Bi-Amp and External Crossovers

Discussion in 'AV Receivers & Amplifiers' started by Mark Ward, Apr 17, 2002.

  1. Mark Ward

    Mark Ward
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    I'm back on the Bi-Amping/External Crossovers debate again.

    To summarise from my limited understanding.

    When using 2 or more amps per speaker each amp channel is still processing the full frequency range, irrespective of the fact the speaker's internal crossovers will ignore frequencies not required for that particulur driver.

    With this in mind....How can Bi-Amping add anything? Low Channel amp's High Freq and High Channel amp's Low freq parts of the signal are, I assume discarded.

    In order to give each amp channel more "headroom" and hopefully improve sound quality the only realistic option is to filter the frequencies after the pre-amp stage but prior to the receiver's power amps.

    Am I correct?

    If so... What external crossovers would anyone recommend? Specific makes/models would really help please, I'd be using them with a Denon AV-A10SE/POA-T10 combo and some Kef Reference speakers (Either 1.2s or 3.2s)

    Does any manufacturer make a single unit (1 box) piece of kit that can handle the external (either bi-amp or tri-amp) crossover requirements for all three fronts?

    Cheers,

    Mark.
     
  2. Reiner

    Reiner
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    You need to remember that when bi-amping (or even bi-wiring) you remove the link at the speaker terminal, thus one amp will only "see" the mid/bass driver, the other one only the tweeter.

    With this in mind....How can Bi-Amping add anything? Low Channel amp's High Freq and High Channel amp's Low freq parts of the signal are, I assume discarded.

    Well, not discarded but rather blocked out. Due to the high- or low pass in the tweeter or bass/mid driver respectively the other frequencies cannot pass through, the x-over components represent an infinite resistance to them so no current releated to that frequency will travel via that wire.

    In order to give each amp channel more "headroom" and hopefully improve sound quality the only realistic option is to filter the frequencies after the pre-amp stage but prior to the receiver's power amps.

    This would be the icing on the cake, car hifi often uses that option, i.e. active x-over before the amplification stage. Note that in this case the drivers won't need a x-over at all!

    If so... What external crossovers would anyone recommend? Specific makes/models would really help please, I'd be using them with a Denon AV-A10SE/POA-T10 combo and some Kef Reference speakers (Either 1.2s or 3.2s)

    Not sure if this makes much sense to a hifi-stereo or AV system. I would just split the signal and let the speaker do the work. As mentioned above ideally you should have no x-over or x-over components in the signal path - so are you prepared to rip them out?
     
  3. Mark Ward

    Mark Ward
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    Hi Reiner

    You need to remember that when bi-amping (or even bi-wiring) you remove the link at the speaker terminal, thus one amp will only "see" the mid/bass driver, the other one only the tweeter.
    I understand this, but my query is to do with the work the AMP has to do. The amp has still done the work processing the part of the signal that never gets heard.

    Note that in this case the drivers won't need a x-over at all!
    I know they don't NEED the internal crossovers, but I can't see me butchering some Kef refs. My assumption is that even with the internal crossovers left intact the fact that the Amp channel didn't have to process frequencies irrelevant to that driver should yield some improvement?

    Any thoughts?

    Mark.
     
  4. Reiner

    Reiner
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    I understand this, but my query is to do with the work the AMP has to do. The amp has still done the work processing the part of the signal that never gets heard.

    Processing like in "a small current running through the pre-amp stage"? Well, not a big issue me thinks, the pre-amp probably couldn't care less and since the power amp stage is reliefed of some of the frequencies there should be a benefit.

    I think as a rule of thumb the load is divided 70 (mid/bass) to 30 (tweeter) when using a single amp, now you take away either thus resulting in more headroom / dynamic which is what you want to achieve.


    I know they don't NEED the internal crossovers, but I can't see me butchering some Kef refs.

    I thought so (didn't know what speaker though ...) :D


    My assumption is that even with the internal crossovers left intact the fact that the Amp channel didn't have to process frequencies irrelevant to that driver should yield some improvement?

    I honestly don't know but the I could argue the double-filtering would also affect the sound - horses for courses?
     
  5. Mark Ward

    Mark Ward
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    Me:- The amp has still done the work processing the part of the signal that never gets heard.

