V72 input pad configuration

[Hanseat]

Hi all,

I want to rack my V72 preamp and I want to add a PAD on the input. Source-impedance should be 150 Ohm and input impedance 2000 Ohm. I know that I could have someone calculate or build them for me, but I would really like to learn some more and DIY is much more interesting.
I read a lot through different forums but I'm still not sure, that I found the right way to do it. Maybe someone could share some details?

A) Output Impedance is more important.
U-PAD with constant shunt resistor - That's what can be found in the V76 circuit diagram and the values of the resistors conform to the calculations / explanations from the Uneeda-Audio-Page http://www.uneeda-audio.com/pads/. My calculation:

B) Input Impedance is more important.
U-PAD again. A calculator for this can be found here: http://starfishcoffee.de/u-PAD_calc_edit2.xls
My calculation:

C) Input and Output impedance are important
H-PAD - low attenuation seems to get difficult with this one as calculated values for the input-series resistor become negative (I did note 0 Ohm in my calculation below).

(using this calculator: http://www.nu9n.com/tpad-calculator.html)
My calculation:

Someone also mentioned a balanced bridged-T-attenuator, but my understanding is, that this is only designed to work between impedances of the same value.

I would really like to learn some more, so If anyone could share some insight on thisissue that would be great!

 

[Hanseat]

After reading some more I found out that not only the PAD integrated into the V76, but also the Telefunken W77 PAD-Switch (used in front of the V77) both use the same principle, a balanced U-PAD with a fixed shunt. The value of the shunt is representing the assumed source impedance (200 Ohm). The load for the microphone varies, but stays above 1kOhm to not put too much load on the microphone. PAD-values below approx. 15dB would result in a load much lower than 1kOhm, so that's the reason why those steps are avoided in both of these examples.

The input-impedance of the V76 and the V77 are somewhat lower than the V72 (because of their higher ratio transformers), but the PAD-circuit should work for the V72 nevertheless.  That seems to be safe.


IF someone would like to get lower PAD-values the k-factor has to become smaller. This is possible, if the value for the shunt gets higher. If the shunt would be changed to 500 Ohm, even a 6dB PAD should work (as the load for the mic would stay around 1k). But then the ratio between source and input impedance would get smaller - to what that would lead? I'm not sure.

P.S.: Found a great link explaining impedance and impedance matching: http://repforums.prosoundweb.com/index.php/topic,1151.msg529857.html#msg529857