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Design Philosophy

How do I build one?

Dual Input Card

Motherboard

Dual Output Card

Power Supply

Processing Card

Control Systems

FAQ

Welcome!

This website describes a high performance audio preamplifier design project for home constructors. This is aimed at hi-fi and pro audio use for selecting audio sources for listening, recording or distribution around a house/building, where maintaining highest audio quality is paramount. The system is flexible allowing stereo and surround sound (5.1/7.1) configurations to be built, with the number of inputs and outputs scaled to suit the user's requirements. For example, a single case will house PCBs for 16 stereo inputs with 8 stereo outputs. 4-layer PCBs are available for DIY construction of this design.

Key Features

The system design is based around the principle of using a passive motherboard/backplane PCB card, into which are plugged input, output or processing PCB cards.

The system has been carefully designed taking into account factors which will affect the audio performance perceived by a listener. Appropriate grade components are used to meet the design's performance without needing any "adjust on test" changes. No esoteric (or very expensive) components, which would not improve actual performance, are used.

The key features of this system design are:

• Balanced (differential) inputs and outputs on XLR sockets - the input stage uses a technique suitable to remove hum and interference from both unbalanced and balanced sources
• RF (Radio Frequency) rejection filters on the inputs, allowing a wide and flat audio band frequency response
• Adjustable gain/attenuation on the input cards (+9dB to -18dB in 3 dB steps) using jumper plugs, allowing the user to accurately balance the signal levels from different sources
• Passive motherboard/backplane card designed to fit in a standard 19" rack case, with two stereo busses (eg. for separate listening and recording feeds) and DC/data control lines to each card
• Output cards with high quality volume control ICs, allowing remote control
• Two stereo inputs on each Input card
• Output card can be configured for two independently selectable stereo outputs or for one stereo output plus one/two mono outputs (normally electronically balanced outputs, but transformers may be fitted to the card if required)
• All PCBs are 4 layer to minimise (and guarantee) crosstalk and interference rejection performance
• On-board ±15V regulators for guaranteed low noise DC supplies on each card.

This project was instigated because the designer could not find all of these features on commercially available equipment (at any price!) - the system design has evolved from that original cause. As it is aimed at the home constructor, the system's modular format allows it to be built in stages as time and money permits. For example, a smaller system can be expanded into a much larger system in the future.

Even though this design does not use expensive esoteric components, the cost of all of the components does add up (especially for a large system) so please do calculate the total cost of the project before deciding to start. Similarly, the time required to build and test a large number of PCB cards is significant - but you will have built a high performance system to use for many years!

PCBs available

This system is an ongoing design project - currently the following PCBs are available:

• Dual Input Card - two stereo balanced inputs with XLR input sockets, preset gain adjustment jumper plugs, electronic audio switches to two stereo busses and local voltage regulators
• Motherboard - two stereo busses, distribution of DC power to all cards, marshalling the data/control line signals and slots for up to 12 cards.

The Dual Output Card (two stereo balanced outputs on XLR sockets, with independent selection from two stereo busses, electronically controlled volume control, jumper plug mono/stereo selection and local voltage regulators) is currently being designed. Software for a PIC microcontroller is already available for download (see Control Systems page for more details).

The Dual Input Card and the Motherboard have been designed first because they can be used as a preamp system with a passive volume control/output buffer and a simple input selection switch - with the Dual Output Cards added later, etc.

It is intended that future designs will include microprocessor (eg. PIC) control cards, main power supply regulators, etc.

Website Layout

Individual pages describe the design philosophy, points to consider before building a system, and details of the PCB cards. All pages have an identical menu bar for navigation. High resolution photos/images can be downloaded from the Photos/Downloads page, or (in most cases) by clicking on the images within the pages (use a graphics display package such as PaintShopPro to print the high resolution files).

Summary

We hope that you will find this design interesting and that it might meet your requirements too! Even if you choose not to build a complete system, some of the design ideas may be useful, as the cards have been designed with the expectation that some users will have different applications for which they are also suitable.