[sdiy] to opamp buffer or NOT with ota's

[nicolas]

Hi Dan

There shouldn't be much difference in the two buffering techniques at low-mid audio frequencies.  The built in ones are non-inverting.  If you want it DC coupled to be able to go down to low bass frequencies you need to use op-amp buffers.  But if you are mainly interested in the high end the op-amp option will give you roll-off and phase shift which will affect your filter design, so it is simpler to use the built-in buffers in that case.

Resonance harsh-or-smoothness is mainly affected by exactly what feedback scheme you use.  The feedback topology is the biggest difference from one OTA filter to the next.  The key to smooth resonance is to put some kind of smoothing overdrive/fuzz type circuit in there so that as the voltages start to multiply at the resonance peak there is something to keep them in check in a musical sounding way.  Usually some back to back diodes or LEDs or similar is used (a-la tube-screamer), but a deliberately limited class-A common emitter stage (a-la fuzz-face) could also be used.

Cheers,
Nicolas


--- On Sun, 26/4/09, Dan Snazelle <subjectivity at hotmail.com> wrote:

From: Dan Snazelle <subjectivity at hotmail.com>
Subject: [sdiy] to opamp buffer or NOT with ota's
To: synth-diy at dropmix.xs4all.nl
Received: Sunday, 26 April, 2009, 2:41 AM



ok

so having some trouble get clean resonance out of the OTA filters i try and build....everything is coming out very piercing with high Q values

not very smooth sounding.

now...could this be because i am using the INTERNAL buffers of the 13700 instead of using opamps?

do the 13700 buffers even invert?

i see some schematics use em and some dont. just wondering if this could make a difference

thanks





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