Practical_Electronics-May_2019

Fig.20. The board can be mounted with right-angle pots.

be powered off another supply, such

as from a power amplifi er. R9 would of

course have to have the correct value

for your choice of Zener diode voltage.

Construction

The PCB follows the increasingly pop-

ular method of home construction,

whereby a CAD-generated file (eg,

from Autodesk Eagle) is sent to a Chi-

nese fabrication house, in this case

http://www.pcbway.com (see Fig.18). On

these boards, plated-through holes,

component ledgending and solder re-

sist are standard. The top and bottom

track layouts and overlay are shown in

Fig.19. These features are diffi cult to

achieve with a typical home workshop

using a UV light and ferric chloride

etching set-up. However, for Chinese

PCBs there is the problem of a turna-

round of a few weeks. I must confess

that despite the mess, I still like the

immediacy of home etching for R&D

and prototyping. Checking and mod-

ifi cations can be done in an hour or

so in a serendipitous uninterrupted

cognitive loop. Normally, when one

commits one’s design to a fab house it

has to be right fi rst time. Both routes

have pros and cons. For this project,

the relevant fi les are available on the

Practical Electronics website.

Components

The board was originally designed

for right-angle PCB-mount pots, as

shown in Fig.20, which are very pop-

ular among the guitar pedal building

community. With plated-through holes

they are strong enough to support small

PCBs directly without the need for

R 8 R 711 R R 2 R 1

10 RR 4

R9

R6

R 5 C 3 R 3

CW VR2 C5 CW VR1

C4

C7

C 2

C 1

C9+ C8+

C6

D 1

IC1

In Gnd 9V Out

Treble Bass

Fig.18. PCBs fresh from PCBWay.com

Fig.19. Component overlay for the PCB

designed by Mike Grindle. Only the top

layer of track is shown. The design is

avilable for download from the EPE /

Practical Electronics website.

There is also a little bit of power sup-
ply housekeeping, such as decoupling
capacitor C8 and current-limiting resis-
tor R9 in Fig.8 in Part 1 of this article.
Power supply reverse-protection diode
D1 is also fi tted because guitarists are
notorious for subjecting equipment to
random power supplies. This could be
replaced with a Zener diode if you want
to stabilise the supply at a particular
voltage; for example, if the unit is to

Fig.17. Frequency response curves for the Mike Grindle Baxandall tone control circuit in
Fig.8 and Fig.15: (top) 800Hz version; (middle) the pots being incremented, illustrating
a small amount of interaction (gain and frequency shifts) between the pots – you can
clearly see the movement of the pivot point in the bass; (bottom) the 1kHz version.