Sweepable high pass filter schematic

[John Lyons]

I'm looking for a sweepable high pass filter from about 50hz to 500hz.
12db cut or more.
Anyone have a link for something like this?

I'm using this for acoustic guitar live.

[stm]

Boss CE-5 (chorus ensemble) has such filter in the LOW CUT filter knob.  It is 2nd order, and uses a dual pot.

[chi_boy]

There are reports that R.G.'s lofi telephone pedal could be adapted to such duty. Mark Hammer made some sugestions to modify the circuit including eliminationg the noise and adding sweep pots. The builder said the mods worked great. I don't have links now, but can post from home tonight if you can't find it.

[John Lyons]

George
I could not find it in a couple searches.
If you can did it up that would be great.

thanks
John

[Quackzed]

http://www.diystompboxes.com/smfforum/index.php?topic=78740.msg649014
you may need  to tailor it a bit, bigger input caps for instance, but it seems close.

[John Lyons]

Thanks for the link.



So is this a -6db filter correct?.
I assume the only way to get to -12 db would be to
add another series filter?

If not can amyone shed some light on how I can get more cut.?

thanks

[chi_boy]

George
I could not find it in a couple searches.
If you can did it up that would be great.

thanks
John

John,

These are the two posts I was talking about.  I built mine with a pair of 1458's to get a more lofi sound, but whe I breadboarded I used 5532's.  With the 5532's it is notably better sound.  I didn't build mine with pots as Mark suggested, but rather build it with fixed ranges per the schematic.  I am planning to build Marks version some time soon though.

George

http://www.diystompboxes.com/smfforum/index.php?topic=26214.msg173796#msg173796

http://www.diystompboxes.com/smfforum/index.php?topic=25185.msg164559#msg164559

[JKowalski]

Thanks for the link.



So is this a -6db filter correct?.
I assume the only way to get to -12 db would be to
add another series filter?

If not can amyone shed some light on how I can get more cut.?

thanks

That's actually a -12dB filter. It's a Sallen-Key filter.


Use Falstad's analog filter applet to develop op amp / passive filters and see the response in real time. His apps are so amazingly useful. Plus, you can use em anywhere you have an internet connection.

If you've never heard of his apps, check out his "Spice" style app where you can build a circuit using an enormous selection of common components and see voltage, current, power, V/I curves, and more in real time. Really easy to learn how to use. And again, can be used anywhere with an internet connection.


I think I might write up a "getting started" type thing for his apps because they are so helpful and I don't think many people here know about them.

[spasticteapot]

I'm thinking that a VCF of some sort is in order? Most of the ones I've seen are quite complicated, though.
Here's an example:
http://www.cgs.synth.net/modules/cgs35_syntha_vcf.html

[John Lyons]

Spastic
I'm not looking to sweep it in as much as have it variable and somewhat set an forget.
I should have clarified 

JKowalski

So If I put a dual gang pot in place of the 110K resistors I could sweep the
frequency range up and down (depending on the cap values for the cut off point).
Does this seem logical?

John

[JKowalski]

JKowalski

So If I put a dual gang pot in place of the 110K resistors I could sweep the
frequency range up and down (depending on the cap values for the cut off point).
Does this seem logical?

John

Yes, it will work if you do that.

You can even sweep the filter with one potentiometer but in doing so you will have less range, and the characteristics of the filter will alter as you sweep (at one extreme you will have a reallly gradual cutoff before the 12dB slope, the other extreme you will have a hump of gain at the cutoff that immediately goes into the 12db slope.

You can see all of this by opening up that filter applet, selecting Circuits - Active Filters - High Pass and then adjusting the circuit component values or the sliders & menus on the right.

[John Lyons]

Chris
Yeah! That is a great link, I missed it somehow. Did you add it later?
I can't seems to find the active filter section you mentioned

[JKowalski]

Chris
Yeah! That is a great link, I missed it somehow. Did you add it later?
I can't seems to find the active filter section you mentioned

Woop wrong link.

Go here for the filter applet

http://www.falstad.com/afilter/

[John Lyons]

Perfect! thanks.
This is great stuff, very useful applets there!

[petemoore]

  I suppose it would be.
  If I could only download the download that allows me to download the download that allows downloading of the programs and then allows the download that allows them to download/run on Vista.
  Was hopin' to jump off at some point during the first phase cycle, but it kind of developed into a frustration LFO cycling.

[Gus]

look for the active filter cookbook
http://en.wikipedia.org/wiki/Don_Lancaster

[John Lyons]

To use this with guitar?instrument level etc should I buffer the input?

http://ok1ike.c-a-v.com/soubory/tipy/HIGH%20PASS%20ACTIVE%20FILTER.JPG

Is the impedance partially set with the 110K?

[PRR]

> Is the impedance partially set with the 110K?

Blunt-hammer analysis:

Caps are open at DC and short at infinite frequency.

So at DC, and very low frequency, C1 is open, input impedance is infinite. (We may not care much, since this is the cut-band of the filter.)

At very high frequency, C1 C2 are dead short. The input must drive R2 110K to ground. Does it have to drive R1? This is a bit tricky. The amp is unity gain. With C1 C2 short, the signal at pin 6 (out) is the same as at pin 3 (+in). Therefore the signal voltage is identical on both ends of R1. No current flows. Effective R1 impedance is infinite. Effective signal input impedance is just R2 110K to ground.

So falling from infinity to some point, then 110K above that point.

Where that point be? Considering how we picked these Cs and Rs, probably around 100Hz. Probably slightly above 110K at 100Hz, rising at lower frequency.

Source loading is one issue. However this type of filter can be sensitive to source impedance. Generally they are optimum for zero-Z source, and sometimes have quite large errors if source resistance is not much-much-much lower than RC impedances.

The Idiot Assistant tells me 110.3K at 1KHz, 113K at 300Hz, 136K at 100Hz, 288K at 30Hz, 800K at 10Hz, and who-cares below that? (About 8Meg at 1Hz.) IA also shows that 50K of source drops the pass-band (obviously) and makes corner less-sharp, but nothing major, and not a problem for musical uses.

[soggybag]

Thanks for the link.



So is this a -6db filter correct?.
I assume the only way to get to -12 db would be to
add another series filter?

If not can amyone shed some light on how I can get more cut.?

thanks

Use Falstad's analog filter applet to develop op amp / passive filters and see the response in real time. His apps are so amazingly useful. Plus, you can use em anywhere you have an internet connection.

If you've never heard of his apps, check out his "Spice" style app where you can build a circuit using an enormous selection of common components and see voltage, current, power, V/I curves, and more in real time. Really easy to learn how to use. And again, can be used anywhere with an internet connection.

Wow that is the most amazing site! Thanks for posting that I had not seen it before.

[Quackzed]

sorry for the ot..
..

Use Falstad's analog filter applet to develop op amp / passive filters and see the response in real time. His apps are so amazingly useful. Plus, you can use em anywhere you have an internet connection.

but that is an amazing site indeed!
I think i'll post it in a new thread so people learning will have better access to it.
for us 'visual' thinkers.