Fig.21. If you use standard vertical-mount pots then some
capacitors may need to be bent over a bit. (Note the PCB files
have now been stretched a bit to avoid this.)
Fig.22. Dual-gang pots are needed for stereo and two boards
are used with the pots only mounted on one. The other board
can be linked with stiff wire.
pillars. Note they are mounted on the
underside of the board if they are to
turn the right way. These are available
from online supplier Tayda, which has
become one of the main suppliers for
electronic music constructors. If you
have to use the standard vertical mount
pots, capacitors C2, C3, C9 and C4 will
have to be bent over a bit as shown in
Fig.21. Later boards and the PE files for
download will have the pot positions
moved forwards to avoid this.
Modifications
For stereo, two PCBs will be needed
with the rear gang of a pair of dual-gang
pots fitted into one PCB, as shown in
Fig.22. The other PCB will have to be
connected with wire links. For lower
noise, use the 5532 and increase all
capacitors by a factor of five and re-
duce all resistors by a factor of five, as
shown in Fig.16. This impedance re-
duction technique is a standard audio
trick popularised by Baxandall. He ad-
vised speaker manufacturer KEF when
their active speakers were hissing, ‘to
up the level, drop the impedance’.
Component List
Resistors (All 5% 0.25W carbon film)
R1, R2 1MΩ
R3, R4 4.7kΩ
R5 33kΩ
Bass Treble18kΩ2.7kΩ68kΩ470μF
6.3V100kΩ33kΩ1kΩOutputNFBcw cw100kΩ
100kΩ A-log
A-log220nF27nF2.2nF22nF22pF0V3.3kΩ8.2kΩ39kΩ 18kΩ1.2kΩ33kΩ+15V
32746100kΩ –15V47μF
Bipolar+Input1μF
+ 10μF+IC1
5534Fig.23. The ‘Japanese’ Baxandall tone control that is non-
inverting, has high input impedance and thus needs no buffer.
Only one op amp is needed, so good news if one wants to use
a discrete amplifier circuit. Also, it has nice single pivot curves.
On the down side, unfortunately, it uses unusual, (very) hard-
to-find anti-log pots.R6 10kΩ
R7 100kΩ
R8, R9 100Ω
R10, 100kΩ (or reduced to 82kΩ for
TL072 or 91kΩ for TL062)
R11 100kΩPotentiometers
VR1, VR2 50kΩ or 47kΩ linear 16mm.
Vertical, Alpha RV16AF Rapid 65-
0725 or Tayda right-angle mount PCB
type A-2393.Capacitors (All 5mm polyester)
C1 100nF
C2, C3 33nF (47nF for 1kHz)
C4, C5 4.7nF (2.7nF for 1kHz)
C6 2.2nF (reduce to 68pF for Hi-Fi)
C7 470nF (increase to 10μF for Hi-Fi)
C8 100μF C9 4.7μF 2.5mm 10V or
greater radial electrolyticSemiconductors
TL072 or TL062 dual Bi-Fet op amp
D1 1N4001
*D1 can be Zener to suit power supplyJapanese ‘Baxandall’
By putting the passive tone control
circuit in the negative feedback loop
of an op amp. It is possible to obtain
the sonically superior passive curve
shape with the desirable active gain
characteristic of the normal Baxan-
dall. It also avoids phase inversionand low input impedance. There is
one major problem, anti-log pots are
needed because the boost/cut direc-
tion is reversed. Getting dual-gang
anti-log centre-detent pots can be
difficult. A typical circuit is shown
in Fig.23, similar to that used in the
Sansui A80 and NAD 3020 amplifiers.End note
Peter Baxandall was the consummate
analogue audio engineer whose influ-
ence has not yet been fully appreciated.
It’s time his biography and technical
legacy were published, although this
would be a research challenge because
his consultancy reports and letters are
scattered among numerous engineers
and companies. Much of this material
resides with those who’ve worked at
KEF, QUAD and Calrec, to name a few.
Douglas Self has published some of
the correspondence he had in a Line-
ar Audio publication, Baxandall and
Self on Audio Power, and that’s it for
the moment. Luckily, all of Baxan-
dall’s Wireless World articles can be
viewed at: americanradiohistory.com
There are also articles he wrote for
the Journal of the Audio Engineering
Society, but these can only be accessed
via the AES Electronic Library, for
which you’d need to pay. Alterna-
tively, you could join the AES – Peter
Baxandall was a lifelong member.