Recording - June 2010 - 72

Now adjust the gain—uhmm, wait a minute. We can’t... there’s no gain control! The schematic specifies an 1800 ohm level control (or pot), and for reasons I won’t go into, it should really be a “reverse-log-taper” device. Well, nobody listed in the Mouser catalog makes an 1800 ohm pot, and it’s really hard to find any reverselog-taper pots. So we’ll compromise on a cheap 2000 ohm linear taper pot ($1.21). Oh, and it’ll need a knob ($1.33). We’re getting there. Connect a cable to the preamp’s output XLR-M jack (after buying one for $3.04). Now, this circuit has an unbalanced output, but we can make it “impedance-balanced” with just two resistors ($0.10), a trick used by a lot of budget gear makers. Since the “giant-killer” design doesn’t include the extra amplifier on the output (if you’re interested, it’s a “servo”, to get rid of unwanted DC), we really need a couple of coupling capacitors on the two output legs, and they need to be non-polarized (another $1.08). To avoid picking up radio stations, a common-mode choke on the input is a good idea ($1.20).

Guest editorial by Paul J. Stamler

The $5.00 Preamp Urban Legend
Every couple of years, an urban legend goes around the Internet audio world about a fabulous five-dollar homebuilt preamp that outclasses much pricier units in listening tests, with the implied corollary that this proves commercial preamps are overpriced ripoffs. If you haven’t seen the legend, go look at it now: www.tinyurl.com/5preamp I won’t go into the “listening tests” here— subjective tests are always a can of worms. Instead, I want to address why the claim of a $5.00 “giant-killer” preamp is nonsense. Because, well, it’s not a $5.00 preamp. Take a look at the circuit diagram on the website. The preamp is based on the INA217 chip from Texas Instruments; in fact the design is from their application notes. The chip costs $4.50; add the half-dozen resistors as specified on the diagram, plus the four protection diodes (which keep the chip from getting fried), and the cost is indeed $5.00. (All prices are from www.mouser.com, for small-quantity orders.) That’s not quite the end of it, though. There are two capacitors on the input, and those need to be non-polarized electrolytics (because the phantom power won’t always be on). Add $1.08. There are also a polarized electrolytic cap bypassing the phantom-power supplies ($0.39) and a couple of film caps on the chip’s pins ($1.06). We’re up to $7.53. Okay, so it’s a $7.53 preamp? Except it’s not. To see why, power it up—oops, batteries not included. Nine 9V ProCells at case prices run $13.95, and battery clips add $2.61. In addition to the SPDT switch for phantom power shown on the diagram ($3.15), you’ll really need a DPDT to switch the chip’s batteries on and off ($5.27), unless you want to open the case each time you want to power up your preamp. Now plug in a mic—oops again, there’s no XLR-F input connector ($3.23).
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Now for the mechanical bits. A piece of 4" x 5" perforated board to build the circuit on adds $4.50, and it’s a good idea when building a gadget like this to put the main chip in a socket, to avoid soldering on the chip’s pins ($0.46). Unless you plan to let the thing sprawl on your bench, you’ll need a box to put it all in; the cheapest appropriate one I could find (not a rack-mount unit) costs $10.32. Finally, add about half a buck’s worth of screws and spacers to mount the board in the box ($0.50). The $5.00 preamp is now really a $59.48 preamp, nearly twelve times as expensive as stated. But isn’t that a pretty good deal? Well, yes; certainly the cost is less than most commercial preamps. Sweat equity adds real value. It’s not quite that good a deal, though. I picked the absolute cheapest level control that would work, and cheap level controls don’t last long. I’d expect the control to get noisy in a year or two—of course, you could always replace it now and again. And battery cost will become an issue; you’ll be replacing the chip’s batteries every 21 hours of use. (Battery life for the phantom circuit will depend on the microphone’s draw.) Still, it’s one way to get yourself a low-priced utilitarian preamp which will sound okay. Before you decide that, since you can build a preamp for $59.48, commercial preamp manufacturers are running a racket, let’s talk briefly about the economics of making and selling audio gear. A typical product sells for between five and eight times its parts cost. That would make this a preamp which sold for between $300 and $475, give or take a bit. (There are fudges in both directions. Manufacturers get parts for less than we do, because they buy them in quantity. However, a commercial product would run from at least a wall-wart power supply, instead of batteries, and that adds to the cost.) Where does the extra money go? Well, manufacturers need to pay the people who drill holes in the chassis, put parts on the board, assemble the product, order the parts, keep track of inventory, keep the books, write the manual... you get the picture. They also have to pay rent on the space where they do business, along with insurance and utilities. Then there’s the tricky issue of profit; if they don’t sell the product for more than their costs, the owners can’t stay in business, and if the product is sold in stores, the stores have to make a profit too, or switch to the dog grooming business. So $300–475 is a good back-of-the-envelope price for an entry-level preamp like this, and in fact you’ll find lots of them in that price range at your local International House of Banjos. Welcome to reality: the legendary $5.00 homemade preamp is really a $59.48 homemade preamp, or a $300–475 commercial preamp... and legitimately so. So are preamps that cost much more than that amount ripoffs? Ah, that’s a story for another day. Paul J. Stamler (stamler@recordingmag.com) wrote this issue’s article on the insides of mics... and it turns out he knows a bit about the insides of preamps, too.

http://www.tinyurl.com/5preamp http://www.mouser.com

Recording - June 2010

Table of Contents for the Digital Edition of Recording - June 2010