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Thread: Power Amps for JBL 4675C

  1. #31
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    martin_wu99, I am curious, what do you mean by "dynamics and others"? Can I find evidence for that?

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  3. #33
    Senior Member martin_wu99's Avatar
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    Quote Originally Posted by gameuno View Post
    martin_wu99, I am curious, what do you mean by "dynamics and others"? Can I find evidence for that?
    Dynamic Compression,it is measurable if you have some instruments
    46 lover

  4. #34
    Administrator Mr. Widget's Avatar
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    Quote Originally Posted by martin_wu99 View Post
    Dynamic Compression,it is measurable if you have some instruments
    Dynamic Compression as I know it relates to loudspeakers. Dynamic Compression is the condition where a loudspeakerís sensitivity is reduced after the the voicecoil heats during higher output playback. Essentially the speaker will not respond as it did when the voicecoil was cool limiting the output level on higher energy peaks. Typically lower quality speakers exhibit this more than better designs where cooling techniques are built into the drivers along with the use of better materials.

    Dynamic headroom on the other hand usually refers to an amplifierís power supplyís ability to provide short term peak current in excess of the steady state current required for its rated output.


    Widget

  5. #35
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    Mr. Widget is on target.

    The JBL document linked to by Mctwins is correct. But did you all read it? The overall emphasis on it is to not run an amplifier into clipping. That is absolutely true. I know that nothing I've written on this (and any other thread) has advocated or suggested it is okay to run an amp into clipping (or other high-distortion).

    However, if you are interpreting the document to mean to have an absurd amount of power reserve that you never use, you'd be mistaken. If you have an amplifier that delivers a clean 100w (wide-band, not just 1KHz) and you are not driving up to 100w, even on peaks, there is no good reason to go bigger on the amplifier except for ego or mysterious perceived improvements. The document even states that it presumes that for a given speaker that you want to get all of the SPL it has to give and thus bases its power suggesting on that. Playing a 4675C at its full SPL potential in a typical household situation borders on absurdity.

    There is a finite amount of level that a recording is going to give and it can't go above. There is a comfortable/desired listening level for the person that is selecting the equipment. With these two and the speaker's sensitivity, one can figure out how much amplifier is really needed.

  6. #36
    Senior Member Ian Mackenzie's Avatar
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    Itís about context and relate what is relevant to the OPs application.

    You donít have enough data from the OP on his max spl required so speculation is not going to provide a conclusion. In the document exceptions 1 and 2 are useful.

    Looking at it on a practical level most home users have no idea they are at the onset of clipping an amplifier unless the amplifier has accurate metering of the power output. But clipping will damage a loudspeaker and or the amplifier where the peak output voltage approaches or exceeds the supply rails. The result is a flattening of the sine wave towards a square wave.

    On a simplic level of the OP is using the system or hifi and he sits 2 -3 meters from the loudspeakers itís not difficult to calculate how much power is required.

    Back to the original post concerning how to improve the system if the loudspeaker is relatively sensitive which it is the first watt is the most important for the hifi user.

  7. #37
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    I agree, that the document has no justifiaction for running a 600W+ amp in my situation.

    My room is 3*4,5m, so for example 300W are definetly going to be enough. Bigger amps just arent needed, but I will take the damping factor of the amps into account, since that seems to have an impact on sound quality. (Getting the membranes to stop at the right time?)

    Id rather focus on improving sound quality with - as Ian says - EQ and DSP, maybe room correction. Now that I have found out that it wont be necessary to spend as much money as I can on amp, I might considering using a sub or woofer at the opposite site of the room, to get rid of room modes.

    I am aware of the theory behind this and I also know that I will have to take measurements and be very careful if I want a good result, but at least one can dream.

  8. #38
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    Quote Originally Posted by gameuno View Post

    My room is 3*4,5m
    And the 2360/2446 horn/driver are about a meter deep on their own so you are down to just 3.5m for listening...but wait, you probably have some comfy recliners or a sofa so take another meter off and down to 3.5m from horn to listening position. So what are we up to? 39W for 105dBc (playing pink noise). But yeah, if you want 115dBc (out of just one speaker) break out the 400W amp...unless you only wanted 115dBc from the pair, in which case your 300W amp will be MORE than adequate!

