On Fri, 8 May 1998 08:13:06 +1000 Dennis Plunkett writes: >At 11:22 AM 7/05/98 +0100, you wrote: >>Hello all >> I'm thinking about making an attenuator ('volume control') >for a >>hifi preamp. I don't want to use semiconductor devices for this [...] You could use the standard "Pi" or "Tee" attenuator networks used in RF work. A relay would either switch each attenuator in or bypass it. With attenuators for 1, 2, 4, 8, etc. dB then the overall attenuation could vary from 0 to 2^(n-1) dB. For each stage you need 3 resistors and a DPDT relay. You can also buy multi-stage attenuators already built, but they have a 50 ohm impedance and are expensive. One nice feature of this sort of setup is the input and output impedances are constant regardless of the attenuation setting. This is essential for most RF circuits and unnecessary for most audio circuits. But it may help sell it to the "hi-fi types." Another configuration with resistors to ground could save on resistors and require only single-pole relays instead of double. But you'll still need at least N relays for 2^N attenuation steps. Because the impedance of the common node, and thus the effect of each resistor, keeps changing, such a network is more complicated to analyze. If you play with it for a while, soem sort of general rule should emerge. It may not be possible to have all the attenuation steps evenly spaced. This will not be noticeable to the user as long as the attenuation increases by about the same amount for consecutive steps. > >Use a Dallas serial LOG pot! Exactly the answer the original poster has already removed from consideration, due to the peculiar demands of the "hi-fi" set. Using a semiconductor switch in the signal path can and does produce distortion, but with some of the newer parts such a small degree that no one would notice it. There are many people who think they can notice it though. Actually, relays can cause distortion (and lots of it) too, if the contacts become oxidixed. It may be good to provide a way to switch a small DC current through the relays once in a while to burn off any tarnishing of them. Reed relays should be better for this sort of thing. Keep the impedance in the attenuator network fairly low so the capacitance of the open relays doesn't affect it. _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866]