How to test an amplifier without speakers

May 01, 2020

This is a very simple project that takes only a few hours to make. It is a pair of 8 ohm resistive loads which can be used to test a mono or stereo power amplifier. If you have ever wondered how to test an amplifier without speakers, you need something like this to do it.

If you are working on an audio amp, you often need to see how it will behave when it is delivering power to a load. Using real loudspeakers is not practical, especially if you want to test at high levels and with sine waves. The noise would be unbearable, also you might damage a speaker. So you need a load to use instead of a loudspeaker.

8 ohm dummy load

This resistive load is made from six 50 ohm resistors in parallel. That gives a resistance of 50/6 = 8.3 ohms, which is close to the nominal 8 of most loudspeakers. I used an old oven tray to mount them - you could use any large piece of metal. The resistors should not be mounted too close together, to help them dissipate heat. They are rated at 60W each when on a heatsink, but here they will be capable of handling a bit less than that because this flat metal backing is not as good as a large heatsink with fins. But even if we guess at 100W per side, that is enough for testing small to medium size amps. The sensible thing is to keep an eye on the temperature in use.

Use heatsink grease to thermally bond the resistors to the mounting plate, and 4mm posts for termination points. Heavy tinned copper wire or wire reclaimed from solid core mains cable is OK to wire it up.

As well as amp testing, the load can be handy for checking power supplies. You can use the two halves in series or parallel, 16, 8 and 4 ohms are possible.

Here is the load being used to stress test a KEF PSW3500 active sub woofer. The amplifier output connects to the two loads in parallel, forming a 4 ohm load. The PSW3500 is driven by sine wave of around 50Hz, and a multimeter and a scope monitor the output. The multimeter says that the output level is about 15VAC RMS, a continuous output power of about 55W. This would be unbearably loud with a real speaker.

After about half an hour, the front panel of the KEF was almost too hot to touch. As I was not trying to “destruction test” the amp, I decided to stop there. (I only wanted to test that it would handle being subjected to sustained continuous use.)

(We say that this load simulates a loudspeaker, but that is not completely true. A loudspeaker is a complex reactive loud that is considerably more complex than a pure resistance like this. But this is good enough for most purposes.)

If you use this for heavy testing of an amplifier, be very careful where you place it. It can get pretty hot if you use it on a big power, so do not leave it unattended.