HMV Model 328A

Power Distribution

Tapping into the Computer Power Supply

The +12V rail is used to power the LED lighting and audio muting relays. The maximum current drawn from this rail is less than 1A.
The +5V rail is used to power the LED Controller Board and Peripheral Controller Board, both described later. The current draw for these guys is minimal.
The +5VSB rail is used by the Computer Power Sense Board to power the Standby LED. The current draw is around 1.5mA.
The Power Good signal is generated by the Computer Power Supply and is used by the Computer motherboard reset circuitry. Here, it is also used by the Computer Power Sense Board to detect mains failure.

The Power Good signal (grey wire) appears on pin 8 of the ATX power connector. The +5VSB rail (purple wire) appears on pin 9 of the ATX power connector. The only way of gaining access to the +5VSB rail and the Power Good signal is either on the back of the motherboard or by cutting into the Computer Power Supply wiring harness. I used a 5-pin Acme "microphone" connector, wired to the Computer Power Supply wiring harness to gain access to all of these rails, plus the computer 0V rail.

Tapping into the +5V Standy and Power Good wires

Modified Computer Power Supply harness with the additional Mains Ground wire and the power connector to the main chassis

Doing this is potentially dangerous - don't try this at home kiddies!

Chassis Power Distribution

Computer Power Supply Connector

1. Computer 0V (To Digital Ground)

2. Computer +5V

3. Computer +12V

4. Computer Power Good

5. Computer +5V Standby

±15V Power Supply Connector

1. Mains Ground (To Analogue Ground)

2. Audio 0V (To Analogue Ground)

3. Audio +15V

4. Audio -15V

The two connectors for the Computer Power Supply and the ±15V Audio Power Supply are mounted on the chassis and go directly to a tagstrip. All peripheral board power is derived from this point. With the exception of the Computer Power Sense Board and the 10 Channel LED Driver Board, all peripheral board power connectors are wired in the same way.

Power Distribution Tagstrip

Left to right:

1. Analogue Ground

2. Digital Ground

3. +5V

4. Analogue Ground

5. +12V

6. Digital Ground

7. +5VSB

8. -15V

9. +15V

10. Analogue Ground

Standard Power Connector

1. +12V

2. +5V

3. (Dig Gnd)

4. Dig Gnd

KEY

6. +15V

KEY

8. -15V

9. Chassis Gnd

10. An Gnd

In addition, the Computer Power Supply Connector provides power to the hard disk mounted on the chassis.

View schematic

System Grounding Scheme

Enlarge

In a normal computer box, the Mains Earth is bolted to the power supply chassis. The remainder of the box is grounded via physical contact with the power supply box.

In this system, the wooden cabinet effectively insulates the computer power supply from the chassis. The Mains Earth from The Dog Box connects to the Chassis Ground via the Audio Power Connector. If anything, this arrangement is better than relying on the metalwork connections in a conventional box. Other metal items mounted in the cabinet such as the optical drive are also connected to the Chassis Ground Terminal.

For safety reasons, an additional earth wire runs from the Computer Power Supply case to the Chassis Ground Terminal. This ensures that if the Computer Power Supply is powered separately, the main chassis is still grounded. Note that although the chassis is grounded via 2 different paths to the Mains Earth, no ground loops of any significance are created.

The common (0V) rail for the ±15 Volt Audio Power Supply is isolated from the Mains Earth in The Dog Box. Pins 1, 4 and 10 of the Power Distribution Tagstrip, The Chassis Ground Terminal and the ±15 Volt Audio Power Supply 0V are all directly connected to the Chassis Ground and this is used as Analogue Ground for the entire system.

In standard computer power supplies, the common (0V) rail is isolated from the Mains Earth inside the power supply. Instead, the 0V (black wires) from the Computer Power Supply is connected to the ground plane of the motherboard, which is in turn bonded to the chassis via the metal mounting posts. This arrangement prevents ground loops. This means that the audio grounds coming from the sound chip on the motherboard should be very close to Analogue Ground (barring any design limitations of the motherboard).

An additional wire connects directly to the Computer Power Supply 0V and is used as Digital Ground for the system. Due to the heavy currents in the 0V wires between the Computer Power Supply and the motherboard, there will be a slight difference in potential between Digital Ground and Analogue Ground. These two grounds are wired separately to the peripheral boards and are isolated from each other. Any difference between these two grounds is taken into account by the electronics.

This arrangement gives the cleanest possible grounding for both the computer and the audio system.

GROUND SYMBOLS USED IN THIS DOCUMENTATION

Analogue
Ground

Digital
Ground

Chassis
Ground