Sweepable mid values with Maleficent Mids

[RFreeman]

Is it possible to mod the sweep frequency range with the Maleficent Mids board? I plan on using this with my bass and wanted to have range sweep between around 200hz to 2khz.

Link to schematic

 

[Chuck D. Bones]

Yes. The Haunting Mids freq range is advertised as 400Hz to 7.5KHz. You can move that down by a little over an octave by increasing C3 to 22nF and C4 to 3.3nF. The new freq range would be 180Hz to 3.3KHz.

 

[RFreeman]

Excellent! Thank you.

 

[RFreeman]

Would it be too much to ask how to formulate this? And if so, is there a reference that you can point me towards that would help me learn?

Im trying to use the amxfx bandpass calculator to figure this out but I'm not doing calculating right.

 

[Chuck D. Bones]

In this circuit, the center freq and bandwidth (Q) are controlled by two time-constants:
C3 * Rx and C4 * Rx where Rx = R9 + SWEEP = R10 + SWEEP

The center freq, Fc = 1 / (2 * pi *Rx * sqrt(C3 * C4))

All of the stuff between pin 2 of the MIDS pot and pin 5 of IC2.2 form a Wein Bridge that can be described by a 2nd-order differential equation. The coefficients of that equation tell us Fc and Q. The HI/LO switch alters Q without affecting Fc.

 

[RFreeman]

In this circuit, the center freq and bandwidth (Q) are controlled by two time-constants:
C3 * Rx and C4 * Rx where Rx = R9 + SWEEP = R10 + SWEEP

The center freq, Fc = 1 / (2 * pi *Rx * sqrt(C3 * C4))

All of the stuff between pin 2 of the MIDS pot and pin 5 of IC2.2 form a Wein Bridge that can be described by a 2nd-order differential equation. The coefficients of that equation tell us Fc and Q. The HI/LO switch alters Q without affecting Fc.

After staring at this for hours, I have a working spreadseat going. Thank you!

 

[Chuck D. Bones]

Spreadsheets are gooooooooooood.

 

[RFreeman]

If I wanted to try to have an adjustable Q, would R11, R12, and R14 need be potted?

 

[Chuck D. Bones]

You already have a switchable Q, is that not good enough?

 

[RFreeman]

It absolutely is. I was just interested in what it would look like. I'm learning more and starting to see the separate blocks of circuits. Just curious, really.

 

[Chuck D. Bones]

Looks like the HI/LO switch varies both the Q and the gain, so that when Q goes up, gain goes down to keep the peak gain low enough to maintain the headroom. Take the HI/LO switch out and vary R11 to change the Q. Vary R14 to adjust the maximum cut/boost.

 

[RFreeman]

So the build is done, and I'm happy with the results. I used Chuck's cap suggestions, but also swapped their respective resistors with 10k's to tighten the sweep. The range of sweep is now 170hz-1.86khz.
I am, however, picking up radio signals. It's only audible when I have the volume and mids cranked, and more so with higher sweep ranges.

 

[giovanni]

Question for @Chuck D. Bones
How does the Wein bridge affect the frequency response of the second stage? And how does it either boost or cut? What does IC2.1 do? I assume it makes the bridge active?

 

[Chuck D. Bones]

I am, however, picking up radio signals. It's only audible when I have the volume and mids cranked, and more so with higher sweep ranges.

Check your grounding & cables.

 

[Chuck D. Bones]

Question for @Chuck D. Bones
How does the Wein bridge affect the frequency response of the second stage? And how does it either boost or cut? What does IC2.1 do? I assume it makes the bridge active?

If by 2nd stage do you mean IC2.2? You need to be specific. Please call out ref des.

 

[giovanni]

Yes, sorry IC2.2.

 

[giovanni]

Also if my math and my Python script are right, I get these frequencies as a function of the sweep rotation:

frequency at 0%: 389.1 Hz
frequency at 10%: 429.8 Hz
frequency at 20%: 480.1 Hz
frequency at 30%: 543.6 Hz
frequency at 40%: 626.4 Hz
frequency at 50%: 739.1 Hz
frequency at 60%: 901.2 Hz
frequency at 70%: 1154.3 Hz
frequency at 80%: 1605.2 Hz
frequency at 90%: 2634.2 Hz
frequency at 100%: 7338.1 Hz

I believe we are in TS territory around 50%, Klon around 65%.

 

[Chuck D. Bones]

IC2.2 mixes the signal coming out of IC2.1 with the dry signal from IC1.1. The mixing is either in our out of phase, depending on whether the MIDS pot is above or below noon.

IC2.1 wraps some gain around the Wien bridge to boost the Q.

 

[giovanni]

So the input gets to IC2.1 via pin 2 of the mids pot? That looks like the only path to the negative input of IC2.1. How does the phase flip work?

 

[Chuck D. Bones]

We'll set the HI/LO switch to LO and MIDS at 10. Doesn't matter where SWEEP is set. With MIDS at 10, C3 is connected to IC1-1. Under these conditions, the signal at IC2-3 is +7dB above the level at IC1-1 and shifted 180° at resonance. IC2.2 subtracts the signal at IC1-1 from the signal at IC2-3. Since those two signals are 180° out of phase, they are effectively added. The gain at IC2-7 is therefore 14dB at resonance.

Now let's look at the opposite case. Set MIDS to 0. Now C3 connects to IC2-7. At resonance, the signal at IC2-3 is 7dB greater than the signal at IC2-7, and it's 180° out of phase. Because we're feeding the + input of IC2.2 with a signal that's 180° out of phase, it's negative feedback. so now, instead of magnifying the signal by 14dB at resonance, we're reducing it by 14dB.

It's easiest to see what's going on if you either use a scope or simulate it.