On Thu, 3 Dec 2009 23:52:03 +0100
fons@email-addr-hidden 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 Linux-audio-dev@email-addr-hidden http://lists.linuxaudio.org/mailman/listinfo/linux-audio-devReceived on Fri Dec 4 04:15:02 2009
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