70W Attenuator Wattage - Headroom Required?

Is a 100W 70V attenuator actually designed to pass 100W of 70V amplified signal to 100W*1.5 of networked 70V speakers?

I have asked other questions on the Xchange regarding EPR ratings, and this question is related:

To date, I have always added 70V attenuators based on the combined taps of the speaker networks they are attenuating.  For example, if I have four speakers each tapped at 25W in a paralleled network, I would put a 100W attenuator in line to control volume (and then make sure my amp channel has 100W plus headroom.)

My colleague asked a question today that had been bugging me:  Are attenuator ratings intended for this use, or should they be specified to allow for headroom like the amplifiers?

Of course I avoid attenuators like the plague when budget allows it...  but when my hand is forced I want to do what I can to preserve the audio quality.

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Chris, This is an interesting question and how this can be answered is first knowing what your use case is an what level of SPL you are trying to achieve. For example, if you have a background music application using distributed audio based on 70 V distribution then the Volume Controls could be from 10 to 35 W or more. Using your example with parallel wiring for each speaker (assuming 8 ohm spkr with a stepped transfomer) and a 100 Watt amp with 70v output the calculation would be: Zt = Z1 + N Zt = total impedance of speaker system  Z1 = the impedance of one speaker N = Number of speakers in the distributed system wired in parallel. From what I could find there is a very small insertion loss of only .5 dB on the Volume Control that uses a stepped tranformer design. So, I would not be too concerned about a insertion value for the Volume Control however the rating for wattage needs consideration.

So, the headroom on the amplifier should be selected on the basis of having at least 2-4 times the Continuous Power rating per channel. However this headroom need will depend on the music or voice application so the multiplier of output power would vary if the program is background music application all the way up to a rock band application. 

Below is a article that may help the overall understanding of distributed audio and amp selection.

I hope this helps... 

How Much | Crown Audio - Professional Power Amplifiers | English

Go to the profile of Christopher Eloranta
2 months ago

Thanks John.  I've done plenty of reading on the amplifier specification side (I meant to hand-wave it away by saying "plus headroom.")  I always factor in 15dB-20dB of headroom for background/foreground music when calculating my tap requirements, so when they are summed to estimate the amplifier EPR (Watts), it has already been taken into consideration.  I also add an additional 20% or 50% to the wattage total to handle insertion loss of transformers.


Here's a bit more about that 100W rating.  Looking at a spec sheet for an attenuator, ATS001063D-Commercial-Attenuators-Datasheet.pdf (atlasied.com), we can see that the power rating is really the maximum power or current the transformer can pass, without over-heating and burning up.

The question then goes to, how much power needs to go to the loudspeakers that are fed from the attenuator?  A simple calculation is to add up your tap settings on the loudspeakers and make sure you are below the rating of the attenuator.  In practice, you will probably far less power than the tap settings.

Here is a video on how to calculate how much power you need for your loudspeakers.


Go to the profile of Christopher Eloranta
2 months ago

Thanks Paul,

I'm very clear on calculating EPR for the amplifiers... no questions there.

If I understand your comment on the attenuator rating, the 100W rating suggests that the attenuator will heat up (and intrinsically create much more resistance) once the signal passing through approaches 100W RMS.  If I have correctly calculated my EPR using 15dB or 20dB of headroom (for background/foreground music) in the speaker tap calculations, the RMS value should be well below the total of the speaker taps.

Did I understand correctly?

Yes.  All transformers have a limit to what power they can handle.  It just comes down to wire gauge really.  The thinner the wire, the less current it can handle.  A 100W transformer will be bigger and heavier than a 20W transformer due to the wire size.