Page 52 of 56
deliver. R24 and R11 form a voltage divider that applies 0.5 volts to the positive input of
U1. The negative input of U1 (pin 6) is driven by a low pass filter that senses the power
supply output current. The DC current limit is set by 350 mV/0.1 Ohms (R12) at 3.5
Amps. Thus the power supply can deliver 3.5*76=266 watts continuously. It delivers
more than that for short periods of time, as determined by the R3-C3 time constant, and
aided by the large size of C6, the 10,000 µF output filter capacitor. When the DC current
exceeds 3.5 Amps, pin 7 of U1 goes low, robbing gate drive from Q5 by way of D10.
This protects both the power supply and the amplifier in the event of fault conditions.
Start-up Circuits
The power supply starts up in a constant current mode, delivering about 0.42 Amps. This
can be calculated by 7/R22*R4/R12. Q2 and U1 create a collector current in Q2 that is
1/10000 (R12/R4) of the output current. That current balances the current flowing in R22,
driving the gate of Q5 to a level that makes about 0.42 Amps of current in pass transistor
Q5.
The power supply remains in constant current mode, supplying the amplifier modules and
charging C6. This slows the rise of the output voltage. When the output voltage is more
than about 54 volts, D12 and D1 are forward biased. That turns on the Q6-Q7 error
amplifier, which changes the power supply from constant current to constant voltage
operation. From 37 volts to 60 volts, the output would rise quickly were it not for the
effect of C1, which continues to give the output voltage a slowed rise-time. This limits
the charging current in C6. Without a limit on the rise time, the charging current for C6
might be large enough to cause troubles.
D14, R10, and R13 limit the current that the power supply delivers in start-up mode.
When there’s more than 33 volts across the pass transistor (at startup, there’s almost 90
volts across the pass transistor), we limit the current. With 0 volts across C6, there are
about 67 volts across R13, 3 Megohms, which diminishes the current from R22 by 22 uA.
That leaves 7/165000-22uA=20 uA of drive current for Q5. Multiplied by the gain of
10000, given by R4/R12, that gives 200 mA available for startup.
R23, 100 Ohms, is used to stop high frequency oscillation in Q5. D9 prevents more than
10.7 volts from being applied to the gate of Q5, protecting it from damage.
Amplifier Modules
This discussion refers to Figure 51.
The amplifier modules are based around discrete transistors with complementary
MOSFET output devices.
R3 and C1 make a low-pass filter to eliminate radio stations that otherwise might be
picked up on the inputs. Q15-Q18 form a Darlington differential input pair that keeps the
input impedance high, making the GT-108 a friendly load for most tube preamps. C2
blocks the input stage bias voltage from appearing on the input RCA jack.
