Best place to add a high pass filter in the feedback loop of a Rebote 2.5?

[ianmgull]

Looking at the Tonepad schem here:

http://www.tonepad.com/project.asp?id=51

It is clear that there is one lowpass filter before the feedback loop formed by the 12k resistor and the .027uf cap to ground. This puts the corner frequency at 491.5Hz.

There is another lowpass filter (now mixed with the feedback from pt2399's output) formed by the 12k and .01uf cap to ground. This corner frequency is at 1.3kHz.

I want to add a highpass filter to get a sort of "telephone effect delay". For this reason I don't want to adjust the input cap as this would affect the dry signal.

Would adjusting the .1uf cap in the feedback loop accomplish this (the one in series with the 15k)? If so, would I need to add a resistor to ground?

I used Jack Orman's RC Calculator here:

http://www.muzique.com/schem/filter.htm

[ianmgull]

Ok, by my calculations if I added a 8.2k resistor to ground between the 15k and .1uf in the feedback loop of the pt2399 that would give me a highpass filter with a corner frequency of 175Hz. Does this sound right?

[ianmgull]

Ok so after adding a 8k2 resistor to ground at the junction of the 15k and .1uf the wet signal disappeared completely. I'm stumped. Anyone?

[JKowalski]

Lower the value of the capacitor in the feedback loop. Try something like 4.7nF.

[ianmgull]

Thanks, I'll try that next. Excuse my ignorance but can someone explain why this would work? It's my understanding that there would have to be a resistor to ground in conjunction with the cap to create a high pass filter. How would this create a filter?

[JKowalski]

It's an active filter:



The resistor after the capacitor goes into virtual ground, at the input of the op amp. (well, it passes through some more filter components first but it still gets there eventually... If you think about it, you know how you change the value of the resistor trimmer in the feedback loop to change the repeats? This is beause you are changing the input resistance of an op amp gain stage and thus the ratio, and thus the gain. Well, the capacitor in series acts as a frequency dependant resistance - the op amp gain stage resistance ratio changes based on frequency, and that's how it filters.

[ianmgull]

Chris,

The 4.7nf cap worked beautifully. I particularly like how each repeat becomes a bit more filtered. Thanks for your help.

[JKowalski]

Chris,

The 4.7nf cap worked beautifully. I particularly like how each repeat becomes a bit more filtered. Thanks for your help.

No prob!

[ianmgull]

OK...WTF...


So after dropping the 4.7nf cap in the feedback portion of the delay I noticed that I'm no longer able to get run away repeats. (I had previously changed the 15k to 10k to do this) I thought this was crazy so I switched the cap back to 100nf and sure enough I had run away repeats again.

How is this possible? Do different caps have different resistances? Or am I just filtering out all of the frequencies that would regenerate in runaway delay mode?

[JKowalski]

Caps have resistance based on the frequency going through them, this changing resistance is called "reactance". Reactance is less at high frequencies and greater at low, eventually reaching infinite resistance at DC. (Remember, caps cant pass DC!)


Now, you are creating a RC filter with your capacitor. RC filters have a very very gradual falloff. You are attenuating the low frequencies alot every time it goes around, which is why you end up with a nice trebly repeat, but you are ALSO attenuating the high frequencies a tiny bit too! That means that your total resistance at all frequencies of interest is going to be slightly greater. And you don't get infinite repeats anymore. To get infinite repeats with each repeat losing bass content, you would need a sharper rolloff filter than the simple RC one you have now. That way you still would get the frequencies you want attenuated the right amounts, but you won't have as much of the slight attentuation of the frequencies you want passed.

If you took a look at the frequency response of the filter with the 100nf cap, all frequencies of interest (guitar frequencies) are nearly totally unattenuated. With the 4.7nf cap, you get 3dB attenuation at 2kHz, which is above even the highest note the guitar can play, and more then that as you go lower. However, you have to live with this because it's the best the RC filter can do and still give enough noticable attentuation to what you want attenuated.

You could lower the resistor in the feedback loop, much simpler, to get greater gain and make up for your slight attenuation of frequencies of interest with the simple RC filter. You'd have to maybe switch in a trimpot parallel with the other trimpot, adjusted for inf. repeats with the lower capacitor.

HOWEVER, With this filter on infinite, and the gain upped to compensate, the repeats would eventually degenerate to only the very highest frequencies contained in the note you play. If you play anything higher than that note it is set to repeat infinitely on, then it's gonna get gain everytime it goes through (because unlike the 100nf the infinite response point is not on an almost straight line it is on a slope, and there points of significantly less attentuation above it) and will be a runaway repeat. (ear piercing shriek, no doubt)


Here's a diagram showing what I mean:



Remeber, the infinite repeat points are arbitrary - you adjust where they are by adjusting the trimpot resistor.




