BOSS OD-1 DIY Project - page III
Now look at the first opamp section. We have to think
of a way to connect the two diodes in series
( D3, D4 ) without wasting too much PCB space. 1N4148
diodes' lead spacing is approx. the same
as the width of our opamp. So, I decided to simply 'staple'
them. See [here].
Pin 7 of the opamp connects directly to lugs 1&2
of a 1M pot. Pin 6 is also connected to the pot, to
the remaining lug, through R5 ( 33k ). But, pin 6 connects
to Vb, as well, through R6 ( 4.7k ) and C3
( 0.047u ). As we already have Vb somewhere down there,
we just have to find a way to connect
them. Relatively easy done, pull the trace 'behind' Q1.
[here]
Between the traces which connect
to pin 6 and pin 7 I have added pads for a 'mod capacitor'. Just in
case I want to experiment. If you want the original
circuit, you don't need this.
All done? OK. Let's move on to the second opamp stage.
Pin 7 and pin 2 are connected through a
10 K resistor ( R7 ). That fits quite nicely in our design,
helps us to overbridge the V+ trace. Now,
the part which follows is really straight-forward
- a resistor ( R8 ) and a cap ( C4 ) in parallel, between
pin 1 and pin 2. [Easy...] And the pin 3 of the opamp is connected
to Vb through R9 ( 10k ). [here]
As no further components will be added on this
side of the opamp, we can draw a straight line there
and connect it to our V- trace. Cool. It already
looks like a real thing :-) Now check out that C9 (47uF).
You see, it connects between Vb and V- ( GND ).
We'll place it in this empty space which is left between
R7 and R9.
A propos C9: note that it
has the following designation - 47uF / 6.3 V. Hey! 6.3
V !! Since the expected
voltage in this part of the circuit is ~ 4.5V (
1/2 of V+ ), the original designer of this circuit has calculated
correctly, and has choosen a cap which can stand
6.3V. But we are not using the same logic. Why? Because
it happens only too often that people try to get
more 'juice', 'cream', 'power', 'mojo', 'drnde', 'mrnde' from a
circuit by connecting it to e.g. two batteries,
instead of one. Hm... What happens is that 1/2 of 18V tends to
be a bit more then 6.3V and the cap suffers.
6.3V cap is surely physically smaller in size then e.g. 16V or 50V,
but we have enough space on our PCB, anyway...
So go ahead and add two pads for C9. Cap leads
are very close, so have that in mind, if you want the cap
to sit nicely on the PCB.
Now we go towards the output. We have a coupling
capacitor C5 ( 1uF ) and a resistor R10 ( 4.7k ) next
in the line. That R10 is going to be very helpful,
much like R7, to work out a path over the V+ trace. Simple.
Connect C5 pad directly to pin 1 of the opamp.
Then place R10 over the V+ trace. From R10 we place a
trace to the pad which will be used for lug 3 of
the LEVEL pot. [see it]
Next to pad Nr.3 we put two more pads, for lugs
2 & 1. OK? Now note that Nr.1 connects directly to Vb.
We shall have to pull that Vb trace all the way
up here. Lug Nr.2 of the LEVEL pot connects to the base of
Q2 through C7 ( 0.047uF ). Again, check the size
of your 0.047u cap, and place two pads for it. Between
C7 and base of Q2 there is R12 ( 1M ), which connects
to Vb. [see it]
We're almost done. Just Q2 'environment' is left
for us to do. See this R13 ( 10k )? It connects emitter of Q2
and V- ( GND ). Do it. And then R14 which we are
going to use to overbridge the Vb trace. From R14 the
path goes to C8 and from there to the circuit output.
R15 can be placed either on the PCB or on the lugs of
the output jack, between TIP and GND. Done...
Hey, but wait! Remember the voltage divider which
consists of two resistors? We only found place for one of
them till now. The other ( R6 ) connects V+ and
Vb. So connect them... And there is one more thing: C17
(100uF / 16V). It connects V+ and V-. Plenty of
room 'under' the opamp. Measure the size carefully, otherwise
you'll possibly get a very tight fit, and you'll
have to move the components around when the PCB is already
etched, and that's uncool. [PCB here]
D1 - reverse polarity protection diode I didn't
manage to squeeze in this time. But it's makeable. It goes between
V+ and V-, and it's there to protect the circuit
when you accidentaly connect battery + to battery -. Original
circuit had a zener diode, which also levels the
voltage ( from the power supply ). Any 1N400x will work for us.
Left side Vb trace and right side Vb plane I connected
using a [piece of wire].
Here are some [tips on PCB layout]
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