Improved 'Rat' Style Tone Control

[liquids]

http://sites.google.com/site/liquidselectronics/circuit-blocks/-rat-style-tone-control

This is nothing new or interesting to those already well familiar with active tone controls/filters. 

However, a fair majority of the the tone controls in pedals I've seen are passive, which surprises me more and more, as I've been studying, and hence begun using active tone controls in my experiments/designs a lot lately, an really liking the benefit/cost ratio over passive in my designs/mods.   Active tone controls do offer a lot of flexibility and far less (or far more, if you prefer) interaction with the 'core' tone of a design, all at the 'cost' of just one or two additional op amp stages.

As for this particular circuit block--it seems that more often that not, if given only one tone pot in a pedal, most of us want a way to control the high end.   So this one is a rather simple, non-threatening active filter in that vein, for starters.  No frequency boosting is available even, just active high-frequency cut as compared to your bog standard passive tone control. 

For the curious, it's a bog standard "Sallen-Key Low Pass Filter" with control.  It can function as a 'direct' replacement for anything from designs that use a Rat-type tone control or in your designs, to an improved tone control for your guitar, be it on-board or in-line.

The schematic does ask for a bit of understanding of active filters (read: encourage a bit more learning/study about filters), as I left the schematic as I realize I left this 'building block' without any starter values, and likewise assume a bit of knowledge about op-amps since I drew it as working off a bipolar supply.  It also stipulates use of a dual gang linear pot, though two single gang pots of the same value can work for the expirimenter on the breadboard.   

So, it's not exactly for the uninitiated on a few different levels.  However, it can be tinkered with quite simply if you have a little bit of experience and understanding of filters under your belt, utilizing a breadboard and a few simple resources like these as you go:

http://sound.westhost.com/dwopa2.htm
http://www.muzique.com/schem/filter.htm

I do hope this inspires new designs and maybe some new mods to 'old' designs, and adds to the pool of all the great info available here on the forum.  Hopefully more to come as well.  Cheers!

[DougH]

I've played with active filters and for most tone-shaping in dirt boxes I think steep rolloffs can be way overkill. They sound good for a synthesizer-like "low pass filter" effect. But IME, the gentle rolloff of a passive filter tends to work more smoothly for getting the high freq content "just right", whereas an active can sound quite jarring in comparison.

Not to discourage experimentation, the TS uses an active tone control after all. It's just that this has been my experience with trying to do a simple LPF with a steep rollloff. Now, for something like a parameteric EQ or speaker simulator- where you need to carve sharp frequency contours, active is definitely the way to go.

Oh, and I don't care for the passive Rat tone control much, BTW, as it affects volume as well as frequency response. Mark Hammer's SWTC is far superior in many ways as is the simple shelving tone control that Jack Orman has posted on his site.

[liquids]

But IME, the gentle rolloff of a passive filter tends to work more smoothly for getting the high freq content "just right", whereas an active can sound quite jarring in comparison....

Oh, and I don't care for the passive Rat tone control much, BTW, as it affects volume as well as frequency response. Mark Hammer's SWTC is far superior in many ways as is the simple shelving tone control that Jack Orman has posted on his site.

The SWTC is basically the same as a passive 'rat' tone control, it just keeps it from sounding like it interacts with a volume control if properly placed since the resistance is constant rather than variable.

Likewise, lots of effects-like the Box of Rock, for example--utilize two passive filters for getting the high freq content 'just right.' Often a separate tone stack is in there as well for even more filtering of highs and/or lows.   That's a lot of filtering too.  With all that passive filtering, oftentimes not only is impedance a factor, but overall there is a lot of loading, and interaction when knobs are turned.  Likewise, the filters are often dis-similarly tuned with heavy-handed roll off points, which can likewise be jarring and hard to dial in, IMO.  That's why, after wrestling with many passive tone controls in my expiriments, I've starting trying actives, and it's been an an enlightening experience.  

And hey, have you tried this particular one?     Don't knock it till you try it...

[wampcat1]

I personally think it's not possible to say that either active or passive is always easier/better/etc - I think it depends on what you are going for. Personally, I think boosting frequencies post gain can make a pedal start sounding very synthetic, at least as far as overdrives and distortions are concerned. Just my personal opinion though... if boosting frequencies, a little goes a long way
bw