Contents

Page 1 - Idea and circuit choices
Page 2 - P3A Amplifiers
Page 3 - Crossover and Surround Channel Amps
Page 4 - Power Supply details
Page 5 - Testing
Page 6 - Improvements

Original Case Construction

Time constraints prevented me from making my own case, so I splashed out on a 2U rackmount from maplin electronics. I decided £35 wasn't too bad if I wanted to get this project in a good case and completed as soon as possible.

Construction commenced very quickly.. as soon as I got the case in fact!

Having the wood template was handy - it meant all I needed to do for over half the required holes was to drill through the holes made by by self tapping wood screws and into the metal base of the rack mount case. Unfortunately my centre wasn't centre enough and I did end up with the whole plot slightly too much to the left - meaning my hopes of squeezing in 80mm fans for the P3A's was out of question.

I did not take many pictures during construction - so this is limited. Construction was just a common sense approach anyway, at least intially.

This is how far I got on the first day. Most components were in, and the hole was cut for the heatsink, but no hole was made for the mains power in, no heatsink had been added (meaning no rear or centre amps) and no holes were drilled on the front panel.

At this stage of construction - I thought 'oh bugger'. I had suddenly realised that there was absolutely nowhere for all 7 loudspeaker binding posts to go.. at all. Now I remembered why I wanted a 3U case!

The heatsink also was proving very slow and annoying to cut with a power jigsaw and I did not want to waste my time attempting to cut it from 100mm to 80mm (the same height as the 2U case). It would have also meant a loss in performance too.

The idea I came up with was to get another piece of wood, about 20mm in thickness and use that to hook to the bottom of the case and mount my loudspeaker terminals on.

The idea worked rather well, and did not take very long to implement. Here are further results:

Front, still incomplete as it needs something to make it look a little better

Back - you can see the purpose of the wood. It works very well indeed. The 4 controls on the left are for centre, rear, front high and front low volumes. They are back mounted for adjustment only and keeps the fiddlers away. The 80mm provide plenty of airflow onto the heatsink. There are 8 phono sockets for L/R in, rear L/R in, centre in and subwoofer out. The other two will function as triggers for the soon to come internal relays.

A very simple bottom - using door stops as feet (its very effective). Speaker cables come out the bottom of the case and wire to the binding posts, and the rear fans are also connected this way. You can also see the bottom of the heatsink.

Yeah, its a little bit messier than I hoped - lots of wires! This was unavoidable though, and the point is it still looks good and meaty and like it will blow many other amplifiers away!

The design was good, but I was not quite happy with it and knew this project was really meant to go into a 3U rack mount case...

Finally there was a little hum to be eliminated too, I suspected this to be down to improper star earthing (I cheated and sent one wire from all loudspeaker terminals to ground). With the loudspeaker terminal posts now being screwed onto the case itself, wiring them up for both inside the amp and outside was much easier.

New Case!

Whilst the amp worked quite happily in the 2U case, when I moved back from university I wanted a better case design without the tacky looking wood bottom, so I brought a new case.

The system went under quite a rebuild into the new case. The Linkwitz-Riley crossover was rebuilt using a better choice of (5% tolerance) capacitors and lower value resistors! I also brought and used a board designed and sold by Rod Elliot himself as I was convinced that my design could be better. After the rebuild I did notice less distortion and hum - so I did something right!

Since the regulated PSU was squeezed on the original crossover board, that also had to be designed separately and I decided that it should mount comfortably on the rear heatsink.

I also designed a new fan controller, since the orginal refused to work - I had put up with a year of fan noise when using this amp and that needed to be changed! This new controller worked for a while, but was quite difficult to set right, and since I was seriously looking at the idea of some advanced amplifier control, like the PIC micro solution in my subwoofer described here, I was going to move fan control to the digital world.

Inspired by my sucess with my advanced microchip controlled digital audio preamplifier, I really like programming the PIC micro so I can make my projects do exactly what I want them too - which can involve quite complex analogue electronics if done the so called 'proper' way.

My features for this new amplifier control circuit would be quite simple, but wide too.

The first thing anyone would notice is the two seven-segment digits now on the amplifier front. This would be current amplifier temperature. By that I mean the temperature of those metal heatsinks the P3A amps were mounted on.

A PIC micro cannot sense the temperature on its own, and I wanted something that would give a readout in degrees C. This is where the Texas Instruments [PART NUMBER] chip came into use.

They are cheap and come in a small SMD package and require just one other component (a SMD 100nF capacitor is ideal) to form a working serial temperature sensor. Accuracy at worst (normal temp range) is +/-1.5 C. This is good enough for my needs and in most cases it is better than this.

I would need two, one for each amplifier heatsink. The values returned from them would go directly onto the LED display, and the temperature would be checked to see if the heatsinks were running too hot (say 50 C).

Other features would be switch on mute, fan control (on/off) and control of an RGB LED to show the amps current status. DC fault detection is planned later, but at the moment the circuit is without that feature.

The LED is a nice feature. It is a bright LED with four leads, a common cathode and one for each colour. This allows me to represent conditions - such as flashing red/blue for power on mute, blue for on and normal, green for on and fans running and red for overheat condition.

Because I only used a PIC16F84A too, quite a bit of circuitry was involved as I had to connect two, 7-segment displays which would require a BCD to 7-seg decoder chip, used to drive each display via a multiplexing system - and also a shift register to cut the 4 BCD lines, display select line and fan control into a 3-wire serial system. This is all in order for me to get all the I/O into the limited pins the PIC16F84A provides you...