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[drpat]

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[NewYorkDave]

http://www.microwaves101.com/encyclopedia/calcattenuator.cfm

 

[drpat]

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[drpat]

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[Boswell]

[quote author="drpat"]So... a "U" pad for a 10db drop @600 ohms would be...
604R shunt resistor
649R series resistors

Right???[/quote]
Those values would certainly give you 10dB attenuation when driven from a zero Ohm source and driving an open circuit. However, the output impedance is 412 Ohms. Your decibel calculations are OK, but you didn't allow for the source resistance.

Does your 600 Ohm requirement apply to the input or the output? If you want to present a load of 600 Ohms to the source, you need series resistors of 205 Ohms and a shunt resistor of 190 Ohms. If you want your driven circuit to see a source resistance of 600 Ohms, you need series resistors of 949 Ohms and a shunt resistor of 878 Ohms. All these calculations assume a zero source impedance and an infinite load inpedance.

 

[drpat]

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[Boswell]

[quote author="drpat"]The 600 ohm requirement is based on presenting a load of 600 ohms to the source.[/quote]You don't need to present 600 Ohms to the output of a pre. It just shouldn't be less than 600 Ohms.

The actual output impedence of the preamp that this will be fitted into is listed as <85 ohms.

OK, so we'll take 85 Ohms as the source impedance.

For the sake of education:
1) how did you arrive at those values
2) and what is a good method to allow for source resistance?

It's a matter of resistors in series and parallel. If the source resistance were zero, the two input resistors are effectively in series, and their series combination is in parallel with the shunt resistor. So that parallel combination gives the load as seen by the preamp. The attenuation is the ratio of the shunt resistor to all the resistors in series. Solve the simultaneous equations and then subtract half the source output impedance from each of the input resistors.

If R1, R2 are the series, shunt resistors respectively,

Attenuation = R2/(2*R1 + R2)

Input resistance = 2*R1 + R2

Output resistance = 2*R1*R2/(2*R1 + R2)

I'm also wanting to build a fixed universal Line level (+4) balanced 10db pad to interface between two unknown devices. Something like a barrel that I could just insert between a couple of devices when I need it.

1) How would one go about creating such a device when neither source nor load impedence is known. Is there a common figure for such a calculation, or is every commercially available line level pad constructed completely different.

2) Which values would you use to construct such a device.

Yes, it's not possible to get a device that is universally correct. Basically, the input resistance should be as high and the output resistance as low as the attenuation allows. If you know your preamp will drive 600 Ohm and its output impedance is 85 Ohms or less, try those as starting values. If the output resistance of the attenuator is much less than the input resistance of the load, you can ignore the load.

Hope that helps.

 

[drpat]

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