User manual

VMDA Series User Manual P/N 821115 Rev-C.5

6) IC15 selects which analog channels are monitored by setting the control lines ACH1, ACH2, ACH3 and ACH4

(pins 30, 29, 28 and 27). . The logic level output of IC15 is translated to the level required to control the analog

switches (IC4, IC5, IC6, IC7, IC10 and IC11) by the quad op amp IC14.

7) IC15 reads the push buttons on the front panel and lights the LED’s inside of these buttons. These buttons are

connected to the following pins; pin 36 is the channel 1 button, pin 35 is the channel 2 button, pin 33 is the channel 3

button and pin 34 is the channel 4 button. The analog/digital/mute switch is connected to pin 37.

8) IC15 reads the state of the headphone sense line (pin 39). If a headphone is inserted into the ¼” jack on the front

panel IC15 will mute the speaker amplifiers (IC1 and IC3). The logic level output of IC15 is translated to the

level required to mute the speaker amplifiers by the transistor Q2.

9) IC15 presently does not use pins 10, 25, 26 and 40.

The Codec

IC20 is a Crystal Semiconductor CS4226 codec. It has a stereo 20-bit A/D converter, six 20-bit D/A converters

and a S/PDIF receiver. Each of the six D/A outputs has an independent volume control.

1) IC20 has four digital audio input signals on pins 1, 42, 43 and 44. These signals either come from the BNC’s on

the rear panel (J7, J6, J9 and J10) or the mini-terminal block connectors (J8 and J11). The BNC’s accept 75 ohm

unbalanced digital audio. The mini-terminal block connectors accept 110 ohm balanced digital audio. The 110

ohm balanced digital audio signal is converted to 75 ohm unbalanced digital audio signals by the transformers

T1, T2, T3 and T4. These digital audio signals can be terminated by the user by closing the appropriate section

the DIP switches S1 and S2. IC15 controls which digital audio signal is selected by means of the serial control

buss (pins 3, 4, 5 and 6).

2) IC20 has a balanced analog audio signal on pins 10, 11, 12 and 13. This signal comes from the analog input select

circuitry.

3) IC20 has six signal ended analog outputs. AOUT1 and AOUT2 (pins 21 and 22) are summed, sent through a low

pass filter (IC18) and then to the center speaker amplifier (IC3). AOUT3 and AOUT4 (pins 23 and 24) are filtered

to reject out of band noise (IC23), sent through a high pass filter (IC21) and then to the side speaker amplifiers (IC1).

AOUT5 and AOUT6 (pins 25 and 26) are filtered to reject out of band noise (IC26) and then sent to the balanced

audio output (IC24 and IC28).

Analog Input Select Circuitry

Balanced analog audio signals are connected to the unit via the mini-terminal block connectors J2, J3, J4 and J5. This

signal passes though the analog switches (IC4, IC5, IC6, IC7, IC10 and IC11). The left side of the selected channels

are summed together by IC8. The right side of the selected channels are summed together by IC12. The potentiometers

P2 and P3 are used to adjust the signal to the correct level for the codec (IC20).

Refer to the interconnect block diagrams on he following pages, for assistance in tracing the signal path through

the detailed schematic.

MAIN PCB (719141) CIRCUIT DESCRIPTION (Cont.)