Epic Polar Boost v1.0
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10
When you switch the bright switch on (
SW1B
) the resistance is cut to 1k since the switch bypasses
R6
. So the cutoff frequency changes to:
𝑓
𝑐
=
1
2𝜋𝑅𝐶
=
1
2𝜋 ∙ 1000 ∙ 3.3 𝑒
−9
≈ 48𝑘𝐻𝑧
So now the signal cuts off in the
very high treble
section which is almost not audible, but something
else also changed.
By lowering the resistance the gain of Q2 also changed! The effect is now brighter and also a bit
louder (approx. 3db).
By adding a tone control to the effect you can change the bandwidth of the cut off frequency.
Mark Hammer introduced the “
Stupidly Wonderful Tone
Control
” (
) in 2005 which combines a
simple adjustable RC filter with the ability to keep the gain constant. This is done in the optimized
version of the boost by introducing
P2
(B50k). If you want to remain close to the original boost but
still want to be able to change the cut off frequency, I changed
R6
to 22k.
This way the 50 pot and 22k resistor in parallel give
𝑅
𝑡
=
1
1
50000
+
1
22000
≈ 15𝑘
which is about the same
as
R6
in the original. Now the tone control gives you a sweep between 3kHz and 48kHz while not
changing the level of boost while turning the knob! The bright switch (SW1B) will act a bit different
than the original as is will take out R6,P2 and C5 all at once, just leaving
R5
in the circuit. This way
there is no RC filter left, just a small extra boost. Changes are that high frequency interference is
more audible when you switch
SW1B
on.
If you use a jumper in
J1
the filter will act more like the original and will cut some volume when you
turn it up (=cut highs). However the bright switch will function exactly as in the original. To end this
lecture, some suggestions to make the tone control even more usable:
Author
R5
R6
P2
C5
fc
min
fc
max
LG
1k
22k
B50k
3n3
2,9kHz
48kHz
LG
1k
NC
B25k
3n3
1,85kHz 48kHz
LG
1k
NC
B50k
3n3
945 Hz
48 kHz
LG
1k
NC
B100k
3n3
477 Hz
48kHz
F Briggs 470R
NC
B10k
10n
1,5kHz
33,8kHz
NC= not connected
NB. When using J1 “jumped”, the higher P2 the bigger the
volume drop when turned down.
Transistors
The original uses a
2N5457
as
Q1
and
2SC1815
as
Q2
. A lot of people prefer a
J201
as
Q1
since it has
a bit more gain and a
2N5088
as
Q2
. Check the pinout of your transistors before you solder them!
The correct pinout is printed on the PCB and insert the transistors accordingly. You can always
experiment with other types of transistors and if you find better combinations, please let me know!
