Passive mid cut/"boost" wiring diagrams?

[drew_half_empty]

So... a wee while back I decided to make the G&L treble/bass cut circuit in a box with 5-way switches for different cap values, and it's been awesome as a tool to figure out what values to put in a guitar

Recently I learned you can also have passive mid cut/"boost" circuits as well--have even seen one that cuts below 5 and "boosts" above 5 on the pot. Only issue is I have no idea how it works and generally suck at this type of stuff.

Would anyone happen to have that wonderful combination of generosity and free time necessary to map it out for me?

Bonus 1--re: the 5 switchable values... any thoughts on what they should be?

Bonus 2--i've also seen circuits that cut below "5" on the pot and boost above... how does one do that?

thanks

 

[freefrog]

Hi,

What you evoke seems to be the good old Torres mid control, wired like that:

https://music-electronics-forum.com/filedata/fetch?id=815064

On one side, there's a LRC circuit (inductor+cap+ resistor) acting like a "mid cut". Many variations of this circuit inspired by Gibson VariTones are available under different names: Bill Lawrence Q filter, Gresco Tone Qube, Armstrong Tone Choke...

On the other side of the pot is simply a low value capacitor acting like a "mid boost" by shifting the resonant peak of pickups in the hi-mids. The schematic mentions a capacitance of 6nF to 10nF but actually, one can go as low as 2.2nF or 1.5nF for a more subtle effect (my Strat number one hosts an even lower mid enhancer, measuring 1nF).

TBH, this dual circuit is not very interesting IME / IMHO... Even it it doesn't cut nor boost the mids at 5/10, it changes the resistive loading of pickups and therefore their tone.

If you absolutely want to try it, consider the possibility to use a Fender TBX control: put the "mid cut" LRC thing on the 250k no-load part of the dual pot. Add the "mid boost" capacitor on the 1M side. It will have less effect on the tone at 5/10 and this kind of pot has the advantage to feature a center detent position...

FWIW. More later maybe for the "bonus" questions. I lack of time right now. See ya.

FF

 

[Chistopher]

Any time you do a passive boost, you are always making some sort of tradeoff. I don't know how well that plays out in passive circuits. A James filter (ie a passive baxandall) can give you a passive treble and bass cut, an apparent treble and bass boost, and have a broad enough cutoff that they can be used to boost or cut mids as well. It will attenuate the output though. Here's a link. You could easily breadboard it out and try different cap and resistor values to figure out how exactly the controls interact, and when you find values you like you can put them on a switch

 

[Chistopher]

If you absolutely want to try it, consider the possibility to use a Fender TBX control: put the "mid cut" LRC thing on the 250k no-load part of the dual pot. Add the "mid boost" capacitor on the 1M side. It will have less effect on the tone at 5/10 and this kind of pot has the advantage to feature a center detent position...

It may just be a product tolerance thing, but my TBX was not no-load in the middle position, but roughly 1 meg of resistance

 

[beaubrummels]

Technically, there's no such thing as a passive boost. In order to boost, you need gain, which means additional power, e.g. a battery. Otherwise, to 'boost' mids passively, you need to cut bass and treble.

 

[freefrog]

It may just be a product tolerance thing, but my TBX was not no-load in the middle position, but roughly 1 meg of resistance

Well, there's the 250k part which is no-load and the 1M part measuring... 1M so what you mention seems normal to me, as long as the two parts of the pot are used.

 

[freefrog]

Technically, there's no such thing as a passive boost. In order to boost, you need gain, which means additional power, e.g. a battery. Otherwise, to 'boost' mids passively, you need to cut bass and treble.

In this case, the "boost" comes from the pickups themselves, which are resonant filters... If one shifts their resonant peak in the midrange, they actually boost mids.

 

[drew_half_empty]

Technically, there's no such thing as a passive boost. In order to boost, you need gain, which means additional power, e.g. a battery. Otherwise, to 'boost' mids passively, you need to cut bass and treble.

hence the quotes, lol. I assumed it was just cutting bass and treble, but that's cool with the resonant peak thing people are talking about

 

[drew_half_empty]

Hi,

What you evoke seems to be the good old Torres mid control, wired like that:

https://music-electronics-forum.com/filedata/fetch?id=815064

On one side, there's a LRC circuit (inductor+cap+ resistor) acting like a "mid cut". Many variations of this circuit inspired by Gibson VariTones are available under different names: Bill Lawrence Q filter, Gresco Tone Qube, Armstrong Tone Choke...

On the other side of the pot is simply a low value capacitor acting like a "mid boost" by shifting the resonant peak of pickups in the hi-mids. The schematic mentions a capacitance of 6nF to 10nF but actually, one can go as low as 2.2nF or 1.5nF for a more subtle effect (my Strat number one hosts an even lower mid enhancer, measuring 1nF).

