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I'm trying to build a High Pass Filter box to go at the end of my fx chain just before preamp. The only thing it needs for control is a pot for adjusting the cut off frequency. I've done some research and ordered the parts. I wanted the highest cut off frequency to be around 400Hz. Found an RC calculator and determined I wanted a .33uf cap and a 1K pot. If I calculated right, I'd get a cut off freq up to 482Hz. That's fine. So, I bought the parts and they arrived today.

I guess going from schematic to hands-on soldering has me a bit mystified (well, the schematic doesn't include variable resistance). The way I have it now, the tip of the input goes immediately through the .33uF polyester film cap, then to the wiper terminal of the 1K pot and then to the tip of the output. Reading about how to use a potentiometer as a variable resistor, I believe then I needed to connect post 1 (clockwise) to ground to the I/O jack sleeves. This would give me the 482Hz cut off freq at full clockwise position and no pass filtering at the counter-clockwise position.

The reality though is, turning the pot fully clockwise I'm hearing what I would expect my guitar to sound like with a cut-off frequency around 482Hz, but when I turn counter-clockwise I get volume attenuation (fully counter clock-wise results in no volume at all). I've tried reworking this 5 times and each time I get volume attenuation (sometimes, just volume attenuation and not HPF). I'm so new at this, I'm hoping someone has an illustration or can tell me where I've gotten it wrong based on my description.

Thanks!

Here is a pic of what I have right now that seems to give me HPF in the 400Hz range clockwise on the pot and volume attenuation counter-clockwise on the pot.

The frequency roll off will also depend on what comes before the passive filter (eg buffered bypass, straight guitar through true bypassed pedal etc) but I think the problem is that as you turn it counter clockwise, you are effectively grounding the output, hence the silence. I'm not completely clear on how you want the control to work: I would assume you wanted a control that left the signal flat at minimum setting, then progressively rolled off more bass as you turn it up.

This is how I would do it
The final freq rolloff will include any series resistance with the pot on the input side.

Groovenut wrote:This is how I would do it.

That's a low pass filter.

Nocentelli wrote:

Groovenut wrote:This is how I would do it.

That's a low pass filter.

Woops! I misread the op. Sorry

Lets try this. Swap your pot for a 100k and use a 1k resistor. And I suppose if you were crafty you could put both together in one box and have a passive LP/HP filter box

Nocentelli wrote:The frequency roll off will also depend on what comes before the passive filter (eg buffered bypass, straight guitar through true bypassed pedal etc) but I think the problem is that as you turn it counter clockwise, you are effectively grounding the output, hence the silence. I'm not completely clear on how you want the control to work: I would assume you wanted a control that left the signal flat at minimum setting, then progressively rolled off more bass as you turn it up.

Thanks.

It's a buffered bypass before the filter. Actually, I have 2 choices. Before the preamp at the end of the front-end fx chain (buffered) or after the preamp at the beginning of the fx loop. The goal is to have something like an amps Tight switch, but variable on the cut off frequency. Your assumption is good, I wanted flat to approx 400-500Hz cut off.

After reading a few things and talking again to the guy who pointed by to getting a .33uF cap and 1K resistor, I think the calculation was incorrect and I'm sending 482Hz - 482kHz to ground, which on one end is all the frequencies ... hence a volume attenuator. This certainly sounds like the most common result I was getting from the various ways I tried to wire it up. Sound right to you?

Groovenut wrote: Lets try this. Swap your pot for a 100k and use a 1k resistor.

The attachment high.jpg is no longer available

The attachment low.jpg is no longer available

Thanks, that's definitely what I'm after. What if I used a 10k pot alone, or is the 100k pot and the 1k fixed resistor the way to go? The guy who set me off on this task who's advice I was originally following (who I appreciate getting me going, but has corrected himself a couple times now) has given me this image now (the cap would still be .33uF). The schematic is nothing new to me, but is the wiring illustration correct? Because, since I was using wrong values and didn't know it, I tried a half-dozen different wirings looking for a correct solution. My head got pretty well screwed up on how to wire the pot. I thought I was settled on the input going to the cap and the wiper terminal (joined to terminal 3 and the output) and terminal 1 going to ground, but now today's info has me questioning that.

BTW, a combined passive HPF and LPF in one box would be cool. I was already thinking of making that a project next.

