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schematic

simulate this circuit – Schematic created using CircuitLab

Hello!

I have constructed my very first amplifier with 2n3904 transistor, based on the diagram and suggestions from comments from here: Class A single transistor amplifier with 2N3904.

I know that I'm working close to it's thermal limits, but that's not the problem. You see, when using it everything sounds like it is a earrape remix of the thing i'm playing. Also, RL (47ohm) is getting very hot, almost to the point of it getting dark, while the transistor is only a little bit warm.

At first I constructed it with R1=2.2k, R2=1k, RL=22ohm and the same caps and potentionometer, sound was louder, as expected, but RL(22ohm) started smoking and darkened.

After increasing the resistances to those stated on the schematic the bass problem started. How can I get rid of it?

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    \$\begingroup\$ According to your schematic, C2 is shorted out. \$\endgroup\$
    – Barry
    Commented Apr 21, 2018 at 12:43
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    \$\begingroup\$ missing an emitter resistor... \$\endgroup\$
    – BeB00
    Commented Apr 21, 2018 at 12:43
  • \$\begingroup\$ Barry - The C2 is chorted because there is an potentiometer set as a resistor where the wire is, but its set to 0 ohms, so I just skipped it. \$\endgroup\$
    – thegamebusterpl
    Commented Apr 21, 2018 at 12:45
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    \$\begingroup\$ You're also running a DC bias through your speaker. It will be permanently biased away from its neutral, at-rest, position and so will distort at much lower volume levels than normal as it can travel less in one direction before reaching the limit of travel. If you have a pot in there then it should be shown in the schematic. You can set the component designation to 0 if that's the way you're running it. \$\endgroup\$
    – Transistor
    Commented Apr 21, 2018 at 13:05
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    \$\begingroup\$ Maybe put the volume control on the base of the transistor rather than the emitter \$\endgroup\$
    – BeB00
    Commented Apr 21, 2018 at 13:05

2 Answers 2

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Your circuit is missing an emitter resistor. You need the resistor to set the bias voltage and also reduce the low frequency gain. It's also the reason that your RL is getting so hot. In your setup, the transistor is fully on almost all the time, so it wont dissipate much heat, but it will be letting a lot of current pass, which will ignite your RL.

enter image description here

enter image description here

Input - Left output - Right output

Link to simulation

Notice that theres still some distortion in the output of the left one, although its not nearly as bad

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    \$\begingroup\$ I see, i'll try to replace the pot with 22ohm and i'll update on hat happened! \$\endgroup\$
    – thegamebusterpl
    Commented Apr 21, 2018 at 13:04
  • \$\begingroup\$ Ok, no bass boost now, 47ohm still heat up and the volume stays the same as input. Also there is a high pitched noice and no of sound, curiousle, the earphones are heating up. \$\endgroup\$
    – thegamebusterpl
    Commented Apr 21, 2018 at 13:27
  • \$\begingroup\$ So i saw that the circut you simulated is quite diffrent from mine (exept the emmiter, which was meant to be changed), ther is the fixed one: tinyurl.com/y8a58umg \$\endgroup\$
    – thegamebusterpl
    Commented Apr 21, 2018 at 13:31
  • \$\begingroup\$ If you look at the currents here, you'll see that the input is 100uA peak to peak. To bring your output to half the supply, your poor transistor would have to pass 100ma, so a 1000x current gain. This is too much to expect for your single transistor. try a darlington cofiguration \$\endgroup\$
    – BeB00
    Commented Apr 21, 2018 at 13:37
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    \$\begingroup\$ You still need a (large) capacitor in series with the speaker. \$\endgroup\$
    – JRE
    Commented Apr 21, 2018 at 13:37
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well, there is excessive current going on. first, you need to put a electrolytic cap in series between the collector speaker. something like a 2200uf @12V. This will get rid of the dc current going through your speaker. also, just like what is mentioned above, there is no emitter resistor, however the proper bias ratio would be somewhere 10:1,so if the collector's bias current should be 10 times greater than the emitter's bias current, you would need a 22 ohm resistor on the lower resistor with the 220K on the voltage divider with a grounded emitter and a collector resistance of 200. Now this is not including ac loading effects that going to occur with the speaker/headphones. But lets use this simulator with the total circuit in mind (with the speaker circuit)

I do want to keep you in mind that an emitter resistor help prevent thermal runaway when intrinsic resistance of the base-emitter junction changes with heat causing the current to go up in the collector circuit due to the base circuit's forward biasing more.

I've built a simulation here, a screenshot of which is shown below.

Screenshot of Simulation

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  • \$\begingroup\$ I added which is hard to see, a .22 ohm resistor instead of the wire to stabilize base current in real world application \$\endgroup\$
    – drtechno
    Commented Apr 21, 2018 at 21:06

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