TBH, this dual circuit is not very interesting IME / IMHO... Even it it doesn't cut nor boost the mids at 5/10, it changes the resistive loading of pickups and therefore their tone.

If you absolutely want to try it, consider the possibility to use a Fender TBX control: put the "mid cut" LRC thing on the 250k no-load part of the dual pot. Add the "mid boost" capacitor on the 1M side. It will have less effect on the tone at 5/10 and this kind of pot has the advantage to feature a center detent position...

FWIW. More later maybe for the "bonus" questions. I lack of time right now. See ya.

FF

My man. Thank you.

Seems like wiring up the mid "boost" pot/switch is gonna be easy... just 2-4-6-8-10nF on the switch and a 1 meg pot.

Cut looks a little more complicated with the LRC... Interested to find out how each component affects the tone

 

[Chistopher]

You absolutely can do a passive boost. As I mentioned earlier, using a transformer can increase your signal voltage at the expense of increasing your impedance as well. So a 1:2 transformer will double the voltage of your signal (6 dB of gain) at the expense of only getting half the current and have 4 times the impedance. This means that depending on the quality of what you are plugging your guitar into, your guitar will be less predictable. While your signal may be louder plugging into a modern amp with good input impedance, it will absolutely wreak havoc on your Fuzz Face.

 

[freefrog]

You absolutely can do a passive boost. As I mentioned earlier, using a transformer can increase your signal voltage at the expense of increasing your impedance as well. So a 1:2 transformer will double the voltage of your signal (6 dB of gain) at the expense of only getting half the current and have 4 times the impedance. This means that depending on the quality of what you are plugging your guitar into, your guitar will be less predictable. While your signal may be louder plugging into a modern amp with good input impedance, it will absolutely wreak havoc on your Fuzz Face.

Yeah, it's the principle of the Villex mid boost if memory serves me. ;-)

 

[freefrog]

My man. Thank you.

Seems like wiring up the mid "boost" pot/switch is gonna be easy... just 2-4-6-8-10nF on the switch and a 1 meg pot.

Cut looks a little more complicated with the LRC... Interested to find out how each component affects the tone

You're welcome.

On the cut side, "schematically":

-the higher the inductance of the choke, the lower in the spectrum will be the dip in the mids. Big inductors also limit the depth of this dip but leave the high frequencies intact, while it's the contrary with low inductance chokes (for the record, vintage VariTone's had chokes with TEN times more inductance than the 1.5H inductor in the schematic that I've shared).

-the cap in series with the choke cooperates with it to set the frequency of the mid dip. Other factors being unchanged, the higher the capacitance, the lower in the midrange is the dip...

-the resistor in parallel with the cap limits the depth of the dip. It's not necessary with big inductors but becomes useful with low inductance chokes.

 

[drew_half_empty]

You're welcome.

On the cut side, "schematically":

-the higher the inductance of the choke, the lower in the spectrum will be the dip in the mids. Big inductors also limit the depth of this dip but leave the high frequencies intact, while it's the contrary with low inductance chokes (for the record, vintage VariTone's had chokes with TEN times more inductance than the 1.5H inductor in the schematic that I've shared).

-the cap in series with the choke cooperates with it to set the frequency of the mid dip. Other factors being unchanged, the higher the capacitance, the lower in the midrange is the dip...

-the resistor in parallel with the cap limits the depth of the dip. It's not necessary with big inductors but becomes useful with low inductance chokes.

sounds intense... I guess I'll need 3 switches for the "cut" side

Any thoughts about what 5 values to go with for each?

 

[freefrog]

sounds intense... I guess I'll need 3 switches for the "cut" side

Any thoughts about what 5 values to go with for each?

Most inductors are cumbersome so five of them wouldn't fit in a typical electronic cavity.

Switchable parallel resistors would be redundant, since the pot is already there to vary the deep of the mid dip in the schematic shared above (the parallel 220k makes sense only @ 0/10).

What remains is the idea to choose between 5 capacitors... which is concretely close to the recipe of a Gibson Varitone.


Hard to recommend precise values, since the mid frequency dug by each cap would depend on the inductance of the choke in series with them...

Gibson Varitones involved 5 caps from 220nF to 1nF with initially a choke of a much higher inductance than mentioned in their schematic (this vintage draw evokes a 1.5H inductor but intentionally or not, it's a mistake : the "real" value would be either 7.5H, either 15H, since that's one can find in vintage Gibson's with Varitones).

If one wants to focus on the midrange, the capacitive scale must be narrowed. For instance, a big 15H inductor would have to be paired with caps from 47nF to 2.2nF, min/max (which would locate the mid dips between 190hz and 940hz approximatively). Under or over that, the frequencies affected would/will be in the bass or high ranges.
With a small 1.5H, the caps would have to measure 470nF(!) to 19nF to dig the same frequencies from 190hz to 940hz...

FWIW... HTH.