DIY Guitar Pedals Oil Tanker Fuzz, Инструкция по эксплуатации, Страница: 14 / 29

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As a rule of thumb, Zin should be at least 1 MΩ. In other pedals with similar input stages like the one in the Big Muff Pi a
series resistor is placed at the input in order to higher the impedance (at the cost of creating a voltage divider that
reduces the available input signal).
The Oil Tanker Fuzz low input impedance will load the guitar pickups. This is the reason why they do not respond well
when they are placed after other pedals, it is best to place it first, or before them, in the pedal chain. However, due to
this property, it responds to the guitars volume knob very well.
4.2 Voltage Gain of the Input Stage.
In a Common Emitter transistor the voltage gain does require a bit of math to calculate and requires some assumed data
beforehand.
•
First, we need to assume our pedal is being powered by 9V.
•
Second, we need to know the thermal voltage of the transistor, which is approximately 25mV (sometimes
expressed at 26mV, depending on assumed temperature)
•
Third, we need to know what the collector voltage for Q1 is at when no signal is going into the pedal. Typically,
this is found to be at 2.54V when using 2N5088 transistors on this circuit. They can range from 2.5V – 2.7V
depending on tolerances.
With this information, we can now calculate the gain of the first stage. First, we need to calculate the I
E
, which is the DC
emitter current. To calculate, we use the following formula:
I
E
= (V
CC
-
V
C
) / R
3
= (9 V
–
2.54 V ) /
100,000
= 0.0000646 A = 0.0646 mA
Next, we need to get the g
m
, or measure of conductance of the transistor in this state. To calculate, we use the following
formula:
g
m
=
I
E
/ V
T
= 0.0646 mA / 25 mV = 0.002584
Lastly, we can now calculate voltage gain. To calculate, we use the following formula:
A
V
= − g
m
⋅
R
C
= − g
m
⋅
R
3
=
0.002584
⋅
100 K = 258.4 (48.2 dB )
In the real life, the input stage will not reach 48dB of gain, the feedback network will reduce this levels to 47 dB approx,
assuming R4 is 100K and the 1K fuzz knob is maxed. It reduces the first stage gain to 36 dB when the 1K fuzz knob is at 0.
4.3 Input Capacitor Frequency Response.
The C1 22nF input cap creates a high pass filter together with the input pedal impedance (5KΩ approx.), removing
dangerous DC levels, hum and overloading bass.
fc = 1 / (2 π RC ) = 1 / (2 π
⋅
Z
in
⋅
C
1
) = 1 / (2 π
⋅
41 K
⋅
0.022 uF ) = 176 Hz
All harmonics below 176Hz will have 6dB/oct of attenuation