Adventures In Audio
Giant-killing $5 mic preamp - its secrets revealed

David Mellor

David Mellor is CEO and Course Director of Audio Masterclass. David has designed courses in audio education and training since 1986 and is the publisher and principal writer of Adventures In Audio.

Monday January 23, 2006

If you're not up to speed on this topic, you need to read this article, and listen to the audio examples carefully. When you have made your judgment on which is the most expensive preamp, you can see the results here. In summary however, significantly more people judged the $5 preamp to be sonically the most expensive sounding. To be fair, the test was limited, but even so, surely a $1500 preamp should have proven its mettle in comparison? Surely it should have blown away the $5 home-made one?

Since the tests took place, several requests have been received to see the schematic of the $5 preamp. Well it certainly isn't a secret. The preamp is based around a Texas Instruments INA217 integrated circuit, using the schematic provided on the manufacturer's data sheet. You can see the schematic right below - click on the image for a full-size version.

But the $5 preamp doesn't even use the full circuit. R3 and its associated capacitor are intended to reduce any click when phantom power is switched on. The $5 preamp doesn't bother with that. The four diodes you can see are protection for the IC against unexpectedly high input voltages - omitted. A2 is a secondary op-amp intended to eliminate any DC offset in the output. Well Rupert Neve didn't always bias the output of his push-pull stages to zero volts, so why should the $5 preamp do that either? So basically the circuit is what's left. Not much is it? Oh, and the power supplies... the power for the IC was from four PP3 batteries, wired to give +18 and -18 volts, which is the maximum the INA217 is specified for.

Phantom power was supplied from five PP3 batteries wired in series to give 45 volts nominally. One commentator remarked that this "seriously under-powers the Neumann U87 microphone". This is not so. For one thing, new batteries will give higher than their rated voltage. Secondly, a microphone that couldn't operate on 45 volts instead of 48 simply wouldn't be up to the job of meeting real-world conditions. They're not that stupid at Neumann - they rate the U87 for 48 V +/- 4 V, and I suspect there is some margin beyond that.

It might be that it's the battery power supply that gives the $5 preamp the edge over the others. In my earlier tests, I found that I was spending an inordinate amount of effort getting a power supply derived from AC to be quiet enough, so using batteries eliminated that problem and perhaps produced a benefit.

One thing's for sure. The cat's among the pigeons and feathers are flying. Would anyone care to repeat my test?

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