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JBL 708i Monitor Review (DSP: Part 2)

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  • 1. Poor (headless panther)

    Votes: 28 22.8%
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    Votes: 69 56.1%
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    Votes: 23 18.7%
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    Votes: 3 2.4%

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sarumbear

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For the tweeter in 708i the 2409H 8ohm they are using a condensator with 1uF. It is always active regardless of single wire or bi amp mode.

In bi amp mode the passive crossover for the woofer is bypassed
and it is using a 1,5khz LR24db crossover both in the BSS presets and Crown DCi N
A 1 micro farad capacitor with an 8ohm load you are well into 20kHz. That value cannot be true.
 

sarumbear

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It’s really not hard, especially if you just run it in single amp mode. Which is what the engineers responsible for the speakers recommended anyway.

After all this I’d be taking measurements of any B-stock 7-series purchases to make sure the tweeters aren’t out of spec. Unfortunate but true.
You just say it's not hard, it works as intended, or irrelevant but never explain why? Unless you explain or answer to clear questions, and not only from me, I can only assume that you have no idea.
 

Tangband

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A 1 micro farad capacitor with an 8ohm load you are well into 20kHz. That value cannot be true.
Hmm…Maybe its 10 uF for protection ?
A single capacitor can be had in series to do some ( unperfect ) equalisation of the tweeter horn , but the value would be something like 3,9 uF or such. If the compressiondriver has a flat response and as low sensitivity as the bassdriver.

Or….

If the 1 uF is true, then its doing some heavy equalisation all the way up to 20 Khz, more than 20 dB compared to the crossover frequency. Maybe its needed….. the horn is probably 20 dB more efficient than the bass driver, at least between 1,5 - 6 KHz. And the high frequency response of an unequalized horn is probably terrible.

If this is correct then its one of jbl:s horn-secrets then - a 1 uF capacitor and a 11 Ohm resistor in parallell.
 
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beatelund

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Hmm…Maybe its 10 uF for protection ?
A single capacitor can be had in series to do some ( unperfect ) equalisation of the tweeter horn , but the value would be something like 3,9 uF or such.

Or….

If the 1 uF is true, then its doing some heavy equalisation all the way up to 20 Khz, more than 20 dB compared to the crossover frequency. Maybe its needed….. the horn is probably 20 dB more efficient than the bass driver, at least between 1,5 - 6 KHz. And the high frequency response of an unequalized horn is probably terrible.

If this is correct its one of jbl:s horn-secrets then - a 1 uF capasitor and a 11 Ohm resistors in parallell.
1uF i correct, it works as intended and sounds great!
I use that in my diy 708i as is a perfect copy of the original.

In some way the jbl engineers thinks that it works better than padding down the compr. driver and use a steeper slope
 

Tangband

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1uF i correct, it works as intended and sounds great!
I use that in my diy 708i as is a perfect copy of the original.

In some way the jbl engineers thinks that it works better than padding down the compr. driver and use a steeper slope
Its a nice way to lower the noise from the amplifier also. And to do both eq, protection and lower the sensitivity for the horn, using only one crossover component.
I did this in my Hybrid project, but I didnt need to lower the sensitivity and eq as much because I used a dome in a smaller waveguide, thus the value was 5,6 uF. This speaker was a hybrid of passive and active dsp crossover.

Thanks for this info.:)
 
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beatelund

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Its a nice way to lower the noise from the amplifier also. And to do both eq, protection and lower the sensitivity for the horn, using only one crossover component.
I did this in my Hybrid project, but I didnt need to lower the sensitivity and eq as much because I used a dome in a waveguide, thus the value was 5,6 uF.

Thanks for this info.:)
my nearfield rew measurement IR window 20ms.

Roon is doing the dsp in its convolution processing.
I have exported thoose from REW, where i did a measurement of the bss outputs from the 708i settings.

I think they line up very well with amirs on axis measuerments
 

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Robbo99999

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Tweeter filter is 20khz HF Butter 6db.
It also has a 11ohm resistor in parallell so i dont know how that converts. But i suppose it Will be lower Maybe 12-15khz But 6db.
WUT!! 20kHz 6dB, that's no kind of a crossover between a woofer & tweeter, you've done a typo?! also 12-15kHz is also nonsense.
 

sarumbear

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1uF i correct, it works as intended and sounds great!
What is the intention?

A simple calculation shows that a 1 microfarad capacitor to a 8ohm load forms a HP filter at 20kHz. I.e. the FR will slope down starting at -3dB at 20kHz and reaching to -18dB at 2.5kHz.

If you use that 11ohm resistor in parallel to the driver the frequency will go much higher.

If a clever and unorthodox crossover is utilised I hope someone with the knowledge of the speaker will come up with a scientific explanation other than the slowly becoming the norm, “it works as intended” reply.
 

jhaider

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You just say it's not hard, it works as intended, or irrelevant but never explain why?

