Amp for Sennheiser HD800 headphone
Reply #16 – 2013-11-15 04:21:17
Back to the topic, what's the typical "output impedence" of a mid fi level home audio reciever, such as a denon, marantz, or premier? You'd have to look up the specs or measure it, but usually in the hundreds! Yamaha for example specifies many of their headphone outputs with 470 ohm. Your guess with internal resistors connecting the headphone jack to the speaker taps internally is right and still a common practice today.How about a typical car audio amplifier that's designed to push full range speakers into 4 ohm stereo or subwoofers at 4 ohms mono? Are those sorts of amplifiers also designed with very low (< 1 ohm) output impedences? Yes, usually very close to zero ohm. This is true for most receiver's speaker taps, solid state power amps.Are headphone jacks typically the only "amps" with high output impedence There are some power amps, especially tube ones, that may have an output impedance up to a few ohms. This causes some peaks in the speakers frequency response, bass boost, flabby bass etc. which audiophiles that are into tube amps praise as <insert flowery BS here>. Powering a headphone directly with an opamp can lead to instability. The easy (but imo wrong) way to fix this is by adding output resistors that raise the output impedance. Such resistors are also used to limit output current. Another use is to limit the output level depending on the load: a high impedance headphone may get 50% of the voltage, but a low impedance IEM that is highly sensitive may only get 10%. So when you switch from headphone to IEM your IEM won't blow up. That's why you see high output impedance with headphone jacks. The standard really should be 0 though...