Info on pads?

Hi Guys,

Im looking to build a simple standalone pad i can use bewteen mics and pres. Im guessing this should be pretty simple to build on veroboard and would ideally like something with a 10 / 20 / 30db cut.

Anyway, I was wondering if anyone has built anything like this before.. or if you could point me in the direction of some info on pads?

Search only seems to throw up info on go between and similar. Thanks a lot

http://www.uneeda-audio.com/pads/

OR

http://www.gcarter.us/Articles/attenuators.pdf (i like this one)

The google search your looking for is "U Pad" or "H Pad"

/R

thanks mate.

appreciated

snipsnip said:

Hi Guys,

Im looking to build a simple standalone pad i can use bewteen mics and pres. Im guessing this should be pretty simple to build on veroboard and would ideally like something with a 10 / 20 / 30db cut.

Anyway, I was wondering if anyone has built anything like this before.. or if you could point me in the direction of some info on pads?

Search only seems to throw up info on go between and similar. Thanks a lot

Click to expand...

Be aware that there are some constraints in designing pads.
The most important are:
The load presented to the mic should be high enough, typically 2kohms, in order to maintain its frequency response
The source impedance presented to the mic pre should be low enough, typically 200R, in order to maintain its frequency response, its distortion spec and its noise performance.
This makes it virtually impossible to design a pad with less than 20dB of attenuation that would work consistently with a variety of mics and pres.
A 10dB pad has to be designed knowing the properties of both mic and pre, and will work satisfactorily only with the specific combination it's been designed for.

In many cases, it is just simply impossible to design a 10dB pad for a specific combination.

"Universal" 20dB and 30dB pads are easier.
20dB pad:
+in--1k-- +out
          2
          2
          0
          R
-in--1k-- -out

30dB pad, you have more options; you can choose to increase the input impedance (many mics love that)
+in--3k-- +out
            2
            2
            0
            R
-in--3k-- -out

or reduce the output impedance (mic pres love that),
+in--1k-- +out
          6
          8
          R
-in--1k-- -out

or a combination of both, such as:
+in--2k-- +out
            1
            2
            0
            R
-in--2k-- -out

10 dB pad with compromises:

- the load presented to the mic: ~ 1,3 kohm
- the source impedance presented to the mic pre: ~ 300 ohm

See the description of the construction at: http://www.uneeda-audio.com/pads/

Something else useful to know in designing pads is:

(R sh // R L)  / (R sh // R L) + R ser = V out

R L is the total load resistance of chosen pad resistor values.  R ser is the total series resistance.

This equation gives you the actual output voltage and thus db attenuation value. You may find situations where smaller differences in attenuation are more critical. Or one <20db pad may work better than another of a different resistor configuration.

I made a chart based on common shunt values from 150 - 390ohms and series values from 510 to 15K.  Hundreds of combinations and easy to see at a glance both attenuation value and resistor combo.

If you're feeling really industrious, pile a stack of turret boards together in a box and make a nice multi-pad set.

Wow - thanks for the info. Bit more to it than I had first imagined.

Thanks again guys!