All About Pads
A question that pops up frequently is that of building atteunator pads. Here is what you need to know, in one place.
What is a Pad? Where can I use them?
A pad is nothing more than a network made of resistors that creates loss (attenuation) in a transmission line. Pads can be designed with many different attributes: matched impedances, unmatched impedances, etc. You might use a pad to reduce the level of a +4dBu source to -10dBu, or to allow a microphone preamp to handle the signal from a hot microphone in front of a loud source (even with the preamp’s gain trim control at minimum, it still clips).
A thorough treatment of the subject can be found in the following (among others) references:
- The Audio Cyclopedia, 2nd ed. Howard W. Tremaine
- Handbook for Sound Engineers, the New Audio Cyclopedia, Glenn Ballou
- Motion Picture Sound Engineering, Research Council of the Academy of Motion Picture Arts and Sciences
- Sound System Engineering, Davis Davis
All of these texts deal with the subject from the classic matched-impedance standpoint; where the line is driven from a specific impedance, and terminated by that same impedance. These are power-based transmission systems. The concept is that maximum power transfer occurs when the load impedance matches the source impedance.
Today, most audio systems operate on the voltage transmission model. Sources have low impedances, and loads are many times this impedance so that they bridge the source. You can model the connection of source and load as a voltage divider. Looking at it this way, you can see that the majority of the source voltage appears across the load, and only a small fraction is lost in the resistance representing the source impedance.
The usual application for a pad is to attenuate the output of a microphone that is too high for the dynamic range of the following microphone preamp. Using a matched-impedance pad here is not optimum (this is not to say that it won’t work) because even microphones expect to have their output bridged by the input impedance of the microphone preamp.
Another issue here is one of coloration. The microphone and preamp operate together as a system. The input impedance of the preamp (which is not resistive) varies with frequency and this interracts in a complex manner with the output impedance of the microphone (also not resistive). If there are transformers involved at either end, that’s just an additional factor in the equation. This complex interraction causes coloration, which may be good or bad, beneficial or harmful. It’s one of the things that make different preamps and microphones sound different. The point here is that you can minimize the change in coloration caused by inserting a pad by paying attention to this detail and designing the pad to mimic the conditions present before its insertion. The easiest parameter to mimic, and the one that is the biggest contributor is the impedance that the pad presents to the microphone, and the source impedance that it presents to the microphone preamp.
Although there are many different configurations (L, T, bridged-T, etc,), they all boil down to a form of a voltage divider. Two resistors in series, with the input across both, and the output across one. The resistor at the input is in series with the output; the resistor at the output is in shunt. The output voltage is divided by a factor equal to 1 plus the ratio of the resistor values (ignoring source and load impedances). You can prove this for yourself using Ohms law, and then comparing this result with that obtained by using the resistor ratios.
All of these configurations may be made into balanced configurations by simply mirroring them. For example, a balanced T-pad (known as an H pad) has resistors in series with both sides of the input, and both sides of the output, but with a common resistor (R2) bridging the midpoints of the series legs. The series resistors are half the value used for the unbalanced case. In the following diagrams, the notation R1/2 means “the value of R1 (in the unbalanced case) divided by 2.”
Note: K is the voltage loss ratio for the pad. The resistors for the L and U configs follow the ratio K-1.
Universal Pad PCB The documentation for this product has formulas for calculating other configurations.
Discussion/explanation of the standard values used for electronic components, and a computer program to convert from ideal to standard values.
If 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.
Copyright 2001, 2002, 2003, 2004, and 2006 by Rick Chinn. All rights reserved.
Last modified 02/19/2014.
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