What is a passive crossover?

This may seem an obvious answer, but I’m still unclear. If speakers have separate inputs for bass and treble, or even bass, mid, treble then has the passive crossover been removed as the separate frequencies already have their own input??

As a corollary of this, a passive crossover is therefore speakers with ONE set of inputs and the speaker splits this to bass and treble.

Am I correct??

Don't spoil what you have with what you wish for!

Original Post

It's simpler than that - a passive crossover is one which contains no active (powered) components, only "passive" ones - these are resistors, capacitors, inductors.

The seperate inputs are for those who'd like to bi-wire or bi-amp the speakers (depending on whether the design makes both possible) - they are still passive if not powered.

Consciousmess posted:

This may seem an obvious answer, but I’m still unclear. If speakers have separate inputs for bass and treble, or even bass, mid, treble then has the passive crossover been removed as the separate frequencies already have their own input??

As a corollary of this, a passive crossover is therefore speakers with ONE set of inputs and the speaker splits this to bass and treble.

Am I correct??

In general I think you are correct. However if speakers have separate inputs for bass, treble etc. it could, in principle, be that between input and driver there still is a branch of a passive crossover left. E.g. to protect the tweeter from bi-amping. I don't know if there are speakers like this.

Regarding Naim speakers, you are correct: if you have separate inputs, then the passive crossover has been removed and the amp directly moves the driver. It's your responsibility then to make sure that only the correct frequency band is passed through to that driver (especially important for the tweeter). The way to ensure this is the use of an active crossover between pre and amps.

Crossover is probably a misleading word,  they are high & low pass filters in the case of a two driver speaker & additional band filters when there are more drivers.     

A passive filter simply means the filter circuit components, predominantly capacitor(s) & inductor(s), block signals above or below specific frequencies.   In the most simple form an inductor connected to the bass driver allows low frequencies to pass & a capacitor connected to the treble driver blocks them.    Then as frequency rises the inductor begins to restrict flow to the bass driver & the capacitor begins to allow flow to the treble driver.   

Tweeters are delicate devices, designed so the diaphragms can vibrate effectively at the highest frequencies in music, with small, lightweight diaphragms and fine wire on the voice coils If you feed low frequency signals to a tweeter, the very much greater energy normally present at lower frequencies in music relative to the energy at high frequencies will readily overload the tweeter, either overheating and causing deformation of the voice coil, or burning it out, or mechanically damaging the delicate diaphragm or its suspension. The crossover filters out low frequencies from the signal passed to the tweeter.

At the other end of the scale, a woofer is capable of reproducing frequencies above its optimum range, albeit gradually falling off as frequency rises. Higher frequencies from the woofer can interfere with the same from another driver (midrange, or tweeter if a 2-way design), causing peaks and dips in the overall response due to phase cancellations, which varies as  the woofer cone changes its position relative to the high frequency unit however many times a second the woofer cone is moving in and out with the low frequency signals. The effect of this can be avoided by removing the high frequencies from the feed to the woofer, which is what the crossover does.

With a midrange unit, both of the above effects apply, one at each end of its range, so the crossover removes both higher and lower frequencies than the mid unit’s range.

A passive crossover does this by using capacitors, which inhibit lower frequencies but not high frequencies (the frequency above which the effect is negligible depending on the capacitance, and inductors, which work the other way round. So a capacitor in series with a tweeter can remove, or rather reduce, low frequencies, and an inductorreduce high frequencies from reaching the woofer. And a capacitor in parallel with a woofer will pass the high frequencies a bit like shorting them out more than the low frequencies, reducing the high frequencies getting to the woofer’s voice coil, and similarly an inductor in parallel with the tweeter. So, using inductors and capacitors selectively in series and parallel, and multiple stages of them, the degree of removal of unwanted frequencies can be increased, usually referred to as increasing the ‘slope’ of the crossover, expressed in dB/octave. In addition, additional attenuation of one (or more) driver relative to another can be achieved with resistors, balancing outputs, and other filtration can modify the response, e.g to compensate for less than perfectly level response of a driver.

(An active crossover does the same thing, though in a different manner, in the signal line before the power amp, when a separate power amp is then used for each of the speaker’s drivers.)

A passive crossover can be internal to the speaker, with just one pair of terminals externally connected to the power amp, or a speaker can have each driver connected directly to a pair of terminals, usable either with an external passive crossover, which will have a separate output to be connected to each of the speaker’s inputs, or with an active crossover with a different power amp connected to each speaker terminal. Whether using an external passive crossover, or an active crossover, it is vital that they are connected correctly, as otherwise destruction of at leat the tweeter and very possibly the midrange unit may be very quick.

