The Kit Is It

Feb 1, 2008 12:00 PM, By Eddie Ciletti

EVOLUTION OF THE D.I.Y. MOVEMENT

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Figure 1b: The completed circuit board with solder mask and silk screening. Students stuffed and soldered all of the components.

Figure 1b: The completed circuit board with solder mask and silk screening. Students stuffed and soldered all of the components.

YOU WILL NEED…

At one time, CAD and PCB-layout software cost hundreds — or even thousands — of dollars. Manufacturing circuit boards requires lots of chemicals, both photographic and for etching copper. The elements required to fabricate a front panel — engraving and the drilling of holes — now come under the umbrella of CNC (Computer Numerical Control).

Now it's easier to make a D.I.Y. project look less like a prototype, thanks to Internet-savvy companies that realize more business by “giving the tools away.” I'm talking about free software that, while somewhat limited (and mostly designed for the Wintel OS), is more than enough to get you started. A year ago, one of my grad students took advantage of Express PCB and Front Panel Express. I was so impressed with the end results that I used his circuit board layout for this project and designed my own front panel. Twenty panels like the one in Fig. 2 cost $500, plus shipping. It really helped inspire my students — building a mic preamp that looked manufactured (at least from the outside) and gave them a sense of pride.

A LITTLE HELP FROM OUR FRIENDS

The most recent kit was made easier when a local manufacturer provided a finished chassis (a scratch and dent sale) and power supply components that required minimal modifications to work with our electronics and custom-made front panel. It was not quite as ambitious as Fig. 2 might imply; students were only required to connect the phantom-power switch, hi-Z input, sensitivity and output level controls — the additional holes were for future options. It's better to have nicely drilled, unused holes (filled with hole plugs) than to have to drill by hand and label later.

EXTRA CREDIT

Figure 2: This front panel was designed using online software from Front Panel Express. It inspired students to work extra hard on the guts.

Figure 2: This front panel was designed using online software from Front Panel Express. It inspired students to work extra hard on the guts.

In addition to power supply and amplifier electronics, I felt it necessary to include input and output transformers so students would have a better grasp of their benefits. Their ability to convert a signal from unbalanced to balanced increases the Common Mode Rejection Ratio (CMRR). In addition, they isolate the ground from one piece of gear to another, further reducing power- and ground current — related noise.

The other thing that transformers do is impedance matching, with ratios of 1:1 and 1:10 being a typical range for line and mic transformers, respectively. The power is equal on each “side” of a transformer — primary and secondary — a 1:10 (pri:sec) ratio matches the microphone's low impedance (lo-Z) to the high impedance of JFETs or vacuum tubes. Voltage gain is the result on the secondary side, with a corresponding loss of current (power = volts × amps).

In addition to a transformer's obvious benefits, one of the most interesting differences is that each type has a sonic signature. Lower ratios are more sonically transparent, higher ratios require more care (in manufacture and implementation) to maintain transparency. Transformers are not “one-way” devices. To reap the benefits of impedance matching, it's important to understand, for example, how the load on the secondary can be reflected back to the primary; this effect was demonstrated by the load switch on the prototype preamp.

Fortunately, the Websites from most transformer suppliers provide application notes on how best to optimize the circuitry around the transformer. The Jensen Website (<a href="http://www.jensen-transformers.com" target="_blank">www.jensen-transformers.com</a>) is perhaps the best for &#8220;white papers&#8221; on how transformers work.

QUALITY CONTROL

After creating this course, I can say that these are great times to be a geek. All of the tools are out there to help make your next custom project look professional. All it takes is a bit of discipline, some time and the motivation to get started.

<i>Eddie would like to thank all of the transformer manufacturers for just being: Altran, AMI, CineMag, Edcor, Jensen, Lundahl and Sowter. Additional thanks to Great River Electronics for the chassis.</i>






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