June 30, 2010

Tweeter pad - a simple tweak that might save your speakers

Here is a simple tweak that will make many budget speakers sound more natural (and even some not so budget speakers), as well as increase the power handling and reduce the chances of tweeter failure when pushed hard. It's called an L-pad and it's a very simple mod that anyone who is soldering iron handy can do. You simply need two resistors added to the crossover.

Too late: a TDL speaker that needs a pad

Recently I fixed a TDL RTL3 speaker that needed fixing. The tweeter often goes on this speaker. A late night party was too much for the tweeter, being pushed way too hard. I replaced the tweeter with a new pair and put an L-pad on to make it a bit more natural and also more robust. Does this look like too much power to you?

Read more about it here >

Why do I need one?

Most well designed speakers of conventional design have a tweeter that is more efficient than it needs to be. Often the designer will allow for this and use a pad, but in some cases the tweeter level is left unaltered so that the treble is emphasised. Often in a retail store with a short demo, customers will choose the speaker with slightly more treble or bass. Over time this becomes less appealing. If you have a speaker like this, a pad is a quick and easy fix.

You may prefer less treble than the designer.

You might also listen at higher levels, where it can be better to have reduced treble. This will also increase the power handling. A typical 3db pad will double the power handling of the tweeter.

How to calculate it

You need to know the nominal impedance of the tweeter. Often this will be shown on the tweeter. If you know the tweeter brand and model number then you can perhaps find a datasheet on the internet. You can measure the resistance of the voice coil with a multimeter when the tweeter is disconnected from the crossover. Please note that this is the DC resistance and gives an indication of the impedance which will be higher.

Vifa D27TG R= 4.6ohm, impedance = 6 ohm
Peerless HDS R= 5.6ohm, impedance = 8 ohm
Peerless 100 HDT R= 6.8ohm, impedance = 8 ohm
Scan Speak Revelator R= 3ohm, impedance = 4 ohm

Use an L-pad calculator to work out the value of the two resistors.

L-pad calculator >

How much power?

Obviously you don't want 1/2w resistors. The power required depends on the amplifier used and the proportion of that power that the tweeter will see. Higher crossover points and steeper slopes will result in less power going to the tweeter. Let's work out an example.

100w amplifier
2nd order crossover at 3k

Above 3k, the tweeter will see 15% of the power so in this case 15w is the most that the tweeter will see. In normal use, the continuous power is more likely to be less than this so a pair of 10w resistors will be fine.

Soldering it in

Conect one in series and one in parallel with the tweeter. The pad should be inserted after all components involved in the high pass filtering components.

Amplifier > tweeter high pass > L-pad > Tweeter

Amplifier > L-pad > tweeter high pass > Tweeter

If done incorrectly, the L-pad will now see a full range signal and thus it's power rating will be inadequate. It will overheat and most likely fail.

Now evaluate by ear and see what you think. You might need to try different values. Most pads will be in the 2 - 4 db range. Less will be too subtle and more will rarely be required.

Ensure the resistors are securely soldered with a strong bond, then fix them to the crossover board with a hot melt glue gun, or with straps.

Conclusion - give it a go

Sometimes this simple tweak can save an upgrade. If you think your speakers are a little too bright, this could be the answer. It's simple and cheap. If your speaker already has a pad, but you want to increase the value, first find out their values, then with a pad calculator such as the one linked above you can work out the attenutation. Then work out the values for more attenutation.

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