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I try to make a little project for school.I used the following schema to create an audio amplifier.

For the circuit I used:

C1:104j63 capacitor (1uF)

C2:103j (0,1uF)

My problem is that the sound is lower than direct input.

I changed both capacitors with a 104 and the sound was same as the input but more distorsioned.

PS:The transistor is connected correctly.

enter image description here

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  • \$\begingroup\$ Where did you get that circuit? \$\endgroup\$ Commented Dec 23, 2015 at 18:48
  • \$\begingroup\$ What is the purpose of R1? \$\endgroup\$ Commented Dec 23, 2015 at 18:48
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    \$\begingroup\$ What is your load? An 8-ohm speaker? Or a 10Mohm oscilloscope? If it's a speaker, the 10k R3 is your culprit. \$\endgroup\$ Commented Dec 23, 2015 at 18:49
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    \$\begingroup\$ I remember this schematic.I posted it in a question and also built it physically.Note that the transistor will be very weak and useless,much as Matt Young states.I had to put my ear tightly on the speaker to hear the outputed sound.Why all this?Because this is a PREAMP not an amplifier.It is supposed to boost very low signals so they can drive an amplifier stage.You might want to modify it if you want it to work. \$\endgroup\$ Commented Dec 23, 2015 at 19:02
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    \$\begingroup\$ also, with no emitter resistor, i am not sure how the transistor will bias. it could be saturated if the \$\beta\$ is high. or it could be in cutoff if the \$\beta\$ is low. \$\endgroup\$ Commented Dec 23, 2015 at 19:33

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Simulation shows that this will work just fine - as long as you use it as a preamp, and do not try to drive a speaker. Driving a 10k load it will produce about a gain of 60. Driving an 8-ohm speaker it will have a gain of about 1/8.

So don't try to drive a speaker with it, OK?

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  • \$\begingroup\$ for what range of transistor \$\beta\$ does this simulation work? lacking an emitter resistor (it could be bypassed with an electrolytic cap if you don't want it to kill the amplifier gain), i am not sure how you can be sure it biases up correctly. \$\endgroup\$ Commented Dec 23, 2015 at 23:53
  • \$\begingroup\$ and the OP can put in another transistor (or two as darlingtons) in the back end in voltage-follower mode, if they want to drive a speaker. \$\endgroup\$ Commented Dec 23, 2015 at 23:56
  • \$\begingroup\$ @robertbristow-Johnson - I use TINA. The 2N3904 model has a default beta of 679, and results varied a few percent for a range of 100 to 1000. \$\endgroup\$ Commented Dec 23, 2015 at 23:57
  • \$\begingroup\$ a \$ 100 \le \beta \le 1000 \$ and no emitter resistor? i use pen and paper and i will just go check on where that Q point ends up. \$\endgroup\$ Commented Dec 24, 2015 at 0:00
  • \$\begingroup\$ @robertbristow-johnson - Keep in mind that I'm assuming a 1 mV input. \$\endgroup\$ Commented Dec 24, 2015 at 0:12

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