Adjustable High Pass Filter

[hugh]

I want to build an adjustable high pass filter.

I'm thinking of a rotary switch with at least 5 different cutoff frequencies.

I'm guessing it will have to be preamped, because I will have to push the signal through a cap to get the desired effect. I'm afraid passive will just be too weak of a signal when I'm done.

To pass treble, I just have to select caps such as those normally used for tone controls, but instead of grounding that signal, I send that signal to the output... right?

Does anyone have any idea of actual cutoff freq's for various capacitor sizes? I'm assuming I will just have to choose a range of them and experiment.

One more question: There isn't anyway to drain bass to ground like a regular tone control, is there?

I'm thinking of starting with a high pass cap like .047 and working my way smaller from there.

[sumgai]

hugh,

An adjustable high-pass filter is another name for a low-cut tone control. For that reason, you and I need to keep ChrisK's prime admonition in mind:

[glow=red,2,300]Search is.[/glow]
That said, we've done this before, and not distantly in the past. Try searching back for 600 days, and 30 (or more) posts (not the default 7 days/10 posts). Look for "bass cut" or "bass control", and see what you find.

Here's the first thing I dug up:
guitarnuts2.proboards45.com/index.cgi?board=wiring&action=display&thread=1142462210

As usual, your mileage may vary! ;D

HTH


sumgai

[hugh]

Thanks for the tip. I will use those search terms.

I called it a high pass because thats what my electronics education has taught me.

[sumgai]

hugh,

Your terminology is correct, to be sure. However, as in nearlly all things in life, there is an equal and opposite corollary. In this case, members have wanted to cut the bass, or control the bass, just as they were controlling the treble. To them, a treble control is a high-cut filter device, so they thought that the opposite would be called a low-cut filter.

It all depends on your point of view, that's all.

Good luck with your search, and don't be afraid to ask for clarification, either on some old thread, or right back here, makes no difference to us.

Oh, and you might also search on "varitone", that's actually pretty close to what you're contemplating, what with the rotary switch and all. Might be worth looking into.

HTH


sumgai

[hugh]

Alright, been reading tons on this and now I've decided to continue with my original plan of a rotary passing through various caps.

I guess I need a clean signal bleed past the cap, and I haven't tried to draw it out yet... if somebody has a minute to explain that one, feel free to post a diagram or short explanation. I'm thinking a clean bleed will give me adjustable bass attenuation and the selectable cap will pick my cutoff freq.

I could make it 100% cut, but it might be more fun to make it adjustable.


Also, the Varitone uses this list of caps (as I've found)

C1 = .001 micro farads
C2 = .01 mf
C3 = .022 mf
C4 = .047 mf
C5 = .22 mf

Seems like using those as high pass caps could have a nice result... although the .22mf will be a little too big to have any effect on bass. So I will select something smaller, I guess.

Possibly a size in between .01 and .001?

[ChrisK]

I believe that series low-cut filters work best with smaller caps. I might start at 0.022uF, but suspect that values in the 0.01uF to 0.001uF will give the best effect.

While the Varitone values are a nice range to look at, it is a series resonant tone circuit to common. Anything in series with the signal to the output will not be in the same value range.

Go to the G&L website and look at their schematics.

The S-500 model shows bass as well as treble cut tone controls (1Mohm [taper unknown]/0.0022uF).

A good place to start!

Bear in mind that different values of caps will give you different cutoff frequencies, but these are better described as "knees" or resonance points in the curves. The cutoff is not binary or absolute, but just the region where the slope of the response curves change.

Since these are passive components, they ALL interact with each other (the pickup with the hi-cut with the low-cut with the cable with the amp.....).

You have to listen to judge and model (pSpice) to "see".

If you can describe the pickups that you want to use (resistance, inductance, and interwinding capacitance) and the tone control components as well, I can model it and post. If you have an engineering background, you can do the same.

pSpice is often freeware (I get it from the Linear Technology web site, it's called SwitcherCAD). Another one is 5spice.

Search aboot.

There isn't anyway to drain bass to ground like a regular tone control, is there?

Yeah, it's called an inductor. Duality is (xL<>xC).

ELI the ICEman! (Go Harbaugh that!)

[hugh]

Sorry, I don't have an engineering background. I have formal electronics schooling (10 months, 40 hours a week in the Marine Corps) and 13 years of experience both military and civilian. I understand the basics and I learn very quickly through research... but engineering math goes over my head pretty quickly. Because of this, most of the guitar circuits and pedals I've built are all based on guesswork and trial & error. (but its served me pretty well so far)

I would love to play with pSpice, but I'm afraid I'll just waste a couple hours and get nowhere with it. As far as giving you specific numbers... I'm starting to lean towards building this into a stomp box so I can easily add a preamp before and possibly after the cap. If its in a box, then each guitar will change the results.

