Filter "quality"

When people talk positively about the quality of a filter, like celebrated VCFs on Moogs or the MS-20 one - I’m wondering - if you take the dynamics out of the question (so when it’s not being modulated by a VCO, envelope or by hand or whatever) - is there still something to get excited/opinionated about?

I guess another way of putting it - does the eq/ timbre you hear at any given setting on a particular filter have something unique that you couldn’t achieve with pretty much any decent eq, or are people really always talking about the dynamics across the cutoff and resonance ranges of a filter?

I’m thinking here as much about digital / software emulations as analog gear…

Unlike an equalizer filter, a synth filter aims for different results and handling from a circuit design viewpoint. More often than not, a certain form of nonlinear behaviour is desirable in a synth filter, whereas in an equalizer filter design, nonlinear behaviour can also be considered undesirable (if the original EQ design brief is for the equalizer to sound ”transparent”, that is).

Dunno if that answers your question, just some food for thought…


Isn’t it just the same as when people talk about nice EQs?
I always understood that what is “better” about a given filter/EQ is the careful selection of certain curves and slopes, and the precision (or imprecision) with which they are generated.
Each curve or slope, and their various artefacts from particular components, may or may not sound more musical and pleasing to different ears.

But I definitely take your point. The real magic often tends to happen when you modulate it.

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Not exactly IMO. Some EQs deliberately try to sound as transparent as possible. Such a design goal is very rare in synths IME.

My replies are more towards the ”cant I just mimic a synth filter with an eq” part than anything else in the OP.

Sure, but that’s still to do with the slopes you apply to the EQ bands
A filter is technically just an EQ with one band and a drastic slope isn’t it?

Agreed though. You could maybe get close with a very versatile EQ and a bit of tweaking, but I agree, a lot of what will make some analog filters sound good is actually the subtle imprecision in the components, and that would be impossible to emulate with an EQ.

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that “drastic slope” is probably part of the answer though, right?

like… a digital EQ or filter can have a 96db slope, most analogue ones won’t, and how they achieve the slope they achieve won’t be perfect. Not perfect usually means “character”

so I guess the answer for plug ins is probably “are they trying to be transparent?” if they’re not and they’re trying to recreate an analog filter style, they probably will sound a little bit different at the same cutoff/resonance.

agree that it’s probably more apparent when they’re moving, but even “all pass” filters cause coloration.

Nerd out:

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The answer to both your questions is Yes.

However, My answer is entirely dependant on the fact I own 2 Moogs, an original MS20 and a decent Equaliser.

The only way for you to answer your question is to sit down with some filters of note (MS20, Moog, SH101 etc) and play with them. Then play with an equaliser of your choice.


Definitely there is for me.

I do the following, if I want to “understand” a particular filter better. I am using a low pitched saw tone, pitched like a baritone, because this seems to make all the various changes in the sound charcter to stand out better. I sweep manually the filter between the closed and open settings, apply different levels of resonance, and study the change of tone.

  • some filters just get brighter or duller and keep it quite neutral
  • some filters generate a coloration, which changes between closed and open
  • few filters begin to “talk” like building very simple formant like tones
  • few filters create some kind of “sing-sang”

Sometimes those effects are not so easy to notice, if the filter is used with fast modulations. If the filter is set to one of those “sweet-spots”, it can generate a very interesting sound. It’s interesting to check out different filters of different manufacturers, because often the results are very different too.

Also digital filters are very interesting in a fixed position, like setting a comb-filter to a particular sweet-spot.

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I might be misunderstanding, but an example of what could be exciting about a filter over an eq - I’ve made sub bass using the lo-pass filter on the Analog Heat… didn’t run any audio thru it… just used the filter on it’s own. Window rattling sub bass.

The Analog Heat & Ms-20 (I have the mini) have self oscillating filters & I’m not 100% if this is the reason, but the filters on both are great.

