Simple Passive 3 Band EQ with Sweepable Midrange


2016-01-03 5:13 am
Hi all, I wanted to get any thoughts or suggestions about this circuit I made. It has an input and output impedance of 1k and 1k, so it works ok with mp3 player/phone to auxillary input in car, etc (10k input impedance). I also multiplied all the resistors by 100 and divided all capacitors by 100 and used it in a guitar distortion preamp pedal. The midrange sweep is a bridged t filter, and as the frequency increases, so does the q but the notch depth decreases. Practically speaking, it does seem to have utility as it will sweep a fair amount of the midrange even though its simple. Has anyone seen this used? My goals were to make the simplest passive 3 band with mid sweep. I also wanted the very low loss when all knobs dialed up.
I also put directions on how to make one without as much bass loss, published on page: Page not found - Gary Davenport Electronics
Thanks all for looking and suggestions.


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2017-12-28 12:06 pm
Hi Gary,

I've been searching for info on super simple sweepable eq, not much around.

This looks perfect for me, I need to remove a 2.2Khz peak from my violins valve amp.

I see it's no longer on your website, did it work ok in the long run?

Why did you us a bridged T instead of twin T?



2016-01-03 5:13 am
passive eqs with sweeps

Hi Ben, sorry about the delayed response I had some things come up and haven't been able to get online much.

Hmm, I think it took it down because I was concerned it might load the source down too much under certain conditions. I looked at input output impedance at 1khz, but not the whole spectrum. Like if the treble is turned down all the way, the sweep and the mid. So here is a fix I applied, its for a low impedance source like a cell phone etc a newer device that would drive headphones.

Also, I will attach a second style I made today after I saw your post. This one would probably need logarithmic/audio taper pots. It just uses a "bleed" capacitor after a 100ohm resistor with a much larger pot to limit any bleed. Then it goes into same bridged -t.

Then it gets a little trickier. For the bass control, I use a bass blocking capacitor and then use a pot to go around that capacitor. The thing is, that capacitor and the input of the next stage form the high pass filter, and so the bass rolloff frequency (and bass loss) is dependent upon the input impedance of whatever your plugging eq into.

I'm expecting this to be fed into 10k input impedance, so I put a 22k resistor there. If for some reason you plug into high impedance input 100k-1Meg etc, it will parallel with 22k and you will still get bass roloff, just different frequency and depth. If you plug into 10k input impedance you get 10k||22k -> about 7k so it should work reasonably well plugged into anything. I have not tested this circuit, I think you would have to play with different pot values to get it to taper ok.

However (other than the sweep control) none of the pots are critical if you build I would try by ear.

Why the bridged -t? Just because of simplicity. I built some other types with dual ganged pots to change frequency etc, but this seems to get the job done.

If you really need to remove 2.2khz signal, bridge-t is a wider q, so may or may not be good for you. I would probably change/lower caps in bridged -t changing in pairs at the same time, and raise treble cutoff frequency.

You could try .33u on treble control, .33u/.033u pair of caps on bridged-t, that'll sort of raise the high-mid cutoffs. It won't be a narrow notch you get.

(The mid control when all the way down may pull down treble and bass, the bridged-t is kind of wide)


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