Yorkville AP404 0 Power Amplifier M1129 “THE INPUT BOARD” The input board processes the audio signal from the input jacks to the volume control board, (M1128). Each channel consists of a balanced gain stage, switchable subsonic filter, and a stereo / mono / bridge switch. Looking at the left channel, the balanced input, (XLR Jack) and unbalanced input (phone jack) are wired in parallel to the input of a balanced operational amplifier, (U4). The gain of this stage is 0.82 (-1.3dB) balanced and 1.6 (4.0dB) unbalanced. Resistors R25, R27 along with capacitors C11 and C12 form a radio interference elimination filter. Switch S1 selects the cutoff frequency of the hi-pass subsonic filter. The subsonic filter provides a 20Hz or 40 Hz high pass filter. The filter consists of a tee network on the input of U3 along with R10, R28, C29 and C30, C33 and C34. The gain is 1 (0dB) in the passband, (above 100Hz). The audio signals from the input board M1129 pass through the 14 conductor cable to board M1128. M1128 “VOLUME CONTROL BOARD” This board contains: • the front panel audio gain controls • the front panel indicating LED’s (power, protect, activity and clip). • the audio limiters Circuit Explanation: • The left channel of the circuit is explained. (Refer to the schematic of M1128 as the sections of the circuit are explained.) The audio signal out of M1129 passes through volume control P2 and the desired level enters U2 through pin 6. U2 is set for a gain of 5 (14dB) when the volume control is in the fully clockwise position. The AP4040’s defeatable limiter is built around LD8. LD8 is an opto-resistive cell comprising of an LED that shines on a photocell. As the LED in the LD8 becomes brighter, the resistance of the photocell decreases, placing more of the audio signal on pin 5 (non-inverting input) of U2. This audio voltage gets subtracted from the signal on the inverting input and less signal appears on the output of U2. Transistors Q5 and Q6 along with the surrounding passive parts provide the attack and release time constants of the limiting function along with the drive currents for the clip LED and the LED inside LD8. When an audio signal on the output of the power amplifier section (on board M1146) enters clipping, pulses representing the duration of the clipped portion appear at LCLIP. These pulses turn on transistor Q6, and Q6 provides current pulses to turn on clip LED LD6. The pulses also pass through R7 and D6 to charge C3 and C36. When the voltage across C3 reaches 0.5 volts then Q5 turns on providing a cur- rent into the LED of the LD8 limiting the audio signal at U2. The charging (attack) and discharging (release) times of the limiter are 80mS and 3.5 seconds respectively. Resistors R50 and R7 provide the charging path, and resistor R51 provides the dis- charge path. The limiter can be defeated by placing the limiter switch (S2) in the in position which disconnects Q5 and the charging / discharging circuitry from V+. The activity LED circuit consists of Q1 and the surrounding circuitry. The audio sig- nal enters the activity LED circuit through R2. R2 and C21 form a differentiator that turns Q1 on illuminating the activity LED whenever the audio signal increases in ampli- tude. A constant current flows through R55A, R55B and when Q1 is off, the collector current then flows through D1. From M1128 the audio signal passes through a 12-conductor ribbon cable to circuit board M1147. On M1127 an operational amplifier U201 re-references the ground for the audio sig- nal from LREF or RREF to the corresponding LOG (left output ground) or ROG (right output ground). U201 also provides DC correction for DC offsets appearing on the out- put binding posts. Feedback from the output binding posts appears on LFNB or RFNB. Through R203A or R203B the DC offset achieves a gain of -1 from U201. The DC off- set of opposite polarity on the output of U201 will compensate for the DC offset in the amplifier section on M1146 resulting in 0 volts DC on the output binding posts. • The audio signal continues to M1146 via an 8-conductor ribbon cable. M1126 “THE VOLTAGE AMPLIFIER AND CURRENT AMPLIFIER” This board contains: • a voltage amplifier section • a current amplifier section • amplifier current limit section • DC output protection • heatsink temperature sensing Voltage Amplifier Section The voltage amplifier amplifies the audio signal’s voltage from 6.8 volts peak (at the output of U201) to approximately 98v peak, which is required to drive the current amplifier section. The current amplifier provides the current required for the 98v peak signal to drive 1200 watts into 4 ohms out of the binding posts. Before the circuit is described in detail here is a quick rundown on the signal’s path through the voltage amplifier stage. Refer to the schematic of M1146. Let’s consider that a positive going AC signal is present at the SIG input. The positive going signal will turn on the positive side of the voltage amplifier. The signal at the SIG input turns on Q12A (through R40A, D14A and D13A). The collector of Q12A pulls down on the base of Q14A turns this transistor on further and allows a greater current to flow out of Q14A’s collector. This increase in current passes through Q15A and it’s collector to emitter voltage decreases. The collector of Q15A now being more positive in voltage turns the base of Q18A on causing an increase in Q18A’s collector current resulting in test point 1 going positive. As the positive side of the amplifier was turning on the negative side would have been turning off. This is how test point 1 was able to move positive following the input signal. The reverse would hold true if a negative going signal were present on the input of the voltage amplifier.