[LAU] Filter envelope

[Q]

Hello

I'm looking for a resonant filter that's switchable between lowpass, 
bandpass and highpass and which has a traditional ADSR envelope 
controlling it. I'm wanting something that can be used in JACK Rack or 
something like that, or which could be used as a plugin in Ardour, in 
order to process incoming audio in real time.

There are plenty of LADSPA filter plugins, but they all seem to be 
either static or swept by an LFO or similar. I thought someone, at some 
point, might have made a filter and envelope triggered by an incoming 
signal, in the way that a synth's filter does.

It looks like all the building blocks are probably there in various 
LADSPA plugins, but I can't figure out how you'd connect them together.

I'd also consider a hardware solution (but only a cheap one, as finances 
are very tight at the moment) and in some ways it would have some 
advantages. I thought there was bound to be a guitar stompbox that would 
do that (and no doubt a cheap Behringer knock-off version as well!), but 
so far I've not turned up anything suitable.

Does anyone have any suggestions -- either for software or cheap 
hardware -- that would help me achieve what I want to do?

Thanks in advance

Q