Chapter 6 - Theory Of Operation; Amplifier Theory - Lexicon RV-8 Service Manual

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CHAPTER 6 – THEORY OF OPERATION
RV-8 AMPLIFIER THEORY
The RV-8 amplifier sections are designed using the most advanced linear power technology available.
Using this technology allows maximum amplifier performance under the most demanding and extreme
conditions.
In the RV-8 there are two amplifier modules. One module consists of three amplifier channels and the
other consists of four amplifier channels. Each individual channel is identical schematically, but not
identical in the way they are laid out.
For this portion of the theory, only one channel will be referenced since all channels are the same
schematically. All references correspond to the SIDE RIGHT (Channel 2) channel on the four-channel
module.
Features:
•
Full-complementary output topology
•
High performance error amplifier to provide extremely low THD
•
Ultra high 140V rails to maximize dynamics
•
Critical protection circuits local to each amp channel
Input Stage
Op amp U1-A (MC33078) is used to set up a conventional balanced to single-ended conversion stage.
The overall gain of this stage is approximately –6 (15.6dB), which is set by R2 (2.49k ), R4 (2.49k ), R3
(15k ), and R5 (15k ). C1 (22pF) provides high-frequency feedback.
Power Amplifier Overall Gain
For this portion of the theory, consider the power amplifier as one large op amp. The output of U1-A
(MC33078) feeds a single ended audio signal into the power amplifier. The power amplifier is set up as a
conventional inverting amplifier. The gain of this stage is approximately –10 (20dB), which is set by R7
(2.49k), R9 (2.49k), and R10 (49.9k).
Bias
Bias is set by R14 (9.09k), R17 (9.09k), R15 (825), and R16 (825). This causes a small equal current to
flow through R19 (825) and R29 (825) via steering diodes D4 (1N914) and D7 (1N914). R19 drives Q5
(MPSW92) and Q6 (MPSW92), while R29 drives Q9 (MPSW42) and Q10 (MPSW42). These four
transistors also serve as voltage translators. Emitter resistors R20 (95.3), R21 (95.3), R30 (95.3), and
R31 (95.3) set a current used to bias Q8 (MMBT3904). R84 (1k potentiometer) is then used to precisely
tune the bias level to a point where output devices are just out of conduction. This provides the AB+B
mode of operation for the output stage. Bias voltage is measured across R25 (30) and is set to 0.350Vdc
± 0.01 @ 25 degrees ambient.
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