Help understanding the Fulltone OCD HP/LP setting

[kraigen]

I'm having a little trouble understanding how the High Peak and Low Peak settings work in the OCD. From what I can see in the schematic, when the LP setting has the output of the opamp go through the 22k and 33k resistors in parallel,this means it goes through a 13.2k resistor, much lower than the 33k resistor it goes through in the HP setting. How does going through a smaller resistance produce a lower peak and a warmer tone with a decreased distortion than going through a higher resistance as in the HP.

this is the schematic I used:
https://tornadoalleyfx.files.wordpress.com/2015/07/ocd_schem.gif

[KarenColumbo]

Hi, have a look at this: http://www.muzique.com/schem/filter.htm

[kraigen]

So, a higher corner frequency means that I will have a low peak distortion?

[KarenColumbo]

It looks like this to me. As I read it it's a simple low pass filter with a shallow "knee", so it cuts off more or less of the high frequencies AFTER all the overdrive. But frankly: I'm an absolute n00b - I hope some of the genius gentlemen here will chime in to bail me out

[kraigen]

I think this could very well just be a treble control, but I'm not sure about it either. Thank you very much!

[slacker]

It's a shelving low pass filter, there's an example of it here http://www.linkwitzlab.com/filters.htm with the maths if you're interested. The corner frequency is at the low end for guitar, so I think it's designed to give a the impression of a bass boost. I don't know why the switch is called HP/LP it's a low pass filter in either position, the switch just changes the corner frequency and the minimum amount of cut.

[GGBB]

I'm having a little trouble understanding how the High Peak and Low Peak settings work in the OCD. From what I can see in the schematic, when the LP setting has the output of the opamp go through the 22k and 33k resistors in parallel,this means it goes through a 13.2k resistor, much lower than the 33k resistor it goes through in the HP setting. How does going through a smaller resistance produce a lower peak and a warmer tone with a decreased distortion than going through a higher resistance as in the HP.

this is the schematic I used:
https://tornadoalleyfx.files.wordpress.com/2015/07/ocd_schem.gif

It's the other way around - HP uses the parallel combination, LP uses 33k. Less resistance raises the corner frequency of the low-pass RC filter, thereby allowing more high frequency content at the output. In this case a little over an octave. Therefore a "high" peak.

[Mark Hammer]

It's a shelving low pass filter, there's an example of it here http://www.linkwitzlab.com/filters.htm with the maths if you're interested. The corner frequency is at the low end for guitar, so I think it's designed to give a the impression of a bass boost. I don't know why the switch is called HP/LP it's a low pass filter in either position, the switch just changes the corner frequency and the minimum amount of cut.

Assuming that schematics I have seen for it are accurate, the toggle simply changes the overall range where the treble-cut is applied.  So, not really a switch between highpass and lowpass, in the sense of filter topology.  Just a higher or lower range.

Wouldn't be the first time a manufacturer labelled/legended a control as something other than what those with more electronic knowledge knew it to be.  It happens.

[kraigen]

I'm having a little trouble understanding how the High Peak and Low Peak settings work in the OCD. From what I can see in the schematic, when the LP setting has the output of the opamp go through the 22k and 33k resistors in parallel,this means it goes through a 13.2k resistor, much lower than the 33k resistor it goes through in the HP setting. How does going through a smaller resistance produce a lower peak and a warmer tone with a decreased distortion than going through a higher resistance as in the HP.

this is the schematic I used:
https://tornadoalleyfx.files.wordpress.com/2015/07/ocd_schem.gif

It's the other way around - HP uses the parallel combination, LP uses 33k. Less resistance raises the corner frequency of the low-pass RC filter, thereby allowing more high frequency content at the output. In this case a little over an octave. Therefore a "high" peak.

Really? The schematic led me to believe that when the switch is open, you are in HP mode using the 33k resistor.

[GGBB]

Really? The schematic led me to believe that when the switch is open, you are in HP mode using the 33k resistor.

That's not how I read it. The open position is labeled "LP" and the closed position is labeled "HP."

[Ben Lyman]

That switch is just a marketing scam because the average consumer wants more switches and knobs.
Here we have a tone knob "range limiting" switch to fool the buyer into thinking it is a "range expanding" switch.
Do away with it in your own pedal and simply turn the tone knob for the desired sound 

[PRR]

I see it as a low-pass at a low frequency or a low-pass at a higher frequency.

Which-is-which is unimportant.

[Mark Hammer]

That switch is just a marketing scam because the average consumer wants more switches and knobs.
Here we have a tone knob "range limiting" switch to fool the buyer into thinking it is a "range expanding" switch.
Do away with it in your own pedal and simply turn the tone knob for the desired sound 

Nah.  Paul/PRR nailed it, I think. 

Sometimes it is more convenient for the user to have a toggle that switches ranges, than it is to have a pot that provides the full scope of adjustments.  The Tone pot is a fairly modest value - 10k in Version 3 - which doesn't provide a wide range of adjustment (still some treble bleed even at max brightness), but does allow nice dialability within each range.

Think of it this way:  Imagine you had a Rate pot on a phaser, whose speed range went from 12hz to .12hz.  So, faster than most phaser LFOs, to slower than many LFOs.  Cool. Only trouble is, you now have a 100:1 range of adjustment in one puny little knob.  Are you ever going to be able to replicate a speed setting precisely?  Probably not.  Far better to have a Rate control with a toggle that switches between fast and slow ranges.  Not a gimmick.  A convenience.

So the OCD uses two ranges of treble cut adjustment with a Tone pot that has a limited span of adjustment within each range, but easy repeatability of setting.

Sometimes, I'll grant you, manufacturers do throw on controls to make things snazzier.  Sometimes, it's just a freebie or easter egg that the designer recognizes could be had for the price of a SPST toggle.  Sometimes they do it out of consumer pressure, though.  When my buddy Tim started producing the Retro-Sonic compressor (a Ross clone) some years back, we discussed having a gain-recovery option and I worked out values for him for a 3-way toggle to give slow, medium, and fast recovery.  He manufactured the compressor with the 3-way toggle for a while (as EHX still does with the Soul Preacher), and then I saw he started making it with a pot instead.  When I asked why the switch he said it was simply customer pressure.  Even though I'll bet few set the gain recovery to anything other than 7:00 or 5:00, they felt like they were hemmed in and missing out, via 3 fixed presets, so they kept asking for a continuously variable control, and Tim yielded.

I don't own an OCD, but I do have a Joyo clone of one that someone gave me.  The toggle/pot combination is effective at doing what it attempts to do.

[Ben Lyman]

hmm... you have a point there. I still stand by my own personal motto:
"More switches and knobs equals less music" 

[PRR]

> The Tone pot ... ... doesn't provide a wide range of adjustment

The pot sets the shelf-depth.

This has little to do with the cutoff frequency.

If you love knobs, replace the 22k/33k with say 100k pot and 10k fixed, so you can dial 110k-10k and shift frequency about 10:1. Ah, but then the "tone" (depth) pot's action changes from a lot to hardly-any. And fine control of frequency is not so very important on a soft-corner filter like this. A 2-way switch is a reasonable design, does make "a change", lets you shape your racket to fit into the overall band sound (mellower or brighter), costs little.

[kraigen]

Thank you so much for taking the time to answer, this is very interesting.