Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!brutus.cs.uiuc.edu!apple!mips!mark From: mark@mips.COM (Mark G. Johnson) Newsgroups: sci.electronics Subject: Re: Tone control transfer function Message-ID: <28476@obiwan.mips.COM> Date: 29 Sep 89 16:17:35 GMT References: <9723@thorin.cs.unc.edu> Reply-To: mark@mips.COM (Mark G. Johnson) Organization: MIPS Computer Systems, Sunnyvale, CA Lines: 49 In article <9723@thorin.cs.unc.edu> rohlf@pooh.cs.unc.edu (John Rohlf) writes: > > I need the transfer function for the basic 3-band tone controls >diagrammed below: Here's an input "deck" for a circuit simulator (such as SPICE) that'll give you the transfer curve from 1Hz to 1MHz. john rolf's tone control, case2 (input node 10, output node 20) * for case 1 just set rout to a small value like 1 ohm * * .ac dec 20 1.0 1e6 * vin 10 0 ac 1.0 * * rout emulates the output impedance of the preceding tube stage rout 10 1 (rout value) * c1 1 2 (c1 value) r1 1 3 (r1 value) c2 3 4 (c2 value) c3 3 6 (c3 value) * treble control Pt. Be sure (r2+r3)==Pt r2 2 20 (r2 value) r3 20 4 (r3 value) * bass control Pb. Notice that (0 < r4 < Pb) r4 4 6 (r4 value) * midrange control Pm. Notice that (0 < r5 < Pm) r5 6 0 (r5 value) * .print ac vdb(20) vp(20) .plot ac vdb(20) (-37.0, 3.0) * .end If you want to do it analytically, just draw the ckt again but instead of capacitors temporarily put dummy resistors RD1, RD2, RD3, and for the potentiometers put k*Pt and (1-k)Pt, xPb, and yPm (where k, x, y are between 0 and 1; they represent the % rotation of the pot shafts). Now, using KVL and KCL, write the expression for Vout/Vin. When you have that, replace RD1 by (C1/s), replace RD2 by (C2/s), and replace RD3 by (C3/s). Voila, you've got the transfer function. -- -- Mark Johnson MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086 (408) 991-0208 mark@mips.com {or ...!decwrl!mips!mark}