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47 Labs 0347 "Treasure" stereo amplifier

Posted 15th December 2011 at 11:33 PM by rjm
Updated 17th December 2011 at 01:33 AM by rjm

This is interesting.

A stereo amplifier.

Yes. Well, think of it as a bulked-up headphone amplifier. The design is one of those "ah, I should have thought of that" moments, but the details are quite tricky, as usual.

If the extend the basic "voltage stage + current buffer" topology of the 47 Labs 0147 headphone amplifier out to 40 wpc territory, you end up with the 0347.

The high power means than unlike the headphone amp design you have to run it out of class A, and that means closed loop operation.
While most people might be tempted to wrap the voltage feedback around the whole shebang, this basically ends you up with a big op amp. Op amps, with their massive negative feedback design, are not so happy driving high inductance/capacitance loads like speakers.

The trick 47 labs uses is to have two feedback pathways, with heavy "local" feedback around the op amp and a small secondary global loop from the buffer output.

The op amp is a simple discrete circuit, so using higher voltage transistors its quite simple to run it at higher voltages needed to obtain sufficient voltage swing to give 30 W into 8 ohms (rail-to- rail is probably about 50 V)

Damn, more grey hammertone. Must ... resist...

Update: Resistance was indeed futile. I'm now on the waiting list for one each of the 0347 and 0247.
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