Quad VCA crackle on gate input

Eurorack
#1

Hi,
pretty new to eurorack, works fine so far. Only problem I have right now:
Any sound source I plug into my quad vca seems to crackle when a gate is applied to the cv input. The osc (in this case a sine wave from dpo) sounds fine itself, but when used with a gate there’s a noticeable crackle applied that sounds like voltage. You may need to use headphones to hear it…

If I use an envelope instead of just the gate itself, its fine.
I tried with different gate (metropolix, midi 1u) and sound sources (dpo, plaits) and cables. Can somebody explain me how to use it correctly?

I made a video to illustrate the issue.

Patchnotes:
metropolix → pitch → dpo 1v/o → sine out → quad vca in 1
metropolix → gate → quad vca cv 1 in

Thanks. Andre.

#2

Hi Andre,

Analog circuits (like those found in Quad VCA, other analog VCAs, mixers, filters, etc.) tend to saturate or distort at higher levels of gain. This is often called “driving” or adding “drive” to the signal, and can result in added harmonics. This is a normal behavior and is often actually desirable, depending on the circumstance. However, in this case, it may be that the CV signal the Quad VCA is receiving is too high and is pushing the VCA beyond it’s intended limits, so some noise is being introduced into the audio signal.

In your video you will notice that the crackling sound becomes apparent when the response curve knob is set to extreme exponential values on the left. This is to be expected because you’re getting a much higher boost to the signal level (compared to the boost you would be getting with a linear response, the right side of that knobs range) with the same CV signal. If you still want to use a more exponential response, rather than a linear one, and reduce that crackling, I would recommend trying these things in different combinations and amounts: 1. Turn your Level 1 knob all of the way down, 2. Use a less extreme exponential response by turning the knob to the right by small amounts until you find something that you like, 3. Turn down the CV input attenuator (the leftmost knob) to lower levels until you hear the crackling go away.

You also mentioned that this isn’t an issue when using an envelope rather than a raw gate. This is probably because the envelope’s level doesn’t go as high as the gate’s level. It is less common to send a raw gate signal to control a VCA that is processing an audio signal, and the Quad VCA may have been designed with that in mind, and so, is expecting a particular voltage range at the CV input. If you still want to use a raw gate, rather than an envelope, in this particular patch, you may be able to decrease the level that the gate goes to in the Metropolix’s settings (although, I have not used a Metropolix, so don’t know for sure). But in the end, it would be simpler to just turn down the CV input attenuator on the Quad VCA, as mentioned above, and you’d probably get the same result.

Also, I can’t see what kind of output you’re using. If you’re not using some sort of module or other method of bringing the signal down to a standard line level from Eurorack levels, you might be getting the noise in your headphones/monitors from the high signal level, rather than from the VCA itself.

If you try sending that audio signal through a low-pass filter (LPF) before or after it goes through the Quad VCA, you may find that crackling to disappear or even add a pleasing character to the sound, depending on your tastes. From what I can see of your rack in the video, I don’t see any type of standard filter present. (I know that Rings is tagged as a filter on Modular Grid, but that is a bit misleading and can be especially confusing to people newer to synthesis as Rings works quite differently than an LPF, for example.) If you do not already have one, I would highly recommend getting at least an LPF for your setup, especially if you are using an oscillator like DPO - to tame down some of the harsh and harmonically rich waveforms that it’s capable of putting out.

I hope some of that helps! Hopefully I wasn’t misleading either, and if someone does see something that I’ve said incorrectly, please correct me.

Chace

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#3

Dear Chace,
thanks so much for your detailed response.

I will try out your suggestions asap. I am still waiting for a filter module.

Also an attenuverter module might help to tame down the raw gate signal. There might also be a setting for the gate in the metropolix…

I had a low pass gate, but wanted to save some space, so I got the Quad Vca.

I use a 1u headphone out for standard level output of my system.

I still need to learn how the gate, envelope and VCA work together in terms of CV in level, response curve and level knob. It’s all pretty intense how they interact with each other.

Thanks again.
Andre.

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#4

First thing I see is a 10 dB level boost when you switch to the gain CV input. Plus, the knob you are turning is the least commonly adjusted knob for that device, at least for me. I leave mine @ linear (far right) and use the envelope to adjust the curve (Stages and Quadrax). The knob that controls the amount of CV applied to the VCA is the one to the left of the one you are turning in the video. That is the knob to turn to reduce the overall signal and thus the crackle (I’m guessing). I’m also guessing you’re sending a 10v rather than 5v gate, and the envelope you use is 5v.
TL/DR: Turn down the CV Trim knob when using the gate CV. Use any audio meter to roughly match the level, and your distortion should go away too!

#5

Are you referring to the click and the onset and offset of the note press? This is because you are using a gate signal and not an envelope. With a gate, the square shape is so abrupt that it can cause discontinuities in the source audio unless it happens to occur at a zero crossing. Envelopes ease the sound in and out with more gradual shapes to avoid these clicks.

1 Like
#6

Hey Andre,

You’re welcome! I hope it helps when you get a chance to try them out!

You shouldn’t need an additional attenuator before the Quad VCA because that’s what the attenuator knobs at the CV inputs are there for. Although, if you wanted to invert the gate, then an attenuverter would help with that.

I wouldn’t quite consider a VCA to be a replacement for a low pass gate (LPG). (But that would of course depend on what you’re trying to accomplish.) An LPG does control the amplitude of the signal like a VCA, but it also simultaneously controls the harmonic content of the signal, like a filter. It’s like having a VCF and VCA combined into a single unit. They can be great for achieving particular sounds (like staccato notes or percussive sounds) in a simpler package. However, they do not allow the dynamic range that is possible with a separate VCF and VCA in series because the VCF and VCA can be controlled individually and with different CVs.

Yes, that’s where I find a lot of the fun of working with modular comes from: experimenting to see how everything interacts and affects each other! As far as the Quad VCA goes, I generally leave the level knobs turned all the way down when using a signal at the CV input to allow the widest range of dynamics. However, if you’re using it as more of a mixer, or just need to hear the signal passing through the VCA, then I would turn the CV attenuator down and turn the level up to suit the occasion. As far as gates and envelopes, most commonly a person would send the gate to trigger the envelope, and then send the envelope signal to control the VCA. Envelopes (attack, decay, sustain, release, etc.) are much more dynamic and flexible than a raw gate (on, off… on, off) and will allow you to sculpt your sound in many more ways. Also, remember that an envelope without a “sustain” section will trigger off of a gate the same regardless of the gate’s amplitude (in most cases) or length. If you are using a “sustain” in your envelope, then the gate length becomes important because it will affect how long the envelope is held at the sustain level. (Make sure to read your Stages manual! It will help a lot if that’s your primary source of envelopes!) I realize you may already know some of that stuff, but I thought I’d put it on here anyway in case it helps someone. =]

Chace

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#7

No, not on/offset. I was talking about the crackle during the actual sound. Might be only audible with headphones. Don’t use raw gates on the vca, so I’m fine. Thanks for all the great help you guys.

#8

OK, yeah I gave it another listen. It’s probably clipping distortion from being set fully exponential and sending too hot a signal.

#9

On a lot of modules the gate outputs are typically fed from digital pins on a microcontroller, through a buffer or amplifer, so they aren’t as precise / regulated as typical CV outputs and in some cases can be a bit more noisy. Since digital and gate inputs are typically only concerned with whether or not the level is beyond some threshold it’s usually not an issue in most uses, but if you’re feeding it directly to the CV of a VCA you may be able to hear some of that noise. If the VCA is operating in exponential mode that effect will be amplified.

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