Let’s start over, as simply as possible. Ignore the random postings of screenshots from hacked files that may or may not be complete and accurate, and focus on the known and relevant information - such as the published spin and @amirm’s Klippel scan of the complete system.

The raw speaker is designed to be used in conjunction with files loaded into a Crown DCIn amp (as @amirm tested it here) or a BSS processor, or using presets available in JBL’s Intonato processor.

It can be run off of one processed channel or two. Their engineers recommend the former. Note they call the latter case is called “bi-wire” not “biamp,” which is IMO a considered demotion.

If you look at the measured frequency response of the DUT here, it’s largely as expected based on the published spin, except for a deviation in the region around 7kHz. That means the highpass and lowpass filtering are working as intended, because the cross is almost 2 octaves below that. The errant treble is explainable by variances in the raw tweeter response. The measurements I posted show the tweeter on my 708i’s have lower output in that region, which would sum flatter with the filter.

And that’s what actually matters here. All the inside baseball stuff about crossover parts and such is a pointless sideshow.
 

Tangband

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What is the intention?

A simple calculation shows that a 1 microfarad capacitor to a 8ohm load forms a HP filter at 20kHz. I.e. the FR will slope down starting at -3dB at 20kHz and reaching to -18dB at 2.5kHz.

If you use that 11ohm resistor in parallel to the driver the frequency will go much higher.

If a clever and unorthodox crossover is utilised I hope someone with the knowledge of the speaker will come up with a scientific explanation other than the slowly becoming norm, “it works intended” reply.
Being no expert on this specific jbl compression driver ( except I know the one used in jbl studio 5 ) , their compression drivers seems to drop sharply above 12 kHz , so it might need the eq from a 1 uF. The horn also amplifies everything between 1,5 - 7 kHz, more than 6 dB.

It would surely be nice If a JBL constructor could participate.
 

sarumbear

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Let’s start over, as simply as possible. Ignore the random postings of screenshots from hacked files that may or may not be complete and accurate, and focus on the known and relevant information - such as the published spin and @amirm’s Klippel scan of the complete system.

The raw speaker is designed to be used in conjunction with files loaded into a Crown DCIn amp (as @amirm tested it here) or a BSS processor, or using presets available in JBL’s Intonato processor.

It can be run off of one processed channel or two. Their engineers recommend the former. Note they call the latter case is called “bi-wire” not “biamp,” which is IMO a considered demotion.

If you look at the measured frequency response of the DUT here, it’s largely as expected based on the published spin, except for a deviation in the region around 7kHz. That means the highpass and lowpass filtering are working as intended, because the cross is almost 2 octaves below that. The errant treble is explainable by variances in the raw tweeter response. The measurements I posted show the tweeter on my 708i’s have lower output in that region, which would sum flatter with the filter.

And that’s what actually matters here. All the inside baseball stuff about crossover parts and such is a pointless sideshow.
Thank you.

I’m sure you know that bi-wire is a single amp connection type. However, we are discussing the bi-amp use of the speaker.

I read the manufacturer documents too but cannot see anywhere it says it’s bi-wire. Maybe you can correct me. Everything I read shows a bi-amp configuration where two amps are used. It’s normal to use an active crossover where the drivers are fed by an active crossover. What is the benefit of using a separate amplifier for each driver then? However, so far not a single bit of information or proof was posted that shows information about an active crossover, i.e. show settings for the filters not for the EQs. Have I missed it? If so please be kind enough to show me.

I do see a faster slope on the tweeter but we do not know how that is achieved.

However, do you see an active crossover, or in fact any crossover used on the woofer in this chart? The driver LP slope is slower than 6dB/octave. It looks like there is no crossover on the woofer. The slope is most likely achieved mechanically (inherent slope of the driver).

1673193660862.png


The purpose of speaker reviews on ASR is dissect all aspects of the unit, understand it and praise or shame accordingly. We do not know enough about this speaker to judge it.
 

Robbo99999

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Being no expert on this specific jbl compression driver ( except I know the one used in jbl studio 5 ) , their compression drivers seems to drop sharply above 12 kHz , so it might need the eq from a 1 uF. The horn also amplifies everything between 1,5 - 7 kHz, more than 6 dB.

It would surely be nice If a JBL constructor could participate.
Well this could contribute to it making more sense re what looks like silly high reported crossover points chosen by the passive filters. For what it's worth, as I've said in earlier post, it's the woofer nearfield measurement that looks the most weird as it doesn't look like it has any steep crossover point, the tweeter crossover point in nearfield looks more reasonable. I thought maybe Amir's nearfield measurment of the woofer was not totally accurate and maybe contaminated by the tweeter. I'd quite like to know the in's and out's of how all the DSP and passive filters have been put together, so your comment about asking for a JBL constructor/designer to chime in is indeed welcome, but I guess it doesn't change the end result of what Amir measured in terms of his final spin......but it would go a ways to understand the workings of the speaker and perhaps some of the reasons for some of it's shortfalls.
 