    Reiner:- Processing like in "a small current running through the pre-amp stage"? Well, not a big issue me thinks, the pre-amp probably couldn't care less and since the power amp stage is reliefed of some of the frequencies there should be a benefit.


    Perhaps "processing" was the wrong word... I'm no expert here...Doing what power amps do is what I meant; If the power amp is doing it's thing to parts of the signal that is never going to be heard, will filtering unwanted frequencies prior to the power amp improve overall performance?

    Apologies, I know what I mean but seem to have trouble conveying it to others.

    Mark.
     
  6. Electric Mayhem

    Electric Mayhem
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    Presumably the power amps will only be driving the signals sent to them from the amp after they have gone through bass management in the processor.

    If the amps crossover is set at 80hz. The amp will then send signals above 80hz to the pre outs for front left/right etc and all other frequencies below 80hz to the sub.

    If we´re looking at the signals the power amps now have to play with it´ll will depend on where the crossover is in the speaker between mid bass and treble. If you´re power amping the tweeter it might be everything above 10kz (or something??) whilst the other power amp will be running everything below this but only down to 80hz (the initial crossover set by the processor).

    If you´re sending full range signals to the power amps again one power amp will have to run everything up to the crossover point of the tweeter (lets say 10kz). The other amp will then takeover from there onwards and upwards.

    Thats the way I see it but I dont know if its right!!! Feel free to shoot me down in flames!!! :)
     
  7. MarkB

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    I think we are almost there – I can see what mark means.

    Think of it like this: If you use one amp to run both the bass and treble then the amp is running the load of these two drivers. Say you bi-amp, if you take the amp that is running the treble, the load of the bass unit is gone; the load "seen" by the amp is just the treble. The same will be true of the amp running the bass – the load of the treble will be gone. If you run an amp with no drivers on at all it runs totally idle and draws no energy.

    Feel free to flame me too!

    Mark
     
  8. rob_w

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    mark b

    the way i see it is that the treble will still draw the whole (100 etc)watts of the signal, but the x-over will lose the sound bit (in heat/resistance or whatever). the bi-amp setup just isolates the drivers, allegedly eliminating cross talk/interference between the bass x-over and the treble x-over. also meant to improve the damping/control of the drivers.

    btw the mid/treble x-over is approx 2khz.

    using active x-over and passive(inside kefs) will probably mess up the phasing(timing) of the treble/midbass drivers - also could possibly put a filter slope on top of a filter slope ie: 4Db per octave becomes 8Db per octave - very messy.....

    in answer to the active x-over query, there are professional companies for nightclub pa`s etc that do active filters. these cover simple x-over settings to speaker delays (for phase matching)and ,i think, parametric eq etc to set up the speakers perfectly..

    i saw one reviewed in sound on sound magazine about 3 years ago ........i thing it was by jbl (not certain) and it cost about 3k!!!!

    stick to bi-amping methinks!

    hope this is helpful
    robwells
     
  9. GaryG

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    Hi Mark

    You have an enquiring mind, I would recommend you buy the following book:

    The Loudspeaker Design Cookbook by Vance Dickason, it will answer all of your questions.

    You can purchase the book from here
    https://secure.wilmslow-audio.co.uk/acatalog/Wilmslow_Audio_Books_on_Loudspeakers_9.html

    I think I mentioned this the last time you raised the debate but check out
    www.williamshart.com

    Sean Williams formerly the technical director of Wilmslow Audio sells all the stuff you need for active filtering for bi-amping etc. If you telephone him he will give you all the advice you could ever want about crossovers and he'll even design crossovers for you.
     
  10. Reiner

    Reiner
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    Apologies, I know what I mean but seem to have trouble conveying it to others.

    No worries, I think I understood you, Mark. ;) Just wanted to point out that "processing" wasn't the right term.

    The bi-amping will not affect the bass management here, so I leave that aside.

    What Mark thinks is that both amps still amplify the entire signal / frequency range since there is no filter / x-over before, thus still producing the same load - which would speak against the benefit of bi-amping.

    I will try to explain below why that isn't the case, taking a quote from robwells to make it clearer:

    the way i see it is that the treble will still draw the whole (100 etc)watts of the signal, but the x-over will lose the sound bit (in heat/resistance or whatever).

