passive mid "boost"

[El Heisenberg]

With this mod:






If I wanted to do this without a pot and just a switch, would I need to increase the value of the 1 meg resistor??

[petemoore]

  You don't have to change a thing.
  But...having never seen/tried/read about this circuit I got all curio-spicious, and decided to focus on "Pickup drives Inductor?" as an aside to changing the 1meg, which would change the ratio between the R/C, the "C" value being quite high for a ground shunted capacitor for guitar pickup. Or removing the pot as a starting point.
  I would say a higher resistance in parallel [what the pot is doing when turned >right] with the .02 cap would tend to let more signal [HF's in particular] through.
  By removing the pot, ~500k is removed from between the left and right shunt rolloff circuits [inductor>R/cap on the left, R/C on the right].
  Maybe start by putting a 470k [or less] at the pot position to see what CCW and CW would have sounded like. Then parallel smaller resistor to reduce the R where the pots outside lugs were, and add a resistor to form a fixed divider that'd be like some middle pot settings...iow diddle with resistors to form something R or R-divider, like what'd be like what the 500k pot shown would do.
  Starting off as shown or with any pot or some big resistorances [maybe form a 'switch pot' with a string of "add and divide" by choosing tap point] resistors there] between the I>R/C and R/C.

[PRR]

Is this a "boost" at all?

Turn pot to 0.02u||1Meg side. You have 0.02u loading the pickup. This wipes highs off, may exagerate pickup top-resonance. Network impedance never gets close to 1Meg, the resistor seems unnecessary.

Turn pot to 0.04u+L side. The L-C network goes to a low impedance at one frequency. This wipes out a band of middle frequencies, a mid-dip. For a 0.8H inductor, 880Hz will be very weak. Inductor Q is perhaps 5, reactance about 4K, dip impedance about 800 ohms. Against the ~~5K+5H pickup impedance you get a fairly narrow dip, probably leaving everything below 400Hz or above 1600Hz nearly full strength.

And it sure looks like a commercial mid-dip product discussed here recently. Their snazzy network analyzer plot resembles my one-thumb analysis.

[PRR]

me> Is this a "boost" at all? ... 0.02u loading the pickup. This wipes highs off, may exaggerate pickup top-resonance.

After some more study:

Actually the 0.02u across the pickup will give significant pickup top-resonance bump, about 7dB boost near 500Hz, a real "mid boost". It also clobbers everything over 800Hz.

This very narrow spectrum -may- be useful for fuzzing-up. Losing the highs reduces IM hash and spectral clutter.

The 1meg resistor still seems unnecessary. So the project is a switch and a cap.

[El Heisenberg]

So it really DOES boost mids? I thought it just cut lows and highs leaving only mids.



The problem is, I'd actually already stuck it in a 45 dollar guitar I bought at walmart. I used a center off SPDT switch. On one setting is just the inductor and resistor+cap to ground and on the other is a 1M resistor andthe .02 cap in parallel to ground. I didn't realize till after that that I was doing was different from what the circuit with the pot does.

I like the mid "boost". It cuts highs, but I like the sound of it. I have it in a cheap squire strat. Gets QOTSA sounding.

[El Heisenberg]

is what I did doing what the circuit with the pot is doing?

[PRR]

> I like the mid "boost". .... I like the sound of it.

So be happy. Nothing wrong.

[fuzzo]

looks interesting !

I'm never really happy with the "classic" tone control on guitars (I never use it , like a lot of people I guess).

That thing seems simple, and can be remove easily . Just find an inductor to do it,

what value for it by the way ?

[Gus]

Look at this recent thread check the links

http://www.diystompboxes.com/smfforum/index.php?topic=84527.20

[Ben N]

I wonder how the inductor or transformer from a CFL might do for one of these...   

[El Heisenberg]

The inductor is only needed for the mid scoop. The mid boost doesn't need it if I'm not mistaken....but I could be.


PRR, I didn't think there was something wrong with the circuit. I'km not trying to change it I'm just trying to do it without a pot. I can't fit another pot in the guitar Im doing this in. I wanna do it with an SPDT switch. A center off one, so the center will be untouched.

[Paul Marossy]

The inductor is only needed for the mid scoop. The mid boost doesn't need it if I'm not mistaken....but I could be.

