Part 2 - Building
Here is where I describe a little about building the amp. All the challenges I came across and ways of getting round problems.
Width controller and decoder
The first part of the build process was to design the circuit boards. I have
extensive information on how I design and build circuit boards
here, so I won't provide much
information.
Things were quite simple for me, as I had PCB's for rear amp, centre amp and tone controls from the previous amp (and the headphone out too). This meant the only boards required was the decoder board, the front amps board, the power supply board and a small board for mounting switches and LED's on.
As mentioned, the first board I started on was the decoder board. This was the most complex, because I tried my best to squeeze the width controller, the rear decoder and the centre decoder all on one small board. The design I came up with in actual fact was very impressive, I was quite happy.
For your information, I used the ESP stereo width controller deluxe version for both the front width controller and the rear decoder. The rear decoder use 2 trim pots which are set in the position for maximum for maximum "split". This effectively gives me the rear speaker requirement of no centre image at all, with a bit of stereo image on the rear. The centre decoder is basically the small part from the ESP simple surround sound decoder, without the 100k pot level adjustment or the 1n capacitor frequency cut-off - as I had the pot later in the circuit and I prefer the centre sound not to be cut-off at high frequencies, its better sound quality to my ears. This means all the centre decoder consisted of from that project was R3, R4, R9, R10, U1B, C2 and C3. A very simple circuit indeed :)
This photo was taken before the 1uF capacitors were added, but that is basically how small I made it, and there were only a few jump wires too. Those 1uF capacitors were a tight squeeze when I got them however. Its always a good idea to wait till you have all the components before you construct the board.
Once I knew where I was going, from my design, I started cutting the case up. My old amplifier had the same size heatsink as the one I planned to buy (it wasn't reused because I couldn't face the fact it would look ugly from all the holes). With all this information - the case was cut and the first two boards went in fine:
The board on the right is the tone controls, from the old amplifier. The original holes for those pots lined up fine with the boards (actually there was some extra space), so I was happy there. I came across a problem when it came to mounting the boards however. My old amplifier used a very small amount of spacers, all of different heights but I wanted to do a better job. Unwilling to get ripped off by proper spacers, an idea came to mind.
Those old knackered felt tips! This has to be one of the cheapest ways to make spacers ever, seeing as in places you can pick up something like 50 felt tip pens for £1. They are not strong, but strong enough to bolt down the circuit boards. The spacers are barely stressed in any way at all.
Care has to be taken when cutting them - it can't be done with a hacksaw, they just snap. I used a small rotary tool (like a dremmel) to slowly ease my way through the plastic. It produced nice results and my measuring was accurate enough to have all the spacers the same height. Warning however - the plastic is very cheap, do not use it for circuitry where heat is common (not an issue with my op-amp boards).
PSU
My next challenge was the power supply board. The aim of this board was to
provide an earth loop cut (by a large 10 ohm resistor and a 0.1uF 250VAC
capacitor), regulate the 12V transformer output to positive 12V and negative 12V
for all those op-amps and provide a mounting point for large smoothing
capacitors for the rest of the power supplies.
Capacitors:
Two 4,700uF 35V capacitors for my regulators, one for each polarity (+/-)
Another two 4,700uF 35V capacitors for the 18V unregulated power supply to the
TDA2040 amps (rear and centre)
Two 10,000uF 35V capacitors for the 25V unregulated power supply for the TDA1514
amps (front)
Regulators:
Based on the ESP Simple
pre-amp power supply, only with 12V regulators rather then 15V, and with my
own 4 diode rectifier to give me the 12-0-12 AC connection a +/-18 DC output.
Earth loop breaker:
This loop breaker is based on the one in the
ESP Earthing your hi-fi
article.
Unfortunately I did not take a better picture of the PSU so the above is all I can show. This picture does however show the location of those huge capacitors, the regulators can just about be seen below the two largest capacitors, and the loop breaking is on the left, and is adequately earthed to the case chassis.
Front Amplifiers
The next board to make was the all important, all powerful, all great
quality TDA1514 front amplifier board. I designed this board to be able to fit
both TDA1514 chips at the back of the board (for heatsink mounting) and so that
the width of the board so it would be less then the total width of the heatsink
with the centre amplifier next to it. This didn't leave lots of space so my
TDA1514 PCB design was a bit more cramped then my TDA2040 designs (very spaced
out). Saying that, the design was fine and the tracking is wide enough to carry
those essential high currents to and from the TDA1514 chips themselves. A major
overlook was the smoothing capacitor which jumped from the positive 25V rail to
the negative 25V rail. The design was for, and initially used a 35V capacitor
across here. It came as no surprise that after about 20 minutes that it blew up
with 50V across it rather then the 35V maximum. Fortunately, I could squeeze a
larger 50V capacitor in its place. The board looks as below:
After the new heatsink came, I used two TO220 mica washers on each chip to provide insulation from the TDA1514 chip and the rest of the heat sink since my supplier no longer stocks the proper washers for these devices :/ Its works though, and two screws tapped into the heatsink for each chip was enough to secure the whole assembly in place.