There is another variant with an internal passive crossover, which is wher a pair of terminals is  still provided for each driver, intended for bi/tri-wiring. (An idea I have never really understood). So three pairs of terminals does not necessarily mean no internal passive crossover! 

If a speaker has no internal crossover, IMO it should be clearly marked to indicate such, to avoid someone thinking it is just a set of bi- or tri-wire terminals and connecting accordingly. This certainly should be done if a crossover is removed where normally would be present.

Bi/Tri-wiring terminals also allow for what is known as passive bi/tri-amping, sort of a halfway house to active bi/tri-amping, with some benefit arising from the restricted frequency range of the power amp’s output that is used, but still passing the high energy signal through the passive crossover components. (Not something I personally think is worth bothering with.)

In order to understand crossovers you've got to look at the whole picture.

Start with a stereo amp driving 2 speakers with single inputs. Here you would run a single speaker cable to each speaker.

The speaker +ve and -ve  input sockets connect internally to a passive crossover , which is in fact a frequency divider, which splits the signal between the speaker drivers, according to frequency, sending only high frequencies to the tweeter and low frequencies to the woofer/midrange driver(s).  The crossover also helps to integrate the 2 drivers by 'shaping' the amplitude of the signals at the crossover frequencies to ensure a relative flat response over the entire spectrum.

Next up is bi-wiring. Essentially the frequency divider is split into 2 separate components...hf and lf and each is given its own set of speaker terminals. 2 sets of speaker wires are connected to each amplifier output, one set connecting to the speaker's hf input, the other to the speaker's LF input....so essentially each driver gets its own speaker cable. 

Following that is bi-amping, where 2 stereo amplifiers are used to drive the speakers.  This may be in a 'horizontal' arrangement where the L & R channels of 1 amp connect to both speakers' hf or lf inputs, or in a vertical arrangement, where 1 amp is used to connect to both the lf & hf inputs of a single speaker while  the 2nd amp connects to the other speaker. Here each driver gets its own amplifier channel and speaker cable

In all the above cases, the full signal is present at the amplifier output and passive components within the speakers divide the frequencies between the different drivers.

Next we move on to active speakers. Here the signal is divided before it goes to the amps, so each amplifier output only has the frequencies it needs for its dedicated speaker driver. Essentially, the pre-amp sends the unamplified full range signal to an active crossover unit, which divides the signal into different frequency bands suitable for the speakers' various drivers. The crossover passes those signals to dedicated amplifiers, which add the appropriate 'power' to the 'tailored' signal before sending to its dedicated driver. The advantage of active is that the amplifier sees  its driver load directly instead of seeing it indirectly through a reactive frequency divider (usually called the crossover). As a result, the amplifier has much tighter control over its driver, allowing it to stop and start with far greater precision. 

So there you have it. Multiple speaker inputs with a passive crossover allow bi-wiring or bi-amping, while multiple inputs with a dedicated external 'active' crossover are used for active operation.

 

 

 

 

 

 

Just to add to the discussion, some loudspeakers do not need any form of crossovers as they use full range drivers such as Lowthers and few others. This would be an ideal situation if there would not be some form of drawbacks to this approach at the moment.

Removing the need for a crossover with a "full range" driver is a great idea in theory, but such drivers are usually far from full range, at their best in the middle, and at their poorest at the extremes (which aren't so extreme - horn loading helps in the bass though). Of course, if you value speed and coherence in the midband, above all else, they may be ideal for you.  Anyone on here remember of get a listen to the Rehdekos?  Definitely a "marmite" loudspeaker...

My first ever speakers used full range units - Eagle FR8 In home-made reflex cabinets. They did pretty well at the bass end, going comfortably below 40Hz apparently undiminished, producing, as a HiFi magazine ‘reader’s casebook’ recorded, mind-numbing sounds from Pink Floyd’s Saucerful of Secrets, but as the same article noted, and I realised when a second home build relegated this drivers to midrange, a certai lack of anything high!

Against those, drivers like the Lowthers are in a very different league, but as Richard noted they really need horn loading, especially for decent bass output - or any at all - which is what they are designed for. In that mode their very limited  cone excursion ceases to limit bass output - though ver few people have the space for horns going below around 40Hz, so very bottom end will still be restricted in most applications, while I am unclear how the very top compares with dedicated tweeters, somehow not expecting quite as good (which of course may be fine for people in a certain age group.

