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Nigel, you are too kind. It looks like crap to me. There is no AC or DC feedback (well, a little local feedback in the emitter of the input transistor). As you said, the output stage is a common emitter with a constant current load. What kind of luck would you have to have for the output stage to be biased in the linear range? Even if it were, you would wind up with a very high output impedance - not exactly ideal for driving headphones. Whiz needs to find a better circuit to study. Unfortunately, I can't recommend one. I'll bet Audioguru can.Nigel Goodwin said:That's a tiny class A amplifier, so not really a good subject to try and learn from.
But essentially the first transistor is a boot-strapped common emitter amplifier, and the second is a normal common emitter amplifier. The third transistor (at the top) is a constant current load for the output transistor.
Can't say I think much of it, no negative feedback at all.
Roff said:Nigel, you are too kind. It looks like crap to me.
Yes.whiz115 said:0,47uF is a coupling capacitor for cutting any DC and for not letting any resistive loads interfere with the bias of the Q3 transistors.
No. It doesn't have the transistor's DC base current in it. The 390k negative feedback resistor biases Q3. The 27k resistor helps set the gain of the amplifier.27K is a the resistor to bias Q3
yes, and it also biases Q3.390K is the negative feedback resistor
It isn't a preamp but it is the only stage with voltage gain.Q3 is the pre amplfication stage (voltage amplification)
They turn on the two output transistors slightly to avoid crossover distortion. The diodes voltage changes the same way the transistors voltage changes with temperature.the diodes i don't know exactly what they are doing there
No. The 2.2k resistor is the collector load resistor for Q3. It also supplies base current to Q4.The R2 regulates the emitter/collector voltage
Yes.Q4/Q5 is the final stage... two transistors in push/pull which each one
of them handle half period negative or positive.
It prevents DC in the speaker. Its cutoff frequency (half power) into an 8 ohm speaker is 43Hz. Use 2200uf for a cutoff frequency of 9.1Hz.470uF is an output capacitor to remove DC offset...(but probably destroys the frequency responce?)
audioguru said:The 27k resistor helps set the gain of the amplifier.
audioguru said:No. It doesn't have the transistor's DC base current in it.
audioguru said:It isn't a preamp but it is the only stage with voltage gain.
audioguru said:They turn on the two output transistors slightly to avoid crossover distortion. The diodes voltage changes the same way the transistors voltage changes with temperature.
audioguru said:No. The 2.2k resistor is the collector load resistor for Q3. It also supplies base current to Q4.
If the voltage gain of Q3 was thousands without negative feedback then its gain would be reduced to 390k/27k= 14.4 with negative feedback. The gain of Q3 is much less so the voltage gain will be less than 14.4.whiz115 said:oh! it's gain resistor! it regulates how many mV of AC signal will go to base
right?
The DC base bias current for Q3 is supplied through the 390k resistor, not through the 27k resistor.you mean it doesn't draw any current from the source right?
No, a buffer has no voltage gain.works like a buffer?
Look at crossover distortion in Google. Both push-pull transistors must conduct a little to avoid crossover distortion. The diodes are almost the same as a base-emitter transistor junction so they regulate the small current in both transistors to eliminate crossover distortion.hmm farther explanation? because i don't understand it...
When Q3 turns on more then its collector voltage drops. When Q3 turns on less then its collector voltage rises. The current in the 2.2k resistor moves from the collector of Q3 to the base of Q4.hmm if i understood correctly it is both regulates the collector and also the base at once?!
Don't make it. It is crap.I liked very much this simple discrete amplifier... and i think i'll build it... can i learn specifications?
Most transistor amps have a frequency response to at least 400kHz.freq response/SNR?
I simply replaced the 2.2k resistor with two 1k resistors in series. Then added a 47uF capacitor from the output to drive where they join.about the second schematic with the bootstrap thing... i'm still looking at it! you changed everything...
speakerguy79 said:I think a great learning amp that works as a headphone amp is here:
**broken link removed**
Real common way of doing things from what I understand. Should drive any headphone on the planet. I've got parts for a (modified) version I am working on now.