[sdiy] Passive filters and impedances

[Richie Burnett]

> That's how the Roland Diode Ladder filter works.

True, but it also has much additional circuitry that puts negative feedback around the sloppy un-buffered ladder to tighten up its response into something more musically usable.  (The poles of the 303 ladder are scattered over more than 3 octaves without resonance, due to the loading effects in the un-buffered ladder! That gives a really soft gradual transition from the passband into the stopband of the lowpass filter without resonance.) Fortunately a little bit of negative feedback around the filter makes the poles move together and tightens up the response to something that is much sharper.

> And slight variations of simple RC ladders are used all over guitar amplifiers.  Like the "slope" control on the Marshall JCM 800 bass amp.  Or the weird tone controls in the Sound City amps.  The Gibson GA-40.  There are several other examples I'm not thinking of right now.

Ok, perhaps unbuffered filters *are* useful in applications like guitar amps if you don't mind complex interactions between the various tone controls. Fender tone stack for example!

-Richie,