Blues Driver
MODS
C26
220P
R37
330K
R38
100K
C34
0.1
C25
0.047
R32
2.2K
R51
18K
resistors with trim pots, just connect
one hole to pin 1 on the trim pot, and
the remaining resistor hole to pin 2
on the trim pot. Leave the third lug
untouched.
The BD- 2 EQ before clipping looks like
this due to this filter [see Figure 1]:
Notice how there is a ton of bass present? That is before the majority of the
clipping is happening, so it’s no wonder
the pedal sounds fuzzy when the gain is
turned up!
A good mod at this location is to make
R50 a 100 ohm, and change R36 to a
47k. That will give you a much flatter
EQ response.
After that the signal is clipped by
diodes connecting to ground (D7, D8,
D9, D10) with two diodes on each side
and fed into another discrete opamp.
This opamp is nearly identical, except
the frequency response is a little different. There is more gain in the bass
(set by R34 and C24, frequency is about
C1
220p
R2 [Treble]
330k
R1
100k
Out
C2
100n
R50
1M
Zsrc
38k
C3
47n
R3 [Bass]
1M
R4 [Mid]
15k
R5 [load]
1M
72hz) but it works exactly the same.
Notice that since the bass is boosted
yet again here it’s really no surprise
that the BD- 2 would be so fuzzy with
the gain turned up.
C17, R25, and C19 form both a high-pass and low-pass filter, which will get
rid of some high harmonics about 5k
or so, as well as to get rid of some of
the bass content that was created by
boosting the lows so much previously.
From here, the signal goes through
a fairly standard tone control very
similar to that of an old Fender tweed
Princeton. It acts as a high-pass filter
with the tone knob turned up, and
a low-pass filter with the tone knob
turned down. So, you increase the
highs as you turn up the tone, and lose
some bottom end once you are past
about halfway or so. Changing C100
will change what high frequencies
are filtered with the tone control up,
and changing C101 will change what
frequencies are filtered with the tone
control turned down. The volume con-
trol is next before going into the next
stage—the EQ stage.
The next stage is a simulated inductor,
which is boosting the bass content at
about 120hz or so by 6dB. Even though
there are diodes in this stage (D1 and
D3), they aren’t really clipping much
at all like diodes usually do in an overdrive or distortion circuit—it is actually
more to protect this opamp from being
slammed with a loud and hard signal.
It also does a little bit of filtering as
well as help to output a low impedance
signal. Changing these diodes to a different type will give a little bit different
“feel.” However, it becomes a little
less compressed if you use LEDs, and
becomes a bit more compressed and
filters out a bit of the highs if you use
germanium type diodes. This is probably due to varying degrees of harmonics being ever so slightly clipped.
If you want to mess with the EQ, there
are several ways to do it. You can try
changing the cap sizes of C9 and C16,
or changing R21. Changing the
