Needing help understanding filters in Delays Circuits - Rebote Delay 2.5

[percyhornickel]

Hi,I need help trying to descifrate the filters  in delay circuits.

I´m trying to understand the topologie used in the tonepad´s rebote 2.5 and the way how the filters are used/calculated. (It´s almost the same than others delays like the wampler faux echo and some others)

Question 1:
In the tonepad rebote 2.5 diagram, as far as I can see after the first buffer there is a high pass filter (1uF - 12K) and then a multiple feedback low pass filter (12k, 12k, 12k, 47k, 27n, 10n, 560p). Is this correct? .  I need help trying to descifrate the filters.

I just finished the Rebote Delay 2.5 circuit and works pretty good, I really like it and the hiss repeats are present from the middle of the delay pot till the end (which is more audible), maybe the fact is not fitted in a enclosure yet helps to increase the "dirty feedback".


Questions 2:
What frecuencies are cutting with this filter?.  This filter calculator seems to work with this topologie:

http://sim.okawa-denshi.jp/en/Multiple3tool.php

Results: 1103 Hz and 2672 Hz. How  do I have to interpretate these frecuencies? are they decreasing the signal in two places in bode´s diagram? How many decibels/decade ?

Questions 3:
The HPF located in the feedback line (0.1uF - 15K) which cut frec results somehing like 10,6Hz and the line goes right beetwen two 12k resistors. Why this feedback line is going right beetwen these 12k resistor before? or afterthis point?.

I guees it is the same with the other LPF for the out.

Thank you

[Keppy]

Question 1:
In the tonepad rebote 2.5 diagram, as far as I can see after the first buffer there is a high pass filter (1uF - 12K) and then a multiple feedback low pass filter (12k, 12k, 12k, 47k, 27n, 10n, 560p). Is this correct? .

The multiple feedback filter is correct, but the 1uF is really just a coupling cap. With just the 12k resistance that cap would pass all audio frequencies, and the frequency response goes even lower when you factor in the other resistors which raise the impedance. It's still technically a high pass filter, but it's not affecting audio.

Questions 2:
Results: 1103 Hz and 2672 Hz. How  do I have to interpretate these frecuencies? are they decreasing the signal in two places in bode´s diagram? How many decibels/decade ?

There is a 1-pole rolloff beginning at 1103Hz and an additional 2-pole rolloff beginning at 2672Hz. Each pole is 20db/decade, so you end up with 60db/decade rolloff above the higher cutoff frequency. You can see this on the page you linked in the Magnitude graph in the Frequency Analysis section.

The three-pole multiple feedback filter is really a basic one-pole RC filter in front of a two-pole multiple feedback filter. The two sections interact, though, which is why the first cutoff frequency doesn't match a basic RC filter calculation for 12k/.027uF.

Questions 3:
The HPF located in the feedback line (0.1uF - 15K)

That 15k is in series with the 2.7k coming out of the delay chip, some parallel combination of resistors in the three-pole filter, and some amount of the repeat pot depending on how it is set. The real value for the sake of the filter is something like 25-50k depending on the pot setting.

Why this feedback line is going right beetwen these 12k resistor before? or afterthis point?.

The idea is that the delayed signal gets filtered again each time it is fed back through the delay, so the repeats gradually lose treble. If you ignore the first 12k resistor and the .1uF feedback cap, you'll see that the feedback signal goes through the same three-pole filter, except that the first resistor is the 15k feedback resistor (really 25-50k as mentioned above) instead of 12k. The feedback cannot be mixed in earlier because that would put it right at the IC2b output where the opamp would overwhelm the feedback signal. If mixed in later, it wouldn't pass through the whole filter.

[percyhornickel]

That is the kind of explanation I was expecting for, thank you. I forgot to mention I am using a 4558 instead TL072 for the rebote delay, the only one I had in hand. Could be this affecting the hiss in repeats?.  I don´t know if the slew rate or something else could affect in a bad way this kind of circuits. Most of buffer/delay circuits uses TL072 but with this 4558 putting aside the hiss, the sounds is very good and clear (RC4558P is the one I am using).


I still have some points to clear:

The multiple feedback filter is correct, but the 1uF is really just a coupling cap. With just the 12k resistance that cap would pass all audio frequencies, and the frequency response goes even lower when you factor in the other resistors which raise the impedance. It's still technically a high pass filter, but it's not affecting audio.

