Bruce wrote: A more general question would be: Is it possible to design a chopper stabilised null detector using modern components with a performance equivalent to the 845A? Whilst the noise of most chopper stabilised opamps is much larger than that of the 845A, perhaps something like the approach used in: http://cds.linear.com/docs/LT%20Magazine/LTMag-V16N01-11-FETAmp-Williams.pdf could be used. Since the low noise preamp is AC coupled a lower noise input using a single ended JFET could be substituted for the one using the LSK389. --------------clip----------------- Good question/suggestion Bruce. Maybe that approach could be used to re-design the first "half" of a HP 419A, building a modern circuit into an existing HP 419A to save time/money and to come up with a nice little box that works well. (The HP 419A *was* a small form-factor, cute, attractive instrument, with built-in supply & batteries, so it might yet still have a use?) I think the 419A can be purchased for practically nothing, if it can still be found. ...again, I'm afraid many of them have already gone to graveyards for scrap metal recovery. Greg

Does anyone know where and how to obtain the LK389 mentioned below.  Linear Systems has a high minimum order but i would like to pursue rebuilding my HP-419 since the chopper doesn't work (as usual). Randy ----- Original Message ---- From: Bruce Griffiths <bruce.griffiths@xtra.co.nz> To: Discussion of precise voltage measurement <volt-nuts@febo.com> Sent: Sun, September 19, 2010 2:17:51 PM Subject: Re: [volt-nuts] Fluke 720A Kelvin Varley Divider Questions Charles P. Steinmetz wrote: > Bill wrote: > >> You're kidding, right ? > > I've never used one, and I'd be interested to know what you (and others) think >its failings are. > > Best regards, > > Charles > A more general question would be: Is it possible to design a chopper stabilised null detector using modern components with a performance equivalent to the 845A? Whilst the noise of most chopper stabilised opamps is much larger than that of the 845A, perhaps something like the approach used in: http://cds.linear.com/docs/LT%20Magazine/LTMag-V16N01-11-FETAmp-Williams.pdf could be used. Since the low noise preamp is AC coupled a lower noise input using a single ended JFET could be substituted for the one using the LSK389. Bruce _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.

Since the dc gain of the amplifier using the LSK389 is unity a precise match of the JFET input offset isnt critical so one can use a selected pair of 2SK369BL. These are readily obtainable on eBay. I purchased about 20 and easily found several pairs with a Vgs match to within a few mV. The amplifier that uses the low noise JFET's as its input stage is ac coupled with a dc gain of unity so that it could easily be redesigned to use a single FET input device with current feedback to its source. Bruce Randy Evans wrote: > Does anyone know where and how to obtain the LK389 mentioned below. Linear > Systems has a high minimum order but i would like to pursue rebuilding my HP-419 > since the chopper doesn't work (as usual). > > Randy > > > > ----- Original Message ---- > From: Bruce Griffiths<bruce.griffiths@xtra.co.nz> > To: Discussion of precise voltage measurement<volt-nuts@febo.com> > Sent: Sun, September 19, 2010 2:17:51 PM > Subject: Re: [volt-nuts] Fluke 720A Kelvin Varley Divider Questions > > Charles P. Steinmetz wrote: > >> Bill wrote: >> >> >>> You're kidding, right ? >>> >> I've never used one, and I'd be interested to know what you (and others) think >> its failings are. >> >> Best regards, >> >> Charles >> >> > A more general question would be: > > Is it possible to design a chopper stabilised null detector using modern > components with a performance equivalent to the 845A? > > Whilst the noise of most chopper stabilised opamps is much larger than that of > the 845A, perhaps something like the approach used in: > http://cds.linear.com/docs/LT%20Magazine/LTMag-V16N01-11-FETAmp-Williams.pdf > could be used. > > Since the low noise preamp is AC coupled a lower noise input using a single > ended JFET could be substituted for the one using the LSK389. > > Bruce > > > _______________________________________________ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. > > > > > > _______________________________________________ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. > >