BlackmoreC, you missed a step in your description of the distinctions

Active system:
preamp (full range) -> Active (i.e. electronic) crossover -> multiple power-amps (1 amp for each speaker drive unit)

Active speakers:
preamp (full range) -> Low level connection to each speaker containing an Active (i.e. electronic) crossover and multiple power-amps (1 amp for each speaker drive unit).

 

Richard Dane posted:

Removing the need for a crossover with a "full range" driver is a great idea in theory, but such drivers are usually far from full range, at their best in the middle, and at their poorest at the extremes (which aren't so extreme - horn loading helps in the bass though). Of course, if you value speed and coherence in the midband, above all else, they may be ideal for you.  Anyone on here remember of get a listen to the Rehdekos?  Definitely a "marmite" loudspeaker...

Nearly bought a pair many moons ago. I listened to them several times and on certain music they were the best speakers I think I've ever heard. However, get the wrong type of music and they could sound atrocious. They always reminded me a bit of Linn Sara's, very musical but very coloured. Often paired with Lavardin amps if my memory serves me correctly? This was a pretty devastating combo!

That's right Geko, they didn't need much power and the Lavardin was allegedly an ideal partner.  I agree, they worked well on some music, and not so well on other stuff.  

Quite different to Saras though - I still have a secret hankering for a good pair of early Saras, but knowing that the drivers will be past their best and the vacuum seals will likely be long gone, I'll stick with my happy, rose-tinted memories...

Thanks for the responses. So in short, a passive crossover uses capacitors/resistors to filter out unwanted frequencies? If a full speaker already had two or three separate inputs dedicated for each frequency of speaker, then the full range signal (all frequencies) to each subspeaker still gets filtered down with the capacitors/resistors before the tweeter or bass speaker gets the signal??

(I know that wasn’t short, but I’m trying to understand why active sounds better than passive and inter that passive still filters out all frequencies but the necessary ones for the e.g. tweeter AFTER its already been fed with its own speaker input (due to the bass also having its separate speaker input).

Consciousmess posted:

..........   So in short, a passive crossover uses capacitors/resistors to filter out unwanted frequencies

Not quite right,    capacitors & inductors are the low, mid, high band pass filter components.  Resistors are used to attenuate individual driver volume level and/or change some aspect of capacitor or inductor performance.       

Consciousmess posted:

 

(I know that wasn’t short, but I’m trying to understand why active sounds better than passive and inter that passive still filters out all frequencies but the necessary ones for the e.g. tweeter AFTER its already been fed with its own speaker input (due to the bass also having its separate speaker input).

That wasn’t the question you asked!

The  primary difference is that with passive crossover the high power signal from the power amp has to go through capacitors and/or  inductors and or have part of the signal shunted by one or other (and often multiple stages), before reaching the voice coil of the speaker driver, whereas with active the power amp is directly connected purely by wires to the voice coil, or sometimes high frequency units may be through a single capacitor to protect the unit from destruction under certain conditions, depending on the amp). At the bass end in particular that maximises the ability of the power amp to ‘grasp’ the driver, providing maximum control.

In addition, each power amp only has to amplify a limited frequency range, which can reduce inter modulation distortion. It can also allow different power ratings of amps to be used, as there is less energy in music as you go up the spectrum, and it is common to use a high power amp on bass, lower on mid, and lower still on treble.  N.B. This is purely to do with amp available power, not sound quality of the amp - for that the best should probably be on ,midrange. This is where the dichotomy occurs when actively driving with amps that are designed for use with passive speakers, when the better quality amp is often also higher power (as in Naim range) - a manufacturer of active speakers will generally use amps of identical sound quality but different power delivery capability, not providing unneceesary capability at higher frequencies.

What I don't really understand is how active crossovers are different.

A passive crossover (say an SBL one - let's not complicate things with multiple inputs which still lead to a crossover) splits one incoming signal into one track per driver, and filters each track such that only appropriate frequencies are passed to the driver.

Whereas an active crossover does.... what that's different?

I mean, reading back over active threads here shows people for example being really pleased with the difference made by having a supercap on their SNAXO rather than a hicap.  From a naive point of view that sounds a bit mental, even though I don't doubt that there could be a difference.

Edit, cross posted with IB, that'll teach me to get distracted by the telly!

I'm just guessing here..

Active crossovers need a power supply because they contain active components such as transistors? 

A bit like another pre-amp in between the pre-amp and power amp? maybe?