    Actually not. The signal is AC (alternate current) but if the x-over resistance would convert the not-needed signal into heat your speaker would probably burn down.
    (A real resistor for 100 Watt would be very big not to say huge.)

    The x-over comonents (some capacitors and coils) will represent ("simulate") an infinite resistance like an open circuit to the AC signal, thus you can expect that there is no current from bass going into the x-over components of the tweeter and no current from the tweeter going through the components of the mid/bass driver.
    If there is no high frequency signal in the mid/bass path the amp won't need to amplify this, thus the load is reduced.
    The same applies for the amp serving the tweeter, low frequency signals are not amplified.

    All the above refers to the basic theory, ignoring component tolerances, actual (accurate) functions of x-overs (sloping etc.) and amplifier design (Class A, A/B etc.).

    Well, if you still want to try and not spend much money look around at car hifi dealers or big electronic stores selling car audio, they usually carry (active) x-overs and IMHO they are technically suitable (since all happens on line-level) for your purpose ...
     
  11. nfordenfield

    nfordenfield
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    Agree with Reiner's final point.

    Many year ago now, i used to endulge in Car Hi Fi and used to run an active system.

    Simply, the Treble, mid bass and sub bass is split prior to the amps which consentrates the actula signal needing to be amplified.

    This type active system is only seen in domestic situation from the likes of Meridian.

    It must be remebered that twice the power is required to produce the same volume each octave that is dropped.

    From memory i ran the following system.

    Sub 200 watts
    Bass 100 watts
    Mids 75 watts
    Tweeters 50 watts

    As for ripping out internal xovers, you would first need to know the freq of each driver and then get a Car audio 3 three or fourway active x over.

    You would also need an equal amount of application.

    Personally, I run my KEF 3.2 bi wired from a pair of bridged Meridian 557 which delivers over a Kwatt to each speaker, and back them up with a KEF THX AV1 sub each. This now gives me the bass that actually moves you. That's what I call true 3D sound.

    Regards

    Nathan
     
  12. Lowrider

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    As someone already mentioned there is more than frequency splitting to the passive crossover, like level matching and phase correction, so I don´t think you could replace them easily with active external ones...

    Stick with bi-amping, although I would rather buy a single, better power than do it...
     
  13. rob_w

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    quote "The x-over comonents (some capacitors and coils) will represent ("simulate") an infinite resistance like an open circuit to the AC signal, thus you can expect that there is no current from bass going into the x-over components of the tweeter and no current from the tweeter going through the components of the mid/bass driver.
    If there is no high frequency signal in the mid/bass path the amp won't need to amplify this, thus the load is reduced.
    The same applies for the amp serving the tweeter, low frequency signals are not amplified.

    All the above refers to the basic theory, ignoring component tolerances, actual (accurate) functions of x-overs (sloping etc.) and amplifier design (Class A, A/B etc.)."

    is the impedance of the speaker a measure of its resistance? - usually 8ohms - 4 ohms (varies with frequency)
    reiner - there has to be a full range signal present for the x-over to filter (?) (doesnt there?)if there is no current from treble going to mid/bass components,why do you need a x-over?

    "If there is no high frequency signal in the mid/bass path the amp won't need to amplify it"
    this is on active filters, and not passive..

    youre bang on with the car setup though - i use active x-over to feed 2 power amps in my car nice full range sound for not much money (2x 6x9`s fronts /1x10" sub)..
     
  14. Reiner

    Reiner
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    is the impedance of the speaker a measure of its resistance? - usually 8ohms - 4 ohms (varies with frequency)

    The impedance is the AC resistance which, as you say, varies depending on the frequency.

    What is normally quoted and perhaps wrongly called impedance (say a manufacturer claims this is a '8 Ohm speaker') is the DC resistance. You can measure the DC resistance with a normal multimeter in the Ohm range; to measure the actual impedance you will need an oszilloscope (sp?) - and normaly it's measured over a frequency range giving a graph rather than one value. (I.e. in audio systems you will measure across the range of 20Hz to 20kHz.)
    This will show that a e.g. 8 Ohm speakers may go down to 5.2 Ohms and up to 10.4 Ohms (just giving some example) and anywhere inbetween.


    reiner - there has to be a full range signal present for the x-over to filter (?) (doesnt there?)if there is no current from treble going to mid/bass components,why do you need a x-over?

