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Low Pass Gates
MUFF WIGGLER Forum Index -> Modular Synth General Discussion  
Author Low Pass Gates
raccoonboy
Hi everyone

I've never owned a LPG. I'm wondering though, most of them have a CV in (such as the Pittsburgh LPG) I think this modulates the VCA/Filter. So you just feed this an envelope right?

Well what happens if you do this and at the same time you trigger the vactrol via a pulse, does it combine the two envelopes?
noisejockey
To your ears, it'll sound about like that, yes, depending on what you're doing or trying to achieve.

I do this on the Optomix (which has separate CV's and knobs each for the filter and VCA, as well as a "strike" input for triggers into both) and the Buchla 208/Easel (which has a no-input slider for VCA and/or filter, and another slider for incoming envelopes or pulses); Wide open with a sustained gate or knob, a drone is produced, and you don't hear the rhythmic pings. When that CV control is pulled down, the drone diminishes and you hear those plucked gates triggering. Nice way to do builds or breakdowns.

But normal envelopes, and not just pulses or triggers, can add a lot of articulation to the use of LPGs as well, not just short pulses or triggers.
raccoonboy
noisejockey wrote:
To your ears, it'll sound about like that, yes, depending on what you're doing or trying to achieve.

I do this on the Optomix (which has separate CV's and knobs each for the filter and VCA, as well as a "strike" input for triggers into both) and the Buchla 208/Easel (which has a no-input slider for VCA and/or filter, and another slider for incoming envelopes or pulses); Wide open with a sustained gate or knob, a drone is produced, and you don't hear the rhythmic pings. When that CV control is pulled down, the drone diminishes and you hear those plucked gates triggering. Nice way to do builds or breakdowns.

Great ideas. thanks

noisejockey wrote:

But normal envelopes, and not just pulses or triggers, can add a lot of articulation to the use of LPGs as well, not just short pulses or triggers.


Oh really? So the LPG can work like a really sensitive envelope filter too? I assumed it could be a trigger only as this seems to be the only thing mentioned on many places I've read, or does it have some kind of other quality that I'm missing!?
MRoyce
The Vactrol will slew any CV that get's put through it. It's non-linear in a nice way though, and will therefore behave differently than a standard slew.
noisejockey
MRoyce nailed it, raccoonboy. Most non-passive LPGs I've encountered can be opened by any CV, from a trigger to an envelope, often with the amp and filter addressed individually (e.g., Optomix). But when you do hit it with a pulse or trigger, a vactrol will ring/decay in a lovely way on its own, without an envelope. Just experiment to see what expressivity you can wring out of it! (As I said before, mixing LPG pings/strikes with envelopes or even LFOs can really mix things up nicely).
raccoonboy
noisejockey wrote:
MRoyce nailed it, raccoonboy. Most non-passive LPGs I've encountered can be opened by any CV, from a trigger to an envelope, often with the amp and filter addressed individually (e.g., Optomix). But when you do hit it with a pulse or trigger, a vactrol will ring/decay in a lovely way on its own, without an envelope. Just experiment to see what expressivity you can wring out of it! (As I said before, mixing LPG pings/strikes with envelopes or even LFOs can really mix things up nicely).


sounds perfect. Thanks for the explanation, I'm glad it has these options.. Think I'm going to get the Pittsburgh LPG Mk II for Eurorack grin grin

edit: I also wonder, how it would react to audio in? hmm
nickgrys
There's some good advice here, thanks. I have a related question. Is an AD envelope generally better to use with a LPG than and ADSR? Will a "slow" vactrol be able to articulate the 4 stages of an ADSR well? Does the length of each stage make a difference in how well the lpg reposnds?
thetaflux
I think the slow decay time of the vactrol won't allow it to react to all 4 stages of the ADSR. Hence the slewing of any cv signals sent through it.

Make noise has just released a new line of transistor based LPGs in the Dynamix and 0 Coast, which have a faster decay time and can react to the contours of a 4 stage envelope.

