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Thread: Active Loudspeakers - The what, where, why and how of your active loudspeaker project

  1. #1
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    Active Loudspeakers - The what, where, why and how of your active loudspeaker project

    This is a new thread that picks up on the discussion of active crossovers in the 4345 - 3 way project thread over on the Lansing DIY forums.

    https://www.audioheritage.org/vbulle...-with-JBL-2447

    https://www.audioheritage.org/vbulle...l=1#post442634

    Given this new thread is a technical reference l have posted it in the Technical Help Forum.

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    Looking over my shoulder.

    To the thousands of lurkers watching this thread try and interpret each post from your own perspective.

    Everyone has has their own level of hands on experience, insights and accumulated technical understanding when it comes to active crossovers.

    Context is a powerful thing in the face of ambiguity and rhetoric.

    Putting the above paragraph into context it’s about where you yourself are starting from (1), your goals, your expectations and what information (2) you have available to implement your bi amp or fully active system correctly. (See (1) and (2) is the following posts below for why this is important).

    Getting started.

    Try and avoid aiming for perfection at the outset.

    The best approach is to take it step by step starting off with bi amp mode of operation. This will prepare you and pave the way for tri amp mode or full active operation should you wish to do that. Taking a step by step approach will provide you with experience, new insights and a more realistic view of what you want to achieve with your system.

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    Everyone is at a unique station in life.

    Reading on the forums about how someone was able to set up their bi amp or active system is only really valid from their own perspective.

    In this post Rob discussed his experience with active crossovers:

    https://www.audioheritage.org/vbulle...l=1#post442639

    (1) At the end of this post Rob comments on ease of which you can create filters using DSP is impressive so you can try and make changes almost on the fly. But that is an individual perspective and we all exist in a different station in life.

    Rob has years, decades of diy loudspeaker experience behind him and number of important skills he has picked up along the way. So Rob’s comments are valid in his situation.

    Yes you can make changes quickly with a DSP active crossover but unless you have an in-depth understanding of crossover networks and how dsp active crossovers work you most likely will be shooting into the darkness of the black hole.

    If your curious with what this is all about and new to the scene it’s not going to a walk up start. I have personally met someone who spent a fortune on state of art jbl drivers and attempted an active system with a DEQX active crossover believing it would be easy based on DEQX marketing at the time.

    DEQX active crossovers have employed an auto wizzard that promise according to DEQX marketing to work the magic for you. Please do not believe the hype.

    This person ended up selling everything because he gave up trying to make it work.

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    (2) Obtaining complete and accurate information to make your bi amp or fully active JBL system work the way it should.

    Let’s first look at the pre determined JBL passive loudspeaker systems

    Unfortunately grabbing complete and accurate information to bi amp or convert a passive system to full active mode is not as straightforward as we would like.

    If your starting from scratch looking at a graph and some technical numbers such as shown in the attachments this is not enough for you to grasp exactly how to realise the true potential of a bi amp mode or a fully active system.

    JBL contemporary passive systems

    JBL do provide some guidelines in the owners manuals on connections to the binding posts for bi amp mode but the technical information for the crossover characteristics is sketchy. Unless the user has a detailed familiarity with loudspeaker crossover technical “jargon” the information provided is like reading a foreign language. Refer to the attachments.
    Attached Images Attached Images   

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    The work around

    In the absence of complete information on the crossover characteristics the work around is to obtain detailed frequency response measurements of the passive system and that of the individual drivers. This can be done with REW and some technical guidance. REW have a very good online Q&A and there are tutorials on the www. These measurements are the templates for obtaining the optimal crossover characteristics with an active crossover. In some cases the acoustic response measurements will closely match a 4th order Linkwitz Riley crossover characteristic. But this is not always the case.

    Be aware however that the crossover characteristic that you program in the DSP active crossover will most certainly NOT be a perfect 4th order Linkwitz Riley crossover curve. This is due to the fact that most loudspeaker drivers do not have flat text book response curves in the crossover region. Many people have not understood this. You may also need to account for some equalisation incorporated in the passive crossover characteristics. A DSP active crossover may have PEQ filters to obtain the correct acoustic crossover curves.

    As a recommendation shifting the crossover point +- 1/3 octave is one approach to obtaining a reasonably close match to your acoustic driver measurements with the passive crossover. If this does not seem to work on the horn high pass curve try a 2nd order Linkwitz crossover character and repeat adjustment of the crossover frequency. Failing this you may need to raise or lower the filter Q of each filter section. This will create a peaking effect at the crossover point.

    If you are able to match the passive acoustic crossover characteristics it is not necessary to apply a dsp delay in an attempt to time align the woofer and the horn compression drivers. This is because the crossover frequencies and the acoustic response in the crossover region accounts for the driver phase alignment.

    A test to confirm or validate the crossover characteristics is to reverse the electrical connections of the mid or high frequency driver. Then measure the system response in the crossover region. A uniform null of 20 db should occur at the acoustic crossover frequency. Re adjust the crossover points of this is not the case.

