43 pm
starthief wrote:
Wed Mar 04, 2020 4:09 pm
Got any of these for sale!?! :cloud:
Yes please! I would love to know if any of these are available!
I don't have any for sale but the schematic is very simple and you could DIY one up in no time with no PCB, and parts soldered directly to the jacks. For a mainly passive version (except for the output voltage), the LED side of the vactrol is wired just like you would wire a JackLED or LED indicator in a module - the input jack would go through a resistor to one leg of the LED with the other going to ground. The LED polarity would choose whether it responded to positive or negative side of the input signal. Adjust the resistor value for the LED in the Vactrol (start around 1K). The other side of the vactrol would be wired as a simple voltage divider with the power supply being controlled by it. Just use an online voltage divider calculator resistor value site and the value of the resistor side of the vactrol when the Vactrol LED is at rest as one one the calculator values (you might want to fire up the LED if you wanted to calculate a range). Then the input signal controls the vactrol, the vactrol opens and closes off the DC power supply and there you go. (just use "voltage divider calculator" in a google search and you will get sites with the circuit and inputs for values like whether you want to use the 12V or the 5V as a input voltage).

Variations would be things like a switch for the legs on the LED side to switch polarity, normalizing the power through a jack so that you can get VCA like stuff with a signal instead of the DC input there. You could replace one of the other resistor in the voltage divider circuit with a pot and have a crude range control.

To add more bells and whistles you have to add more circuitry of course. The FSR-4C I mentioned is a FSR-4 motherboard that connects to a remote panel (a FSR-4B) that has four large FSR sensors on them. The FSR acts like a large resistor that changes value when pressure is applied and is totally passive. I was going to make a panel that replaced the four FSRs with four of these vactrol circuits. Then you would get the FSR-4C's added advantage of scaled output voltages and threshold controlled gates. I never thought it would be in much demand so never made a production version.
-James

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Re: Passive possibilities?

Post by gasboss775 » Sun Mar 08, 2020 7:15 am

I recently made a passive lowpass filter

The cut off frequency is selected with a rotary switch ( The diagram shows only a 2 way switch due to not having a rotary switch symbol ) My circuit had 6 frequencies but with more switches and capacitors more frequencies could be selected. The pot varies the resonance, which is highest at minimum resistance.

The 4 Henry inductor was made from the primary winding of a mains transformer, the secondary is just left unconnected.
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