filters in modulation

[POTL]

Hello.
At the moment I'm studying how modulation works and was interested in some nuances in MXR schemes

1) The first thing that seems to me strange is the 2 second-order filters in the mxr flanger scheme (upper picture) U1 - half is switched on nonstardardly, 2 additional 30K resistors (R5 & R13) are used for normal operation, why is not the standard filter used?

2) MXR Chorus (bottom image) uses 2 halves of U1 as input buffer and active filter.
I did a simulation and realized that you can get a similar result using only 1 half of an operational amplifier, as is done in the MXR Flanger scheme, just change several ratings to get a similar filter.
Is there any advantage in the form of a separate filter and input buffer against the classic non-inverting amplifier in the MXR flanger circuit, both have high input impedance and the same cutoff frequency, what reason to use more components?

[noisette]

@1 R5 is standart, look up multiple feedback filters, R13 is because the archaic opamp used needs it, if you´re using modern opamps just connect neg.pin to gnd...
@2 the buffer is needed for the dry path?

[ElectricDruid]

1) The first thing that seems to me strange is the 2 second-order filters in the mxr flanger scheme (upper picture) U1 - half is switched on nonstardardly, 2 additional 30K resistors (R5 & R13) are used for normal operation, why is not the standard filter used?

One's a Multiple Feedback filter (MFB) and the other is a Sallen-Key filter. Both are "standard" designs, as Noisette said. One difference is that the MFB is inverted and the SK isn't. Sometimes that can be a reason to use one and not the other. Another potential reason is that the MFB has better high frequency performance without putting such heavy demands on the op-amp. For filtering clock noise, that's not a bad idea.

2) MXR Chorus (bottom image) uses 2 halves of U1 as input buffer and active filter.
I did a simulation and realized that you can get a similar result using only 1 half of an operational amplifier, as is done in the MXR Flanger scheme, just change several ratings to get a similar filter.
Is there any advantage in the form of a separate filter and input buffer against the classic non-inverting amplifier in the MXR flanger circuit, both have high input impedance and the same cutoff frequency, what reason to use more components?

The chorus uses three filter stages rather than the two in the flanger, so I don't think they're really comparable. The flanger also uses a pre-emphasis/de-emphasis scheme which the chorus doesn't seem to bother with.
The other thing that's worth remembering is that reducing the op-amp count by one doesn't really help - a single op-amp is an 8-pin chip just like a dual. So unless you can remove *two* op-amps from a circuit, often there isn't much point.

[POTL]

@1 R5 is standart, look up multiple feedback filters, R13 is because the archaic opamp used needs it, if you´re using modern opamps just connect neg.pin to gnd...
@2 the buffer is needed for the dry path?

for dry and wet
After all, the signal is divided after it.

[POTL]

1) The first thing that seems to me strange is the 2 second-order filters in the mxr flanger scheme (upper picture) U1 - half is switched on nonstardardly, 2 additional 30K resistors (R5 & R13) are used for normal operation, why is not the standard filter used?

One's a Multiple Feedback filter (MFB) and the other is a Sallen-Key filter. Both are "standard" designs, as Noisette said. One difference is that the MFB is inverted and the SK isn't. Sometimes that can be a reason to use one and not the other. Another potential reason is that the MFB has better high frequency performance without putting such heavy demands on the op-amp. For filtering clock noise, that's not a bad idea.

2) MXR Chorus (bottom image) uses 2 halves of U1 as input buffer and active filter.
I did a simulation and realized that you can get a similar result using only 1 half of an operational amplifier, as is done in the MXR Flanger scheme, just change several ratings to get a similar filter.
Is there any advantage in the form of a separate filter and input buffer against the classic non-inverting amplifier in the MXR flanger circuit, both have high input impedance and the same cutoff frequency, what reason to use more components?

The chorus uses three filter stages rather than the two in the flanger, so I don't think they're really comparable. The flanger also uses a pre-emphasis/de-emphasis scheme which the chorus doesn't seem to bother with.
The other thing that's worth remembering is that reducing the op-amp count by one doesn't really help - a single op-amp is an 8-pin chip just like a dual. So unless you can remove *two* op-amps from a circuit, often there isn't much point.

[/quote]


1) I thought about inverting, but I'm not sure if this is important 
2) Yes, I am aware that the dual operational amplifier is indicated on the diagram by two elements 
In fact, I did some simulations of this scheme (without taking into account BBD) and thought that it would be a good idea to keep the tone but remove some nodes that in my opinion (but I'm not sure for sure and interested) can be simplified.
Failure of the stereo output, some changes in the filters and the input output stage (starting from the flanger circuit), this will remove 2 two-channel operational amplifiers from the circuit.
I have many ideas for modifying the circuit.
But there are nuances in which I'm not so sure, in particular - whether it makes sense to use an input buffer and a filter or a non-inverting amplifier (as in a flanger) will work just as well in terms of stability of the circuit, in terms of sound the simulator says that I can get almost identical results with minimal deviations in decibel fractions

[POTL]

There were a few more questions.
1) Regarding the Flanger. In many Flanger circuits, I see an inverting amplifier with a symmetrical clipping(U2).
What role does it play in this type of effect. I saw a similar cascade in the BOSS BF-2 and Ibanez FL9 & Airplane.
I understand that it limits the peaks of the signal, but from a technical point of view I do not understand why this is done. Clipping and feedback is the main difference between Flanger and Chorus (apart from delay time)?
Why is this amplifier inverting?
Why change the phase, because you can just put a non-inverting amplifier.
2) What does the Manual controller do?
Does it change the frequency on which the LFO works? Or does it change the delay time?
3) I'm confused about what Q1 + Q2 + Q3 + Q4 do.
Please tell us in detail their principle of operation or what is the name of this design.

[Mark Hammer]

A lot of questions, but I'll respond to the first one.

Feedback is never used in chorus pedals, so there is no risk of the combined amplitudes of the feedback and input signal exceeding the headroom of the BBD.  The soft-clipping simple limiter you will often see in flangers is to keep higher feedback settings from being too hot for BBDs.  Without any feedback, that risk does not exist for chorus pedals.

Why inverting?  Because negative feedback sounds more impressive than positive.

[POTL]

Thank you. You clarified the situation.
I did not think that feedback could cause distortion and exceed the stock of BBD.
About transistors, I partially clarified the situation. I was told that this current mirror, at least in the scheme Ibanez FL9, here I think about the same.
These transistors are the last (sort of like) a question in modulation schemes, to which I did not find the answer =)