Bridged-T attenuator

Kingston said:

glbaudio said:

the value of 66,5 ohm is correct? or should be 6,65?

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66,5ohm is correct. It's because the scale jumps directly from -20db to off/inf. Otherwise it would be divided to lower ohm -30dB, -40dB etc. values.

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thanks a lot for the explanation!

All potential users of this circuit must keep in mind that Bridged T type attenuators may only be used when equal impedances in both directions are involved. The Bridged T attenuator will operate correctly only when working between impedances of equal value, while a T configuration may be designed to work between equal and inequal impedances. Check your circuit for actual source and load impedances before implementing this attenuator.

Does anyone happen to have this wonderful little diagram of New York Dave's saved? As the link is dead. Or New York Dave yourself? I would really love to get it as it will save me a lot of math!

vinyvamos said:

Does anyone happen to have this wonderful little diagram of New York Dave's saved? As the link is dead. Or New York Dave yourself? I would really love to get it as it will save me a lot of math!

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It shouldn't be too hard to recreate. Just rememeber for a 600 ohm one there are a pair of 600 ohm resistors in series between input and output. One pole of the switch connects various resistances between input and output and the other pole connects other resistance values between the junction of the two 600 ohm resistors and the cold terminal. When fully off the input/output switch is open circuit and the junction to cold switch is short circuited. When fully on the input/output switch is shorted and the junction to cold switch is open circuit. Values in between depend on the attenuation you want. You can use this link to work out individual values:

https://www.pasternack.com/t-calculator-bridged-tee.aspx

Cheers

Ian

Thanks for your reply Ian. In the past hours I decided to get down to it and started doing out a spreadsheet with the formulae punched in. Now I have the series of values, and it makes total sense the way you describe the arrangement of the resistors and switches.

Cheers! And Happy New Year!

I don't know how you have arranged your spreadsheet but rememeber the calculator I linked to calculates the actual value of resistances required. When you wire up a switch it is common to wire it as a ladder network (resistors across adjacent pins) so you need to work out the incremental resistor values. You probably already know this but I thought it worth mentioning for others following along.


Cheers

Ian

Well worth mentioning Ian. Although I am going to go for this configuration as shown below rather than series resistors or a ladder. Diagram taken from the schematic for the Chiswick VK1 compressor, where in that case the impedance is 200 ohms. I will have to recalculate all values depending on the project anyway, as sometimes I need to load-down an output trafo to get the correct impedance for the circuit