This Week on the Breadboard: The FreakZEQ

[Chuck D. Bones]

This EQ pedal kicks ass. My breadboard is built pretty much per the build docs, but I subbed a few parts when I didn't have a few oddball values. A word of warning, this circuit does not take kindly to mods. Don't ask me how I know that. If I had to guess, this design is based on an earlier design that used 4558 opamps or something like them. TL072 opamps are much closer to an ideal opamp and don't need a bootstrapped bias circuit (C3) on the input. It works fine if you follow the build docs.

Parts I subbed:
R16 = 220Ω
R17 = 330Ω The Q still goes plenty high
R18 = 24K

I didn't have a B10K dual, so I subbed a B50K dual for the FREQ pot and made these changes to accommodate the change from 10K to 50K:
R14 & R20 = 22K
R15 & R19 = 10K
C8 & C10 = 1.5nF
C9 & C11 = 6.8nF
With these changes, the Bass tuning range is 100Hz to 850Hz. The Treble tuning range is 600Hz to 4.8KHz.
This tuning range is about the same as stock, but doesn't go quite as high. There's not much going on above 5KHz anyway. The tuning range is easily altered by changing C8-C11.

The Q knob goes from mild to extreme.

The GAIN knob sweeps from notch (0 to 5) to bandpass (5 to 10) with 5 being flat freq response. I think of it as a Filter Intensity knob. It does affect the gain, but only near resonance. The gain everywhere else is 6dB no matter the GAIN setting.
You're likely to saturate the filter if you dime both Q and GAIN.

While it definitely does the "Stuck Wah" sound, that's just the tip of the iceberg. With variable Freq, Q and Filter Intensity (GAIN), this pedal does everything the VarioBoost or Haunting Mids does, and then some. In fact, it bears some resemblance to the VarioBoost and Haunting Mids circuits, except it has a State Variable filter inside the feedback loop instead of a Wien Bridge filter. More features requires more parts, which is why the FreakZEQ contains three dual opamps.

Run it on 18V for additional headroom.

Knobs (L-R): Q - FREQ - Bass/Treble sw - VOLUME - GAIN

 

[Blackboarcult]

I wasn't aware of this pedal actually, thanks! This looks familiar... Yamaha G100-II:

 

[Chuck D. Bones]

I like that circuit! It can be tuned to work just like the SoK and has some advantages over the SoK.
It's DC-coupled throughout, so biasing is simple.
No Vref buffer req'd.
No interaction between GAIN & Q.
The only downside is it requires one more opamp for the input buffer.
One could implement the BASS/TREBLE freq range switch the same way as the SoK, which lets us make the FREQ knob have a smoother sweep.

Definitely gonna breadboard that one!

 

[Chuck D. Bones]

Here's the latest, based on the Yamaha G-100-II Parametric EQ circuit. Very similar to the FreakZEQ, but has a little more tuning and Q range. Both work well. I re-familiarized myself with Bill's Law a few time on this one.

 

[Blackboarcult]

That's a beaut'!!! I really need to breadboard this one when I'm done with my current preamp (or when I scrap the whole thing in frustration, 50/50 odds...). I will have to scale up values, since I only have C100K ganged. I've been wanting to do the perfect 80's boost with a compressor into a tight control (BEOD style) into a paramid like this one. Will definitely riff on yours!!

 

[Chuck D. Bones]

C-taper will have a funky sweep in this circuit because it was tailored for B-taper. For C100K to have a decent sweep, leave pin 1 open on both pots (delete R16 & R19). change R17 & R19 to 15K and scale C6-C9 up by 1.5x. Not quite as smooth of a sweep as the circuit above, but definitely good enough for experimentation.

If you want Q to go lower, change R12 to 100K.

 

[Blackboarcult]

Oooh, I see what you did now with the freq pot. I can't f**** visualise things without spicing them...
I might indeed lower R12 to 100k for a broader Q, and change C6, C8 to 2.7nf & skip the switch - I don't think I'll ever want to boost under 400hz or above 3k.
I did find a ganged B20K among my wrecks, might adjust to that instead.
Thanks again!

 

[cooder]

Indeed quite a surgical frequency shaper... lots to dial in, before or after drives and all those options.
At present I prefer it before drive and to get that cocked wah push that can be very well fine tuned with this.
Noice!
Because of the flexibility and somewhat interactivity of controls not as straightforward as the Varioboost for example, however a fair bit more capable. Finding out all the different settings that hit my sweet spots will keep me going for awhile.
Great circuit design again!

 

[Chuck D. Bones]

Another killer build!

 

[Cybercow]

It would be interesting to see a waveform range chart for this circuit. And a question: can it be easily modified to exact a variable Q notch filter as well? I want to put a crunch circuit in front of this, but with a Q that can notch as well as bump.

 

[Chuck D. Bones]

The FILTER knob goes + or -. At the minus end, we get a notch. This is with Q at 10, FREQ at 5, RANGE at LOW, FILTER at zero.

 

[Cybercow]

Indeed quite a surgical frequency shaper... lots to dial in, before or after drives and all those options.
At present I prefer it before drive and to get that cocked wah push that can be very well fine tuned with this.
Noice!
Because of the flexibility and somewhat interactivity of controls not as straightforward as the Varioboost for example, however a fair bit more capable. Finding out all the different settings that hit my sweet spots will keep me going for awhile.
Great circuit design again!

 

[Passinwind]

Indeed quite a surgical frequency shaper... lots to dial in, before or after drives and all those options.
At present I prefer it before drive and to get that cocked wah push that can be very well fine tuned with this.
Noice!
Because of the flexibility and somewhat interactivity of controls not as straightforward as the Varioboost for example, however a fair bit more capable. Finding out all the different settings that hit my sweet spots will keep me going for awhile.
Great circuit design again!

Really sweet! This is the second prototype of my problem solver / Swiss Army Knife EQ pedal:



I'm doing a revamp of my old PEQ format for v3, I will definitely have to throw this one into LTspice and see how it compares to my latest try. Right now Q interaction with frequency is a "feature" that I may want to lose, but there are some upsides to it as well. V2 works especially well around drive widgets, especially the resonant LPF section. I'm running +/-15V internally via a Traco brick, so tons of headroom. I've also contemplated subbing a bass control in place of the HPF, are maybe just adding one.

My current PEQ acts like so at a fairly broad bandwidth setting with the HPF wide open and PEQ boost full up:



...which are actually some great sounding curves for some use cases. Freq is set by a single section C taper pot, which was what originally attracted me to this topology. I've used in a several DIY bass amps and preamps over the years with nice results, but for may other applicatiosn I don't necessarily want all that interactivity.

And yes, this will be yet another one of my open source modules eventually, with a board and BOM share. So suggestions gratefully accepted, as always.

 

[Chuck D. Bones]

Just to be clear, this is a completely different circuit from the FreakZQ and Yamaha.