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Spiny
diyAudio Member

Join Date: Feb 2008
The input resistor is the upper part of the divider network.
I attach a simplistic circuit of the attunator with the formula. The actual attenuation is also affected by both the output resistance of the driving circuit and the input resistance of the following circuit. These two are normally termed impedances as they vary with frequency to a degree.

All the stepped attenuaters do is switch different bottom resistors. the output then gets a different portion of the signal. One problem of the simple form is the output resistance varies vastly and can cause problems in the next stage.

You may find this book a good starting point. Covers the basics and more.

Electronic Circuits, Fundamentals and Applications. By Mike Tooley

Those forum members with more knowledge will I'm sure provide other books and links to information and correct any errors in the simple explanation above
Attached Images
 attenuator.gif (1.5 KB, 44 views)

 27th December 2008, 03:00 PM #3 jamesk   diyAudio Member   Join Date: Dec 2008 thanks for the reply, i will check that book out ASAP. But as for the resistor, although I am grateful for the formula you gave, I still don't know the actual reason it needs to be there.... I just don't get why it is there... I'm just taking peoples words for it and don't understand the purpose. Lets imagine all the schematics depicted a chicken drumstick instead of this resistor... and there was ohms law explaining how the chicken drumstick's impedance adds up to 7 banana bucks.... I still don't know why I need a chicken drumstick in my circuit! But anyway, when you say the problems in changing impedance can mess up the following circuit (£2000 amplifier) I am worried... how would I make a minimal signal path passive balanced attenuator without resistors? lol
 27th December 2008, 03:50 PM #4 Spiny   diyAudio Member     Join Date: Feb 2008 Passive attenuators imply resistors to loose the unwanted part. You can terminate a network so the input and output resistances are constant - this would I think require more wafers on the switch to add in extra resistors, not my field - I'll have to look up. The top resistance is used as part of the attenuator (its R1 in the diagram) the input signal is shared accross the two resistors and you tap the portion you want at the mid point. Without the top resistor you are just connecting direct to the output any attenuation is then provided by lower resistor against the output resistance of the first stage, this is probably ill defined and may be subject to frequency variation. the result will be unpredictable and poor. (and your first stage may object to having its output taken to ground via a .1 or 0.2 ohm resistor at the maximum attenuation setting. With the 10K in place the minimum resistance the first stage sees is 10K when the max attenuation is applied - the resistance increases from here. Input stages (the next stage ) do not generally mind low resistance drive so the output side is OK. Easier (and cheaper) to use a couple of OP-Amps to buffer the signal with the attenuation between them.

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