[sdiy] help with clicks on VCA

[Magnus Danielson]

Antti Huovilainen wrote:
> On Mon, 17 Aug 2009, Dan Snazelle wrote:
> 
>> it happens with square waves too even with saws
> 
> Square has two sharp edges, saw one. Only difference is the number of 
> clicks per cycle.
> 
>> from what i have been reading this is a very common problem with tremolos
> 
> Slewrate limiting followed by a lowpass should help a lot.
> Perhaps use something like TLC27L2?
> 
> On related note, is there some way to artificially slewrate limit an 
> opamp circuit without affecting the gain-bandwidth product?

If you mean true slew-rate limit, then you can achieve that as long as 
the bandwidth is allowed to be reduced during the limit case, if not 
then you have problem. If you current-limit the output stage with a 
known capacitive load and with a feedback load, then you should be able 
to achieve this. If you hook a cap over a long-tail pair the current is 
limited to that of the common emitter current-mirror, so buffering the 
sense and feedback it should be able to keep track until it is limited 
by it's current.

Slew-rate increases with both frequency and amplitude, and so does 
gain-bandwidth, so they are quite related... but slew-rate does not 
necessarilly relate to sine-signals. A slew-rate limit must reduce 
amplitude when in limit case... at least eventually. There is a form of 
non-linear compression where a sine becomes non-linearly shaper higher 
and higher up to a triangle. Eventually it should be entierly compressed 
for further amplitude as it is almost a perfect triangle with the slew 
of the limit for both slopes. The linear feedback will linearize the 
non-limited regions.

Cheers,
Magnus