High Pass and low pass filters with variable frequency

[Vivek]

What is the best way to implement Pre-distortion low pass and high pass filters with variable cutoff frequency but no interaction with mid band gain ?



Preferably without ganged pots for cutoff frequencies.

[kraal]

Hi. You can check this: https://sound-au.com/project155.htm

[ElectricDruid]

What is the best way to implement Pre-distortion low pass and high pass filters with variable cutoff frequency but no interaction with mid band gain ?
Preferably without ganged pots for cutoff frequencies.

If you want to avoid ganged pots, you've only got limited options. Let's go through them.

1) Single pole filters
These don't need dual-gang poles, because you're only controlling a single pole. Simple. But often, this is enough. Depends on the job.

2) Voltage-control
Any multi-ganged filter that can be voltage-controlled can then be controlled with a single-gang pot. My PDS-1550 article started life this way - I realised they'd used a voltage-controlled filter (ok, current-controlled really) to avoid the need for a dual-gang pot. I'm not recommending their design,. but there are lots of 12dB/oct SVF designs you could use that will give you voltage-controlled HPF or LPF outputs.

3) Switches
Do you really need continuous control? Would "several options" be good enough? 2x6 rotary switches are easily available. The plus point here is that if you use a switch with 1% resistors, you'll have far better matching than you'd ever get with a dual-gang pot.

4) Something else I haven't though of
There's always this option!
PWM, maybe? Vactrols? Getting a bit esoteric...

HTH,
Tom

[marcelomd]

The VFE Dragon uses dual ganged C10k/C100k potentiometers for the filters.

[Fancy Lime]

+1 to what Tom said, as usual 
Dual gangue pots are really too imprecise if you want to have a consistent Q-factor. PWM is the way to go for 12db or more, imo. A single CD4066 plus a CD4069 and a dual opamp plus a bunch of resistors and caps and two pots will get you a Sallen-Key HPF plus LPF. That being said, are you sure you won't be happier with the MUCH easier 6db version? I usually like the sound of 6db filters better.

Andy

[bartimaeus]

most classic distortion pedals use single pole filters, and since this is a distortion design i'd recommend giving them a shot before trying two pole filters with more complex techniques (though the switches method suggested by ElectricDruid is straighforward and effective). maybe just a pot, a capacitor, and a unity gain opamp buffer per filter?

[marcelomd]

Should have said above.

The Dragon uses a pair of single pole high pass filters in series before the gain stage and a pair of single pole low pass after. The filters use the dual ganged C10k/C100k pots. Really simple stuff.

It's one of the ugliest schematics I've ever seen. I'm not sure if I can post them.

[Vivek]

Thank you everybody !!!!!

I need to study all comments above, and come back if I have further questions

[rutgerv]

You could also consider ICs like the SSI2164 to be at the heart of an active filter. A simple pot then allows you to sweep filters of any kind you like. Several poles, high pass, low pass, band pass. The data sheet provides nice examples. Currently I'm working on some kind of buffer and filter pedal to shape my guitar sound and intend to include a wah option with An external footpedal.

[Vivek]

2) Voltage-control
Any multi-ganged filter that can be voltage-controlled can then be controlled with a single-gang pot.

HTH,
Tom

You could also consider ICs like the SSI2164 to be at the heart of an active filter.

That's a great idea !

Some links for future reference :
http://www.soundsemiconductor.com/downloads/ssi2164datasheet.pdf
http://www.soundsemiconductor.com/downloads/AN701.pdf
https://www.youtube.com/watch?v=Qmd_MZ6IHOo
https://electricdruid.net/wp-content/uploads/2020/06/AS2164-Quad-VCA.pdf


I too am searching for other VCA or EQ chips to find out alternative ways to get flexible filtering options for Distortion pedals or AIAB applications.

[niektb]

I think you might be overdoing it... Exploring alternative ways of doing something is never wrong of course, but i'm not sure whether you'll end up with something that is 'more' useful than the stupidly simple standard (even though it might sound slightly different).

[Rob Strand]

I think you might be overdoing it... Exploring alternative ways of doing something is never wrong of course, but i'm not sure whether you'll end up with something that is 'more' useful than the stupidly simple standard (even though it might sound slightly different).

The simple standard filters force you to use pots,  pots normally force a filter with Q=0.5 (quite slow roll-off).   To get around that you have to use crappy Sallen and key filters with gain or State-variable filters which require a lot of opamps.

However, there is a buffered version of the Sallen and key filter, where you put a buffer between the two RC networks.  With that filter topology you can set the Q and use pots.

Edit:
Here,
https://www.scienceforums.net/topic/69559-modified-sallen-key/

[bartimaeus]

the ssi2164 is a great chip, but i agree it's total overkill if you just want to build a distortion with variable input and output filters.

the buffered sallen key linked above would be a more suitable option, though for some distortions even that is a more robust design than you'd need.

[ElectricDruid]

Agree with the above posters.

The by-far-most-likely option is that the filters on the VFE pedal at the top of this thread are simple 6dB/oct rolloff filters using a single pot and a single capacitor. That could be active or passive, it hardly matters. They have a soft roll-off and consequently a fairly "natural" sound - they reduce frequencies moderately rather than weirdly or extremely. Sometimes "weird" or "extreme" is what you want, but many times something a bit more subtle is perfect. It's tone "shaping" rather than total tone rearrangement!