So i'd say that AD envelopes are probably better for traditional vactrol LPGs.
luketeaford
nickgrys wrote:
Is an AD envelope generally better to use with a LPG than and ADSR?


If the LPG has a vactrol, it just can't respond as quickly due to the slewing. If you try to do AM on one, it will sound like vibrato (hint: this is a cool way to patch vibrato). You can get nice ADSR shapes, but it's nothing like a really plucky subtractive synth envelope.

With Optomix, I like to patch a gate signal to the strike input and to the CTRL input and dial back the attenuator. That makes a kind of rudimentary ADSR (and the damp control is effective for shaping it, too). thumbs up
MRoyce
raccoonboy wrote:
I also wonder, how it would react to audio in? hmm


Do it and find out!

Any slew, even a vactrol, is basically a low pass filter. Depending on how fast the vactrol reacts you'll get some audio rate modulation out of it but don't expect to get too high in frequency. Really low audio-rate Saw can be cool to add a ripple effect, for instance.
raccoonboy
MRoyce wrote:
raccoonboy wrote:
I also wonder, how it would react to audio in? hmm


Do it and find out!

Any slew, even a vactrol, is basically a low pass filter. Depending on how fast the vactrol reacts you'll get some audio rate modulation out of it but don't expect to get too high in frequency. Really low audio-rate Saw can be cool to add a ripple effect, for instance.


I will as soon as I own one grin

thanks for the tips
captjrab
One little quickie I would add here is to attenuate the trigger you have going into the ping input. It gives you control over the tone of the vactrol. Blend that with your CV in and you can get a lot of variation with your percussion timbres and some wicked bass sounds too.
starthief
nickgrys wrote:
There's some good advice here, thanks. I have a related question. Is an AD envelope generally better to use with a LPG than and ADSR? Will a "slow" vactrol be able to articulate the 4 stages of an ADSR well? Does the length of each stage make a difference in how well the lpg reposnds?


Different LPGs react differently, but active LPGs can work well with an ADSR. It can sound nice to have a fast attack, a relatively sharp decay and just enough sustain to keep it open a little bit -- basically just adding a tail to the classic vactrol ping sound. Of course the vactrol messes with the timing, so you'll get different results than with a VCA, but it's still worth trying.

Passive LPGs tend to have a minimum CV threshold, below which they're closed. That can turn slow attacks into somehwat sloppy delays, or low sustain levels into silence. Triggers or gates generally work better with them than envelopes.
yeatsvisitslincoln
n00b question here. How hard is it to get a similar sound of out a normal vca? I get that I'd be losing the filter, but is it going to be THAT different?
cptnal
yeatsvisitslincoln wrote:
n00b question here. How hard is it to get a similar sound of out a normal vca? I get that I'd be losing the filter, but is it going to be THAT different?


A VCA will control the amplitude of a sound. Filters control frequencies. Use a filter instead, with the frequency all the way down and ping it with a quick envelope. Instead of increasing the volume it increases the frequencies that get through. Not exactly how a LPG behaves, but it's the workaround I'm using right now. That's how I understand it anyways.

Also, further to Starthief's point about each LPG being different - I've heard tell that because of the nature of vactrols each unit in a batch can be different to the next.
starthief
You can get some pretty good LPG results with a VCA, filter, and envelope, but it can be note fiddly to set up than an LPG which should just sound good right out of the box. Leaving out the VCA can work too in some cases. Leaving out the filter makes a very noticeable difference though.

One of the things LPGs do for you is make higher frequency sounds decay faster, which is generally how acoustic sounds work. Depending on the response, that can be a bit subtle or it can really color the sound.
baremountain
That faster high frequency decay is one of my favorite sounds.

I hadn't thought about vactrols adding slewing to CV, but that makes a lot of sense. That thought has clarified a ton for me. I'd be really interested to sit down with a Dynamix and an Optomix side by side and compare the two.
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