Building a Stereo Pad

[jason porter]

I am trying to build a Tape (-10db) to Mic (-50db) pad.  It is to cut the level from a stereo -10db source to a stereo mic input on a video camera.  The stereo signal needs to be maintained.

Does anyone have a schematic for this?

[John Steigerwald]

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

[Philip Perkins]

I just made a simple dual T-pad to do this--here's a chart for the resistor values.

http://www.scribd.com/doc/8616057/T-and-Pi-Pad-Attenuation-Chart

Philip Perkins

[jason porter]

Thanks Philip,

I will give it a try tonight.

-Jason

[John Blankenship]

If it's unbalanced, use a simple L-pad with a 10k ohm input resistor (R1) and a 100 ohm resistor from there to ground (R2).  The output of the pad is taken from the junction of the two resistors and ground.

See the "L" pad on the site mentioned earlier:

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

If you're connecting balanced circuits, use the "U" pad configuration and make the two input resistors (R1) 5k. ohm each.

A 10k. input is sufficient for a bridging impedance that won't load the circuit feeding it and a 100 ohm output is a low enough value that the input impedance of the succeeding circuit won't affect the pad's ratio much.

For 40dB attenuation, you're after a 100:1 ratio.

Naturally, for stereo, you'd insert one of these pads into each channel.

John B.

[Izen Ears]

  Kind of a tangent and at the risk of sounding ignorant, but did you guys study electronics?  I can solder, but I can't really make much sense of those diagrams...  I guess the question is did y'all take classes or learn from a mentor?

  I wanna learn that shit but it ain't practical to take a real school course.  I bet if I'd taken a basic electronics course in college I'd get it, but I majored in communications and art!

  Dan Izen

[jason porter]

Thanks again guys...

I tried the dual L-Pad...but it was late and I could only get a mono signal to pass.  Will try again with fresh eyes.

Dan-  I am completely self/internet taught (albeit still very green)

[John Blankenship]

Thanks again guys...

I tried the dual L-Pad...but it was late and I could only get a mono signal to pass.  Will try again with fresh eyes.

Jason -- I don't understand your statement, "could only get a mono signal to pass."  Wiring a "stereo pad" is the same as simply wiring two mono ones -- each channel has its own pad.

Dan -- Yes, my uncle (Sam) paid for my training -- I'm a U.S. Navy vet.  However, I was never stationed on a ship.  After training I spent my time on shore bases behind locked doors doing things I can't talk about.

John B.

[jason porter]

Hi John,

Here is what I did-

tip----10k---+-----pin2 (5 pin XLR)

                  |

                100

                  |

sleeve-------+-----pin1

                  |

                100

                  |

ring---10k---+-----pin4

Does this look right to you?

[John Blankenship]

You need to specify more, such as the pinout of each device, in this case, the 5-pin mic input.

If you are coming out of an unalanced output but are going into a balanced input you will probably need to tie the "low" side of each input to ground.  i.e. if pins 3 & 5 are the "low" side, tie them to pin 1.

But, these things are not always standardized, so you need to provide specific pinout information.

Nicely done diagram, BTW.

John B.

[jason porter]

Hi John,

Does this help?

The input is for a video camera front camera mic input (-50db)

1- ground

2- mic in + (1)

3- mic in - (1)

4- mic in + (2)

5- mic in - (2)

the output is an H4 Holophone (-10db)

tip- left

ring- right

sleeve- ground

[John Blankenship]

That looks like it should work.  I'd reverse the tip and ring leads so that the Left input feeds channel One and the Right input feeds channel Two.

You may end up tweaking the values based on actual tests.  If you need more gain make the 100 ohm resistor a larger value and if you need less gain, use a smaller value.  Depending upon the input impedance of the mic preamp, 200 ohm resistors should give you about 6dB greater gain and 50 ohm resistors should give you about 6dB less gain.  Double or halve the values again for a total change of 12dB.

Let us know how it works out.

John B.

[jason porter]

Thanks for the help John.

I reversed the tip/ring configuration as you suggested (my diagram was wrong, but I had it right in my head)

The cable works properly now (passes a padded stereo signal),  but I will have to test it on the actual camera to see if it needs to be tweaked.

Thanks again!

-JP