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JBL 708i Monitor Review (DSP: Part 2)

Rate this speaker:

  • 1. Poor (headless panther)

    Votes: 28 22.8%
  • 2. Not terrible (postman panther)

    Votes: 69 56.1%
  • 3. Fine (happy panther)

    Votes: 23 18.7%
  • 4. Great (golfing panther)

    Votes: 3 2.4%

  • Total voters
    123

ExUnoPlura

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I believe it is a typo. Ran = Fan. The amp has a fan that makes noise when it runs.
Thanks. That makes sense.
 

Buckster

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Did this speaker really measure higher distortion with its tweeter at 96dB than the much much cheaper A130 ? Don't get it - if you look at the A130 tweeter there is hardly anything to it Vs a sophisticated compression driver
 

changer

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Everyone knows that an active crossover is always better than a passive one.

A known art to create them with electro-mechanical components, this you are completely making up. Acoustic slopes are acoustic slopes.
 
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sarumbear

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A known art to create them with electro-mechanical components, this you are completely making up. Acoustic slopes are acoustic slopes.
1) what is a mechanical component in a passive crossover?

2) you forget that a driver is a filter by itself with a frequency dependent impedance. How can you control the slope of the final filter?
 

GXAlan

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Did this speaker really measure higher distortion with its tweeter at 96dB than the much much cheaper A130 ? Don't get it - if you look at the A130 tweeter there is hardly anything to it Vs a sophisticated compression driver

The compression drivers always have high distortion unless you get into really exotic stuff. The benefit is the dispersion characteristics.

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If you look at the JBL 4319 a premium tweeter can do a very nice job, but that was a one-off tweeter that wasn't used anywhere else.


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And the vintage XPL90 with probably dried out ferrofluid even does better than the 708i.
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It does show why you have fans of both compression drivers and traditional tweeters.
 

abdo123

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I cannot for the life of me understand why would JBL designed this speaker for bi-amp use but not use an active crossover? What did JBL gain from that omission?

There will be no extra cost involved. Why not leave the passive crossover for the single wire operation then rewire the existing link so that bi-amp terminals will bypass the passive crossover. There is a DSP on each amplifier channel. Adding LP and HP filters to the EQ is free!

Because they don't want people to blow up their speakers i guess? Lots of ''I don't need to read the manual'' dum dums out there.
 

sarumbear

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Because they don't want people to blow up their speakers i guess? Lots of ''I don't need to read the manual'' dum dums out there.
This is a pro speaker aimed for post & broadcast market. “Dum domes” do not survive in that world.
 

sarumbear

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Are unfiltered tweeters a thing in the pro world? I have no idea.
Almost always an option.


Bi-amp or passive operation shall be selected by a switch at the rear of the enclosure. In bi-amp mode the loudspeaker shall be operated with a separate electronic controller providing a 2.2kHz crossover between mid and high frequencies.
 

changer

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2) you forget that a driver is a filter by itself with a frequency dependent impedance. How can you control the slope of the final filter?
1) Impedance was already measured for the driver without filters in the final enclosure when the engineers acquired a full set of polars and 2) then simulates the crossover. It is hence part of the equation. 3) A target curve for i. e. a low pass filter 24db/oct linkwitz-riley is superimposed on the simulation based on the measured data to control whether or not the text book proposal for a 24db/oct LR-filter does create the actual acoustic slope that is needed. 4) Where this is not the case in simulation and finally after remeasuring, the components of the low pass filter will be changed accordingly, until the acoustic slope of the driver perfectly matches the target curve throughout the effective range of the filter. It is only the acoustic slope/curve that matters, not what creates the filter.

Electro-mechanical might be a German lingo residue for caps, coils and resistors, nevermind.
 
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fineMen

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Looks poor from the perspective of a consumer. I don't know what a commercial user would expect from this. The sound power drops significantly between 1k and 2k. The vertical radiation is to narrow to be useful at 2kHz. The x-over woofer ./. tweeter is way too high as to match the 8" woofer. Alas, even if the passive x/o was circumvented, the tweeter would not allow to be used below that 2kHz, where it shows clear signs of overload.

The advertising says, that the compression driver outperforms any dome tweeter. There one may find a better alternative: https://heissmann-acoustics.de/en/test-vifa-xd-270-f4-waveguide-wg-300/ for just 50$ per side waveguide (!) and dome combined.

