Here are the details of my home-made loudness control as requested by pboser.
The article in Audio Engineering, August 1951, page 15 requires the use of a tapped potentiometer, which nowadays are almost impossible to get. I can't reproduce the article here as I do not have permission of the copyright holders but give the values I calculated for a working loudness control. Note, these are from the 1950's Fletcher-Munson curves.
To create an equivalent potentiometer you can get a superior quality component by using a multi-way rotary switch and good quality resistors, tapping at any point along the chain. The switch I used is an Electroswitch 4-pole 23-way rotary switch from the C7 series. I found the method for calculating the volume control values on page 794 of 'The Radio Designer's Handbook' by F. Langford-Smith, 4th edition, 4th impression 1957.
The picture "Attenuator" shows the attenuator network. The values given are for a total resistance of 100K ohms with 2dB difference between switch positions. The picture "Circuit" shows the resistor and capacitor networks that give the bass and treble boost. The small monochrome picture is copied from the original article and the values given are for a 1 Megohm control. Tapping points are at one sixth total resistance (measured from the ground rail) and one third. The values calculated (in ohms) are as follows for a 100 Ohm attenuator.
Switch position
23 R0 = 20567 no attenuation
22 R1 = 16337
21 R2 = 12977
20 R3 = 10308
19 R4 = 8187
18 R5 = 6503 - One third tapping point here
17 R6 = 5166
16 R7 = 4103 -| One sixth tapping between these points
15 R8 = 3259 _|
14 R9 = 2589
13 R10 = 2056
12 R11 = 1633
11 R12 = 1297
10 R13 = 1030
9 R14 = 818
8 R15 = 650
7 R16 = 516
6 R17 = 410
5 R18 = 325
4 R19 = 258
3 R20 = 205
2 Rf = 794
1 0 (tied to ground rail) infinite attenuation
C1 & C3=33nF
C2=679pF
C4=1953pF
R4 & R5 = 12K ohms
The one sixth tapping point is between switch positions 15 & 16 (counting clockwise). With a 100Kohm control one sixth of the resistance is 16666 ohms. At step 15 the total measured resistance of the chain is around 15770 ohms (using 1% metal film nearest value resistors) so to get the correct tapping point I connected an 820 ohms resistor at step 15 in series with a 3300 ohm one connected to position 16. The tap is taken from the junction of the series connected resistors. The one third tapping point is 33333 ohms which is at position 18. The chain resistance here was around 31400 ohms and in this instance I decided not to add any more resistors.
The bass boost is given by C1, R4 and C3, R5. These remain fixed and do not need altering for different potentiometer resistances. The frequency around which C1 and C3 work is 400Hz (the turnover frequency). When the reactance (resistance to a.c current) of C1 equals the resistance of R4 the turnover frequency is 400Hz. Above 400Hz the reactance decreases thus pulling down the voltage at the junction of R4, C2 and C4 and attenuating the treble. Below 400Hz the reactance of C1 increases effectively boosting the bass. R4 helps to limit the effects of C1 which on its own would have too big an effect on treble frequencies. C3 and R5 perform the same bass boost function on the lower tap of the control. The turnover frequencies for treble compensation are 3500Hz for C2 and 5000Hz for C4.
The picture "Finished Control" shows the loudness control in situ in my line level preamp using 76 triodes. The control's effect is pleasant and subtle with music retaining "body" at low volume levels.
