Sunday, 14 August 2016

Dummy Load

All of the work so far in the design of my amplifier has been away from the workbench... all the reading and increasing knowledge, and all the circuit design work, take place at the computer or elsewhere.

Currently I have no parts at all for the amplifier yet. The mains transformer is being made, and when that arrives, it'll be the first actual component of the amplifier to arrive through the door. So I am a long way from actually starting to build.

Apart from designing the circuit, the next challenge I face before I can start building is that I currently have no idea how I am going to find a chassis. When I do, I need to then work out the physical layout of the components on the chassis, draw that out (so adding a new skill... CAD drawing, which I have never done)... then send it off to the sheet metal workshop to get all the holes cut.

Then I've *somehow* got to make that chassis pretty enough to withstand being on display and (dare I flatter myself) even admired, in the pleasant environment of the living room. No idea how I'm going to do that yet.

So in summary, lots of not much going on at the moment. A test of the patience!

In order to satisfy my action bias in an area that has little scope for disaster, I decided to make up a simple dummy load, since this will be needed when testing the amplifier. Essentially this is like a large box of hammers and about as dumb... what could be simpler than a bunch of wirewound resistors in parallel, that has to dissipate a bit of heat.

This will be an inductive load, so like a real-world loudspeaker, in other words...– if an amplifier can't be stable powering a slightly inductive dummy load then it won't be stable powering a loudspeaker

This one's made up of 10 x 47ohm 10w resistors in parallel. It measures at 4.5ohm. Due to its common-ground construction, it can be used as a 2 channel 4 ohm load or a single-channel 8 ohm load.

To test it I connected a 12 volt SLA battery and observed the (lack of) fireworks. The resistors got warm as expected but no drama.

12 volts 4.5ohms equals 32 watts of heat being disspiated.

then just to give it a bit more to think about I connected another 12V battery in series with the first. Now we have 24v going into 4.5ohm – double the voltage and hence double the current – so 4 times the power. Giving a total of 128 watts. Now the resistors got too hot to touch and a few small wisps of smoke appeared as bits of stray dust burned off, but the smoke cleared and the load appeared to be OK, after three minutes I removed the power and re-assembled the case as the resistors cooled down.

128 watts is more than I would ever anticipate putting into the load from my amplifier, so I feel this load will probably serve the purpose, unless the inductance becomes a problem, in which case a stronger single resistor will be called for.

The dummy load, minus lid:


Not gonna cut it if I start designing kilowatt amplifiers, but I don't think that will happen any time soon. The lid of the case is vented and I could add a cooling fan if needed, or else re-design to mount all the resistors on a heatsink. However, this should be adequate for the 50w or so it's going to be called on to disspiate.

Now I don't have anything much else I can build until the amp itself. :(

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