Mystery circuit snippet

[mth5044]

I'm thinking low pass filtering... only because I'm pretty sure someone told me that at some point    (sorry about the copious amount of white space).



Thanks 

[.Mike]

Sim...

[GibsonGM]

yup, LPF, rolloff (-3dB) seems to occur about 2.4 kHz...

[GibsonGM]

What's it from?

[mth5044]

.Mike - Wow.. I'd never seen a circuit simulated before. That was pretty cool 

GibsonGM - Cool, thanks for the info. It comes from Scott Swartz's PT80 on JD Sleeps site.

[CynicalMan]

interesting symbol for a 2n5088...

It looks like a low-pass filter with an output buffer and some sort of feedback. It's oddly biased too.

[mth5044]

interesting symbol for a 2n5088...

It looks like a low-pass filter with an output buffer and some sort of feedback. It's oddly biased too.

Sorry about the symbol, if it is incorrect. I just grabbed one out of the parts lottery in expressPCB. I can't comment on the feedback or biasing of it, so what makes it so strange? If you don't mind me asking.

[puretube]

Fjallbrant 

[PRR]

Simple well-biased 3-pole low-pass.



The only "odd" is that the response is not one of the "ideal" curves that Fjallbrant, Butterworth, Chebby, Sallen, Key, and other mathematicians derived for technical filtering. As Mike's sim shows, it has "excess bump"; however this is often quite useful in audio. If you are cutting out highs-hiss, a bump before the drop masks the fact that the extreme highs are abnormally quiet, so it won't sound so dull.

[Brymus]

What components would need to be changed and to what value to raise the knee to 4 or 5 khz ?
Could this be used as a noise gate - by replacing the output buffer with this - on for example a TS808 pedal ?

[PRR]

> changed and to what value to raise the knee to 4 or 5 khz ?

Leave the resistors alone. Change the caps, all the same proportion.

> Could this be used as a noise gate -

It's a filter, not a gate. Nothing swings/switches.

[blackcorvo]

can this filter be used as a cabinet simulator, for amplifier emulators like some of Runoffgroove's projects?
or even as a small and simple cab sim for recording purpose...?

[Brymus]

> changed and to what value to raise the knee to 4 or 5 khz ?

Leave the resistors alone. Change the caps, all the same proportion.

> Could this be used as a noise gate -

It's a filter, not a gate. Nothing swings/switches.

OK so change the caps by the same factor /or percentage ? Not just one cap like a simple first order filter ?
I guess I shoulda just said replace the buffer.Or act like a noise gate not work as one.
I like the hump and the steep roll off, I would think it would work well for removing a certain amount of hiss,while still leaving most of the guitar signal sounding good when set to the right corner frequency.

[PRR]

> change the caps by the same factor /or percentage ? Not just one cap like a simple first order filter?

You got it. The simple first-order filter has one tuning cap. Change it. The simple third-order filter has... three tuning caps. Change all three the same factor. Half the nFd moves an octave up (double the frequency).

(The 1uFd caps front and back are not part of the low-pass. They make a sloppy 2nd-order high-pass. The input passes above ~~2Hz, and the output passes depending what is hanging to the right of it.)