My Audio Probe / Signal Tracer is based on this circuit. The difference being that I use an a oscilloscope probe as an input and an old Sansui HIFI amplifier (with one channel broken) as an output.
A Variac is special type of transformer that supplies a variable AC voltage. A variac usually has a big control knob which when you turn it gives a voltage between 0 and 240V out. Why would you want that then? Well with valve amps having lethal voltages all over them it is useful to have a way to turn them on slowly for certain types of problems. Actually the same is true for transistor amps too – just to a lesser extent.
The bench variac that I have built is based on a 7 amp Zenith Variac with a steering wheel on it, an analogue ammeter for keeping an eye on current drawn by the amp under test and a digital volt meter for keeping an eye on the voltage and a double pole isolating switch to keep me safe.
I almost always use a variac to power on an amp, here’s why:
- You can prevent damage to an amplifier from turning it on – especially useful on amps with unknown history – electrolytic caps lose their strength over time, it is argued by some that they can be restored to an extent by turning them on slowly.
- When a fuse would normally blow or smoke would come out. A variac enables you to turn on gradually and identify the physical source of the fault.
This little keyboard amp came to me with a fault of not making a useful sound.
Using the effects loop trick I was able to work out that the pre amp was at fault but the power-amp was fine. Since this amp has three input channels it is inlikely that all three pre-amps are going to be duff – another clue. There is a comining op-amp that mixes the three input channels into a single feed into the power amp – surely that has to be the issue. So replaced the TL072 op-amp fixed the issue immediately, cleaned the flux off the PCB and the amp is ready to go.
It’s actually a great little amp. Three channels. Inputs for CD /IPOD using phono sockets, effects loop and a lovely reverb. I used it for a www.lovevultures.co.uk gig playing bass and it was delicious, the big speaker could easily have handled our keyboard player too.