[sdiy] diode war

[ben]

on 8/10/02 7:56 PM, jhaible at debitel.net at jhaible at debitel.net wrote:

> about output impedance: You normally have a large resistor
> across the diodes. This sets a (rather high) initial gain, which is
> then reduced as th ediodes conduct. No problems with output
> resistance here - th eloop is always closed.

i have seen this resistor, but didn't think too much about till you
mentioned it.

> About the gain-bandwidth-product: This is a small signal parameter,
> and certainly doesn't fit for a distortion circuit once the diodes
> conduct. I'd say you have to choose a time domain description for
> analyzing this case.
> For very low levels, before the diodes conduct, the high closed loop
> gain would certainly mean that an opamp with low GBP would
> cause some HF rolloff and/or distortion. (Which might be wanted or
> unwanted.)
> 
> Many people claum there is a difference in sound for certain opamp/diode
> based distortion circuits, depending on the opamp that is used.
> I'm ready to believe this, but I have not analyzed it. Here are some
> thoughts nevertheless:
> 
> (1) If the opamp is driven so hard (and if the circuit allows it), that
> the opamp gets into saturation when the output approaches the supply
> voltage, or if it goes into slew limiting, then a *very* different sound
> from different opamp types is obvious.
> 
> (2) If the opamp is driven hard, and a diode bounding circuit prevents
> the output from case (1) - this should be most diode clippers! - then
> I expect no big difference from opamp to opamp.
> 
> (3) At low levels (diodes not - or barely - conducting), the GBP of th eopamp
> and the maximum closed loop gain (from resistor across the diodes) of the
> circuit will have a certain influence.
> 
> Does this make sense?

yep, i was thinking along the right track, but i've changed the topic a bit
i guess. thanks.

might have to try some circuits.