Cheap Amp

[_Scotty_]

The current sound system in my R33 consists of:

Eclipse CD5425 Head Unit

Eclipse SC8264 6.5" 2 Way Front Splits

Eclipse SW6123.4 12" Sub running off a bridged EA2000 Amp

Standard Rear Speakers

I've been planning to upgrade the rear speakers at some stage in the future and run those and the fronts off a 4Ch Amp with a high pass filter set at around 55Hz, since the fronts are rated down to 50Hz.

I've been offered a new Sony XM-554ZR 4Ch amp for about $100, so I'm tempted to get it and install some Jaycar 6.5" splits in the rear and just be done with it. The problem is the Amp doesn't have an adjustable filter, just a switch for 80Hz. I can set a 70Hz filter on the head unit, and was thinking of setting the low pass for the Sub at about 90Hz.

Is is worth holding off to buy something better or should I just go with this? Is it worth being able to set a high pass for the front that allows their full range?

Edited March 4, 2008 by _Scotty_

[mikel]

In reality your front 6.5" will be rolling off above 55Hz even though that's what the box says.

Having the sub take over from around 80Hz is fine.

You only want to apply a single stage of filtering which will result in a matched (ie the same cutoff frequency) high and low pass signals. The high pass to drive the front and rear splits and the low pass to drive the sub.

Based on your descripton you can do it at either the head unit or at the amps, but you don't want to do it at both.

Two cascaded filters (eg one in the head unit and one at the sub amp) set at the same frequency will double (or there abouts depending on the "order" of the filter 1st order 2nd order, 3rd order etc) the slope of the rolloff.

Eg a 4th order filter (24db rolloff per octave) is built by cascading 2x 2nd order filters set at the same frquency (each being 12db rolloff per octave).

In other words, if there's already a low pass output at the head unit for the sub, you don't need to apply another low pass filter in the sub amp. The same goes for the additional Sony amp. You don't need to set a high pass at the Sony amp if the head unit drives it with a high pass output.

If you can't disable the additional filters because they're in built, separate them as much as possible. For example set the head unit high pass/low pass combination at 70 Hz and the low pass at the sub as high as possible eg 140Hz and the high pass on the Sony amp as low as possible eg 35Hz.

That way in effect the low pass at the sub, and the high pass on the Sony amp are doing as little as possible.

To minimise the filter interaction of cascaded filters, ideally the crossover frequencies should be at least 2 octaves apart (one octave above 70Hz is 140Hz, 2 octaves above is 280Hz, one octave below 70Hz is 35Hz etc). This may not be possible in your application, but your idea of setting the low pass of the sub at 90Hz is on the right track (ie higher than the head unit).

The other option is to disable the filters at the head unit and use the high pass in the Sony and the low pass at the sub. Set the crossover at the same frequency (or as close as possible, 10Hz won't make much difference)

cheers

[_Scotty_]

Thanks for your help Mike!