This could be why the quality of power supply will make a difference just as it does in a main pre-amp.

 

With an active crossover the preamp output signal is band pass filtered before power (speaker level) amplification.  The active crossover splits the pre-amp signal into high & low pass bands (with two way speakers) or additional mid band with a three way.  The active crossover outputs in each of its different frequency bands to individual power amplifiers for each of the individual drive units.      i.e. the drivers are connected directly to the power amplifier output & the speaker level signal is not affected by any passive crossover filter elements.

Aha, so part of the benefit is that with active, the signal filtering happens before a power amplifier gets its contaminating hands on the signal?

I suppose that'd also make sense of the difference between power supplies - the 'cleaner' the power to the crossover, the less the signal is affected.

Consciousmess posted:

If a full speaker already had two or three separate inputs dedicated for each frequency of speaker, then the full range signal (all frequencies) to each subspeaker still gets filtered down with the capacitors/resistors before the tweeter or bass speaker gets the signal??

Not necessarily, if I understand your question correctly. That would depend on the speaker.

For Naim speakers, if the passive crossover is removed, then the (now accessible) inputs for bass, treble etc are directly connected to the individual drivers. There is no filtering between inputs and driver and if you feed the tweeter input with a full range signal, you'll probably blow the tweeter.

For other speakers it might be like you assumed.

Never having operated or even heard, an active hifi system, I’m also slightly confused by the “dual” use of the word “active”...

In an electronics sense, as has been said, “passive” vs “active” is an easy separation between circuits (eg high and low pass filters, as used in the audio crossover application under discussion here) using only passive components - resistors, capacitors, and inductors - and those that also use active, or separately powered, components - typically operational amplifiers, transistors, or (with a nod) tubes / valves. If a crossover, essentially a combined high pass filter in front of (ie whose output feeds)  the tweeter and a low pass filter in front of the woofer, has only passive components (LRC...), then it’s a passive crossover. On the other hand, if any of the components are active (eg it uses an op amp as well), then it’s an active crossover. That’s the easy bit...

Many of the recent posts seem to use the “active system” term to describe one where the filtering happens after the pre-amp but before the power amp, such that the power amp is then connected directly to the speaker (tweeter or woofer, depending on if it’s after the high pass or the low pass filter). This could, of course, be effected using either an active crossover or a passive crossover.... hence the potential for confusion and talking at cross purposes (haha). 

In spite of my opening disclaimer, I do indeed have an active system at home: the DynAudio studio monitors attached either to our electric piano or our recording/mixing daw are powered and include an active (digital) crossover network to feed the (DAC that connects to) amps and audio drivers. 

No big ideas here, and no judgements on performance, just an attempt to join the conversation and point out that we are using one word in two related-but-distinct contexts. Happy to be corrected if I’ve missed the boat on the evolving inquiry...

Regards alan

Huge posted:

+1 to Mike-B's answer (and he really does know - he designed and built his own speakers, including crossovers!).

I also agree with Mike’s comment with another addition, the filters have to be carefully phase aligned to reduce distortion between the transducers, and provide consistent energy output from the transducers, as well as the filter cut off frequency and pass being carefully chosen to present optimum frequencies for the transducer within the design and provide minimum distortion at the filter cross over points.

Typically a two way speaker will have a low pass and high pass filter, a three way speaker will have a low pass, band pass and high pass filters.

The passive crossover  means the filters are passive, with active element coming from the amplifier. With active speakers, the crossover occurs before the power amplification either within the speaker or sepereately. Sometimes you see the term ‘active cross over’ used for where the cross over and amp is separate from the speaker and ‘active speaker’ used where the crossover and amps are in the speaker itself.. though I  note this terminology is not generally usd with Naim.

Having multiple inputs makes no difference to the filtering... the choice is aggregate the filters at the amp, multiple inputs, or aggregate at the speaker, single set of inputs on the speaker.

A passive crossover within a speaker is generally a big compromise, and certainly when driving any sort of power will start to become far from ideal as the crossover heats up from the resistive as opposed to reactive elements, and it’s critical values at crossoverpoints shift.

Indeed, a crossover between pre and power amps potentially could be ‘passive’, with components similar to those used in the crossover in a speaker - but quite different in practical terms because the values of capacitors and inductors relate to the quite low impedances of the speaker drive units (typically 4 or 8Ω), whereas before the power amp they would relate to the much higher impedances of the power amps (typically 18-50kΩ), so the component values would be much smaller (also physically smaller and cheaper). More commonly (maybe universally?) they are designed either with resistor-capacitor networks altering the frequency response of an amplification stage (analog active crossover), or, increasingly common and much more flexible, using digital signal processing (digital active crossover), either taking a digital music stream as its input, or taking an analog input from a conventional preamp and digitising it first.

