Hi all,

With the amplifier module modified and working, let’s rebuild experiment 89 on page 195 of the legacy Lectron manual.

This circuit is an audio meter which displays the loudness of audio picked up by the speaker.

But now, with the added gain control connection, the device is so sensitive that even my typing on the computer keyboard is registered and also the bird tweets are shown on the meter !

I’ve modified the original circuit a bit to make it perform better and it can be build with legacy blocks or a mixture of old and new. Both examples are included here and they perform equally.

The advantage of using the legacy Germanium transistor is that the 100K resistor is already built in which saves blocks and precious space.

Just pay attention when putting the earphone in your ear ! Since this circuit has such a large amplifying gain, feedback may occur ! Keep the earphone and the speaker as far as possible away from each other.

This is version 1 with the combination of modern and legacy blocks :

Sound meter V1

And here version 2 with a legacy Germanium transistor :

Sound meter V2

A bit of tech info :

The amplifier module with no attached load has a gain of about 180. This means that the output signal is 180 times stronger then the input signal.

With a 100µF capacitor attached at the gain connection (C1 in our circuit), the gain is about 1200 !

The input of the amp module is low ohmic, which means it has a low input resistance.

The output is high ohmic, so the following stage should pose no load to obtain maximum performance.

This is why we use a transistor stage T1 with a 220 Ohm resistor at the emitter to retain this high resistance and which also sets the amplification rate of T1.

The use of the earphone as an inductive load helps to maximize the performance of T1.

Many greetings