[RMSzero]

I stumbled across this design for a very-low voltage headphone driver in a datasheet for a dual-channel opamp. I spent some time investigating how it worked and I had some problems with the design. I found one resistor that should be removed and two R values that I would change (no, not to increase the gain

).

Anyone else think this design has a flaw, or am I not understanding it correctly?

 

[tangent]

Strange little beast, isn't it? I examined it myself a few days ago, while reviewing the AD823, and decided I wasn't interested enough to build and debug one. Me, I'd rather just plop the AD823 into a CMoy type amp and be done with it.

I don't fear dual supplies like some people apparently do, and that's all this design seems to be trying to accomplish. I figure, if you can't afford the extra space to put in a proper voltage splitting circuit, it's either got to be for a mid-fi bit of portable electronics, or a toy for some hardware hacker. I'd like to shoot a little higher, myself....

By the way, I recall from the AD823 datasheet that at minimum supply voltage, the noise floor comes up to about -60 dB. Amplified to normal listening levels, that's probably quite audible.

 

[kevin gilmore]

There is one extra 95.3k resistor. That is all i see that
is wrong. Other than capacitors in the signal path and
a single power supply...

 

[aos]

I use single-supply 5V powered amp in my portable DAC, and achieve excellent results with multilooping an opamp like AD823 or LT (1357 etc.) or OPA (2227/8) with buffer CLC5602 which can supply over 100mA of current and have reasonable voltage swing (3V peak to peak at 75 Ohm or 2V at 25 Ohm). Next version I'm working on will have DC coupling (one needs to buffer the virtual ground).

The amp above has too many coupling caps as Kevin noticed. If external amp is being built, there should be enough space for voltage splitter, which would eliminated caps. If the amp is internal, then it's likely that the voltages will already be referenced to virtual ground, therefore eliminating all but the end capacitor (and even that one if the virtual ground is buffered). At any rate, I wouldn't build the amp above as a standalone amp.

 

[ppl]

That gif file is a copy of a section of the AD-823's data sheet. It is a sugested application for (2) cell 3 volt Headphone amp from one of Analog devices App Eng.

 

[paulb]

Keep in mind the intended context of this circuit. It is for applications like a high quality Walkman. It probably provides better performance than the standard sludgey circuits.
Agreed us DIY types can do much better than this. But for quantities of hundreds of thousands, where every resistor counts, it might provide a pretty good compromise between fidelity and cost.
I'm using an SSM2135, a similarly-intended device, in a Cmoy-type arrangement with active supply splitter and am very happy with it.