Dummy load , high power audio, add ons ?

I also have a box I made which has a stereo input, switchable to paralleled-channel mono, with two switchable and variable basic ranges of zero to low, for low level instrument simulation such as guitar pickup, and zero to high for line level tests. It also contains two 600R line transformers to produce 'balanced' outputs for testing amps with a balanced XLR type input. I have a special Speakon adaptor which has a built in standard 1/4" jack so that the test speaker can have a normal jack plug on it, and if I get an amp for repair that has Speakon outputs, as many of the higher power heads have, it's just a case of plug in the adaptor first.

I also have a little box that I made that has a 'normal' edgewise meter, and a centre zero edgewise meter and a 3 pin plug for setting and checking the bias balance on Fender Twins.

My load tester has a built in very heavily attenuated 4" 'squeaker' with volume control, for monitoring intermittent faults, and jack-switched 'through' output for a genuine speaker. It also has a crude power meter based on measuring the voltage across the load. This stays in circuit if the through socket is used to connect a real speaker. There is a BNC connector to hook directly to my 'scope.

The loads are force-air cooled, and relay switchable for 4,8 and 16 ohms. When the tester is switched off, the relays drop out, and it defaults back to 16 ohm load, so you don't accidentally overload the next one on the bench. I also put in a double bar-graph display, monitoring output waveform

• / - either side of zero, with a level control. The idea was to be able to immediately see any lack of symmetry in the output waveform, but I've only ever had one instance where it did its job. Still, it looks pretty, pumping away on the front panel, and it soon catches your eye if it stops, and you've got the squeaker turned down ...

Arfa

Mine is made from several lengths of electric fire bar 'tuned' to 8 ohms each bar. there`s a tap off via a three way speaker selector switch well resistored down to provide some monitoring using an external speaker. A couple of capacitors provides DC isolation.

I have one of those Velleman electronic VU meters hooked in which gives me a rough guide to the input power - the firebars can handle over a kilowatt, tho they do get pretty hot, the elements glow quite brightly at those sort of powers.

The front of the case has Speakon,jacks and binding posts.

Originally, I was going to install some kind of cooling fan driven from DC rectified from the input signal, but never got round to it.

Ron(UK)

You could always buy a cheap fan heater instead, this has a nice 2 or three speed fan built in. (a tenner from Argos) I tried this but was annoyed to find that the fan motor was actually 110v, taking its tap from the centre of the element.

It should be noted though that the element resistance is dependent on its temperature, so don't bother trying to get any accurate measurements from this arrangement.

Gareth.

True, but most of the time a dummy load is just used for soak testing at high powers, and remember a real loudspeakers impedance/resistance also changes appreciably during use.