hack@umbc5.umbc.edu (03/06/91)
I am tring to build a audio amplifier and I wish to have
it electronically switched (as few mechanical switches as
possible). What I was concidering is using CMOS bilateral
switches to handle this. What I am concerned is they might
produce a great deal of distortion to the signals. Now I know
that audio signals are typically 20-20kHz, still I don't know
if that would be a problem.
I would like to know if there is a better way, or if
the CMOS chips are right for the jobs which ones are better
for this type of application.
Thank you in advance,
J. Hackeacj@theory.tn.cornell.edu (Julian Vrieslander) (03/06/91)
In article <5292@umbc3.UMBC.EDU> hack@umbc5.umbc.edu () writes: > > I am tring to build a audio amplifier and I wish to have >it electronically switched (as few mechanical switches as >possible). What I was concidering is using CMOS bilateral >switches to handle this. What I am concerned is they might >... > I would like to know if there is a better way, or if >the CMOS chips are right for the jobs which ones are better >for this type of application. Precision Monolithics, Inc. (PMI) makes some CMOS analog switches that are specifically designed for audio applications. They feature low distortion and are noiseless when switched. Part number is SSM-something. Siliconix also makes a large selection of analog switches, many of which are useful for audio, but you will have to a bit more work in choosing and designing them in. -- Julian Vrieslander Neurobiology & Behavior, W250 Mudd Hall, Cornell University, Ithaca NY 14853 INTERNET: eacj@theory.tn.cornell.edu BITNET: eacj@crnlthry UUCP: ..cornell!batcomputer!eacj
spp@zabriskie.berkeley.edu (Steve Pope) (03/07/91)
In article <5292@umbc3.UMBC.EDU> hack@umbc5.umbc.edu () writes: > > I am tring to build a audio amplifier and I wish to have >it electronically switched (as few mechanical switches as >possible). What I was concidering is using CMOS bilateral >switches to handle this. What I am concerned is they might >produce a great deal of distortion to the signals. Now I know >that audio signals are typically 20-20kHz, still I don't know >if that would be a problem. > > I would like to know if there is a better way, or if >the CMOS chips are right for the jobs which ones are better >for this type of application. > >Thank you in advance, >J. Hack Don't use CMOS switches, use a single depletion-mode discrete JFET and observe the following: (1) place the switched FET(s) in series with the signal(s) at the summing node (- input) of an op amp. (+ input) of the op amp should be at ground. The input signal is fed through a resistor (say 10 Kohm), through the FET, into the summing node. (2) Use 0 V on the gate to switch ON. (3) Use a large enough voltage (negative for an N-channel FET) to turn the FET off so that any signal swing does not start to turn it back on. For eample, with a -2V pinch-off voltage, a signal that swings down to -12V, and a -15V voltage on the gate, you have 1V of margin which should be adequate to keep the FET off. (4) (this is important) couple the signal controlling the FET to the FET via an RC filter with 100 msec or so time constant. This will keep the circuit from clicking when you turn it on or off. (This is why you don't want to use a CMOS switch such as a 4016, they click). You should measure the distortion of each circuit built -- it should be well under .01% (in fact about .001%), if it is too high replace the FET. The trick is that by putting the FET at the summing node, with its on resistance being very small compared to the resistance in series with it, there is never any appreciable voltage across it, therefore the nonlinear I/V characteristic of the channel does not result in appreciable distortion. steve
dupuie@ee.eng.ohio-state.edu (Scott T. Dupuie) (03/07/91)
In article <5292@umbc3.UMBC.EDU> hack@umbc5.umbc.edu () writes: > > I am tring to build a audio amplifier and I wish to have >it electronically switched (as few mechanical switches as >possible). What I was concidering is using CMOS bilateral >switches to handle this. What I am concerned is they might >produce a great deal of distortion to the signals. Now I know >that audio signals are typically 20-20kHz, still I don't know >if that would be a problem. > > I would like to know if there is a better way, or if >the CMOS chips are right for the jobs which ones are better >for this type of application. > >Thank you in advance, >J. Hack Several companies make special low distortion analog switches for high fidelity audio applications. In particular, I know that Analog Devices, Inc. and PMI (subsidiary of Analog Devices) both make these kind of switches. Don't recall the part numbers off the top of my head. Try contacting your local sales rep. or distributer for more info. Scott T. Dupuie Dept. of Electrical Engineering The Ohio State University dupuie@ee.eng.ohio-state.edu
michael@otago.ac.nz (03/08/91)
In article <5292@umbc3.UMBC.EDU>, hack@umbc5.umbc.edu writes: > > I am tring to build a audio amplifier and I wish to have > it electronically switched (as few mechanical switches as > possible). What I was concidering is using CMOS bilateral > switches to handle this. What I am concerned is they might > produce a great deal of distortion to the signals. Now I know > that audio signals are typically 20-20kHz, still I don't know > if that would be a problem. > Electronics Australia had a 100W amp design that did this in I think 1988 called the Studio 200. It used 4066's coupled with BiFET op-amps to keep the current through the CMOS switch down; apparently distortion arises only when you try to draw current through the things. It worked, or at least they claimed some fairly impressive figures for it... Michael(tm) Hamel, Computing Services Centre, University of Otago, New Zealand LITTLE URSWICK (n.) The member of any class who most inclines a teacher towards the view that capital punishment should be introduced in schools.
grayt@Software.Mitel.COM (Tom Gray) (03/14/91)
In article <5292@umbc3.UMBC.EDU> hack@umbc5.umbc.edu () writes: > > I am tring to build a audio amplifier and I wish to have >it electronically switched (as few mechanical switches as >possible). What I was concidering is using CMOS bilateral >switches to handle this. What I am concerned is they might >produce a great deal of distortion to the signals. Now I know >that audio signals are typically 20-20kHz, still I don't know >if that would be a problem. > > I would like to know if there is a better way, or if >the CMOS chips are right for the jobs which ones are better >for this type of application. > The problem you will find with these divices is not the variation in impedence with frequency but the variation in impedence with amplitude. The on resistance of these devices can very from 3 ohms for large signals to 300 ohms for small signals. You will have to compensate for this variation in resistance or there will be significant intermodulation distortion in your amplfier. Some sampling gate designs, I have seen close a feedback path around the switch to null out any variations in the on resistance. . . . . . . . . .
panek@hp-and.HP.COM (Jon Panek) (03/22/91)
Julian Vrieslander responds: | Precision Monolithics, Inc. (PMI) makes some CMOS analog switches that are | specifically designed for audio applications. They feature low distortion | and are noiseless when switched. Part number is SSM-something. My PMI catalog here describes the SSM-2402/2412 Dual Audio Analog Switch for "clickless" audio signal switching. I quote from the list of features at the top of the data sheet: FEATURES * "Clickless" bilateral audio switching * Guaranteed "Break-Before-Make" switching * Low Distortion 0.003% Typ * Low Noise 1 nV/SQRT(Hz) * Superb OFF-Isolation 120dB Typ * Low ON-Resistance 60-ohm Typ * Wide Signal Range +/- 18V Looks pretty nice to me, and it should meet your requirements. As another poster suggested, the internal topology looks like pass FETs with op-amps providing feedback to compensate for variations in impedance. The catalog lists Authorized Distributors in Maryland (none in Baltimore) Newark Electronics, Beltsville (301) 206-3300 Newark Electronics, College Park 982-9711 Allied Electronics, Columbia 381-1560 Hall-Mark Electronics, Columbia 988-9800 ...and others Good luck! Jonathan Panek Hewlett-Packard, Andover Division panek@hp-and.an.HP.COM (508) 687-1501 X-2866