[sparkymarky]

Passive crossovers. (i.e placing a crossover between your amp and speaker).

A 12db second order crossover is often made up of a capacitor(s) and an inductor(s) coil.
The value/rating you choose of the capacitor (uF) and the inductor (mH) is what determines the cutoff frequency of that passive crossover will be.

This table shows what rating of each will get you what cutoff frequency (hz).

http://www.diyaudioandvideo.com/Tabl.../2ndOrder.aspx

As you can see, a 200hz cutoff does not use the same rating cap and inductor on a 4 ohm load as it does on a 2 ohm load.


So when we use, say a jl audio amp with built in crossovers, and we set 200hz low pass on it for our 2 ohm underseats say, then surely we aren't crossing them at 200hz? The amp will likely have been designed for 4 ohm loads even though it will happily feed a 2ohm load - but how can the crossover in it achieve 200hz for a 4 ohm speaker as well achieve the same 200hz for a 2 ohm speaker?


So on our 2 ohm speakers are we selecting say 200hz low pass on our amps but are in fact getting a completely different crossover point?!

[csu87]

no.

your amp's crossover is essentially trimming out any signal above 200hz. so no matter what ohm load your speaker is at, it is being sent the same signal

[fdawg4l]

Ah, my EE degree finally coming to some use!!

Your amp has a crossover before, essentially, the amplifier circuit itself. So, in effect, there is a passive cross over (probably after some impedance matching network on the input), then the output of this crossover goes to the input signal side of the amplifier. The impedance of your speaker is irrelevant at this point in the circuit.

[fdawg4l]

Originally Posted by fdawg4l

Ah, my EE degree finally coming to some use!!

Your amp has a crossover before, essentially, the amplifier circuit itself. So, in effect, there is a passive cross over (probably after some impedance matching network on the input), then the output of this crossover goes to the input signal side of the amplifier. The impedance of your speaker is irrelevant at this point in the circuit.

Also, you should note how wasteful it would be if the crossover were AFTER the amplifier. The full range signal would come into the amplifier, apply the appropriate gain, then waste the portion of the amplified signal over 200hz (as heat) before going to your speaker.

Alternatively, doing this filter before the amplifier means the amplifier circuit can be simplified because it only needs to be optimized for that sub 200hz band. In an A/B amplifier, that means the A side of the amplifier network doesn't need to be all that efficient since the small signal (high frequency) will likely be filtered out.

[sparkymarky]

Originally Posted by fdawg4l

Ah, my EE degree finally coming to some use!!

Your amp has a crossover before, essentially, the amplifier circuit itself. So, in effect, there is a passive cross over (probably after some impedance matching network on the input), then the output of this crossover goes to the input signal side of the amplifier. The impedance of your speaker is irrelevant at this point in the circuit.

ahhh that makes sense - and actually the manual says: Filtering between
subwoofer systems and satellite speaker systems
is best done with active filters, which cut off
frequency content at the input to the amplifier.


thanks

[sparkymarky]

so now i'll continue to design a passive crossover for my underseats....my amp only does either high or low, not both aka bandpass.....groan.

[sparkymarky]

Thinking to use the amp to high pass (built in 12db/second order butterworth) the hifi 2 ohm underseats at around 65/70hz and build a passive to low pass the underseats at around 200hz, (comprising a 280uF capacitor and a 2.2mH inductor).

Can you forsee any problems with this?

ps. My sub is crossed at 55hz using a 24db crossover (seperate mono amp). (I currently have that sub connected out of phase to blend in with the 12db crossover on the underseats and components).


Will the addition of the new 12db crossover to the amps already in use 12db crossover further affect phase? i.e will i still have a 12db/180 degree shift or now will i have a 24db shift??? confusing lol

[ajaye]

You are making this way too complicated for yourself.

1. Any passive crossover will affect phase. In general, each order the filter is increased causes a +/- 45 deg phase shift at the xover frequency.

2. As already stated, passive filtering is wasting a part of the work your amplifier is doing. Its ok in your house when they can just split another atom or two down at the plant. Power in the car isn't as easy to come by.

3. I see active crossovers on craigs list constantly for like $20-$50. Buy one, bandpass filter your preamp signal, spend the time and money I just saved you on upgrading the components in the passive crossovers that came in your house speakers insead.