I did this before with tone controls (bass, mid, treble) and it sounds fantastic, however the cutoff frequency is not adjustable. This circuit I'm building is supposed to be a one-trick-pony, but adjustable. I'm actually going for a sort of old AM radio sound that will only be used every once in a while... so the effect needs to be dramatic.

To that effect, I was thinking of trying to make the preamp circuit more lo-fi on purpose, or make it switchable. The preamps I've built are very clean, though. Gotta put some time into that as well.


As far as the caps go, I've found with a .022 compared to a .047 as a coupling cap in the beginning of a pedal circuit that the former has less bass response. As a goof I tried a smaller one and the result left it with no bass. To keep it simple, I'd just go on with something like that. Getting into inductors will just complicate things.

[JohnH]

hugh said:

I'm actually going for a sort of old AM radio sound that will only be used every once in a while...

..so are we in fact a Pink Floyd enthusiast who wishes to play "Wish You were here?"

That song made a huge impression on me. I heard it first through my newly finished home-made stereo amp when I was 15. The AM radio intro scared the crap out of me, I thought I'd blown my output transistors!

To get that bass roll off Im thinking you need to set the series cap value in relation to the value of the load after it, ie cap in series followed by a resistor to ground, or else set the cap to match the specific input impedance of the next stage, for the effect you desire.

John

[hugh]

Ahh... I see. That makes sense because the circuit I'm talking about sets the impedance with the resistor following the cap.

Yes, I am trying to get something like Wish You Were Here. I didn't even think of that song, but man that is a wicked song. It's by far my favorite Floyd song. Thats a perfect example of what I want to happen.

[kuzi16]

I have been working on something simaler to this. I was looking to drain the bass off to ground as mentioned in an above reply. i just cant seem to get it to work.

i read in "Electric Guitar Construction" (by tom hirst) that a pot could be used to make a bass cut control pot. He shows the wiring as follows: the output from the volume pot goes into the second lug of the (hopefully) bass cut pot. An output line then goes to the output jack from the third lug. spanning the 2nd and 3rd lugs is a cap that could range from .001uf to .1uf.
the first lug empty.
So i thought i would give this a shot. Needless to say it did not work. Can someone please explain why?

ive been following this thread in hopes of finding an alternative. the Varitone idea seems to be the best so far... minus one problem. I have very limited space and i have routed all that i can.
this page:
alexplorer.net/guitar/mods/varitone.html
lead me to this page:
alexplorer.net/guitar/mods/toneswitch.html

is there a way to have a three way switch to "switch off the bass" to two degrees? If so how would i go about wiring that?

[hugh]

Well, I received my 2P6T rotary switches today so I can start playing around with this. The ones I got are from Mouser, P/N 105-SR2511F-26RN made by Alpha, Taiwan. The quality appears to be pretty damn good for only $2.20 each.

[wolf]

John H
I also thought of "Wish You Were Here" when hugh mentioned trying to get the old AM radio sound.
Still, I always thought it was done by the recording engineers rather than a guitar playing effect.

[JohnH]

wolf said:

John H
I also thought of "Wish You Were Here" when hugh mentioned trying to get the old AM radio sound.
Still, I always thought it was done by the recording engineers rather than a guitar playing effect.

I'm sure it was. But we engineers tend to get a bit antsy when we get built into stomp-boxes.

John

[hugh]

The guy I'm working on this effect for plays live a few times a month or more.

He likes to be able to produce the sound on stage that you hear on the record.

Also, although the concept is something like Wish You Were Here, its not going to be used to play that once I give it to him. (but you can bet I'll try it!) He only plays original stuff.

[kuzi16]

when you do get done with this i would like to know what cap value worked out the best for you.

i have a switch for bass cut. all on or all off. i am using a .001uF cap. it does the trick but not quite as well as i was thinking. I would like to use either a .00033uF or a .0001uF but the local radioshack didnt have those

[ChrisK]

I would like to use either a .00033uF or a .0001uF

For 0.00033uF, put three 0.001uF in series.

[hugh]

I did some testing, and this is what I found:

301K ceramic, (.0003uf) gave a strong bass cut with nice sound.

You may want to give that a try for a single cap setup.

I'm thinking of some smaller than that, and maybe one bigger. I've got a bunch of different sizes sitting here, and that one made the effect I was looking for... although I'd like to go even more drastic. Thats the nice thing about a rotary selector, you can have it all.

[kuzi16]

chrisk said:

I would like to use either a .00033uF or a .0001uF

For 0.00033uF, put three 0.001uF in series.

Brilliant!
Look at me not thinking outside the box.

thanks.