Does it absolutely have to be passive? You'll run into several issues with this approach.
If I were you, I'd build an active filter instead. The "bass control" from Timmy, zen-drive etc. could be what you're looking for

grrrunge wrote:Does it absolutely have to be passive? You'll run into several issues with this approach.
If I were you, I'd build an active filter instead. The "bass control" from Timmy, zen-drive etc. could be what you're looking for

It doesn't have to absolutely be passive, but I'll get this going as a simple utility for adding a tight switch and get more ambitious from there. What I have is an AMT P2 Preamp and I can plug that into a variety of power amps. My favorite combination comes from amps with a tighter bottom end (or a tight switch). I really just want to have some version of a tunable tight switch that I can take with me. I can make up for volume loss or further adjust frequencies in the preamp or power amp. Right now, I have an RC Booster pedal before the preamp to do a bass cut (everything else is the pedal is at unity). I'd like to replace that and use the RC Booster for other things. Plus, I just thought it'd be fun to experiment a bit.

Rockdebris wrote:

Groovenut wrote: Lets try this. Swap your pot for a 100k and use a 1k resistor.

high.jpg

low.jpg

Thanks, that's definitely what I'm after. What if I used a 10k pot alone, or is the 100k pot and the 1k fixed resistor the way to go? The guy who set me off on this task who's advice I was originally following (who I appreciate getting me going, but has corrected himself a couple times now) has given me this image now (the cap would still be .33uF). The schematic is nothing new to me, but is the wiring illustration correct? Because, since I was using wrong values and didn't know it, I tried a half-dozen different wirings looking for a correct solution. My head got pretty well screwed up on how to wire the pot. I thought I was settled on the input going to the cap and the wiper terminal (joined to terminal 3 and the output) and terminal 1 going to ground, but now today's info has me questioning that.

BTW, a combined passive HPF and LPF in one box would be cool. I was already thinking of making that a project next.

You could use the circuit I posted with 10k pot and a 1k resistor, that would put the lowest freq at 44Hz.

The resistor sets the high freq of the RC, the pot value plus the resistor sets the low freq of the RC. Without the resistor, the pot at minimum R would short all signal to ground.

You can use the wiring diagram, just put a 1k resistor between the wiper and ground.

Cool, thanks. Ordered a bunch of new components today and I look forward to make this work.

Groovenut wrote: You could use the circuit I posted with 10k pot and a 1k resistor, that would put the lowest freq at 44Hz.

The resistor sets the high freq of the RC, the pot value plus the resistor sets the low freq of the RC. Without the resistor, the pot at minimum R would short all signal to ground.

You can use the wiring diagram, just put a 1k resistor between the wiper and ground.

Got the parts and some extras with a variety of pot and cap values. I put it together using the .33uF cap, 100k pot and 1k resistor. Seems to work great except that the range from lowest to highest cut off is so large that only the last 10% or so makes any practical difference on guitar. I can set it and get a good sound, but it is touchy.

Say I wanted to have finer control with a range between approx 90Hz to 482Hz. I'm trying to learn to make my own calculations and it seems that if I change the 100k pot to a 5k pot, the range would be 80Hz to 482Hz. Is that correct? I would just try, but I don't have anything below a 25k pot at the moment.

Rockdebris wrote:Say I wanted to have finer control with a range between approx 90Hz to 482Hz. I'm trying to learn to make my own calculations and it seems that if I change the 100k pot to a 5k pot, the range would be 80Hz to 482Hz. Is that correct?

Yes that's correct. I would experiment with different pot tapers too. you may find you want finer control over a specific part of the range.

Groovenut wrote: Yes that's correct. I would experiment with different pot tapers too. you may find you want finer control over a specific part of the range.

Cool. I put the 25k pot in since that is the best I can do with the stuff I have on hand. Gave me about 30% usable range on the pot, much improved. Thanks! I'm not going to be in a rush to get the 5k pot now, this'll take me through the next couple of gigs without too much hassle dialing it in.

Why not reduce the cap size?

Nocentelli wrote:Why not reduce the cap size?

I considered that. What cap size would you say give me the same frequency cut off high and low range keeping the 25k pot and 1k resistor? I'm not terribly fast at figuring this out yet.

Rockdebris wrote:

Nocentelli wrote:Why not reduce the cap size?

I considered that. What cap size would you say give me the same frequency cut off high and low range keeping the 25k pot and 1k resistor? I'm not terribly fast at figuring this out yet.

If you change the cap to .1uF, you would need to change the resistor to 3k3 and use the 25k pot to get similar results. 56Hz-482Hz range with those values.

Here's the formula for figuring the knee point

frequency =1÷(2πRC)