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I've built a very simple VCO based on a UJT transistor.

enter image description here

Source: Sonelec-Musique.com.

It outputs on a 8-ohm speaker but sound is like really, really low. So, I'm looking for an amplifier to build, something simple to amplify the sound. I've tried a few circuits from the web but whether it doesn't sound or gain is negligible.

I've been told it should be doable with 2N2222 transistor or 2N3904 which would be great since I have those. I also have a couple of sound transformers which might be useful. I don't really know, I'm quite a beginner in electronics.

How can I build a simple amplifier from a 2N2222 or 2N3904 transistor to amplify a signal from a VCO? How could I improve this circuit?

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    \$\begingroup\$ Simplest? Use Op - Amps then... very cheap, greater and more flexible performance \$\endgroup\$ – Deep Oct 4 '17 at 18:21
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    \$\begingroup\$ I'm very confused as why one would ever choose to use a unijunction transistor as an audio amplifier? Simply pick a less obscure circuit, and you'd be done. \$\endgroup\$ – Marcus Müller Oct 4 '17 at 18:26
  • \$\begingroup\$ Probably limited by 9V battery.. ah the classic theramin \$\endgroup\$ – Sunnyskyguy EE75 Oct 4 '17 at 18:30
  • \$\begingroup\$ You could try to do a little something by replacing your speaker with a resistor (try something above \$33\:\Omega\$ and perhaps less than \$220\:\Omega\$) that is wired across the base-emitter junction of a 2N2222A, PN2222A, or 2N3904 BJT. The emitter end should be the side tied to the (-) rail. The base side to the Q1 pin that currently connects to the speaker you are removing. With that done, place the speaker between the BJT collector and the (+) rail. Still not good, though. But more audible, probably. \$\endgroup\$ – jonk Oct 4 '17 at 19:15
  • \$\begingroup\$ It's not a VCO, it's an oscillator. \$\endgroup\$ – Andy aka Oct 4 '17 at 19:54
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It can be that your signal is too weak. You will need to build two stage amp. One stage amplifies small signal to the level where another stage can amplify more. Take a look here: hackaweek.com simple AB audio amp There is a simple schematic and good explanation of its principles.

Picture below: schematic Good luck.

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Now this is more like a VCO with a very simple one-transistor amplifier: -

enter image description here

It's a VCO because it has an input (Vin).

Picture highly likely to have come from the same French-speaking site as the original.

Try putting your speaker in series with the collector of the NPN transistor and make the emitter resistor fixed at about 1 kohm or greater. It'll be a little louder.

If not loud enough then use the ubiquitous LM386 audio amplifier connected to the pot's wiper as shown in the picture above.

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  • \$\begingroup\$ What's the FET doing? Are we missing a dot? \$\endgroup\$ – Transistor Oct 4 '17 at 20:00
  • \$\begingroup\$ @Transistor - it's a unijunction not quite a JFET! It doesn't use a dot! google.com/… \$\endgroup\$ – Andy aka Oct 4 '17 at 20:01
  • \$\begingroup\$ I've never used one. Does juice come of of its emitter to bias Q2? It seems to be pointing the wrong way. I thought we might be missing a connection where Q2's base wire crosses R3. \$\endgroup\$ – Transistor Oct 4 '17 at 20:04
  • \$\begingroup\$ @Transistor I've never used one either but I remember the symbol! \$\endgroup\$ – Andy aka Oct 4 '17 at 20:08
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Replace speaker with this

schematic

simulate this circuit – Schematic created using CircuitLab

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