This make sence, with 1uF almost all cut frecuencies calculations start from a real low value, so the audio is not affected.

There is a 1-pole rolloff beginning at 1103Hz and an additional 2-pole rolloff beginning at 2672Hz. Each pole is 20db/decade, so you end up with 60db/decade rolloff above the higher cutoff frequency. You can see this on the page you linked in the Magnitude graph in the Frequency Analysis section.
The three-pole multiple feedback filter is really a basic one-pole RC filter in front of a two-pole multiple feedback filter. The two sections interact, though, which is why the first cutoff frequency doesn't match a basic RC filter calculation for 12k/.027uF.

First Frec: 20 db/dec (one pole)
Sec Frec: 40 db/dec (the double of the first one - two poles)

That 15k is in series with the 2.7k coming out of the delay chip, some parallel combination of resistors in the three-pole filter, and some amount of the repeat pot depending on how it is set. The real value for the sake of the filter is something like 25-50k depending on the pot setting.

I am not too clear in here, do you mean something like this?:

15 + 2.7 + FILTER ARRANGEMENT + 0 REP = (17.5 + FILTER ARRANGEMENT)

TO

15 + 2.7 + FILTER ARRANGEMENT + 25 REP = (42.5 + FILTER ARRANGEMENT)

The idea is that the delayed signal gets filtered again each time it is fed back through the delay, so the repeats gradually lose treble. If you ignore the first 12k resistor and the .1uF feedback cap, you'll see that the feedback signal goes through the same three-pole filter, except that the first resistor is the 15k feedback resistor (really 25-50k as mentioned above) instead of 12k. The feedback cannot be mixed in earlier because that would put it right at the IC2b output where the opamp would overwhelm the feedback signal. If mixed in later, it wouldn't pass through the whole filter.

Ok I get it, just the same thing like the previus point.

I have cleared my point of view about the filters with your explanation, for me there is much more to learn about it yet.!!..   ...I am another kind of engineer (petroleum eng) but numbers are numbers.


P.H.

[Keppy]

That 15k is in series with the 2.7k coming out of the delay chip, some parallel combination of resistors in the three-pole filter, and some amount of the repeat pot depending on how it is set. The real value for the sake of the filter is something like 25-50k depending on the pot setting.

I am not too clear in here, do you mean something like this?:

15 + 2.7 + FILTER ARRANGEMENT + 0 REP = (17.5 + FILTER ARRANGEMENT)

TO

15 + 2.7 + FILTER ARRANGEMENT + 25 REP = (42.5 + FILTER ARRANGEMENT)

Yes, that's right. For low frequencies (which is what we want to look at for the cutoff of the low-pass filter), any current through the feedback cap also has to go through the filter resistors, so they have to be factored in. The feedback point is at the junction of two 12k resistors, so those combine in parallel for 6k resistance. This isn't precise because of the other filter resistors and the fact that they interface with signal currents instead of ground, so I rounded up. This could be calculated more exactly, but since the frequency is variable anyway there's not a lot of practical point in doing so.

[percyhornickel]

Ok I see this better now. Thank you !!!
What do you think about using other opamp (4558) intead TL072 in this circuit?

[ElectricDruid]

What do you think about using other opamp (4558) intead TL072 in this circuit?

Makes no practical difference. You *might* see a measurable change with good equipment, but I seriously doubt you'd hear it enough to identify it in a blind test. The times when you *might* hear differences between op-amps (and I'm not convinced that even then they're as big a deal as people make out) is when the op-amps are pushed to their limits. In this situation, for some basic filtering, pretty much any part that can do the job is fine and you won't know.

Use whatever you've got or like the best. People make a lot of fuss about stuff like this, but then you discover they're trying to sell some boutique pedal with "magic" parts in it for three times what a normal build costs!

[percyhornickel]

You are right, I have tested my DIY ts808 with TL072 instead 4558, my rats with OP07 and LM308N / 308A...     ....and the conclusion is...   ...we are not Vai or Satriani.

Only one advice, chinese opamps often are the same with different labels, happen to me with opamps, NPN transtistors labeled like Jfets, etc...       .....yes I am talking to you AliExpress!!!!!.


Saludos y gracias!!