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    If you want to try a dsp time alignment first obtain a very close match to a 4th order Linkwitz crossover characteristic for the woofer and the horn at identical crossover frequencies. Then with the horn compression driver wired out of phase take a frequency response measurement. Adjust the delay on the woofer until a uniform null of 20 db is obtained. Then wire the compression driver in phase with the woofer. Some adjustment of the crossover frequency +- 1/3 octave maybe required to optimise the subjective performance.

    Analogue active crossovers
    If this is something you don’t feel confident in doing consider using a custom analogue active crossover. Consult with the manufacturer to arrange the correct crossover characteristics. Some measurements of the passive crossover maybe required.

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    The key benefits of active crossover networks

    Depending on exactly what you think your trying to achieve there can be merit in using an active crossover network. If you have or can obtain all the necessary data, access expertise, guidance advice and have the time to implement it correctly you were will succeed.

    A well implemented bi amp and active system exhibits increased dynamic range, improved sensitivity, lower distortion and improved accuracy. This translates into a more vivid life like subjective presentation that brings you closer to the original recorded event.

    The electrical losses in a passive network and the complex reactive nature of passive crossover elements does take the edge off the dynamic performance of woofers and midrange cone transducers. Compression drivers are for the most part a dc load. The horn loading of the compression driver can interfere with a passive network where adequate precaution is not taken to minimise impedance fluctuations.

    The benefit of active EQ is that custom EQ adjustments can be more conveniently realised.

    Time alignment provides an alternative to phase tracking in the crossover region.

    An active crossover can improve low frequency extension and moderate the impact of room modes.

    Improved precision in matching driver levels can improve the coherence of the system. More precise matching of left and right driver level can improve sound stage presentation.

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    Talking

    Myths de bunked

    Sometimes it’s about reaching an understanding of what’s possible and to de bunking some of the myths and over glorifications that seem to float around cyberspace about active loudspeaker systems.

    Most contemporary loudspeaker engineers- designers with real qualifications would prefer to implement their products as active systems. Small active monitors are popular in the home studio market. They are great for getting a much bigger sound than you would expect out of a small bookshelf sized enclosure.

    But not in the consumer hifi home audio market that has a long heritage of separate audio components. Before passive loudspeaker can work and in fact sound any good the loudspeaker engineer - designer has to do a lot more massaging with a passive crossover than you might think.

    What really goes on inside your passive loudspeakers?

    The passive crossover, the drivers and the enclosure are carefully adjusted to work as a “system” to achieve an acceptable subjective outcome. A passive loudspeaker system must incorporate compromises including losses of system sensitivity, bandwidth, system placement and power handling.

    We put up with it because it’s what we know and we accept.

    A full active system can remove some of these compromises. Refer to my comments above on the virtues of an active loudspeaker system. Design of an active system from scratch or converting a passive system into an active system is not a piece meal exercise.

    What you need to know when you remove the passive crossover?

    Referring back to my earlier comments what the loudspeaker engineer-designer did with the passive crossover is now completely removed. What you don’t know is how the system would have been set up if it was an active system in the first place?

    Passive crossovers general work on the basis of attenuation of sounds across the audio spectrum. Active crossovers generally work by boosting and attenuation of sounds across the audio spectrum. They sometimes pull a few tricks too. These modifications to the tonal balance exist to make the loudspeaker subjectively acceptable to listen to. This is worked out not only with paper graphs but with many hours of listening in real listening rooms and a lot of trial and error error.

    Why you can hear but you can’t see?

    Your ears are not just listening to a snap shot or a slice of the loudspeaker sound output. Your ears are also listening to the impact of the loudspeaker distributing sound energy into the listening room. The significance of this arises when attempting to obtain the correct tonal balance from the loudspeaker system.

    It’s important to have the correct tonal balance at frequencies where your ear is particularly sensitive such as in the 2000 hertz region and at low and high frequency extremes for the system to sound tonally balanced.

    As mentioned above sound is being distributed into the listening room by the loudspeaker and does not just stop immediately after it arrives in the room. Sound decays according to the room acoustics. This is referred to the reverberation time and is measured by recording the time required for a sound to naturally attenuate to an inaudible level. The arrival of the next sound will be masked if the first sound has not attenuated quickly enough.

    These aberrations or artefacts are always there whether you are consciously aware of it if not.

    Depending on the room acoustics these artefacts might be perceived at slap echo, flutter, booming and or ringing resonances. The source or cause of the artefacts exist whether the loudspeaker is in the room or not.

    An active crossover be it dsp or analogue cannot remove the source or cause of the artefacts your ear hears in the room. It can only modify the apparent loudness or intensity of the artefacts. Attempting to remove these offending sounds can cause a loss of important program information all together.

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    What’s possible

    The loudspeaker engineer- designer has no control over the issues outlined above. But adjustments can be introduced in the loudspeaker system to reduce the apparent intensity of certain frequency regions sensitive to your ears in the listening room. The loudspeaker does not exist in an infinity large space in the listening room. The shape of the loudspeaker and the size of the baffle will influence the frequency response and the diffraction of sound from the loudspeaker baffle.

