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Topic: Is a headphone amp needed? (Read 44135 times) previous topic - next topic
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Is a headphone amp needed?

Reply #75 –
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The current provided by a typical portable audio player is usually less than 15ma per channel, which can lead to notable distortion with low impedance headphones that require more current draw


My typical audio player(Zen Extra) as a real-world example, has zero trouble providing 27mA per channel of continous current, when driving 32 0hms at it's full scale approx. 0.86VRMS. Distortions as measured under this condition are below any known audible thresholds.

I refer to the overall 'vast majority', regardless of price, because there are few exceptions that are both low impedance and low sensitivity. Examples of low impedance headphones that are not cheap, that operate perfectly on the output voltage/current of a typical portable audio device on most musical tracks: Sony MDR-R10, Sony MDR-CD3000, Grado RS-1, Sony MDR-010, Audio Technica ATH-900. In the case of very dynamic music, as my prior example, some may not have sufficient volume level, rendering the music too quiet. But no clipping should occur in these closed systems, because the gain should be limited to under the voltage driving ability of the internal amplifier when at 0dBFs.
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The Zen Extra is a more powerful portable player than most, so while it is a real-world example it's not a "common" one (though it deserves to be, marketing can kiss my...). The iPod is specifically to what I was referring, with its famously weak output. Sorry for the lack of clarity.

Regarding the headphones mentioned, I'm more bothered by lack of dynamic headroom than I am by clipping (though both annoy me greatly and I'll take my tea without them, thank you), but you definitely have a point.

Is a headphone amp needed?

Reply #76 –
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Further, the capacitors are there primarily for dynamic headroom; they supply current for particularly taxing excursions where an op-amp's steady output would be insufficient. That's the "real purpose" of coupled output capacitors,


No one is claiming that capacitors should not be used in the power supply. But I am not sure what you are referring to as coupled output capacitors, because the 2200 uF caps in the amp you suggested, for example, should be power supply capacitors, not output coupling capacitors. I suspect you made a typo here.

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nothing nefarious there (although I agree that 2200microfarad is rather in excess of what is necessary, especially with a low voltage op-amp, but it extends the flat response considerably beyond the audible spectrum and into sub/hypersonics).


I am not sure what you mean by this. Back to the typo, I think. Most portable amplifiers do not use coupling capacitors on the outputs. Many portable audio players do use output coupling capacitors, however, that may cause very low impedance(<30 ohms) cans to have a slight audibly attenuated low frequency response. Output coupling capacitors server as safety devices, protecting the transducers from DC related problems/amplifier shorts/etc.. The capacitors act as high-pass filters when used in series on an AC signal, such as the configuration of output coupling capacitors.

-Chris
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Let's clear something up here, since neither of you seems to have adequately described the purpose of an output capacitor. Nearly all amps nowadays use split-rail power supplies and do not need output capacitors. Single-rail amps do need them but it has nothing to do with being "safety devices" or providing "dynamic headroom". They're an essential component without which the amp couldn't function. The output of a single-rail amp has a steady DC component of half the supply voltage on it and the output capacitor is simply a DC-blocking component. That capacitor also rolls off the lower frequency response and hence is undesirable (that's why split-rail supplies are more often used) but it can be mitigated by using a large value. The lower -3dB point can be calculated, knowing the impedance of the load.

It's worth pointing out that electolytic capacitors generally have a tolerance of -50/+100% of marked value, so be very pessimistic when deciding a value!