Originally Posted by Baxide
With an average closed loop gain of 30 for a 100W amp I would be expecting to see a massive amount of signal clipping at anything above 4Vrms from a preamp. So 10Vrms won't be doing much good to the power amp output stage.
Preamps require a certain amount of impedance matching with a power amp. To suggest otherwise is fallacy. A low output impedance from a headphone amp might be useful for headphones, but not for a pre/poweramp connection link. You are not driving a difficult power amp input after all.
Sure there are opamps with 600 Ohms output impedance. The NE5532/5534 were the first to be manufactured and widely used. But that doesn't make it a good output stage for headphones. I can't think of a single opamp offhand that has a 600 Ohms output impedance and a high enough current output to drive a set of decent headphones.
If you are saying that a gain of 20 or 30 dB is excessively high for a pre-amp, or not necessary, then I agree. The pre-amp may clip at an output of 10 Vrms, but obviously you just turn the pre-amp volume down to a reasonable listening level.
I agree, the average power amp needs on the order of 1-3 Vrms at the input to drive it into clipping. Anything else from a pre-amp is excessive. Call it pre-amp headroom, if you like.
I don't see any reason why an output impedance of (for example) 1 Ohm for a pre-amp driving a power amp would be a problem. Sure pre-amps often have an output impedance of approx. 100 Ohms (usually some insurance to keep the pre-amp stable when driving very capacitive interconnects), but I can't think of any technical reason why you would not want to drive a power amp with an input impedance of 50 kOhms with a pre-amp having an output impedance of 1 Ohm.
There are a lot of headphone amp manufacturers who use an Op Amp like the NE5534 to drive the headphones:
iBasso, etc, etc.
The NE5534 does not have an output impedance of 600 Ohms. It's less than 1 Ohm Closed Loop; 0.3 Ohms at a gain of 30 dB
It can output up to 38 mA.
25 mA into a 32 Ohm load is 20 mW, more than enough to drive a Grado SR-225i to 110 dB SPL.
Edited by Chris J - 7/9/13 at 3:08pm