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Cheap op amps made everything in electronics work better. In the tube era, an op-amp was a huge device. It's also hard to make a DC amplifier with tubes and have the output voltage be near the input voltage, hard to get enough gain, and hard to keep the zero point from drifting. Tube op amps were used only for analog computers, including ones for gun aiming.[1]

Negative feedback was rare in the tube era, because gain was so expensive. An op amp with negative feedback throws away gain to get linearity. In the tube era, that was wasteful. In the IC era, having a few stages of amplification to get some insane but nonlinear gain, then throwing it away to get linearity, became a win.

Once op amps became cheap, they could be used routinely whenever you needed some gain. So audio gear filled up with them. A negative feedback amplifier behaves like an ideal linear amplifier up to a frequency limit determined by the slew rate of the device. But for most op amps, that limit is up in the megahertz range, so there's no problem using them for audio. (There is an audiophile cult which denies this, of course.)

Op amps with differential inputs can also be used as comparators; the output goes to one extreme or the other depending on which signal is stronger. Comparator ICs are op amps with a logic gate stage at the end. That's how differential signals such as twisted-pair Ethernet get turned into logic levels for processing. Any interference which hits both wires is subtracted out. This is why we can pipe gigabit Ethernet around over rather ordinary wire, rather than coax.

[1] http://www.analog.com/library/analogDialogue/archives/39-05/...




Thanks for this comment! The part on Ethernet in particular made me stop and think.

> But for most op amps, that limit is up in the megahertz range, so there's no problem using them for audio. (There is an audiophile cult which denies this, of course.)

As a hobbyist who does a lot of tube audio, I'm happy and sad. Not because you're right---you're definitely right---but because I know why you bother to mention it.

I love the history and the hands on aspect of the big, clunky guitar and kit amplifier circuits of the 50s and 60s. On my time, point-to-point is so much more fun than checking IC pads under magnification. But I often feel left out. The audiophile tube babble makes me queasy, but at the same time, it's all buggy whips to any real pro.

Thanks again.




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