This is the ideal response you need for your filter in the feedback loop:

[snk]

Hello,
I'm reviving a 10 years old thread, but I am trying to do the same thing with another delay (the Montavillian echo)...
This is the schematic I am working from : effectslayouts.com/product/evil-owl-echo/
http://effectslayouts.com/wp-content/uploads/2021/11/Evil-Owl-Echo-build-doc.pdf

My goal : adding a resistor on a switch to act as a Hipass filter (around 160-240Hz).

From what I understand by reading the thread above :
- I need to add a resistor to ground, between the cap and the resistor in the feedback loop.
- The cap is 100nF on the Rebote delay, and 1µF on the Montavillian/Evil Owl.
- The resistor is 15K on the Rebote delay, and 22k on the Montavillian/Evil Owl.

- To give a Hipass around 180-200Hz on the Rebote, I'd need to add a 8.2K resistor to ground, because the cap is 100nF. To give a Hipass around 180-200Hz on the Montavillian, I'd need to add a 4.7K resistor to ground, because the cap is larger.

- Adding a hipass in the feedback loop will make the echo less prone to self-oscillation, and slightly quieter.
- So, to get more gain  and runaway repeats from the feedback loop, I can decrease slightly the feedback loop resistor value (15k in the rebote, and 22k in the Montavillian) : may I try 15k?
- Also, I should decrease the value of the resistor cap (from 100nF to 4.7nF on the Rebote, from 1µF to...? on the Montavillian) : should I try with 220nF?

Could anyone give the Evil Owl/Montavillian schematic a quick look, and tell me if I identified the right resistor and capacitor, and if the suggested values make sense?
Thank you

[Mark Hammer]

Chris,

The 4.7nf cap worked beautifully. I particularly like how each repeat becomes a bit more filtered. Thanks for your help.

Been doing this for decades.  Instant dub.  Many dub artists use a form of echo that gets each repeat more shrill than the last.  Imperative that one leave some space in the tune to hear the change over time, though.

I like to include the option in a delay of having either the bass progressively removed from repeats, as discussed here, OR having the treble progressively cut from repeats.  The former is more in line with what I describe as "ambience" in another thread, while the latter sounds more like an actual reverberant  physical space.

[snk]

Yes, that's exactly that
I'm not aiming for a transparent delay or an accurate repetition, and the Montavillian already features a wonderful color on its own (I already built one stock, but i'm considering building another one with a couple mods, including a hipass switch).
I wish I'll manage to keep the overall color of the Montavillian, but add the ability to remove the bass from the delayed signal.

[Mark Hammer]

Note that, if you want a progressive removal of low or high frequency content, the trick is to use a simple 1-pole (6db/oct) filter and select a corner frequency such that there is a noticeably change, even with such a shallow filter, yet still leave enough of the unwanted frequency content that there is still more to remove, next repeat.

This is not "better" than strong filtering of any type, merely a different feel.

[snk]

I'd want two things :
- First, to remove the excessive bass content which can occur when I process basslines or drum machines when I use the pedal on the AUX of a mixer.
- Then, a progressive removal of low frequency content after each repeats.
I already have several delays, and most of them only have treble damping. I happen to like vintage delays which cut bass frequencies, so while I am already very happy with my stock Montavillian, I am considering building another one which would keep the same filtering but woudl have optional bass cut on a switch.

Do you think that the mods posted above (and quoted below) would do the trick?

From what I understand by reading the thread above :
- I need to add a resistor to ground, between the cap and the resistor in the feedback loop.
- The cap is 100nF on the Rebote delay, and 1µF on the Montavillian/Evil Owl.
- The resistor is 15K on the Rebote delay, and 22k on the Montavillian/Evil Owl.

- To give a Hipass around 180-200Hz on the Rebote, I'd need to add a 8.2K resistor to ground, because the cap is 100nF. To give a Hipass around 180-200Hz on the Montavillian, I'd need to add a 4.7K resistor to ground, because the cap is larger.

- Adding a hipass in the feedback loop will make the echo less prone to self-oscillation, and slightly quieter.
- So, to get more gain  and runaway repeats from the feedback loop, I can decrease slightly the feedback loop resistor value (15k in the rebote, and 22k in the Montavillian) : may I try 15k?
- Also, I should decrease the value of the resistor cap (from 100nF to 4.7nF on the Rebote, from 1µF to...? on the Montavillian) : should I try with 220nF?

Could anyone give the Evil Owl/Montavillian schematic a quick look, and tell me if I identified the right resistor and capacitor, and if the suggested values make sense?