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Autotat 20th July 2012 08:56 PM

Building active cross-overs into multiple power amps?

I presently have a pair of Cambridge Audio P500 Power-Amps Bi-Aping a pair of speakers (obviously).

I have heard through this forum that active cross-overs give a much more satisfying sound than passive cross-overs. I have also seen a few different ways of making active cross-overs ranging from what look similar in appearance to passive cross-overs to rather advanced complicated looking designs (to a beginner like me).

I would like to give active cross-overs a go at some point but would rather not have another separate box on the rack. As there is a lot of empty space within my power amps could I install suitable filters into the input lines of the power-amps?

Here is a random photo from google of the inside of the power-amps to show the available space:

I've seen planet10's creation on here that amounted to little more than a plastic box with a few components stretched between the input and the outputs, not a circuit board in site! :p Would easily fit but would it be an improvement?

Many thanks for any help.


Speedskater 20th July 2012 10:05 PM

It's the skill of the cross-over designer that determines how satisfying the sound is! There is a whole lot more to designing cross-overs than just putting some values into a design program.

CharlieLaub 20th July 2012 10:11 PM

Passive and active crossovers are equally good at producing crappy, lame sounding speakers. They can both result in excellent speakers, too. They are each just a means of executing the crossovers. Both types have to be carefully designed.

It sounds like you are getting your feet wet. I wouldn't build crossovers into your amp cases at this point. You might consider springing for a Behringer DCX2496 and see where that takes you. You can learn a lot along the way about crossovers.


Speedskater 20th July 2012 10:18 PM

For sure a DSP crossover (like the Behringer) is a good learning tool. You can program several different crossover slopes & frequencies into the unit and quickly switch from one to another and listen to the differences.

Jonathan Bright 20th July 2012 10:21 PM

Getting back to your original question, the answer is YES. They can be quite small and you could easily fit either an active or passive (low level) section in your current amp chassis. You obviously google around and after a bit of that I imagine you'll be quite up to speed. There are some issues to be addressed like grounding etc but nothing insurmountable. Good luck, cheers Jonathan (ex-southern Blighty)
Btw for trivia buffs I learnt the origin of "blighty" this week. I think they said it was a corruption of a Hindi word meaning 'distant province' or something very similar. Our sub-continental members can no doubt fill in the details....or correct me!)

Autotat 20th July 2012 11:10 PM

Thanks very much for the replies, that Behringer certainly sounds like a useful and informative thing to play with. I'll have a look.


Autotat 21st July 2012 09:46 PM

I haven't had chance to research this properly yet but I was initialy thinking along the lines of replicating the cross-over parameters of the original passive cross-overs within the speakers.
This probably sounds a bit lame but I was working to the belief that the speaker manufacturers would have carried out some fairly in-depth testing and development to get the cross-overs to those settings.
Of course that doesn't mean they can't be improved upon to suite individual applications and tastes/hearing. I guess this is where the external programmable Active Cross-over comes into it's own. Then, when happy with the results I am guessing I could make up something to mirror the settings to fit inside the power-amps?
Just a thought.

gfiandy 22nd July 2012 10:31 AM

The passive crossover in your speakers will interact allot with the inductance an resistance of your speakers. When you mover the crossover to before the power amp you remover this ineraction. So you can't simply place a scaled copy of the crossover before the amp, you end up having to redesign.


tvrgeek 22nd July 2012 02:56 PM

It ain't dat simple son....
The passive crossover is dealing with the complex impedances of the drivers. Hopefully to good advantage. I would reference the Zaph SR71 to see how few parts he can use. An active does not worry about that, but you may still need to deal with BSC, a notch or hump here and there, and rolling off the tweeter. I use my DXC exactly for that on the bench. Not cheap though. I can't say I have ever exactly translated one implementation to the other, but I can learn enough on the DCX to get a good start on the passive. Final voicing is always be ear. Even when I use an active crossover, I frequently still have some passive network. At a minimum, you MUST, read that again, MUST still have a blocking cap on the tweeter. MUST. If anyone says not, they are a fool, ignorant, or own stock in a tweeter company. All three are represented within these pages.

A line level passive crossover can work only if it does not mess up the amp input stage. Oh yea, it will. Some strange things can happen as you are playing with the input impedance of the amp. If you model it in Spice, you can see it can do very bad things for distortion. I did not understand this until I got a thrashing over in the Lounge on amplifier design. Easy to give it a try.

I think most would suggest an active crossover. You can tap the mains power for a good low voltage power supply for op amps, or even build one using FETs that runs off the same power as the input stage. You have the space for a fully independent supply.

You are on a very good tract. Do some reading and come back with more specific ideas and I am sure this thread will gladly help, laugh, shred and otherwise provide the usual support. We will suck you into the dark side and make a real DIY out of you. :D

Autotat 22nd July 2012 05:36 PM

Wow, there is a lot more to it isn't there...

I shall certainly do some homework on this, but it might have to take a back seat for a little while. I thought it might be a nice quick little project but it looks like it's going to demand a fair bit more of my time. Unfortunately I've got a car project to finish first.

I will be revisiting this though as I'm determined to get the best out of my components as I can and if I can create something that sounds great and that no-one can simply go down the shops to buy ready made without spending mega-bucks then I'll be as happy as a pig in sh**! :D

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