[17 August 2020] I assembled one of the other amplifiers. I think there are four in total; the first one is project 2 (seen on the previous page), but there are three others in the dedicated Amplifiers section (61, 62, 63). 2 was quite complex compared to 61 as it includes the switch and Control knob/variable resistor, whereas 61 is on as soon as you apply power.

Circuit diagram for project 61.

Both use the 9V battery. I tested 2's power prior to disassembly and setting up 61 and found it to have a peak output voltage of 4V with the Control turned up to 9 (you couldn't turn it beyond that without the sound breaking up). I was able to measure this with both my dedicated circuit tester and the included Meter. 61 instead could only be tested with my circuit tester which reported 1.5V output.

• The variable resistor (Control) in the first amplifier is labelled as a 50K resistor and it connects via three points. I'm wondering how I can connect this in a similar fashion to project 61 to give me some control. (I have looked ahead to the next two amplifier projects and see 63 has the Control resistor across the microphone.

• Strangely, in these diagrams, the input, labelled IPT looks like the earpiece used for output, and the output, labelled OPT looks like a loudspeaker which is a loudspeaker but is used for input! That had me confused for a while and thinking the diagrams were labelled wrong.

Here are the next two projects in the Amplifier series:

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