AUDIO TONE CONTROL CALCULATOR BY ROD ORMSTON ,4/91.
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TONECTRL: 
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This  program  quickly works out the component values  for  the  standard 
Baxandall  tone control circuit.   By eliminating a lot of  very  tedious 
calculator  work,  it  is  possible to  experiment  with  many  different 
combinations  and  examine the effect that changes in the  value  of  one 
component has on others, etc.  Component values are selected from the E24 
series for resistors (10,11,12,13,15,16, etc) and from the E12 series for 
capacitors  (10,12,15,18,22,27,...).   A print option is included  and  a 
circuit diagram can be displayed on screen.  Program does not work in low 
resolution.


GENERAL INFO.: 
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The  Baxandall  configuration dates back to the  1950's,  and  is  almost 
universally used in tone control circuits in one form or another.

The  program  requires  values for the following:

     *  Resistance values for RV1 (Bass pot.) and R3 (see later).
        Both are entered in kilohms.

     *  The frequencies at which the Bass and  Treble  pots.  have 
        greatest effect (boost/cut) - in Hz for Bass and kHz for Treble.

     *  The gain (i.e.,  maximum boost or cut).   This may also be 
        specified in  decibels (dB) if 'dB' is typed in (not  case 
        sensitive) following the figure.         

The  first figure to be entered is for RV1.   If instead,  'q' (QUIT)  is 
entered,  the program terminates.   If 'd' (DIAGRAM) is entered, the tone 
control circuit is shown on screen (Use ALT-HELP to print it) - press any 
other key to continue.

Each  set of results will be printed if a positive response is  given  to 
the prompt at the beginning of the program.  Note that condensed print is 
selected, and horizontal tab stops are set at every ninth column.


IN  PRACTICE... 
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A  typical starting value for RV1 (Bass) is 100K,  and R3 could  be  upto 
12K.   Note carefully that the program calculates a minimum value for the 
treble pot.,  RV2.   The minimum is directly affected by the value of  R3 
and  to  some  extent by RV1.  Try varying the value of R3  so  that  the 
minimum value specified for RV2 is CLOSE to a standard value,  bearing in 
mind that most pots. have +/- 20% tolerance.  So if your pot. is going to 
be  220K,  you could adjust R3 so that the minimum specified for  RV2  is 
about 175K.  

It is suggested that 220K is probably the MAXIMUM value you would want to 
employ for RV2 as 470K pots.  may get a bit 'scratchy' after a few  years 
use.  Note that if the pot. value used is way above the minimum specified 
by  the program,  there will be greatest effect (boost/cut) near  to  the 
ends of the pot's. rotation rather than an even spread.


FREQUENCY RESPONSE:
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The frequency values entered are the points at which the response of  the 
controls  peak (in either boost or cut).   The actual effect  extends  to 
the 'corner' or 3dB points where the graph of the gain response has  just 
broken away from the flat,  rising to +3db in boost or falling to -3dB in 
cut.   The 3dB points are at frequencies about a factor of ten above  and 
below the peak point.


BUILDING THE CIRCUIT:
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The  circuit drawn by the program includes an Op-Amp.-buffered  input  to 
provide a low driver impedance for the tone controls - a driver with high 
output impedance would alter the tone control characteristics.

Use a low noise,  low distortion op-amp.  like TL071/72.   Alternatively, 
LF351/53.  DON'T use a 741 or 1458, etc!!!  

It is advisable, particularly if a stereo version is being built, to make 
a PCB and use PCB-mounting pots rather than stripboard and separate pots.
as  the  latter  arrangement will be very susceptible to  noise  and  hum 
pickup.  

Written with GFA Basic V2.

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