passive mid "boost"

[PRR]

> Would an inductor (with a set DC resistance) in series with the AC source be a better sim? Or an inductor with a series resistor, perhaps?

5K resistance plus 5H inductance will show the general effect of external loads.

[El Heisenberg]

Would putting a 150k or 220k resistor in parallel with one of the caps, say the .033 in my rotary tone control, I should get some lows back in that setting right?

[gmoon]

5K resistance plus 5H inductance will show the general effect of external loads.

Good to know--thanks.

I now return you to your regularly scheduled (original) post...

[El Heisenberg]

So the circuit above, with the pot turned all the way clockwise in "midboost" mode looks like figure 1. here below, correct?








Or is it just the figure 2?? The 2nd circuit is what I put in the guitar. It doesn't sound like the circuit with the pot in my strat. The mid boost doesn't sound like it should at all, and the mid scoop doesn't have as many highs. It's doing what it should tho. I can't remember what cap I used...

Because I'm trying to have the two tone options in a guitar with one DPDT center-off switch.

[El Heisenberg]

?

[gmoon]

Wired like the schematic, looks like a hybrid filter you got there...a combination of band-pass and band-stop filters. The two filters you reference interact into one; there's no isolation between them. There's an LC series shunt, and then a cap forming a LC parallel shunt. Plus the pickup inductance thrown in for good measure...

I think you'll get a freq response like the blue graph on cynicalman's post--a peak - notch - peak. The slope up to the initial peak I'm seeing is probably caused by the pickup sim. With this simple circuit the order (notch-peak) will always be the same. I'll let an EE or a physicist explain why...

Mixing them passively with the 500K POT does isolate the filters, but not in a helpful way, I think. You'll never get the same resonance as the original, just a modified low-pass. Probably giving you the worst of both worlds...

Use the circuit as drawn. Substitute a 1K (or 5K) POT for the 500K R. With the POT much above 1K, the second peak disappears and you'll get a low-pass filter (first peak - loooong notch.) The POT effectively takes the series LC out of the circuit (removes the inductor.)

Have you tried simulating this yourself? (in LTspice, etc.)  It's worth it.

BTW, did you ever mention value of the inductor you're using?

[El Heisenberg]

I don't have any programs like spice or anything. This computer isn't really good for much. 2000 laptop. So I just use it to browse web pages....



The inductor was marked TL021.



gmoon, im not very clear on what you're saying. But I don't really wanna put a pot in. I can't fit another pot in the conrtol cavity. This is done with a center-off DPDT switch. One setting is the mid scoop (parallel cap and 220k resistor in series through the inductor to ground), the other setting is set so it's like the schematic I drew in figure 1. Center position is open. But is I were to play with the value of the 500k resistor it would do what? More highs? Cos doesn't the inductor add the highs back in after everything is cut?

[gmoon]

Apparently, that transformer has ~1.5H inductance. I'll plug that in...

If I simulate that circuit, but replace a 100Meg R (functionally near open circuit) for the 500K R, there's almost no difference in the frequency response. I.E., 500K is way too much resistance to allow the inductive portion to ever come into play.

Notice how the other (Anderton?) filter uses a 20K "depth" POT? Even that's overkill.

Don't add another POT. Replace the 500K POT with a 10K POT (10K seems to work better with 1.5 Henries than the 5K I mentioned before.)

1 ohm:
(There's a real honest-to-God boost here @~1.25KHz. But it would be greater with other, simpler filter circuits. No matter.)



2K:


10K:


See the next graph? That's the one with the 500K. The series LC filter basically isn't present.

Not significantly different than the 10K graph, other than shifting the initial corner frequency, maybe. And lifting the signal a bit closer to unity.




Your current setup below-- you've got the POT wired as a mixer between the two filters.

With the inductive half "full ON", and the RC half mostly isolated, here's the Freq response. I don't think it's anywhere near as interesting as the original circuit with the 500K removed, or reduced... (first two graphs). There's no secondary "spike", and the higher freq response just tables at -25dB.


With the POT turned the other way, the graph looks like the graph above this one. You're "mixing" the filters alright (introducing the notch), but not very effectively.




Whew. That was epic capture/copy/pasting. Sorry.

[El Heisenberg]

Thanks for the explanation. But I'm not actually using a pot at all. I'm only using fixed resistors and a center-off DPDT switch to do this. I want both settings on full. Full mid scoop, and full mid boost.

I already have the circuit with the 500k control pot in a squire strat. I like the mid boost. But not the function of the control. I have the mid scoop half of the circuit in my rickenbacker copy. It sounds great. Or sounded great, I just took it out since the vari-tone gets me almost the same tones.

What would happen if I used two of these inductors. In series or parallell?


Anyway i bought a 40 dollar guitar at wal mart, added a neck humbucker pick-up since it only had a crappy humbucker at the bridge. I put an electra distortion in it and am trying to get this three way tone switch worked out. The settings don't sound like they do in the other guitars. Maybe it's the guitar. The bridge pickup is really dark and boomy.


EDIT: I just replaced the 500k fixed resistor with a 2k2 resistor. Now both scoop and boost setting sound sorta similar. I dunno what's goin on.

[gmoon]

Part of the issue is the "hybrid" nature of the filter--it's a band-stop filter, with a cap (or RC network) in parallel to boost. It's a compromise; the notch is always present. Check out the Wiki LCR page CynicalMan posted earlier in the thread.

But with switching, you can make two separate distinct passive LCR filters with minimal components:



Here's a band-pass (boost) filter (C2 is bypassed.) Break the connections at P1 and P2, and it's a band-stop (notch) filter.

