The Universal Pad PCBThe universal pad board provides a simple and convenient way to make and use single-value attenuator pads. The board accommodates most of the common fixed attenuator topologies: H, T, L, U, O, Pi, Bridged Tee. The wiring layout is flexible enough to be used with either balanced or unbalanced configurations.
The board is small: 2.062 x 0.85 inches, and it is made on a piece of FR4 epoxy-glass material, double sided, and thru-hole plated. A ground plane surrounds the circuitry. It is laid out for 1/4-watt resistors, but half-watt units will fit. The board does not fit inside a Switchcraft S3FM XLR tube. There is a hole provided for a single swage spacer (supplied). You can purchase the board several different ways. You can get the Universal Pad Board Manual here. Click the link for a short article describing how to calculate resistor values.
Prices and Options
Some Small DetailsIf you are ordering a complete kit, with resistors, delivery could take at least two weeks, possibly more. Why? Because if I don't have the resistors on hand, I have to order them. Here's the problem:
1% resistors use the e-96 series of preferred numbers for their values. This means that there are 96 values for every decade. There are 5 decades between 10-ohms and 1-megaohm. There are 480 1% values between 10-ohms and 10-Meg. That's a lot of resistors. As much as I'd like to, I don't stock every e-96 value between 10-ohms and 1-megaohm. It's just too many (nearly 500 resistor values x 100 resistors/value).I stock many of the e-96 values, and I stock all of the e-24 (5%) values. If you request a value that I don't have, I'll have to order it, which takes time. There's no way to foresee which values I have and which ones I don't. I charge a small premium for this service. I'm willing to buy 100 at a time. Are you? If you need this in a giant hurry, then I can have the parts shipped next-day-air, but you get to pay for that privilege. You could also just order the raw (unstuffed) PCBs from me, and order your resistors from Mouser. They're fast, and they don't have minimums. There are many choices, and you get to choose. RN60 metal film resistors fit, so do CMF55 resistors. I like the CMF55 parts because they are color coded. For line level use, the resistor's power rating doesn't matter.
Tolerance: Tolerance is how far the measured value of the resistor is allowed to deviate from the marked value. In the past (long gone), you could get 20% tolerance parts. Why? Because they were cheaper. But think about it, a 1k, 20% resistor could be 10% higher or lower in value. For many circuits, it didn't matter. Today, it is more common to get 5% parts, and because of modern manufacturing, they tend to be more precise than their marked tolerance says. 1% metal-film resistors are relatively inexpensive (unless you buy the uber-mil-spec ones) and come in a wider variety of values, making them ideal for pad building. Still, you can only (usually) get close to the values that the formulas spit out. If you go thru the math, you'll see that what the formulas say gets you right there BANG on the money. If you use 1% values, then you get close, and again, you can grind thru the math to satisfy your curiosity. A spreadsheet program is your friend here.
CMRR: Common Mode Rejection Ratio. This is where really close tolerance parts pay off. CMRR is the ability of a balanced input to reject signals that are the same, or common, to both signal lines. It doesn't take much difference to throw this off. 1% parts are good for 40dB or so of CMR, and if you want better than that, you have to buy closer tolerance parts, or hand match parts with an ohmmeter. If you buy in quantity, it's pretty easy to find pairs or quads of parts that are much closer in value to each other, and this matters more than the absolute value. For an H or U pad, the critical resistors are those in series with the signal path, and it's important for them to be as close in value to each other (for an H-pad, it's the total of the two series resistors) if you want to maximize the CMRR. 0.1% or better resistors are now fairly easy to get without buying a resistor factory. They're readily obtained from Mouser or DigiKey. They are expensive, so buying 100 1% units and hand matching them is a very viable scheme.
If you want 0.1% tolerance parts, then you must say so up front, and be prepared to pay for them because I will only buy them in small quantities because of the embedded cost.
Resistor ChoiceI'm into buying good parts, but I refuse to buy pedigreed parts. If that's your game, be my guest, but don't ask me to be your purchasing agent. My 5% parts are all carbon film resistors, with varying sources, mostly Asian.
My 1% parts are a mix of US and Asian parts. Some are Vishay-Dale, but newer parts are made by Koa-Speer.
If you want to buy pedigreed parts, then you're on your own.
Design ServiceIf you can't make heads or tails out of the calculations, I'm happy to do them for you... For a Price. Design time is $100 per hour. Most designs take 15-minutes. The design process is supposed to be simple, so this should be a learning experience for you. Whatever... Send me your problem, and its application, and I'll tell you how many 15-minute blocks this represents.
If you order the pad board kits from me, calculating the resistor values is included in the price. (within reason, my interpretation). Of course, you're welcome to do it yourself.
Copyright © 2008-2021 by Rick Chinn. All rights reserved.
Last modified 05/28/2021 1500hrs.