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22nd March 2004, 03:19 AM
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#1 |
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diyAudio Member
Join Date: Apr 2002
Location: Singapore
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New JFET switched volume control
Hi all,
what do you think of this? I haven't seen this topology anywhere. Am I missing something or is this a great way to overcome limitations of JFET switches: - no current actually passes the JFET. It is just used as a switch passing voltage to the inverting op amp input - the inv input being essentially a voltage sensor anyway and not a load - R(DS) should therefore remain constant - any minor variations in R(DS) will be insubstantial compared to the input impedance of the op amp. Potential for problems would be - nonlinearity for bipolar voltages - however at virtual ground there won't even ever be any V(DS) *or* I(DS) - leakage of control voltage of the JFET - however, again, at virtual ground on one end and "infinite" input impedance on the other that shouldn't be a problem. Have a found the elusive non-distorting JFET switch application? MBK |
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30th March 2004, 04:04 PM
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#2 |
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diyAudio Member
Join Date: Sep 2003
Location: dongguan
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up
up
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MC_Wilson |
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31st March 2004, 01:48 PM
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#4 |
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diyAudio Member
Join Date: Jun 2003
Location: Connecticut
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Nonlinearity would be my concern too. Isn't that the basic problem with commercial analog switches?
Also be aware that the drain-source capacitance of FET's goes way up when there's no DC voltage on them. Try it and report back.
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dave |
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31st March 2004, 04:21 PM
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#5 |
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diyAudio Member
Join Date: Apr 2002
Location: Singapore
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actually I've tried it
... with 2 stages (not the full R ladder), one JFET "on", the other "off", and a TL072. Didn't listen, but did distortion tests. My soundcard is not the greatest I'm afraid, but at least I see no difference between just resistors, and resistors plus the switch. Photos of distortion spectra below. The source was test CD's with either sine waves (250 and 1000 Hz) or a 200 Hz sine wave modulated tone burst (Linkwitz test CD). The sine measurements show distortion which is equal to the passive R setup and the JFET setup; this may have come from the source recording or another limiting factor. The burst spectra are cleaner. The noise floor especially below 200 Hz mocks things up a little. Regardless - I see no difference at this stage and with this equipment, between passive resistor divider on the inv input, or a JFET switch.
MBK |
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