Not sure what exactly what you are referring to, but you do need the inductor for the mid boost to work. It needs the resonance created by the inductor to do its thing. Otherwise, it's just a variation on the usual capacitor tone control.

[CynicalMan]

The usual capacitor tone control does boost mids because it make a LRC resonant filter with the pickup's inductance and resistance that cuts highs, has a resonant boost in the mids, and cuts bass. The inductor makes it act like a varitone, scooping mids resonantly.

[Paul Marossy]

The usual capacitor tone control does boost mids because it make a LRC resonant filter with the pickup's inductance and resistance that cuts highs, has a resonant boost in the mids, and cuts bass. The inductor makes it act like a varitone, scooping mids resonantly.

I know it's an LRC filter, but I don't think of a standard tone control as a mid boost, I see it more as a high cut control.

I always thought that these passive inductor "mid boosts" used the inductor to boost the mids, and it worked by creating more resonance. I haven't tried messing with this idea, but I guess I got the idea that it was a mid-boost from what this guy said in this thread: http://www.diystompboxes.com/smfforum/index.php?topic=42087.0 - but it's not really?

So can you explain how you "resonantly" scoop the mids? That doesn't make any sense to me. Do you mean that it has some resonant frequency above the midrange frequency so that it gives the effect of scooping the mids? I know the wah circuit is not a passive circuit, but the inductor is the component that makes it a resonant circuit. And the wah patent documents directly state that. And inductor based graphic EQs use various size inductors for each frequency band. So I am confused on how you can use an inductor to cut a frequency in a guitar... I mean normally you dump it thru a variable resistor and a cap to ground. Then the inductor shouldn't even be in the picture. Am I missing something?

[El Heisenberg]

What i've been told about the mid scoop circuit is that it cuts highs, then the inductor puts them back. I've also been told the inductor doesn't do anything with the mid boost function. I mean, the inductor is bypassed when in mid boost mode, isn't it?

[Keysershades]

So it's like the Villex boost ? ( http://www.villex.com/prtb.html ) ...
And it works ?

Simon

[CynicalMan]

So can you explain how you "resonantly" scoop the mids? That doesn't make any sense to me. Do you mean that it has some resonant frequency above the midrange frequency so that it gives the effect of scooping the mids? I know the wah circuit is not a passive circuit, but the inductor is the component that makes it a resonant circuit. And the wah patent documents directly state that. And inductor based graphic EQs use various size inductors for each frequency band. So I am confused on how you can use an inductor to cut a frequency in a guitar... I mean normally you dump it thru a variable resistor and a cap to ground. Then the inductor shouldn't even be in the picture. Am I missing something?

Check out http://en.wikipedia.org/wiki/RLC_circuit#Filters for more equations than you need. This filter is similar to Fig. 13 on that site. Anyway, a simulation is worth a thousand words, So:



This is the inductor circuit with a voltage source and 10k ohms in series before the filter. For this sim and the one below I assumed the inductor was 1H



This is a humbucker model run through both sides of the tone control compared to a unloaded humbucker. V(scoop) is the inductor side and V(boost) is the inductorless side. For V(scoop) there is a mid scoop, but also an upper resonant boost because of the pickup having inductance. The mid boost boosts low mids 4 dB but it would be pretty inaudible compared to the treble cut.

The inductor filter is resonant because the inductor and capacitor have a resonant frequency where, in series, they are at their minimum total impedance. This comes up as a cut because the low impedance shunts the signal at the resonant frequency. I'm afraid I can't give you more of the underlying reasons for this, but I'm sure a few minutes googling rlc band stop filter would turn up some results

[Paul Marossy]

Huh, very interesting. Thanks for explaining that.

[fuzzo]

El Heisenberg, why not using a trimer instead of the pot (if you don't want put it directely on guitar) ?

CynicalMan , thanks for the simulation, it's really clearer than long explanation ! I keep that circuit in my notebook, that could be a cool thing to add in my guitar project !

Otherwise, a french luthier , put a kind of bandpass filter in one of guitars, you can move the frequency with a pot installed on the top of the guitar's body. (That maybe not the only one who made that but it's the one I remember (there's a video on youtube about that by the way)

 

[El Heisenberg]

I didnt think the pot was needed until now. I still havent gotten an answer if it is or not. It seems like a waste of a trimmer if im using an SPDT switch and planning on using the effect on full all the time. Why not juts fixed resistors?