Adding bass control

[Zanius]

Hey guys, I want to add a switch for flat or cut bass on a Demeter Fat Control.


I've been learning some things while building but I better ask because I'm not so sure.
The input caps seems low enough to not let much bass in, so I guess the switch has to be placed in the output. What confuses me is that it doesn't follow the "volume 2 to output" way, or I would just be switching the 220n cap.
So, what is the right way to do this?

Thanx!

[induction]

The Fat Control is a simple boost (about 3dB) followed by an active EQ (low Q bandpass centered around ~350 Hz). The output pot pans between the simple boost output (bypassing the EQ) and the EQ stage output. It gives a flat response with a 3dB boost when the mid control is set to 0.

For flat response: you can easily add a switch to set the control to 0, or you could just step on the bypass switch, which seems like it would be easier.

For bass cut: Switching the input cap would work fine. With the mid pot at 0, a 1n input cap gives you a high pass filter with corner frequency near 150 Hz. A 470p gives you an fc near 300 Hz. At 220p, fc is near 700 Hz. As you turn up the mid pot, you get a wide bandpass filter centered around 450 Hz, 650 Hz, and 1kHz, respectively.

I'm curious why you want to cut lows from your low-mid boost. Understanding how you want it to sound and how you want to use it may suggest other options. For example, yes, you can change the output caps. (There are two of them, one for the flat boost side and one for the EQ side). If you want the bass cut to depend on the mid pot setting, you can change one or both of those caps or put them on a switch. You could also adjust the components in the EQ stage to alter the response.

Lots of options, depending on what you want to do with it.

[Zanius]

Sorry, what I meant was "stock" and not "flat".

Well, I hooked the board on my test box and played a bit with the friend Im making it for and he thought it could be a bit less bass-y. Maybe it is in vain but I wanna try it anyway now

Anyway, thanks for the answer, I'll get on it one of these days.

[induction]

Zanius wrote

Well, I hooked the board on my test box and played a bit with the friend Im making it for and he thought it could be a bit less bass-y.

Maybe your friend would prefer the Blade mid boost. I do.

I find the Fat Control to be mostly unusable, myself. You'll learn a lot from trying to mod it, though.

[Zanius]

I haven't found a schematic for this, but there is the Spackler by Madbean witch seems to be identical except for the gain pot and a missing pulldown resistor. I went ahead and vero-ed it (my 3rd attempt to read a schematic) since a gain pot sounds like it would give more overall control.

The madbean schematic


and my interpretation (I added the pulldown resistor marked "PR")


As read above my goal is to have the option to lower the bass output of the circuit. I can play around with the capacitors on a switch, but in my oblivion about theory I want to learn if I could replace some resistor with a trim or pot to achieve that. That would be easier to adjust for any taste.

Note I mean no disrespect to induction who already gave some answers on the subject. This is more like an learning exercise for me. Thanx!

[induction]

The gain pot is a nice feature. It gives you separate control over bass boost and total volume. That way you could use the effect as switchable EQ with or without a volume boost.

However, with respect to the pot-controllable bass control, I'm afraid the Fat Control is just not a good platform for what you are trying to do. There are no resistors in the circuit that can be turned into pots to make a bass cut control. Besides the input and output caps, C4 in the madbean schematic can be reduced to decrease bass. Values between 10n and 100n give reasonable results. You'll get more mileage from the input caps though.

If you want a low cut you can control with a pot, you could put an active high-pass filter in front of this thing. That will let you control the high-pass corner frequency with a pot. But putting a hi-pass filter in front of a bass-boost is like trying to fine-tune the temperature in your house by running the heat and air-conditioning at the same time. It makes a lot more noise and gives you a lot less control than just using the right circuit for the job.

At that point, you may as well build an EQ that does what you want it to do. A 1-band parametric EQ will give you control over the center frequency of the boost (or cut) as well as the amount of boost/cut, and the width of the frequency band. Breadboard one of those and you'll learn plenty about fine-tuning EQ. There are lots of other pedals that would be good platforms for fine-tuning a frequency-dependent booster. The EWS Bass Mid Control looks like it would be a good candidate. And again, if you haven't breadboarded the Blade mid-boost circuit yet, do yourself a favor and try it out. Google parametric eq and active bandpass filter for more ideas.

I'm going to go ahead and strongly suggest you do this sort of thing on the breadboard instead of building a vero and then trying to mod it. I'm sure everybody is getting tired of me harping on this, so I'll just shut up about it now.

[Zanius]

That all makes sense.
I suppose I'll just experiment with caps for the fat, since that's what the guy wants! I may experiment with the others you suggest when I have the time (tiiime, spare any tiime sir?).

Thanx for all the answers and explanations induction!