And as already identified, ignoring the design of the crossover itself, the term ‘active’ has two implementations, an ‘active speaker’ putting the crossover and amplification (separate for each driver) in or attached to the speaker box, and ‘active system’ where they are separate from the speaker, allowing ‘mix and match’ of components by the user (but latter more complex and requiring greater understanding).

alan33 posted:

Never having operated or even heard, an active hifi system, I’m also slightly confused by the “dual” use of the word “active”...

In an electronics sense, as has been said, “passive” vs “active” is an easy separation between circuits (eg high and low pass filters, as used in the audio crossover application under discussion here) using only passive components - resistors, capacitors, and inductors - and those that also use active, or separately powered, components - typically operational amplifiers, transistors, or (with a nod) tubes / valves. If a crossover, essentially a combined high pass filter in front of (ie whose output feeds)  the tweeter and a low pass filter in front of the woofer, has only passive components (LRC...), then it’s a passive crossover. On the other hand, if any of the components are active (eg it uses an op amp as well), then it’s an active crossover. That’s the easy bit...

Many of the recent posts seem to use the “active system” term to describe one where the filtering happens after the pre-amp but before the power amp, such that the power amp is then connected directly to the speaker (tweeter or woofer, depending on if it’s after the high pass or the low pass filter). This could, of course, be effected using either an active crossover or a passive crossover.... hence the potential for confusion and talking at cross purposes (haha). 

In spite of my opening disclaimer, I do indeed have an active system at home: the DynAudio studio monitors attached either to our electric piano or our recording/mixing daw are powered and include an active (digital) crossover network to feed the (DAC that connects to) amps and audio drivers. 

No big ideas here, and no judgements on performance, just an attempt to join the conversation and point out that we are using one word in two related-but-distinct contexts. Happy to be corrected if I’ve missed the boat on the evolving inquiry...

Regards alan

It's still the same usage...

Active system
Preamp -> Active Crossover -> Poweramp
The crossover is built using active filters,  i.e. the passive components doing the frequency response modifications (capacitors and inductors) are built into the gain control circuitry of amplifiers - the amplifiers are built from active components, so the filter is an active filter.

Active Speakers
The speakers have their own built in amplifier - the amplifiers are built from active components.

Stephen Tate posted:

AFAIK - in Naim active crossovers there no inductors or chokes - inside my IXO crossover there are mainly transistors, tantalum capacitors, resistors with a large transformer - two pots.

The components look completely different to the ones in my PAXOs.

In an active crossover, an inductance can be created using an amplifier and a capacitor.  Think of an inductor as the oposite of a capacitor.  Put a capacitor in the negative feedback loop of an amplifier and it has the opposite effect to it being in the signal path.  The overall effect of this is just like having an inductor in the signal path.

Huge posted:
Stephen Tate posted:

AFAIK - in Naim active crossovers there no inductors or chokes - inside my IXO crossover there are mainly transistors, tantalum capacitors, resistors with a large transformer - two pots.

The components look completely different to the ones in my PAXOs.

In an active crossover, an inductance can be created using an amplifier and a capacitor.  Think of an inductor as the oposite of a capacitor.  Put a capacitor in the negative feedback loop of an amplifier and it has the opposite effect to it being in the signal path.  The overall effect of this is just like having an inductor in the signal path.

Ok, thanks Huge. One thing I could do when I went active was turn the volume up much higher than is usual in passive mode without any distortion or mushing up, oh those were the days!

Stephen Tate posted:
Huge posted:
Stephen Tate posted:

AFAIK - in Naim active crossovers there no inductors or chokes - inside my IXO crossover there are mainly transistors, tantalum capacitors, resistors with a large transformer - two pots.

The components look completely different to the ones in my PAXOs.

In an active crossover, an inductance can be created using an amplifier and a capacitor.  Think of an inductor as the oposite of a capacitor.  Put a capacitor in the negative feedback loop of an amplifier and it has the opposite effect to it being in the signal path.  The overall effect of this is just like having an inductor in the signal path.

Ok, thanks Huge. One thing I could do when I went active was turn the volume up much higher than is usual in passive mode without any distortion or mushing up, oh those were the days!

Also worth considering small signal inductors can also look like capacitors... they are very small and so you might not be recognising them...

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