BOSS OD-1 DIY Project - page II
PCB Design
Turning a schematic into a working circuit, soldered
neatly onto a PCB was
a total enigma for me, for a long time. Somehow it requires
a bit of abstract
thinking, and lots of imagination to make it work. For
me, at least. I am not
EE and have no special electronics background, except
for what I have learned in
the school. All knowledge in this area I collected from
related [websites]
and some [books] I found in the library.
Therefore, my approach had to be pragmatic and systematic.
First, one has to get
a general idea of how the future PCB will fit in the
enclosure. I choose the same
orientation and wiring as used in the original stompbox,
INPUT jack on the right,
OUTPUT on the left side, copper side of the PCB facing
the bottom of the box.
Once we got this right, we start with laying out the
circuit. For this you can use
one of many [free PCB] layout programs, or get [this] handy helper thinggy.
[Start with the opamp and
the two transistors]. I assume that you have checked the pinout
for the transistors already. If not, do it now. 2SC2362
& 2SC1571 have the following
[pinout]. Opamp has a 'standard' [dual DIP8 pinout].
Looking at the schematic, what do we see? The [collector]
pin of the 1st transistor
connects directly to +9V, same as pin 8 of the opamp
and the collector of the 2nd
transistor. You should be able to connect these 3 with
a thick trace (+V and -V traces
should theoretically be a bit thicker, as they carry
voltage, and not signals). Done?
OK. And how about the pin 4 of the opamp? It connects
directly to GND or in this
case V- (these designations vary from schematic to schematic).
Just pull a trace from
the pin 4 downwards. That was the main 'power' part of
the layout. Now let's check out
the signal path.
Note: I made my OD-1 with true
bypass, thus discarding all the FET's used for switching
from the original schematic.
Take a look at the 1st transistor. It is connected to
pin 5 of the opamp through a cap.
The cap is 0.0047uF. Taking care of the cap lead spacing
and overall size of the cap, pull
a trace from the emitter pin of the 1st transistor (let's
call it Q1) to the cap, and then
further from the cap to pin 5. Easy, right? Now we notice
the emitter resistor. It connects
to V- (same as pin 4), so connect one side of the resistor
to the emitter pin, and the other
to the same trace which goes to pin 4. Cool. Now you
see that 100k resistor after the 0.0047u
cap? That one connects to Vb (bias voltage). You get
Vb (usually 1/2 V+) by making a so called
'voltage divider' from two resistors of the same value.
One of them connects to V+, the other
connects to V-. The point where the two are joined is
Vb. For the moment, we will not make
any voltage divider, just imagine that you have it already,
and connect a 100k resistor so that
one side goes between the 0.0047 cap and pin 5 of the
opamp, and the other connects to Vb.
R2 (470k) also connects to Vb, and goes right from the
[base] pin of Q1 to Vb. Connect it.
Look! R7 also connects to Vb. that's one of the voltage
divider resistors, One side goes to Vb
and the other side goes to V-. Go connect. Now decide
where the input pad of the PCB will
be (and we agreed on the lower right corner). Input pad
and the base of the Q1 are connected
through a resistor R1 and a cap C1. Once you have finished
that, you've got the whole input
section ready.
Right... Now you should have something [like this] or similar.
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