# Sherwood EQ



## sdnalloh (Dec 10, 2019)

I'd like to play around with some mods to the EQ section of the Sherwood, but I'm not sure what's going on there.

Could anyone point me to someplace that explains that particular circuit and how to calculate the affected frequencies?


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## Robert (Dec 10, 2019)

The EQ is made up of two transistor gyrators.   These are the building blocks of most graphic EQ circuits. 

There's a good bit of info available about them, and Jack Orman even has a nice calculator tool to work out the resistor/cap values needed for a particular center frequency / Q / gain.






						Guitar Pedals: Gyrator Filter Calculator
					

Calculate freuqency cutoff for guitar effects pedals with vacuum tubes and solid state electronics. FREE calculator.




					www.muzique.com


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## phi1 (Dec 10, 2019)

Robert said:


> The EQ is made up of two transistor gyrators.   These are the building blocks of most graphic EQ circuits.
> 
> There's a good bit of info available about them, and Jack Orman even has a nice calculator tool to work out the resistor/cap values needed for a particular center frequency / Q / gain.
> 
> ...



do these formulas work even though the configuration in the Sherwood is different than the one pictured with that calculator?


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## Chuck D. Bones (Dec 10, 2019)

Here's what you need to know about the Sherwood EQ...
BASS is wired for boost & cut.  Set it to noon and the bass response is flat.  Treble is wired for boost only.  To get to flat, you have to set TREBLE to zero.  If you are getting too much Treble, you might want to read this thread.
Changing the gyrator components will change the frequency ranges for the BASS & TREBLE controls, and the max boost & cut.


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## sdnalloh (Dec 11, 2019)

Looking on the Earthquaker website it says:


> Bass: Selects amount of low frequencies around 80Hz. Clockwise to boost, counterclockwise to cut.​Treble: Selects amount of high frequencies around 2kHz. Clockwise to boost, counterclockwise to cut.​


This suggests that the Sherwood's treble-cut-only EQ is different from the Westwood.


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## phi1 (Dec 11, 2019)

Or the terms boost and cut are relative, and eqd considers nominal position to be noon, so turning it ccw reduces treble relative to the center position, even if that part of the circuit doesn’t technically attenuate the treble frequencies.  

I believe pedalpcb traced this one himself, so it’s up to him if the Sherwood matches the eqd exactly.


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## sdnalloh (Dec 11, 2019)

Plugging values into the calculator I get:

*Bass*

R1: 6200 Ω
R2: 6200 Ω
C1: 1 uF
C2: .1 uF
Freq: 81 Hz
*Treble*

R1: 6200 Ω
R2: 6200 Ω
C1: .047 uF
C2: .0047 uF
Freq: 1728 Hz
Note that C15 on the schematic shows 4n7, but to get ~2kHz I had to change it to 47n.

Both of these give a Q of 0.32, which calculates to 3.5 octaves, which seems pretty wide to me.


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## Chuck D. Bones (Dec 11, 2019)

That is the bandwidth at -3dB (1/2 power) side-to-side.  It's supposed to be wideband because these act as Bass & Treble controls, not as a parametric EQ.  Typically, when these type of Bass & Treble controls are implemented, the BASS is tuned to the bottom of the audio range and the TREBLE is tuned to the top.  Even though the Sherwood's TREBLE control resonates at 5.4KHz, it's effect reaches down below 1KHz because of the broad bandwidth.


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## Chuck D. Bones (Dec 11, 2019)

Not to beat a dead horse or anything, but here is an LTSpice sim of the Sherwood's tone controls in action.  This is the freq response from the input jack to the output jack.  DRIVE is at zero, LEVEL is at 10.  
First pic is BASS sweep from zero to ten with TREBLE at zero.
Second pic is TREBLE sweep from zero to ten with BASS at noon.
Third pic is all nine combinations of BASS & TREBLE at zero, noon and 10.


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## sdnalloh (Dec 12, 2019)

Thank you so much for those graphs!


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## sdnalloh (Dec 12, 2019)

The reason I wanted to adjust the tone controls is because I play electric violin. I was concerned that the bass knob wouldn't affect my low strings (G3 196Hz). But your graph shows I'll have ±8dB at ~200Hz, which should be plenty for me. Also, the treble control seems very reasonably here, even though it's only boosting. 

I'm slowly learning what I can about pedals and audio circuitry, but there's a lot going on.
Thank you for your help.


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