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Hello MoreDat people,

I have a very basic question that has stumped me for the last two days despite countless searching. I'm sure the answer is very basic and a clear explanation would help out.

I am attempting to teach myself electronics.  I studied up on resistors from a couple resources (art of electronics and this site). I then have been trying to apply my knowledge with a very basic circuit: the U-pad. I followed along this website that explains the theory behind pads and has instructions on how to make your own: http://www.uneeda-audio.com/pads/

Here's the problem:
I don't understand how the load (mic preamplifier in this case) sees the shunt resistor as the source impedance (150 ohms in this example) and not the total resistance of the 2 series resistors (1k each in this example) and the the shunt resistor.

I do understand the voltage divider portion of the circuit and that the voltage drop occurs as a product of the ratio between the series resistor and the series resistor.

I don't understand what is happening by bridging the + and - signal in this balanced circuit other than it is allowing current to pass from + to - and vice versa.

I don't understand how to calculate the thevenin equivalent resistance of the U-pad.

Any help, clarification, or links would be greatly appreciated.

Thanks,

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As you learned, pads are used for attenuation and impedance matching.  The impedance matching seems to be the part you do not understand.  First off, the reason for impedance matching is to be able to pass as much of our signal from one stage to another without having a large amount of the signal reflected back and does not load down the circuit. 

I worked in television for many years and most of the video equipment used 75 ohms impedance for input and output.  So, all of our inputs needed to be at 75 ohms and all of our outputs needed to be at 75 ohms for best results.  An example circuit is shown in the picture below.

For your U pad, shown below, the microphone only makes 2 connections and they are across the 150 ohm resistor.  Therefore the 150 ohm impedance of the microphone is in parallel with the 150 ohm resistor.  Anything beyond that is of no concern because we could disconnect the amplifier that the microphone is feeding into and the microphone would still be seeing just the 150 ohm shunt resistor.  The series resistors would have no effect because they would not be connected to the amp.  Our big concern here is getting a good impedance match to the microphone.  The second important thing is the amount of loss you want from your pad. 

In the example 1 on the page you referenced at http://www.uneeda-audio.com/pads/  , they wanted to achieve a 20db loss.  We first convert the 20db to a ratio by using k= 10^db/20 = 10.  Now that we know K, we us the formula K = 1 +(Rseries/Rshunt).  We have already decided that we need the shunt resistor to be 150 ohms to match the microphone impedance.  So we substitute in our known values and solve for the series resistors.

10 = 1 + (Rseries/150)

9 = Rseries/150

9 * 150 = Rseries = 1350 ohms  This is for the 2 resistors combined so we have to divide this in half to find the value of each resistor.  So each series resistor would be 675 ohms.  Resistors are only available in certain values so you may have to add or subtract a few ohms in order to make it a value that is available.

This U pad now would provide you with a 20db loss in signal level and provide you with a very good impedance match for your microphone.

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