Equations for Parametric Equalizer

[Yoshi]

Hello!!! I have this parametric eq (clasic tubescreamer style) but I don´t know what frequency are being affected, nor the boost nor even de Q of the eq...

So plzzzz, can anyone plz provide me the ecuations of this parametric equalizer? for the boost, frequency effect and Q.

I would be very greatfull

[antonis]

http://www.geofex.com/article_folders/eqs/paramet.htm

[Vivek]

https://www.diystompboxes.com/smfforum/index.php?topic=113138.0

[iainpunk]

that snippet isn't a parametric EQ, its more of a WAH-type of filter.

it will boost 7dB of whatever frequency you set it at, with a fairly wide Q of around 1
it ranges from 170 Hz to 2400 Hz

good luck

[Fancy Lime]

I seem to remember that dschwartz posted a bridged-T based mid EQ some years ago. Or was it somebody else? I toyed with the idea with spice but never went further than that. Seems very good for a five or so band EQ but doesn't really work for semi parametric.

[iainpunk]

afaik, the problem with bridge T and single opamp is that its either boost OR cut, not both on one control. for that, you need to have a boosting and cutting filter cascaded and with a single control determine the depth of both, if one goes deeper, the other goes shallower.

i have been playing with bridge-T filters for a few years now. i have also come up with a few cool distortion circuits where one or more bridge-T filters are used to determine the character of the drive. currently doing an HM-2 inspired boost ( /half wave rectifying boost)

cheers

[Fancy Lime]

You can have boost and cut in one filter like so:
Non inverting opamp. One side of a linear pot goes to the inverting input. Wiper goes to the output. Other end of the resistor goes into the next stage. Two caps in series with the resistor to ground in between go between both ends of the pot.

If the pot is turned to one side, the bridged-T is in the NFL, so it boosts, on the other it is just passive to ground after a buffer, so it cuts. It is, as Iain said, like two filters being counter-swept but implemented as one and the same filter. Very cool idea, imo.

Advantages: high input impedance, non inverting. Disadvantages: high output impedance, not really possible to turn into a semi-parametric except by switching the caps. Neutral: it is not a constant Q filter.

Disclaimer: not my idea but I cannot remember or find the original post. I am about 80% confident it was dschwartz who posted it.

Andy

[Rob Strand]

Peavey MK3 era had a three-opamp bridge-T semi-parametric, (IC5)
https://music-electronics-forum.com/filedata/fetch?id=922006

Musicman bass amps had a two-opamp bridge-T semi-parametric, (IC5)
https://irationaudio.files.wordpress.com/2016/06/musicman-2100-b-2.jpg

These days the single opamp (Wein, not Bridge-T) semi-parametric is more common/economical,
https://www.diystompboxes.com/smfforum/index.php?topic=95483.msg827441#msg827441


Something else: to some degree the Musicman circuit can set the peak gain independent of the Q.  Long ago I remember deriving a Musicman type circuit starting with the Peavey circuit (to save an opamp).   Then I found the circuit was used by Musicman and they added an extra resistor to do the peak gain/Q tweak.   [All from memory, it's while since I've gone through the details.]