P3A Power Amplifier

For my project I needed 4 channels of amplification because I am bi-amping each loudspeaker. The crossover is before the amplifiers (unlike passive loudspeakers where the crossover is between the amps and drivers which wastes energy and causes all sorts of problems). Rod Elliot explains the benefits of bi-amping in great detail on his site ESP - Elliot Sound Products. He also designed the P3A amplifier that I built two of for my project. Both the links to these pages can be found here:

Benefits of Bi-Amping (Not Quite Magic, But Close)

P3A Power Amplifier

I built the amplifiers with my uncle as he's very experienced with working with electronics. He restores old radios and builds radio transmitters for Universities and Schools! The last time I did any soldering was in secondary school about 7-8 years ago so I thought it was best to get some help for the amps. He agreed to do one stereo amp each although it ended up more like 60% - 40% as he soldered the transistors and did the mains wiring (I don't want to burn my house down!).

The Power Supply

With the 300VA transformer I'm using, the amplifier is capable of outputting 75W peak (65W continuous) into an 8 ohm load. With bigger power supplies 100W peak can be achieved per channel. The only problem with a more powerful power supply is the inrush current of the transformer and larger capacitance of the capacitors. It causes a sharp spike in current which puts strain on some of the components and will often blow the fuses. A soft-start circuit can be built (again on the ESP website) but I thought I'd keep it simple and just use the smaller (well 'standard') power supply recommended I did increase the capacitance slightly which should prevent the supply sagging during transients. Music is of a transient nature after all.

The supply is not regulated (self regulating?) and consists of a Toroidal Transformer > Bridge Rectifier > Filter Capacitors. For those that don't know what all these things do I'll explain. The Transformer takes the 230Vrms UK AC mains voltage and outputs +25V, Gnd and -25V and Gnd. These are connected to the bridge rectifier. This Rectifies the AC signal which looks like this: v^v^v^ and turns it into this: ^^^^^^. The filter capacitors then smooth this out into a constant DC voltage (usually there's some ripple too).

Heatsinks

The heatsinks ended up costing more than I would have liked (£75 for two). I called a few companies some had a minimum order charge of £100 which wasn't very helpful. Rod Elliot states that a pair of amps will be happy with a 1C/W (1 degrees Celsius per watt) for normal use. He also said that if you intend to push the amp then it should be more like 0.5C/W. I ended up with one closer to 0.4C/W which has even better thermal performance.

The company also messed up the order in my favour. I paid for a 22cm long heatsink, the box they arrived in said 24cm and when I measured them they are 25cm long. 3cm of extra heatsink for free, thanks!

Building the Amps


I went to my Uncles to build the amps as he has a workshop with test equipment and a workbench. We spent about two days building them. 

Some of the parts from Mouser

My Uncles analogue meter

Some of my uncles radio equipment

Radios that are in need to repair

We used a special analyser to work out which leg was what (base, collector, emitter) to ensure we were putting the transistors in correctly

More components

Here they are

My uncles DIY analogue tone generator he built 20 something years ago!

We did some quick testing with a DC bench supply at a lower voltage and a speaker load. Nothing blew up or got too hot so we proceeded to build the power supply.

Like I mentioned earlier, I left this part to my uncle because I wasn't really qualified. I watched him build it and helped hold things steady while he soldered.

Here is another DIY piece of apparatus. Its called a variac and adjusts the mains voltage from 0% to 110%. I had to think about the extra 10% for a moment too! I think he said the casing for the variac is an upturned plastic plant pot. After building the power supply we tested it using this by slowly increasing the voltage to 100%.

We used this large resistor as a dummy load for the amp output. It got very hot!

Here are the two finished amps

This picture shows the power supply connected to the amp, the amp outputs and audio inputs.

Setting the Quiescent Current

Once we got the amps up and running we needed to set the quiescent current. This is done by changing the value of the multi turn variable resistor with a screw driver. The greater the resistance the less current there is flowing through the output transistors. To little and distortion will occur, too much and the output transistors will overheat. We made sure the variable resistors were set to their maximum resistance before any testing for this reason. We played a sine wave 1kHz test tone through the amplifier and hooked up the output to the oscilloscope. There was some clear distortion in the waveform (it looked like a little wiggle). We reduced the resistance until the distortion disappeared on the scope. Later on we measured the current and adjusted it slightly to the recommended setting of about 75mA.






Integration into the speakers

The amps will be mounted in the back of the speaker enclosures with the heatsinks protruding. The power supply will be mounted inside the enclosure.