Simple ANKH schemo question..

[blearyeyes]

Why are there two low pass? Filters on the output of this pedal? Or what is the function.


http://www.madbeanpedals.com/EP/schematics/Ankh.gif

[EBRAddict]

Twice the rolloff? It becomes a -12dB/octave filter vs. a -6dB/octave filter.

[blearyeyes]

I did not know that. If you want a 24db per octave you would run it through 4 sets of rc filters? That seems so odd. There is so much I don't know.

[alanp]

My guess would be that inherent signal loss is why it's not used all over the place.

[midwayfair]

I did not know that. If you want a 24db per octave you would run it through 4 sets of rc filters? That seems so odd. There is so much I don't know.

If you think about this, it happens at several places in your average pedal anyway. Every time you have a capacitor it's reducing a frequency some. Input cap and output cap alone account for a 12dB/octave rolloff at some frequency.

What happens when you put multiple identical filters in a row is that you get them all at the same cutoff frequency.

In the Ankh, though, that's not actually what happens. People always seem to forget that transistor voltage amplifiers have an output impedance. So while it looks like a single 12 dB/octave filter at 7.2KHz, it's actually one filter at 5.4KHz (3.3K+10K and 2.2n) and then a second filter at 7.2KHz. The design also could have skipped R18 entirely and used a 6.8nF for C15.