[mathematics]

just get a Helix P Six DSP or PP82 and be done with it. DSP (crossover's, eq, time alignment) and amp, all in one. simple.

[csu87]

you are trying to reinvent the wheel here.

you have a couple options,

1. Get an amp that will allow you to run active on your underseat speakers (all crossovers set on amp)
2. Get a DSP and you can set all your crossover points/slopes in that.
3. buy a crossover and use that.

what amp do you have that doesnt have a high and low filter?

[sparkymarky]

Thanks for the replies - the reason why i was looking at passive rather than active crossovers before the amp is the (I assume this is a valid consideration) the difficulty in getting budget electronic crossovers which accept our balanced differential inputs?

[csu87]

i must have missed you wanted to use a crossover before the amp...

definitely dont do that. crossover after the amp if anything.

[sparkymarky]

my amp is a jl audio a4300 - it has high and low passes but you can only use one or the other hence i have the underseats low passed at 200hz but need a way of high passing them at around 60-80hz so they aren't trying to reproduce sub bass as they are at the moment.

[sparkymarky]

i also remember why i cant use an electronic crossover before the amp i've just realised:

Head unit front left and right would have to go into the crossover first.
The crossover would have be either:
1. highpassed before the amp- in which case i lose say everything below 60hz and so my daisychined alpine subwoofer amp off the main jl audio amp would not receive any <60hz signals at all

2. lowpassed before the main jl audio amp at 200hz - in which case i lose everything above 200hz to my components.

Obviously there are ways around all this such as different amp which bandpasses or ms-8 etc etc, all of which include significant cost. I'm happy with my kit. Just want a high pass to protect underseats. So this is why i was looking at a passive after the amp.

I'm gonna build one - should cost me about £55 all in.

[ajaye]

Any active crossover you find is going to have at least a 2 way network, usually 3 stereo bands is standard for the car now.

You feed one full range stereo pair in, two if you want to retain fading. Filter front outputs HP, underseat outputs BP, sub/rear outputs LP and feed each amp input accordingly. For 55 GBP I can't imagine you won't find a nice used unit. I haven't seen a passive network in a car since the 90's, and for good reason. The whole point of an aftermarket amp is the increased power and control/linearity, which is significantly undermined with a passive network. But do what you want.

[sparkymarky]

A bit of lateral thinking and this is what i think i'm going to do.

Background info: as i have retained the head unit rear outputs to the stock hifi amp so i retain rear pdc gongs that only leaves me with front left and right preouts. They currently go straight into my Jl audio amp where they are summed to create four separate channels and feed high passed @ 200hz components left and rights, and low passed @ 200hz underseats left and right.

I'm going to:

head unit front left preout - y splitter into (1) the jl audio for full range to front door mid components (they remain high passed at 200hz as before) (2) the Y tap off to an audiocontrol 2xs 24db electronic line level crossover - crossed over high pass at 60hz 24db and plug that into the previously unused (as was summing 1&2) input channels 3 of the jl audio amp. The amp then adds its original low pass of 200hz. So if i now feed channel 3 to the underseat i get 60-200hz.

Then, do exactly the same for the front right headunit preout. Except i plug eventually into channel 4 on the jl audio amp. And that feed the right underseat.

Success The front components are highpassed @200hz, the underseats are bandpassed from 60=200hz, and the the alpine sub amp still gets a full preout signal from the JL (after which it bandpasses between subsonic 15 and 55hz).

Oh and just found an audiocontrol on ebay..... just seeing if the guy will reconfigure/resolder a new crossover point in it or if i will have to


Hope this makes you passive crossover hater guys happy

[sparkymarky]

....and hope i dont get any turn on thump!!!!

What causes that and whats the cure btw? No experience of thump from my installs.

[snaimpally]

Get a Helix DSP or Mosconi 6to8

[sparkymarky]

http://www.e90post.com/forums/showthread.php?t=1132906

[csu87]

Originally Posted by sparkymarky

....and hope i dont get any turn on thump!!!!

What causes that and whats the cure btw? No experience of thump from my installs.

Thump is caused by a hard turn on of the amp. Most good amps have filters to negate this.