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Old 15th Oct 2017, 8:58 pm   #185
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 3,812
Default Re: Audiophoolery. 'Cable Break In' - I never knew that!

Good audio-quality op-amps show an increase in distortion when asked to drive low impedance loads. As a rule of thumb, 2k is a good minimum load impedance (including the negative feedback resistors, of course). Some, like the good old NE5532, are good to perhaps 600-ish ohms, perhaps.

Meanwhile, the output impedance of a "hi-fi" CD player - set by an output resistor - will be anywhere between 50 and 500 ohms, give or take. Just depends on the whim of the designer - there's no formal standards for domestic hi-fi.

So slapping a 30 ohm load at the output of a hi-fi CD player will not only reduce the output voltage (by an amount that depends on Zout, and so will vary from machine to machine), but will increase harmonic distortion. So, a bad idea every way you look at it (if you actively want less level, use a potential divider with higher value resistors that add up to 2-4k). Matched impedance audio circuits went out decades ago. Just try finding 600 ohms in a modern professional installation - if you do, it'll be for legacy gear like vintage valve preamps or similar...

The output stage of a portable device is a bit different, however. The op-amps in these, working from very low supply rails, will be designed to drive 32 ohm headphones (the actual impedance will obviously vary with frequency, so 32 ohms can only be very nominal, just like it is with loudspeakers). These ICs tend to have pretty high distortion values compared to the usual audio op-amps, but for the application, they're usually good enough.

Some output stages can be upset by not seeing a low-ish load. It's hard to investigate exactly what's going on because such devices tend to be hard to take apart, and once you have, you can't see any of the components in there anyway! Perhaps they need a DC path to charge the output capacitors - who knows? Either way, I've found that a load of a k or 2 is enough - 30 ohms would just take current from the battery needlessly.

Needless to say, there are plenty of other areas where matched impedance is useful/essential. But not audio. Ever seen a power amp with 8 ohms Zout?
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