    Of course in normal operation (single-wiring) this applies.

    But when you open the link (metal bridge) at the terminals of the speaker you also split the x-over. The x-over components are still in the signal path, thus blocking any unwanted frequencies due to their electrical characteristics.

    Look at one path at a time:
    For the high frequencies the tweeter (with his x-over component in series) will look like a normal load, thus a current can flow. But for the low frequencies the x-over components will represent an infinite load (open circuit), thus no current can flow.
    Or in other words: where would any low frequency current go if there would be any? You can't "burn" it with a resistor (see also my reply above) or just make it disappear. The only way is to prevent it from flowing in the first place.

    It's the same what happens in single wiring at/after the x-over, you "block" the unwanted frequency from each driver. But the unwanted current is then absorbed by the other driver, so here it can be "burnt", aka turned into sound.

    So it does not matter if the split point is at the speaker (the x-over in single wiring) or at the amp's end (bi-amped with split x-over), for one frequency one of the paths will always represent an obstacle which can't be overcome.


    Okay, this is a bit difficult to describe and my English may not be good enough but I hope you can follow me ...
     
  15. rob_w

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    i can see what you`re saying, but the x-overs are wired in parallel to the amp, so they`d both "see" the full range signal. - youd only pass left over bit of signal if it was a series circuit. 1 x-over,then other.(btw if you could send the rest of the signal after low pass filtering on, then this would not need a x-over surely?)

    i dont know much about how the hi pass filter works, only that a capacitor is put across the +/- in parallel with the driver, and a resistor is used if necessary to attenuate the volume. this means the cap must absorb,damp etc the low freq`s cos the whole signal is present at output stage of amp.

    i know that the coil is an inductor, and my guess is that it "induces" the lowest signals into the output side of the x-over, and the spec of the coil selects freq etc

    this sort of goes back to the bi-wiring stuff i discussed with lowrider, and we ended up going round in circles!

    btw i`ve noticed that my posts always look a bit serious cos i havent figured out how to use the emoticons yet!

    i`m not arguing ,just discussing how i think these things work.
    you`re stuff seems to ring true,but i still think my thoughts are pretty valid too - looks like a trip to amazon for the filter cookbook is in order!

    cheers
    robwells
     
  16. Electric Mayhem

    Electric Mayhem
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    Rob

    When your typing your post, on the left are the smilies

    Just insert them as you go. Click on whichever one you want and its added to the text.:)

    cheers
    Jase:D
     
  17. rob_w

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    tried that - dont work!

    do i need to enable javascript etc? (esp active scripting - leave it off to stop pop ups etc)

    cheers
    rob
     
  18. Electric Mayhem

    Electric Mayhem
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    Check your profile options to make sure vbquick is enabled for smilies etc.

    Its down the bottom of the page above the Avatar section.

    If you still have trouble check with Spectre he´s more computer literate than me!!! Javascript, got me!:confused: :eek:
     
  19. Reiner

    Reiner
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    No worries regarding your posts, haven't found it offensive in any way.

    What I mean is that the x-over components are in series to the driver, not the x-overs in series to the amp.

    The bi-wiring thread, is that the same where I inserted some pics? There you can see the actual components and how it's wired up, just imagine two amps at the bi-wired terminals.


    To summarize this in a simple way:

    The output stage of the amp driving the tweeter will only "see" the high pass / tweeter; low frequencies cannot pass and thus are not amplified (it's like there is no load / an open circuit from the low frequency point of view).

    The output stage of the mid/bass only "sees" the low pass / mid-bass driver, high frequencies cannot pass and thus are not amplified (it's like there is no load / an open circuit from the high frequency point of view).
     
  20. rob_w

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    yeah it is the same post......

    i guess i`m assuming that the output from the amp is an amplified version of the waveform, and is therefore present...as i said i`m not sure about the exact workings so i`m gonna read up on them a bit before i get involved in any more bi-amp bi-wire type stuff!
    trouble is i love chatting `bout stuff like this.....not many people in the real world talk about this, and i find it interesting.....

    cheers
    rob
     

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