I have made this simple class A amplifier with bits and pieces lying around and it works great. I used BC546 instead of 2n3904 and BD139 instead of MJE430.
My question is what bias current should I use? I set it to 1.2A now and it stays perfectly stable with good sound. However I'm not sure it is the correct idle curren.
My question is what bias current should I use? I set it to 1.2A now and it stays perfectly stable with good sound. However I'm not sure it is the correct idle curren.
I am amazed that the current is stable considering the absence of significant feedback.
I would have expected a bit of thermal runaway from the lower darlington pair and some distortion from the 2N3904 stage.
Where did you find the schematic?
I would have expected a bit of thermal runaway from the lower darlington pair and some distortion from the 2N3904 stage.
Where did you find the schematic?
I believe this is a Red Circuits design which has been asked about a couple of times recently. He must be using a rather large heatsink with considerable thermal inertia for the bias to remain stable, otherwise theres no doubt it would runaway.
To get full voltage swing from the output point of view you need around 2 amps, however, I believe the voltage stage will clip before the output. The only sure way to know is with an Oscope but I would not go much more than 1.5 amps and keep a close watch since the thermal stability versus bias is pretty shady.
To get full voltage swing from the output point of view you need around 2 amps, however, I believe the voltage stage will clip before the output. The only sure way to know is with an Oscope but I would not go much more than 1.5 amps and keep a close watch since the thermal stability versus bias is pretty shady.
wont the largish value pot in series with the base in the bottom CCS transistors help stabilize a little? As the current increases, the base current does too, and the voltage over the pot as well, giving some drift compensation. So it is not totally without feedback. Besides, the input transistor will give a lower and lower voltage at it's collector output the warmer it gets. Perhaps a combination between base resitance for the CCS and the resistor values in the VAS will give a pretty stable ckt?
regarding idle current. If we say you have 34V rail, you can at best swing +-17V out. Give some room to keep transistors from satuation and some for asymmetry, so say you will swing +-15V at max. If you have an 8ohm speaker, and it's a nice easy one without scary complicated crossover etc, then 2A should do fine. A compromise between heat, runaway, and target output.
regarding idle current. If we say you have 34V rail, you can at best swing +-17V out. Give some room to keep transistors from satuation and some for asymmetry, so say you will swing +-15V at max. If you have an 8ohm speaker, and it's a nice easy one without scary complicated crossover etc, then 2A should do fine. A compromise between heat, runaway, and target output.
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