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

[David G. Dixon]

> 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?

I realize this is a stale thread, but I just did some simulations yesterday
on my developing 4-pole filter design which were very enlightening, and the
results of which I think are worth sharing.

In a nutshell, the simulations give more or less exactly the same results
whether I use the LM13700's internal buffers (with 10k emitter resistors to
-15V as per the datasheet) or TL072 followers, until I turn the cutoff
frequency way down.  Near and below the bottom of the audio range, at
self-oscillation, the internal buffers force DC levels on the signal and
eliminate the oscillation.  With opamp followers, I continue to obtain
perfect sine waves, at constant amplitude and with almost no DC offset, down
to very low subaudio frequencies.

As I intend my filter to double as a quadrature sine wave oscillator, it is
definitely worth it to use another four opamps in my design.

Having said all that, I don't believe that using the buffers would inversely
impact the sound of the filter over the normal audio range.