[LAD] Milivoltmeter - Was FOSS Ethernet Soundcard

[Folderol]

On Thu, 3 Dec 2009 23:52:03 +0100
fons at kokkinizita.net 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.