Astonishingly the raise in second order harmonic distortion, which is kind of typical for compression drivers, does not lead to equally increased intermodulation. To the contrary, as far as my measurements hold. But here the peak at 2kHz would, I suggest.

Even the bigger-than-life M2 shows some irregularity in the midrange, which are not originating in the ports' resonances. As if they wouldn't care.
 
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Robbo99999

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Did this speaker really measure higher distortion with its tweeter at 96dB than the much much cheaper A130 ? Don't get it - if you look at the A130 tweeter there is hardly anything to it Vs a sophisticated compression driver
Good point! I did try to take a look at the 308p review to see if the tweeter was any better in that one, but I couldn't say it's better (although my own in-room measurements show them to have correspondingly competitive distortion IIRC), but the A130 you mentioned which was reviewed by Amir, the tweeter looks better or not any worse re distortion at 96dB, and of course that's a lot lower price tiered product. Maybe tweeters have hit their limits in what they can do, and they can't do better regardless of what you spend in money on the tweeter driver (but someone will have to correct me on that).
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vs the 708i:
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Robbo99999

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The compression drivers always have high distortion unless you get into really exotic stuff. The benefit is the dispersion characteristics.

index.php

index.php



If you look at the JBL 4319 a premium tweeter can do a very nice job, but that was a one-off tweeter that wasn't used anywhere else.


index.php


And the vintage XPL90 with probably dried out ferrofluid even does better than the 708i.
index.php



It does show why you have fans of both compression drivers and traditional tweeters.
Dispersion is more about the wave-guide isn't it? (I don't profess to know it all as otherwise that wouldn't be a question & I'd be building my own speakers (lol).......but certainly the wave guide has an important influence as one of the factors). Actually I'd really like to have a go at building some DIY speakers some day with some help & knowledge gleaned from ASR, would be quite exciting - getting in some quality drivers for a fraction of the price of a finished speaker and trying to get the most out of them - what could go wrong, lol!
 

changer

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Correct, compression drivers are often used because of their output capabilities. A dome can also have a waveguide control its pattern.
 
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sarumbear

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1) Impedance was already measured for the driver without filters in the final enclosure when the engineers acquired a full set of polars and 2) then simulates the crossover. It is hence part of the equation. 3) A target curve for i. e. a low pass filter 24db/oct linkwitz-riley is superimposed on the simulation based on the measured data to control whether or not the text book proposal for a 24db/oct LR-filter does create the actual acoustic slope that is needed. 4) Where this is not the case in simulation and finally after remeasuring, the components of the low pass filter will be changed accordingly, until the acoustic slope of the driver perfectly matches the target curve throughout the effective range of the filter. It is only the acoustic slope/curve that matters, not what creates the filter.

Electro-mechanical might be a German lingo residue for caps, coils and resistors, nevermind.
Have you seen the filters in this speaker? Do you think they are anywhere near LR4?

1672862890934.png


Have you heard of Linkwitz Transform and why it is needed?
 

sarumbear

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changer

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Have you seen the filters in this speaker? Do you think they are anywhere near LR4?

Judging from the vertical sonogram and the sizing of its distinct interference zones "eyes", this speaker features filters in the magnitude of 4th order. The near-field measurements after DSP that @amirm provided in this very thread furthermore confirm my assumption already put forth in part 1 of an asymmetric crossover with a 3rd order LP and a 4th order HP.

To repeat myself: what counts is the correct acoustic slope, not what causes it, and this is a very simple crossover.
 

sarumbear

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Judging from the vertical sonogram and the sizing of its distinct interference zones "eyes", this speaker features filters in the magnitude of 4th order. The near-field measurements after DSP that @amirm provided in this very thread furthermore confirm my assumption already put forth in part 1 of an asymmetric crossover with a 3rd order LP and a 4th order HP.

To repeat myself: what counts is the correct acoustic slope, not what causes it, and this is a very simple crossover.
You have skirted the issue of cascading two filters at different frequencies which is why the Linkwitz Transport was created.

Simple HP crossovers can never have perfect slopes due to their cut-off frequency being near to the Twitter resonance frequency.

The Twitter HP is clearly showing that there’s at best 12dB slope. If the slope was 24dB the response at 1500Hz would be 10dB lower.
 
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