Recording - July 2010 - 64

equipment from the Symetrix SX-202 preamplifier to the Amek Mozart console, and they provided pretty good sound at a pretty low cost. All this was fine, but then the audio market got bigger and bigger and bigger... and then PMI was bought out by Analog Devices, a large manufacturer of high-performance analogue and mixed-signal ICs. Now, when you’re as big as AD and you’re making large quantities of products, it’s not cost-effective to go with semicustom designs since you can roll your own die from the ground up in-house. Not only that, but AD didn’t particularly like having to subcontract out the manufacture of the PMI integrated circuits to one of its competitors’ semicustom division. You can see where this is going. AD redesigned many of the PMI integrated circuits to be manufactured on its production lines. Many of its products were effectively unchanged from the user’s standpoint. Some of them were improved in performance.

Guest editorial by Scott Dorsey

IC Replacement:
Plug-and-Play Audio Improvement
Back in the 1980s, a company called Precision Monolithics, Inc. was in the business of selling specialized integrated circuits to the audio industry. They had a nice variety of chips from transistor arrays, compressors, balanced input and output circuits, and a couple of really nice mic preamp ICs called the SSM2015 and SSM2016. Now, the thing about PMI (no relation to audio gear firm PMI Audio, by the way) was that they were a “fabless manufacturer.” That is, they didn’t actually have their own silicon foundry, but they contracted the designs out for manufacture elsewhere. Since they started out small, and since they were making small production items for an audio market that wasn’t anywhere near the size of the modern audio market, they actually used a semicustom IC. With the semicustom IC, the IC manufacturer has a standard array of components on the monolithic IC substrate, and the user specifies the top-level metallization that goes onto it to connect those components together. So you may have a die with say, twenty transistors and fifty resistors of standard values, and you can interconnect them in any way you like. The folks at PMI took conventional off-the-shelf semicustom dies and they managed to implement some remarkably high-performance circuits on them. The SSM2015 and SSM2016 mic preamps used front ends that consisted of a lot of transistors in parallel for very low noise. They were a bit odd for the equipment designer to use because the pin layout was strange (due to the configuration of the semicustom die), but they turned up in a large number of pieces of
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The SSM2015 and SSM2016 were discontinued in favor of an 8-pin microphone preamp chip that they called the SSM2017. From the beginning, folks weren’t happy. Equipment designers had to redesign their boards to use the new ICs; the Symetrix SX-202 was replaced with the SX-302 which used the new IC. It didn’t sound as good. In fact, a lot of people were dismayed at the sound of the new IC, but if you wanted a one-chip preamp solution there wasn’t much of a choice.

There aren’t very many times when you can just pop a component out of a device, replace it with a new one, and get a sonic improvement....
But then... the people at THAT Semiconductor came out with the THAT 1510 chip, which is in fact pin-compatible with the SSM2017. And the folks at Burr-Brown came out with their pin-compatible replacement, the INA217 (and they have since been bought out by Texas Instruments in another one of the consolidations that seem to be the state of the semiconductor industry today). So, here we are in the year 2010, and if you happen to have some equipment that uses the SSM2017 mic preamp chip, the THAT1510 and INA217 chips are both inexpensive upgrades that won’t require a lot of work (especially if the originals are already socketed). I did the IC replacement on one of the newer DDA Interface consoles and was really delighted at the improvement. It wasn’t a huge night-and-day change, but it also was less than an hour to change all 16 channels’ worth of preamp and a fairly minimal investment. Mouser Electronics, and a few other electronics wholesalers, carry the chips. There aren’t very many times when you can just pop a component out, replace it with a new one, and get a sonic improvement, so when such a thing does actually exist, it’s worth taking advantage of. Scott Dorsey (dorsey@recordingmag.com) is a recording engineer and lover of good audio electronics, living and working in Williamsburg, VA.

Recording - July 2010

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