On Thu, 3 Dec 2009 23:52:03 +0100 [email protected] wrote: > On Thu, Dec 03, 2009 at 10:29:54PM +0000, Folderol wrote: > > > As promised, PDF attached. > > Mmmm. Just 1 picofarad of stray capacitance on the switch > and wiring, in parallel with 9M, will create a filter with > its 3 dB frequency in the audio band. And 1 pf is really > nothing, expect more.
I use the old fashioned method of wiring resistors directly on standard glass loaded wafer switches. Initial tests suggest the bandwidth well exceeds 20kHz - as opposed to less than 1kHz for many quite expensive commercial units. > This will lead to gross errors at anything but the > lowest frequencies. > > Is there any reason why an audio level meter should > have such a high input impedance ? No reason at all, except that being a cheapskate I was able to convince the boss that a good true RMS meter would be useful in the workshop :) A 10M input is more-or-less mandatory in this case, but anyone else should feel free to make the ladder impedance whatever they like. -- Will J Godfrey http://www.musically.me.uk Say you have a poem and I have a tune. Exchange them and we can both have a poem, a tune, and a song. _______________________________________________ Linux-audio-dev mailing list [email protected] http://lists.linuxaudio.org/mailman/listinfo/linux-audio-dev