    To repeat its important to realise that EQ correction & phase adjustments can only influence or modify the apparent intensity of what your ear hears in the room. Not remove the source of the problem.

    Sometimes these tonal balance adjustments are permanent contours or they can be switched or variable adjustments with the passive crossover. It’s therefore a good idea to obtain some frequency response measurements of your passive loudspeaker system in your room before attempting to convert it to an active system.

    Q & A next

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    Ian,

    This could be a cool thread. I like and agree with your suggested approach of doing things in a very methodical, systematic way. Baseline measurements are super important.

    I think there are lots of us on the LH forums that have pursued a bi-amp setup with a two-output (high-pass and low-pass) external crossover, but probably fewer who have tried a full active tri-amp or quad-amp setup with the big JBL 3-way/4-way systems. This is something I've been wanting to try for a long time, and it might be fun to chronicle it here on the LH forums (if anyone is still around)!

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    Red face

    Quote Originally Posted by mattking52 View Post
    Ian,

    This could be a cool thread. I like and agree with your suggested approach of doing things in a very methodical, systematic way. Baseline measurements are super important.

    I think there are lots of us on the LH forums that have pursued a bi-amp setup with a two-output (high-pass and low-pass) external crossover, but probably fewer who have tried a full active tri-amp or quad-amp setup with the big JBL 3-way/4-way systems. This is something I've been wanting to try for a long time, and it might be fun to chronicle it here on the LH forums (if anyone is still around)!
    I Matt,

    Thanks for your comments,

    A thread without something visual is a bit bland. I plan to put together a case study that illustrates the key points l made in my lengthy posts.

    One thing l missed was that an active crossover is not like buying or using a preamp.

    A preamplifier selects from a known source then amplifies the signal with a defined gain. There is only one control and that’s the volume knob.

    You don’t need to program it or find out and interpret information on how to program it. Then figure out how to adjust it.

    Having analysed the process of using an active crossover there are two roads:

    1. You need to have a step by step approach from the very outset and be prepared to do some
    research and work.
    If that is not something you are interested in refer to 2 below.

    2. You purchase an active crossover that can be programmed for pre determined loudspeakers that
    have facilities for bi amp mode of operation.

  12. #12
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    About 15 years ago l got involved in active crossover and built a diy clone of the Pass Labs XVR1.

    What l soon discovered and then validated with an LHS forum member in CA is that most pro and consumer active crossover significantly impact on the transparency of the original passive system. You get firmer bass but the rest sucks. This is more noticeable with good amplifiers like Bryston, Macintosh, Pass Labs and other high fidelity power amplifiers and preamps.

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    My other earlier post on the myths of active crossovers is to highlight what you can and can’t change.

    The signal processing capability of a dsp crossover might look impressive and make you think it will help you make a perfect loudspeaker and is easy to set up. Neither claim is true.

    As many have discovered if you buy a dsp crossover on the basis of those two claims it’s a road to hell.

    This is illustrated by the number of posts and detailed explanations by Pos and others on how to use a dsp crossover. Secondly people realise it takes an eternity of adjustment and Re adjustments before they start to feel remotely confident they have got it right.

    It ends up being a very bad customer experience. But humans do like to believe in miracles once they are sold the notion of a miracle.

    The forthcoming case study will reveal why option 2 with a high quality design is the pragmatic option.

    The whole idea behind sound reproduction is to listen to the music.

    Not the system because you have no idea if you have got it right.

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    Case study

    Bi amp of the JBL Pro 4343.
    System sensitivity 93-94 db 1 watt one metre
    Power rating 75 watts continuous

    This particular system is now part of JBL’s journey in multi way studio monitors.

    Back then these systems were empirically developed using trial and error measurements. If you can find it JBL comments in an article that the 4343 system exhibited a 2 db bump in its bass mid crossover region due to mutual coupling of the 2231A bass driver and the 2121A mid cone driver. JBL determined the crossover frequencies in bi amp mode using the JBL 5234 active frequency dividing network should be spread from a 230 hertz low pass filter and 280 hertz high pass filter.
    Attached Images Attached Images  

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    But this is the thing

    These days no body makes a stock active crossover network with an off the shelf crossover setting like that.

    So the unsuspecting user plugs in a DBX analogue active crossover or an Ashly analogue crossover purchased off EBay. These are low cost mass manufactured pro audio electronic components with dated internal components and outdated design. They turn the adjustable crossover frequency too 300 hertz. She’ll be right mate.

    The L pads are set up by ear and over a period of time the user becomes acclimatised to the system presentation.

    In the ensuing period the user battles with noisy earth loops by using ground cheater plugs.

    The user mainly listens to the system at 3m away and put up with a constant hissing sound from the horn. They blame the horn sign it’s noisy sea shell sound.

    While the above is not said to offend anyone Is it putting this system in its best light?

    Probably not. Read on and l will reveal the real facts
    Attached Images Attached Images  

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