I have a preamp amplication that's rapidly getting more complex than I want and am wondering whether the lengths I am going to (inorder to prevent phase distortion) are really unusual or not?
I'd like the thoughts of you guys on this.
Roughly the circuit consists of the following opamp gain blocks
Non inverting preamp section (I may stick this after the butterworth)
Unity gain 4th order butterworth low pass in 2 section Sallen-Key topology
2 cascaded first order active all pass filters at the end
My aim is to get a maximally flat frequency response (amplitude wise), steep roll off and minimise any phase distortion at the top end of the audio spectrum and hopefully see 0 degrees on the bode plot in most of the spectrum.
I am wondering whether in pre-amp applications for music designers go to extreme lengths to avoid phase lag at the top end of the audio spectrum?
I've got my design down to a level where the SPICE bode plot reckons I've got only about 45 degree phase lag at a bit over 20KHz and it goes down to 90 degree lag at dog hearing frequencies. Surely that must be good enough? Or do people not really bother with designing out phase distortion, as most people can't tell by ear?
It's not an 'audiophile' application, but I want it to be cheap to make and give good results.
Regarding calculating All Pass filter values, I'm finding this to be VERY non-trivial. I guess if I had a long time I could go through the books and eventually understand it.
It would be great if you could have an all pass filter, one equation, where you choose your cut of frequency (where 90 degree distortion is acceptable), then set some values for capacitors and resistors and bingo - stick that at the end of your signal chain and all phase distortion disappears. It doesn't appear to be that simple, I've got a book called "Analog and Digital Filter Design, 2nd Edition" - this far deeper than I thought.
If you've got any advice on All Pass filter calculation, I'd be very happy to hear it (I mean see it )
I'd like the thoughts of you guys on this.
Roughly the circuit consists of the following opamp gain blocks
Non inverting preamp section (I may stick this after the butterworth)
Unity gain 4th order butterworth low pass in 2 section Sallen-Key topology
2 cascaded first order active all pass filters at the end
My aim is to get a maximally flat frequency response (amplitude wise), steep roll off and minimise any phase distortion at the top end of the audio spectrum and hopefully see 0 degrees on the bode plot in most of the spectrum.
I am wondering whether in pre-amp applications for music designers go to extreme lengths to avoid phase lag at the top end of the audio spectrum?
I've got my design down to a level where the SPICE bode plot reckons I've got only about 45 degree phase lag at a bit over 20KHz and it goes down to 90 degree lag at dog hearing frequencies. Surely that must be good enough? Or do people not really bother with designing out phase distortion, as most people can't tell by ear?
It's not an 'audiophile' application, but I want it to be cheap to make and give good results.
Regarding calculating All Pass filter values, I'm finding this to be VERY non-trivial. I guess if I had a long time I could go through the books and eventually understand it.
It would be great if you could have an all pass filter, one equation, where you choose your cut of frequency (where 90 degree distortion is acceptable), then set some values for capacitors and resistors and bingo - stick that at the end of your signal chain and all phase distortion disappears. It doesn't appear to be that simple, I've got a book called "Analog and Digital Filter Design, 2nd Edition" - this far deeper than I thought.
If you've got any advice on All Pass filter calculation, I'd be very happy to hear it (I mean see it )