    I don't know what music you are listening to or how loud you like it but I strongly suspect that Ian is correct...that first watt will be what you are using for 99% of the time. Gute Nacht

  9. #39
    Senior Member Ian Mackenzie's Avatar
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    Quote Originally Posted by sguttag View Post
    My only comment there is that the passive crossover used in the 4675C was nothing special or particularly good. It was a token piece to make a passive system and it had to work for both the 4670 and the 4675 and did neither particularly well. Depending on the vintage of your 4675C, it would have the 4638TH LF cabinet or the 4648TH cabinet (different drivers and crossovers). While I'm no big fan of the M-series crossovers (never used them...I was using Rane crossovers during that era like the AC-22 and AC-23), I suspect it will do better than the passive that comes with this speaker. They have CD horn compensation and should hit the crossover points as well as the passive. I'd say with proper DSP based crossovers you could do better. That said, I've set up a ton of theatres with the 4675 with various electronic crossovers and the passive ones, the electronic ones generally came out better (less tuning, better sounding).
    I think it comes down to the expectations of the OP.

    http://www.jblpro.com/pub/obsolete/23606566.pdf

    Looking at the curves in this document the OP can obtain the best overall response in the listening window with some REW measurements. That could be just toe in of the system carefully or some form of EQ if absolutely nec.

    Hi fi being what it is not everyone is going to like converting an analogue signal from an analogue source to digital and then back to analogue.

    This system is already a very low distortion design so any degradation of the signal is best avoided.

    I would first get the best response in the listening window without EQ by adjusting the toe in.

    The top end can be easily modified with some passive EQ added in the passive network.

    Then start looking at the bass by way of a sub. Once the right balance is obtained options for improvement to the resolving power can be considered . This might include better crossover capacitors, a better power amp and preamp or design of a very high resolution analogue active crossover network and a small class A power amplifier for the horn.

    The reasoning for an analogue active network is that with such a sensitive loudspeaker your line level signal is going to be tiny. An analogue active crossover is going to be quieter and offer better very low level resolution than a digital dsp network under these conditions.

  10. #40
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    The OP said he already had M500 series crossovers...which are analog. I would agree that these speakers normally need very little in the way of EQ and I've always gone for a minimalist approach. I'm not a fan (through actual usage) of the passive crossover that is part of the 4675C so the use of an active crossover, for that speaker, is encouraged. In fact, that is the speaker that THX started with back in early 80s and designed their crossover (used in cinemas) around. A L-R 24 with appropriate CD horn boost and time-alignment.

  11. #41
    Senior Member Ian Mackenzie's Avatar
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    Yes he did.

    But we here have categorically proved (quite a while back) the M500 series are not strictly speaking a hifi unit in direct comparison to other devices. A pa active crossover in the pa or theatre business has always got a lot of benefits however when the focus is on hifi it has to be done on a better level.

    An active crossover will tighten up the bass a bit but can badly corrupt the fidelity of the rest of the sound.

    So the passive crossover an often win subject to the caliber of the overall system.

    Until now high performance active crossovers have been a rarity in consumer the hi fi audio market.

    I will discuss this on my visit to LA and SF later this month.

  12. #42
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    The OP said he already had M500 series crossovers...which are analog.
    The M552/553 come with a preset for both the 2380 and 2360 family of horns. These are approximations for each family so if you have the capability you may be able to do a superior passive comp depending on the driver horn combo. Certainly a good place to start though as they may work out just fine.

    Rob
    "I could be arguing in my spare time"

  13. #43
    Senior Member Ian Mackenzie's Avatar
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    White Paper

    http://www.jblpro.com/pub/cinema/2360b_wp.pdf

    Thread on 2360 application with 2450 driver with DBX260

  14. #44
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    I really wouldnt mind using a DSP, Im no analog-fanatic or anything like that. Since my source is going to be digital anyway, I could just put a DSP directly after that, which would give me pretty much no signal loss. Then I could go with a digital signal to a good DAC. I guess that would work best.

  15. #45
    Senior Member Ian Mackenzie's Avatar
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    Okay well that works for you.

    Once you start on this in your Man Cave you might want to tell your wife or girlfriend you are going to be pre occupied for quite a while.....Lol

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