The Simple Design Flaw That Could Blow Your Guitar Amp

Amped blogger Neville Ward on why some amps don't make the grade   02-Mar-14

You can make a medium sized amplifier out of a huge amplifier by turning the volume down. If you set the volume to give the same nominal output as a medium sized amplifier and compare them using the same loudspeaker(s), they will sound the same at low volume but different as the medium sized amplifier approaches its maximum output. This is because the medium sized amplifier's protection circuits are operating, while the huge amplifier's are not.

It is not a big difference in sound, but when everything is running at close to 100% (that's all the time, right?) some amplifiers just don't seem to have the "guts" that others do. In fact, it may not be noticeable at all, until one day the drummer re-tunes his kick drum to the exact frequency that the speakers present their worst load and suddenly there doesn't seem to be as much "kick" as their used to be. Either that or the amplifier blows up, leaving everyone wondering what the hell happened.

It's not just P.A. amps; guitar amps have protection too, and although most guitar speakers have their problem frequencies below bottom E, it can certainly cause problems with bass amps. Fancy changing to a 4 X 10 cab? Try it first, then try playing the same song in a different key, then try it with a different amplifier.

Improving a protection circuit isn't very difficult, but it does involve quite a bit of simple maths. Sometimes a number of other components in the amplifier will also need to change, but quite often a huge improvement in the safety margin can be made by adding two tiny, cheap components, and the protection can be speeded up by changing two others. When I have made improvements like these, the amplifiers never come back to me, meaning that they never fail again.

The gist of it is that the protection circuit has to measure the current through the transistors, which most circuits do, and also the voltage across them at the same time. And in particular, the protection has to continue to operate even when the output voltage is positive but the output current is negative, and vice versa, which is where many of them fall down.

I even tried writing to a large amplifier manufacturer, explaining how inadequate their designs were, showing the maths to prove it and describing the necessary improvements. Then I asked them to give me a job. Strangely I never heard back.

To contact Neville about your repair job, check the Zenworks repairs Facebook page by clicking here.