Passive Crossover Padding

I'm about to install a passive crossover (Eminence  PXV2-3K5 Two Way Crossover @ 3.5 Khz) into a cab but I'm wondering if my HF driver will need padding down? I've read on here that its quite common to pad them down as there sound to loud/harsh compared to the main driver. is this info correct? I've only ever used passive crossovers so could turn it down at the amp.

The best place is www.rapidonline.com but they only come in certain values if you go for 6.8 ohm and 4.7 ohm that will give you about 7dB attenuation. Get metal clad 25 watt for up to 400 watt or 50 watt for over 400watts.

Or go for 10 ohm and 3.3 ohm which will give you 5dB as near as damnit. Here's a link to the page

http://www.rapidonline.com/Electronic-Components/Aluminium-clad-wirewound-resistors-65217




Edited by markie - 20 February 2012 at 4:28pm

The best way to determine the values for a pad is to put in a real L-pad and adjust it until it measures good and sounds right. I use a nice 50W Fostex L-pad for this purpose.

Then remove the L-pad, measure the series and parallel resistances, buy the closest values (starting a bit high if need be), and then trim with a higher value to get it down where you need it to be. The trim resistors are usually 10x higher in value than the required value, and thus only need to be a smaller wattage type.

Instead of guessing at the wattage, let's calculate what it needs to be. I've had to use four 25W units in series/parallel (100W total) to handle the heat in a typical two-way network at 1.6Khz and the resistors still get hot enough to melt the solder and burn anything that touches them (and that was with an amplifier only rated at 250W/8Ω driving above 100hz).

Let's make up some examples.

6dB pad with 40V input, 4Ω series, 8Ω parallel. Both resistors would have 20V across them, so the 4Ω would need to be a 100W type, and the 8Ω would need to be a 50W type.

9dB pad with 40V input, 5.16Ω series, 4.40Ω parallel. The series resistor would have 28.3V across it, the parallel would have 14.14V across it. The series resistor would need to be 155W, the parallel would have to be 45W.

12dB pad with 40V input, 6Ω series, 2.68Ω parallel. The series resistor would have 30V across it, the parallel would have 10V across it. The series resistor would need to be 150W, the parallel would have to be 37W.

When I decide to leave and L-pad in a design for user adjust-ability (bad idea in general) I generally use a fixed pad in front of the adjustable one. Example: a fixed 6dB pad followed by the L-pad that would be adjusted to -2dB~-4dB. This allows the use of a 50W L-pad  for a total of about 9dB of attenuation, which would normally require a 200W L-pad (unavailable).

The difference in price between 25 watt and 50 watt resistors is about 25p each. Ask youself what you would prefer to do either spend an extra couple of quid on 50 watt resistors, or possibly have the crossovers pack up in the middle of a gig.

Bear in mind also that 25- and 50-watt metal clad resistors are rated for use with a heat sink. If you don't use a heat sink, drop the rating by half.

Why not build multiple l-pads and use a rotary switch to select which you want to use? And then after a few uses you have the option to take out the switch and keep the level you use most/like.