T1 ToneMatch® Audio Engine / XLR Inputs Sensitivity

Q: What is a Pad?

A "pad" is short for an "Attenuation Pad".

This is a device used to lower the signal level between two other devices.

Padding PS1 Power Stand (Classic) / Model I Inputs

We use pads with our PS1 Power Stand (Classic) / Model Is when running a pro level +4 dBu signal (e.g. output from a mixer) to the XLR inputs on Channels 1 or 2. We want to do this because the input sensitivity for these inputs is set for microphones. To get a better match for levels between the source and our inputs, we can use a pad.

Pads can also help when connecting the PS1 Line Outs to semi-pro or consumer recording devices. If the PS1 Line Outs are too hot for the recorder's inputs, a pad can be used to prevent overdriving those inputs into (clipping) distortion.

A -20dB pad is also a good choice when connecting the PS1 Line Outs to a house PA, or other off-stage system. This would ensure that the house mixing console's inputs can handle the signal level without being overdriven into distortion, especially if the house console has insufficient built-in pads, or none at all. (Beware of the potential for Ground Loops here.)

T1 ToneMatch Audio Engine No Pads Required

The T1® XLR inputs on channels 1-3 were designed to accept inputs from microphones up to normal line-level inputs without needing a switch or a pad. This is in unlike the L1 Model I or L1 Classic) Power Stand where a pad is recommended for line-level inputs.

The T1 ToneMatch^® Audio Engine Channels 1, 2, 3 inputs will accept a wide range of signals. We have not encountered a normal use where someone needed a pad - even using +4dbU console outputs, which is as high as it gets.
— MikeZ-at-Bose

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Daisy Chaining PS1 Power Stand (Classic) / Model Is

Another application (although relatively rare) is if you want to connect the output of one PS1 Power Stand (Classic) / Model I to input of another. Sometimes called daisy chaining, you can connect the Line-Out of one PS1 Power Stand (Classic) / Model I to Channel 1 or 2 of another. In that case you probably want to have a -20 dB pad inline, between the Line-Out of the first System and the Channel 1 or 2 XLR input of the second. See PS1 Powerstand / Daisy Chain for more information.

You can find separate attenuation pads like this:

Cost: $20-30 on the web

Sources:

• http://www.proaudiosolutions.com/product_p/impad20.htm
• http://www.musiciansbuy.com/WHIRLWIND_IMPAD20_IMPEDANCE_CONVERTERIMPAD20.html
• You might find something like this in a local music store if they have a Pro Audio department.

This an example, and you will want to look into the details to determine if you need a pad, and the kinds of connections that are appropriate for your input device.

More Formal Definitions

"attenuator or attenuator pad Electronics. A passive network that reduces the voltage (or power; see usage note under gain) level of a signal with negligible distortion, but with insertion loss. Often a purely resistive network, although any combination of inductors, resistors and capacitors are possible, a pad may also provide impedance matching."

Do It Yourself

An inline pad can be easily soldered, you'll need to insert 3 resistors into your mic cable. The values of these three resistors are determined by the amount of attenuation you wish to achieve. Bose recommends -20dB of attenuation to bring a hot +4dBu balanced line level signal down to a voltage that the Bose PS1 (Classic) Mic inputs can handle.

To build a -20dB attenuation pad insert the following three resistors into your XLR cable path.

• Two 680 ohm resistors inserted into lines (pins) 2 and 3
• One 150 ohm resistor inserted across pins 2 and 3

To minimize the effect on common mode rejection, you should use 1% resistors, but common-ordinary 5% carbon film resistors will work. For microphone pad applications, 1/4 watt or less is fine.

Source: Rick Chinn / Uneeda Audio

References:

• Rane Professional Audio Reference
• All About Pads