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Old 23rd April 2008, 12:58 PM   #1
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Default 3 transistor amp

Hey guys, i found this circuit somewhere on the web and decided to build it, so i got all the parts and put it together, however instead of amping my signal like it should. all i get is a loud buzzing through the speaker, kind of like mains hum.

there are a few points of concern but i cant seem to figure it out.
i've attached the circuit diagram for you guys to look at:

1. im using a 9v wall-wart. it emits 9v AC. the circuit has two diodes which im assuming is a bridge rectifier, converting ac to dc. is the fact that im using AC instead of DC going to be a problem?

2. instead of using the prescribed 2n2905A transistor for the PNP, i used a 2n2907 instead, would this be a problem?

3. lastly, just wondering if this circuit has any errors?
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Old 23rd April 2008, 01:02 PM   #2
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Default 3 transistor amp

9 VDC not AC you must have DC for this circuit. The diods are for bias.
As for the circuit it should work.
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Old 23rd April 2008, 01:19 PM   #3
aurche is offline aurche  Bulgaria
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The circuit has no errors, it should work fine, but the fact that you are running it with a wall-wart, which has an AC output is frightening!!! The diods are not a BR they are used for output transistors pre-voltage! So I think that you should get a BR and try again!!!
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Old 23rd April 2008, 03:45 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
it's not just the rectifier that's missing.
I needs a whole AC to DC PSU.
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regards Andrew T.
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Old 23rd April 2008, 04:21 PM   #5
juma is offline juma  Germany
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jjac7212 my friend, this is what you need (at least) :

Connect U1 point to +9 point on your schematic.
Also, connect the grounds from both schematics together.
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Old 23rd April 2008, 04:54 PM   #6
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Looks like the end result was a real hum-dinger.
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Old 23rd April 2008, 10:13 PM   #7
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wow, thanks guys!
i used a DC wall-wart and it works fine,
however it distorts like crazy which leads me to believe the PNP needed to be that 2905A, i might need to replace some resistors too.

thanks heaps for helping out!!
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Old 24th April 2008, 12:40 AM   #8
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The good old 3 transistor complementary amp! You can make one work well with enough tweaks.

Here's some tips for a newbie to get one working.

The Hardest part IMO of making an amp is bias, and hot transistors blowing up from first making it can be frustrating. I'll offer some help to make it easier, because it took me a long time to learn it by myself.

I'd recommend using big power amp transistors and a heatsink if you can get them. They are harder to blow up then little ones.

1. Put a variable resistor (POT) connected across the diodes where they connect to the base of each transistor. Set it to 0 ohms for no bias to start with.

2. The output where the 3.3 ohm resistors connect HAS to measure exactly HALF of the total DC voltage, each transistor splitting it evenly. This is the key to making the amp work good. Measure this with a good meter to find out.

3. Try another POT +1K in series in place of the 5.6K resistor and adjust the resistance until the output voltage of amp is in the center. Then replace with resistor that matches the resistance.

4. Now adjust the bias POT across the diodes and slowly adjust until output stage pull no more than 50mA-75mA or 15-23mV across 3.3ohm resistors. If that's too hard, then just adjust until transistors get a little warm but not hot.

5. Add a 100-200pf cap across the amp's input and ground, and add another one to the base-collector leads of the input transistor. I'd also recommend adding a 0.1uf cap in series with a 10 ohm resistor from the output to ground.
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Old 24th April 2008, 06:29 PM   #9
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4.The idle current shouldn't be more than around 5-10mA.This is in effect a small amplifier as used in 9V transistor radios.
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