First an apology. There is an error in the drawing (I knew I should have waited a couple of days before posting it).
The bias for the first OpAmp (pin 8) is shown going to +V. It should go to Gnd. As shown it is in the low current - low bandwidtch mode, which is fine for the second one of course. On Fri, 4 Dec 2009 00:51:12 +0100 [email protected] wrote: > On Thu, Dec 03, 2009 at 11:13:05PM +0000, Folderol wrote: > > > 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. > > They are not so 'standard' today, and probably difficult > to find. http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0352250 Cheap modern equivalent, although a bit smaller. http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0327585 Very expensive 'full size' switch. Unfortunately only available as BBM - I prefer MBB for sensitive input stuff. > Bandwidth will not be the problem. What you get is a filter > that will boost HF. 1 pf = +3dB at 17.7 Khz on all but the > 20 mV range. 10 pf means +3dB at 1.77 kHz, and rising. I decided to do some more detailed investigations, because while I totally accept your statements, the actual measured results were quite different. I was getting an essentially flat response right the way up to 99.9kHz, a little wobble between ranges but no more that +2dB, and about +1dB at 40kHz. Taking the cover off I noticed and immediate jump to about +3dB at 99(etc)kHz and taking the unit out of the box completely while being careful to avoid moving anything resulted in a further increase to +4 to 5dB, also becoming quite variable. Clearly stray capacitance to the box shielding was affecting the response. Because I was unable to get 9M 0.1% resistors I used three lower values in series shaped as a 'C' across the tags, so I wondered if the 'spread' of effective resistor body was having an effect too. As a test I temporarily replaced this with a standard 10M resistor. Response was now +2 to 3dB at 40kHz and about +8dB at the top end. While I don't know the whole story of what capacitances there are to where, even the worst of these variants still gave a just about usable performance, so I simply put everything back as it was :) Incidentally total current consumption is a sniff over 1.3mA. -- 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