With a DPDT switch, you could eliminate C2 and route C1 in parallel or series with the inductor. Or neither, with your center-off switch.

(R2 is depth. As is, it's a placeholder. You can buy a 0 ohm resistor, but LTspice won't let you use it...   Some resistance here will lessen the effect. R1 is for the sim, probably not needed.)

[El Heisenberg]

Why are the 220k and 1m resistors in the original circuit designs?

Also what would switching to the middle lead of the inductor sound like?  Just half the effect?

[gmoon]

The two resistors broaden the width (lessen the "Q factor" of the filters.) The notch, being the primary filter, is very narrow without the resistor (220K). It's one of those things you play with until it sounds right.  Wah pedals usually have a resistor or two to do the same.

Changing the inductance shifts the resonance frequency. Cutting it in half would shift the frequency response higher. Changing the capacitance values would also have similar effect.

[El Heisenberg]

This works good. But I'm sure there are more possibilities. What about different inductors? Between my vari-tone, the mid boost/cut pot and this switch control I'm starting to really like messin with passive tone controls. They're alot nicer than the normal cap/pot ones.

[El Heisenberg]

Thanks, gmoon.

These sorta controls are also alot funner. I wanna keep messin with it to see how much I can get out of it, the easiest thing I can see to do right now is put a cap on one side o the switch and remove the jumper to change the caps value when its in parallel with the inductor.

But adding resistors in parallel and changing caps and inductors and inductor leads can have endless possiblities! Itd be different for every guitar. and you could tune it exactly how you like. Well I could if I could get a hande on this.]




More question: Does that 5k pot in series after the inductor in the craig anderton rotary circuit do the same thing as the resistors in parallel to the caps in the mid scoop/boost circuit?

ad i have no idea how to work this LTspice. crap

[soggybag]

Here's another passive mid control. I won't call it a boost. This is meant as a alternative to the standard guitar tone control. This was redrawn from the Stompbox Cookbook. The pot is meant as a dual gang.

[El Heisenberg]

Ever check out Craig Anderton's "Passive Tone Control"? It's a very similar passive LCR filter.

http://i86.photobucket.com/albums/k99/jprak1/Passivetone.jpg

Beats me if you can still get the RS 273-1378 inductor / transformer, but it's spec'ed at 88-100 uH.

The original schematic (from the Ele. Proj. for Musicians book) doesn't have the POT... And some of the caps are different values.

How much different is that inductor from the TL021 1.5h one I'm using. It's what I have in my rotary tone control and the mid control in the strat and the wal mart guitar.






If the 500k resistance is too much, why does this circuit work?? When the pot is centered it's the open setting. All this time I thought it's been attenuating the signal. But I guess not. And the passive mid boost really IS a boost even tho 1 or 2 db isn't all that much volume.









^^^^^ When the switch is set so the signal goes through the cap/inductor in series it just sounds like the tone control is down a bit. I don't like this setting. The other setting, with the cap and inductor in series sounds good. Big volume drop but nice.

[El Heisenberg]

I've messed with the circuit a bit. (it's been in the guitar already this whole time). I only tried a few caps for the series setting, not including the .038. I tried .006, .01, and .1. I shoulda tried .047 but figured it'd sound the same. I settled on .01 cos it sounded like where I usually like my tone control, except I got a little bit a clarity it seemed. I tried putting a 180k resistor across the cap but I couln't hear a difference.

I like the parallel setting alot. like ALOT. it's real cool.





Im guessing if I had tried the resistor across a larger series cap like the .1 uf, I might've heard the effect.

[gmoon]

^^^^^ When the switch is set so the signal goes through the cap/inductor in series it just sounds like the tone control is down a bit. I don't like this setting. The other setting, with the cap and inductor in series sounds good. Big volume drop but nice.

I like the parallel setting alot. like ALOT. it's real cool.

(somewhere in there you found something you like.)

LC in parallel, you should be getting a boost of 3 or 4 dB  @ ~0.7khz with the 0.038uF cap and 5 or 6 dB @ 1.5khz w/0.01uF. Plenty of attenuation above and below that; so there's a net decrease in signal overall, even with the boost. But if the boost is in the right place...it works.  The series connection is a notch, but less complex than the POT filter you already installed (without the highend "pop"). A sim can show you approx what's happening, but it can't tell you if you're gonna like it...

Likewise, you probably like the resonant frequencies of the POT filter. And the "pop" after the notch. I still don't think you're getting all you can with the wiring on that one...

There will certainly be a frequency shift with a 1.5H (lower) vs. a 100mH inductor.

.....
My roommate in college had an Electra with a 5 position tone switch; certainly an inductor was involved. I used to "dis" that guitar, cause it was just an LP knockoff. That was a long time ago, wish I had access to it now... Passive LCR filters might bring something good, especially with humbuckers.

[El Heisenberg]

The guitar Im doing this to has the electra distortion in it that they used to put in those guitars. Maybe that's why this is working so well. With the distortion off there's alot of tone loss. But when you turn it on you can really hear what the effect is doing.

I changed my switch so that it switches between two caps in parallel with the inductor. I tried .057 and .01. I liked both, but since .057 was already in there I just left that. I think I woulda preffered .01, or maybe something like .068. With the electra distortion you can really hear the tonal change. With the .038 cap there are some point on the neck that seem to give me octaves! With the .057 the effect is lower on the neck and doesn't sound much like an octave. I wish I had more possibilities than two.