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6.3 Total Voltage Gain.
The source degeneration resistor RFUZZ creates a local negative feedback, making the second amplifier stage more
stable and immune to gain variations due to temperature, bias current and transistor intrinsic properties.
With this source resistance added, the Common Source N-Channel major parameters (ignoring by the moment the
feedback network) can be determined by the ratio between the drain resistors (R4 + R5) to and the source resistor (the
portion of RFUZZ not shorted to ground through the 22uF cap).
A
V
=
R
C
/ R
E
= (
R
4
+
R
5
) / R
pot
1
A
V
min
= (1K + 5.6
K
) / 1
K
= 6.6 (16
dB
)
The voltage gain (AV) can go from 6.6 to as high as the transistor's basic internal gain (when RFUZZ is maxed out).
If we take into consideration the feedback network, once again the second stage will not reach values as 16dB. In this
case, the total voltage gain measured at Q2 source is around 19.5dB. Remember that the input stage had a gain of 18.6
dB, that leaves the second stage a total amount of 1dB of gain (19.5-18.6=0.9dB). The general amount of gain is
considerably reduced due to the feedback network.
But the output of the pedal is not directly taken from Q2 source, there is a voltage divider created by R4 and R5 (the
power supply is effectively at AC ground). This divider reduces the gain by a factor of R4 / (R4 + R5) = 1000 / (1000 +
5600) = 0.1515 (-16.4dB), so the real gain of the output stage is:
G
VTOTAL
= GV
PEDAL
- Attenuation of R4 / (R4 + R5) = 19.5 – 16.4 = 3.1dB
This voltage divider created by R4 and R5 will greatly reduce the output level. The value usually does not get as low as
3.1dB, the series resistor of the battery should be taken into consideration and will raise the output level.
It might look funny but it has a reason: the output signal is not much larger than the input signal to keep the huge
amount of signal available from over-driving the input of the pedal or amplifier following it. The fuzz is not designed to
overdrive the following system by level.
Oil Tanker Fuzz sounds different with different batteries and with the same battery as it gets run down. The internal
series resistance of the battery is added to the 1K Ω R4 resistor, modifying the value by a significant amount.
Any impedance between C2 source and ground (RFUZZ) will reduce the gain of the output stage, it is a form of local
negative feedback. Increasing this impedance will reduce the gain. If we are looking for high gain it is a common practice
to have part or all of the source resistor grounded with a bypass capacitor.
Capacitors present an impedance that decreases with frequency, the bias (DC) points will remain the same but high
guitar (AC) signals will get higher voltage gain. In terms of design, the bypass capacitor C2 should have a reactance, at
the lowest frequency you are interested to amplify, less than the value of RFUZZ. We can use the formula:
f
c
= 1 / (2
π
RC
) = 1 / (2
π
⋅
R
pot
1
⋅
C
2
) = 1 / (2
π
⋅
1
K
⋅
22
uF
) = 7.2
Hz
All the frequencies over 7.2Hz get full amplification. The 22uF is so big that almost all the frequencies get full
amplification.