I def think filters do something to the sound that an eq couldn’t, but the only eq I use is on my mixers, my MPC’s & the Analog Heat, which all have really basic eq’s. But for example when I use a lo-pass on vinyl it takes a lot of the air out of the sound & the higher pitched frequencies out of the crackle & then I turn the resonance up to bring back some of the high frequencies…AFTER the air & high frequencies in the crackle are removed. Is it possible to do this with an eq (genuinely asking)?


It’s important to realize that in many synth filters, the character and quality of the resonance is of huge importance, as well. Some patches use purely filter resonance as the oscillator source, in fact. Different filter designs will have significantly different resonance characteristics and timbre.

Another difference is the timbre when the filter is overdriven - some filters load up very nicely, almost like a tube amp, others get harsh and break apart (this might be nice in your context, too!). Sweeping the filter when overdriven adds another aspect of dynamic timbre, which could be very different from filter changes when normally driven.

Also, the overtones and harmonics that a filter tends to let through in the stopband or transition band are not just a function of it’s slope (that’s only in an ideal filter) but very slight resonances, harmonics, and other -as previously mentioned nonlinear- tonalities can change the shape of the sound. Even the shape of the transition band is not guaranteed - one 2 pole filter does not have the exact same transition shape as another, even though they both have the same relative cutoff point and slope.

All of these factors are definitely audible if you know what to listen to (some are audible even if you don’t!), and matter more or less to different people in shaping the sound.


Great post / explanation :grinning:

Without question, yes there is. The design of many of the most memorable filters in use is such that they add a recognizable and substantial character to the sound. I rarely use crazy modulation on any of my filters, I control them manually. The character comes from a combination of the components used that may result in slight distortion, a peculiarity in slope response, other odd imperfections leading to nonlinear response, noise, etc. It’s not the clinical sound that sounds right to most ears.

I’m going to assume you’ve never tweaked a continuous variable state filter?

Also, tweaking the frequency on a Pultec is something that will not sound much like your hifi graphic EQ.

Sure is. What is happening in that instance is just a freq boost occurring right below the LPF cutoff point, or around the cutoff point. You can do the same (although in a less drastic way) with an eq. A so-called push-pull eq, something like a pultec, does this by design, but you can also do it with two bands of parametric eq, say with a high shelf cutting the freqs down at a specific Hz value and then boosting with a peaking band (with Q factor/width set to taste) slightly below that cutting shelf eq freq.

You can do the reverse of this as well btw (with a resonant HPF). I often experiment with it on my low freq sounds, sometimes it can work wonders.

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Thanks for answers all!

Wish I hadn’t put the eq comparison in there - not a realistic alternative when drive and resonance are what you’re after. Thanks @delta-c and others!. And thanks for reminders that cutoff slopes are a big part of the filter character -( I was thinking more about dj / kill eqs when I mentioned eq so maybe not so far from a filter as a multiband )

So yeh forget eqs here. But filters enjoyed/ reviewed / talked about without sweeping around?

I really started the topic because musically I want to get away from filter dynamics when I’m playing. Using fast envelope modulation on the filter is really another matter, but freq sweeps and resonance/drive boosts are feeling stale for me.

When I hear a filter praised I’m pretty sure they’re 90% talking about the cutoff curve, or at least the changes in cutoff are what ‘reveals’ the filter’s overall character.

So maybe a different request would be - can anyone share their thoughts on filters (specific or general) used entirely statically - Dialled in and unmodulated?

IMO there is much less variation between different filters when used statically, the only variables are slope, resonance and whether or not there is compensation, frequency response and gain, so assuming a test between filters with similar characteristics then you are much less likely to notice much difference.

As mentioned earlier by others the response of a filter to modulation, input gain, output gain, linearity or lack of, resonance gain and compensation or lack of, and circuit topology are the main things to consider.

Particularly modulation response and type can really make a filter come to life, without getting too weird I think of it like a tickle response, and some filters are much more ticklish than others around the cutoff frequency, especially when the resonance is higher.

