Hi all!
I m using Pd to process the sound of my guitar, and i want to plug my pc into my amp in order to use it on stage. As you may know guitar amplifiers are build to accept only a limited frequency range, mine accepting frequencies above 800Hz and below 13KHz.
What i m currently doing is filter the sound just before the dac~ with a [hip~ 800] and a [lop~ 13000]. I have read Puckette's chapter about filters realized that filters are never perfect and some frequencies actually leak out below and above these limits.
So what should i do? Do i need shelving filters instead? I m not sure how to use these though, and in particular i don't know how to set the frequency limit (the settings in the audio example H11.Shelving are in hundreths...? )
Any help would be greatly appreciated!

what about listening and judging by yourself if you like what you hear or not? From what I know electric guitars have a different freq range than you describe especially in the lows - 800 Hz is a joke you must have misread for 80 Hz. If your amp ignores freq below 800 throw it away to start with.

If you're not happy with what you hear, Gunther Geiger wrote a set of 'front end' controllers for the [biquad~] filter object. This is part of pd-extended and I'm sure you will find it, there are shelving filters and the rest. If you care to know technical details you can easily google for info on filter types, even wikipedia has correct articles.

Are you sure that's not just referring to the output of the amp? I don't think you need to filter it on the way in (unless it's part of an effect you're making). It is not that uncommon for people to run other instruments, like synths, into guitar amps to color the sound, and those instruments obviously have a different frequency range than a guitar. I think the amp will do the filtering on its own. But then again I've never been much of a gearhead so I could be wrong on that.

You'll need a reverse DI to plug your computer into your guitar amp. Guitar amps have a Hi-Z (high impedance) input. Your computer's output is low impedance.

Some amps do have low impedance inputs.

@ Saturno and Maestrom :Thank you both for your replies! But i have pretty much everything i need in terms of hardware. To plug a soundcard's output into an amp, one does indeed need a reverse DI kind of thing (i m using a Shure tranformer, which is much cheaper than a reverse DI and does the job -turning the signal into HI-Z).
Yet a guitar amp does not like just any input (this was confirmed to me by the guy who services mine), especially tube amps. They particularly require low frequencies to be cut off below a certain limit.

@ Ultrasonic : Yeah, it's definitely 80Hz instead of 800 (my filters are actually [hip~ 80] + [lop~ 13000]). I ve tried 150 and 10000 this afternoon, this seems to sound good enough.
I just looked into the ggee library as you suggested, this is really neat! thanks! Now i think i have all i need!

In PD-Extended there's also lp2~ and hp2~ which are two pole filters.