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sarumbear

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Being no expert on this specific jbl compression driver ( except I know the one used in jbl studio 5 ) , their compression drivers seems to drop sharply above 12 kHz , so it might need the eq from a 1 uF. The horn also amplifies everything between 1,5 - 7 kHz, more than 6 dB.

It would surely be nice If a JBL constructor could participate.
Interesting…But why would you not use the DSP to tame such a problematic driver in the bi-amp mode? What do you gain with combining passive crossover with a DSP, where you have dedicated amplifier for the driver?

Also, we know that there’s a connection change between single-wire and bi-amp mode; what does that do if not used to cancel the passive crossover on the horn driver?
 

GXAlan

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It would surely be nice If a JBL constructor could participate.
I'd quite like to know the in's and out's of how all the DSP and passive filters have been put together, so your comment about asking for a JBL constructor/designer to chime in is indeed welcome, but I guess it doesn't change the end result of what Amir measured in terms of his final spin......but it would go a ways to understand the workings of the speaker and perhaps some of the reasons for some of it's shortfalls.

Charles Sprinkle (designer of 7-series) hasn’t logged on here since 2021 and it may be tough to get comments from him since he is at Kali Audio and may not be able to talk about JBL Pro due to confidential information.

The Kali Audio line has the same fundamental strategy of a passive crossover that doesn’t correct to be flat and uses an active DSP.

——
Greg Timbers designed the 5 series and has been interviewed by @hardisj recently. It may be possible to get Greg to get insights on the 5 series. Greg was unceremoniously let go from JBL after a whole career of successes and even if his position was downsizing, a bit more thoughtfulness is probably what all of us would have done. I imagine it is much easier to talk about biradial horns and what was done in the Studio 5 than it is to talk about the current technologies.
 
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beatelund

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Interesting…But why would you not use the DSP to tame such a problematic driver in the bi-amp mode? What do you gain with combining passive crossover with a DSP, where you have dedicated amplifier for the driver?

Also, we know that there’s a connection change between single-wire and bi-amp mode; what does that do if not used to cancel the passive crossover on the horn driver?
It bypasses The passive crossover for the woofer.
The horn driver always has The passive crossover in The circuit.
 

sarumbear

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It bypasses The passive crossover for the woofer.
The horn driver always has The passive crossover in The circuit.
Interesting that there is a passive crossover and that it is switched in and out between active (bi-amp) and single-wire (passive) modes. The woofer resonances are almost exactly the same level between modes. Shouldn't a passive crossover would make an effect?

Do you happen to know what are the LP crossover elements?

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beefkabob

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It’s really not hard, especially if you just run it in single amp mode. Which is what the engineers responsible for the speakers recommended anyway.

After all this I’d be taking measurements of any B-stock 7-series purchases to make sure the tweeters aren’t out of spec. Unfortunate but true.
It really is a lot from concepts to execution. You're into this stuff to a strong degree, writing articles and such, so you might not see how much there is to get when starting from zero. I could do it eventually, but oof.
 

Buckster

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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.

GXAlan - sorry for delay in answering but I wanted to say a big-thankyou for replying and describing so well. Much appreciated
 

NTK

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Greg was unceremoniously let go from JBL after a whole career of successes and even if his position was downsizing, a bit more thoughtfulness is probably what all of us would have done.
To be fair to Harman, Timbers didn't leave them with much of a choice (i.e. he did it to himself).
 

Robbo99999

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Interesting…But why would you not use the DSP to tame such a problematic driver in the bi-amp mode? What do you gain with combining passive crossover with a DSP, where you have dedicated amplifier for the driver?

Also, we know that there’s a connection change between single-wire and bi-amp mode; what does that do if not used to cancel the passive crossover on the horn driver?
Charles Sprinkle (designer of 7-series) hasn’t logged on here since 2021 and it may be tough to get comments from him since he is at Kali Audio and may not be able to talk about JBL Pro due to confidential information.

The Kali Audio line has the same fundamental strategy of a passive crossover that doesn’t correct to be flat and uses an active DSP.

——
Greg Timbers designed the 5 series and has been interviewed by @hardisj recently. It may be possible to get Greg to get insights on the 5 series. Greg was unceremoniously let go from JBL after a whole career of successes and even if his position was downsizing, a bit more thoughtfulness is probably what all of us would have done. I imagine it is much easier to talk about biradial horns and what was done in the Studio 5 than it is to talk about the current technologies.
I don't think we can get much more elucidation on how this speaker is put together until we hear from the designers & company......not unless someone who owns these units (maybe combined with Klippel capability) gets down & dirty & really investigates this one down to the bone. However, we do know it's not an optimal implementation as per Amir's review, so we can Black Box it and say "Ok, it's not the best, we know what we're dealing with here, praps buy another speaker instead, lol!". (Professional installers in the industry may well take their own view, although even there I can't imagine it's optimal)
 
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