Preamp Home Page

Ordering

Photos/Downloads

Links

Design Philosophy

How do I build one?

Dual Input Card

Motherboard

Dual Output Card

Power Supply

Processing Card

Control Systems

FAQ

Main Features

The Dual Output Card is currently being developed.

This card provides two stereo output channels and, when used with the Dual Input Cards and the Motherboard, forms a complete high quality audio pre-amp.

The card has four high impedance unbalanced inputs to connect to the audio busses on the Motherboard, electronically controlled volume control and low impedance balanced output drivers.

The key features of this card are:

• Two independent stereo outputs, each individually selectable from two stereo buss inputs (from the Motherboard) using an Analog Devices SSM2402 audio switch IC
• High quality BurrBrown PGA2310PA Volume Control ICs for each stereo output
• Differential (balanced) outputs on XLRs
• Each output XLR socket can be configured (with a jumper plug) to carry a stereo channel or a mono signal
• Lundahl LL2811 output transformers can be fitted to three of the output sockets (eg. for a house distribution to many preamps/amplifiers - carrying Left, Right and Mono)
• Mute relay option on all four outputs (eg. for power up/down mute)
• Optional PIC microcontroller allows simple control of the PGA2310 volume control with a rotary encoder
• 4 layer PCB, Eurocard 160mm x 100mm (6.3" x 3.94") size
• On-board ±15V regulators for guaranteed low noise DC supplies on the card

Possible Uses

Stereo Output - to loudspeaker amplifiers

This card can be used to feed two sets of stereo loudspeakers (eg. in different rooms in a house). Each stereo output (and actually each channel) can be remotely selected to one of the two stereo busses on the Motherboard.

Single Loudspeaker Output and Recording Output

One stereo output of this card can feed a stereo power amplifier, with the second stereo output being used as a Recording output - feeding one or a number of record inputs. This will provide the "classic" listening buss plus independently selectable record buss facilities, using just one output card.

Distribution Outputs

For a house distribution to pre-amps/amplifiers in other rooms, this card can be set to provide one stereo output and one mono output (two mono outputs if electronically balanced outputs are used). Mono outputs are often useful in noisy environments...such as kitchens or bathrooms. Where the signals will be fed over long cable runs and with many other preamp inputs connected to one of these outputs, it is recommended that output transformers are fitted.

Circuit Description

Buss Inputs - Switches

The two stereo busses from the Motherboard are connected to two Analog Devices SSM2402 switch ICs, to allow the user to select which buss is fed to each volume control. Each switch DC control is wired to the backplane DIN41612 connector, allowing the user to select each left/right channel independently. (As a DC control is used, a microprocessor is not needed to operate the switch.)

Volume Control

The outputs from the two switches (eg. Left Buss 1 and Left Buss 2) are fed, DC coupled, into the BurrBrown PGA2310PA volume control IC (DC blocking capacitors can be fitted if they are required in a particular implementation). The volume control offers independent control of the left/right channels (so a "balance" function is possible), and has a range from -95dB attenuation to +31dB gain, in 0.5dB steps plus "mute". The volume control uses serial data control (see the BurrBrown datasheet) and so a microprocessor (eg. PIC, AVR etc) is needed to control the IC. Optional PIC 12CE519 (8 pin) microcontrollers can be installed directly on the card if required, allowing volume/balance/mute functions to be controlled by a rotary encoder and a single push button switch. See Control Systems page for more details.

Mono Circuit

A conventional "virtual earth" summing amplifier configuration is used, which is followed by a unity gain inverting amplifier to maintain the correct absolute phase of the output. The card contains pads for a DC blocking capacitor on the mono section output if required.

Output Drivers

BurrBrown DRV134 balanced output driver ICs are used to feed the XLR sockets. These driver ICs are fed from either the stereo channel (eg. left) or from the mono output - selected using a jumper plug. The card contains pads for a DC blocking capacitor to be fitted on the input to the driver ICs if needed in a particular implementation. Lundahl LL2811 output transformers can be inserted into three of the outputs - between the driver IC and the XLR. (Note that this method is not the lowest possible distortion drive configuration for an output transformer - but is designed as a "simple" solution for fully isolating a preamp rack from a house distribution cabling network etc. A different technique would have been used if the design relied on the use of transfomers on every output...!)

Mute Relays

A power up/down mute relay is included on each of the four outputs. If energised, this will short together the differential outputs of the driver IC. If this technique is used, it is envisaged that the mute is only held for a short period of time (eg. 1 second). If a dual tracking power supply is used, this may remove the need for the mute relays.

Volume Control Options

1). If this card is used as a volume control/output driver in an implementation where there will not be any microprocessors, then a high impedance volume control may be connected directly to the Motherboard busses - with the volume control output (potentiometer wiper) connected to the output driver ICs on this card.

2). If the balanced output drivers, or mono circuit, are not needed (eg. a short cable run to an unbalanced input power amplifier), then the BurrBrown volume control IC output may be linked directly to the XLR (together with a 0 Volt reference). This has the added advantage that mute relays will not be needed, as the IC includes a power up mute facility.

Technical Specifications (Provisional)

Measurements have been made on the prototype of this card - the predicted performance of the final card is:

• Frequency Response: 10Hz to 60kHz ±0.02dB tbc
• Maximum Output Level: +24dBu tbc
• Distortion (THD 20kHz BW): 0.001% tbc
• Signal To Noise: 115dB tbc