The number of poles can also be a significant factor in how a filter affects a signal, and more isn’t always necessarily better here, I modified my SH-101 to have from 1 to 4 poles and with just 1 pole it sounded really nice, I ended up selling it to a rather well known artist who went on to say it was one of his favourite synths, and this guy has a lot of synths!

I guess I’m kind of a filter fanatic, but I have very specific tastes and technically perfect filters (or more correctly those that are closest to perfect) tend to bore me, I’d take character with flaws any day.


I wouldn’t even recommend to use synth filters static at all. At least for most timbres.

Sounds created in nature, or with mechanical instruments, or with our voice are always living between a beginning and an end - and - most important - they are never static. There is not only the difference between loud and quiet, there is also much movement and change in the frequency domain. If we want to create sounds, which most people might like, we should try to get a most natural feeling and behaviour of the timbre.

One reason, why early digital synths have been called “cold” is that their sounds have been too static, too clean. If it’s not for a particular musical or artistic reason, I always try to get sounds as “natural” as possible. Since synth filters in subtractive synthesis are the most important tools to give a sound shape, expression, and life, IMO modulation of the filter parameters is essential.

Have you ever tried to set up a filter envelope and then only change the intensity of the modulation? It often makes a big difference. I use this often, when playing a synth live, because IMO it sometimes generates more expression rather than moving the cutoff frequency manually.


Absolutely, but not a “fanatic”, just a human beeing … a not-perfect character is natural, because nature isn’t an exact and simple mathematical equation, it’s complex, it’s chaos, it’s order, all simultaneously in various degrees.

I think most humans just prefer a “natural” experience :smiley:

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Well, in the kind of so called super saw territory, however you chose to get there… (inset opening can of worms emoji)

Take the SEM on the OB6. Put the filter on the edge of LP-Notch or HP-notch, and play a slightly detuned 6voice unison, err, “super saw”.
Do the same on a ProphetRev2, with all 16 voices or less, but it only has a LPF.
With fewer voices, the OB6 filter ads so much more richness to the sound, that the Rev2 needs to look for effects to rival it, and can’t.

Usually, in practice, if I can’t get a sound out of my head and into a synth, I usually setup the same patch on another synth and once in the ball park the filter is the first thing I start on. On straight forward basic patches I find the OB6 filter is, well, always perfect. Then the Rev2 is the most forgiving filter but sometimes too forgiving that I wonder if it’s a real 24dB filter (not complaining). Then the Minitaur is just sweet wysiwyg, and then the Subsequent37 filter is straight to the point no questions asked but can go either way quickly.
Those synths I use every day do to speak.
The AK and A4mk1, gathering dust now, I never felt like their filters made a standout contribution unless they were being modulated. The moog Voyager was much like the Subsequent37. The minimoog model D, was “like” the Minitaur.

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Yeh. Suppose so. When you start messing with the dual mode and spacing, the Voyager is a completely different beast.
I just recall, when I had it and the Subsequent37 on the same bench, set to dual LPF with same cutoff, it was responding much like the Subsequent37 filter. Too many days of A/Bing when I got the Subsequent37. Probably just my ears after prolonged saw wave garble.

I might have been too hard on the Elektron filters too. They do their job. What I really meant was, they just didn’t add any colour that I could hear.

To answer the OB: Yes, there’s stuff to like and not like with filters even when taking LFO, sweeps and envelopes out of the equation.

  1. Input drive, how does the filter sound when getting low, normal and high/driven audio levels in. Some sound good others sound like shit with dynamic material.

  2. Some filters have a kind of detached resonance to them like the LP on the Moog Minitaur. Some like that others don’t.

  3. Some filters have sweet spots all over the range, other don’t. High resonance settings can be horrifiC or lovely.

  4. LP filters can muffle the sound and loose punch when cutting away lots of frequencies. Like a volume drop making the signal lifeless. Others just keep the “in your face” attitude on drums and bass when cutting away everything from 200Hz and upward.

  5. Some filters, like the one in the Analog Rytm Mk1, affects the signal even when they are fully open making stuff like lofi samples sound great, other filters have no “coloring” and are very transparent.

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