Lexicon RV-8, Service Manual

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RV-8 Service Manual
Amplifier Operation
At the heart of the amplifier is a high performance error amplifier (U2 - TLE2037). From the error amp
audio signals move through the voltage translators and on to Q7 (MJE15032) and Q11 (MJE15033). Q7
and Q11 form current gain cells, which feed the paralleled main output devices (MJ21194 NPN, MJ21193
PNP). The main output devices are connected in an emitter follower configuration. The devices are
mounted directly to the heatsinks with no insulator, which means the heatsinks are at rail potential
(±70Vdc).
Frequency Response
The frequency response of the amp module is set by several components. The dominant pole of the
amplifier is set by R10 and C5 at approximately 145kHz. All of the stages prior to the power amplifier
stage also contribute to the high frequency roll off of the amplifier.
Amplifier Feedback
Resistor R12 (47.5) and capacitor C8 (22pF) provide a local high-frequency roll off for the error amp. The
error amp also receives split feedback signals from two points at the output. C23 (0.33uF) provides high
frequency feedback through the output inductor L1 (1.5uH) while R58 (107 ) provides low frequency
feedback, which negates the DC resistance effects of the output inductor. R10 and C5 provide overall
feedback for the amplifier.
Time Dependent VI Limiting
For simplicity the positive side current limit is described below. The negative side current limit operates in
a similar fashion.
Current Limiting
The output current of the amplifier is sensed across emitter resistors R26 (0.68), R27 (0.68), R169 (0.68),
and R173 (0.68). This information is summed via R342 (23.2k) and R343 (23.2k) then divided down by
R344 (1.82k) and R345 (100k). The divided voltage information is fed into the base of Q77 (KST5089).
Once the voltage across R344 and R345 reaches approximately 0.65Vdc, Q77 turns on which causes
Q79 (MMBT3906) to turn on as well. This redirects current coming from Q5 and Q6 through D12 (1N914)
and Q79 to the output, thus limiting the amount of current driven into the base of Q7.
Time Dependency
C157 (4.7uF) provides time dependency to the current limit. When no current is flowing through the
emitter resistors, C157 has no charge. When current suddenly flows through the emitter resistors, C157 is
seen basically as a short, which essentially eliminates R345 from the limiting circuitry. As C157 charges
the current is limited more and more until it reaches a steady state.
Voltage Information
D1 (1N914) and R129 (365k) provide the voltage information to the circuitry. As the output voltage of the
amplifier increases, more current is pulled through R342, R343, R344, and R345 causing the voltage
differential across pins 1 and 2 of Q77 to decrease. At the same time, D1 and R129 also pull charge out
of C157, which stretches the time dependency of the limiter.
Other Protection
D14 (ES2GTR) and D15 (ES2GTR) are fly back diodes that steer reactive energy returned from the load
around the current gain cells and back into the power supply. D5 (RLS245) and D10 (RLS245) limit
collector voltages on the MPSW42’s and MPSW92’s, thus reducing cross conduction currents under high
current/high frequency conditions.
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