JBL 708i Monitor Review (Passive: Part 1)

[Tangband]

Sure, i am joking about a few audiophile tabous...

One could argue why having a passive crossover at all in this 708 speaker, if one will use a dsp anyway? A dsp can do the crossover to, with steeper slopes ( this design really needs it ) and feeding the dsp with a digital signal, thus avoiding any unessesary A/D conversion should be a thing that every true audiophile would love ?

 

[ctrl]

My reply was about level matching the nearfield responses as per the emitter sizes. That is only required if you are going to create a summation to achieve the complete FR. As Klippel gives us the full FR, and as there is no summation on the near-field charts why bother with level matching? That is my point. Do you disagree?

Yep...
I thought I showed in post#141 why a level correction is advantageous. There is the following graph of the near field measurements of BR port (pink) and woofer (yellow) and the level corrected port frequency response (cyan) of a 2-way BR speaker:

Without level correction the port resonances would be estimated about 8dB too high. Now, without knowing the phase relationships of woofer and port, one can roughly estimate the theoretical maximum influence of the resonances on the overall frequency response by assuming the worst case of coherent addition (in-phase and out-of-phase addition).

Without level correction the overall frequency response could be influenced by resonances by +-4dB, with level correction only +-2dB - a huge difference.
But because of the relative phase shift of port and woofer, the influence will be much smaller in most cases, as shown in post#141.

The summation of BR-port and woofer frequency responses makes sense, as said in post#141, only if the measurements capture the phase correctly.
But since the NFS measurements also give us information about the decay behavior (e.g. CSD), the near field measurements (preferably with level correction) together with the NFS measurements are sufficient in most cases.

 

[Tangband]

A steeper active crossover is needed in this design. Using a 48 dB/oct crossover in a dsp would have solved the breakup issue ( blue arrow ).
I guess that this is exactly the thing they have done in the true active model, 708p.

 

[dcolak]

View attachment 254108

NOT ALONE! NOT ALONE!!!

 

[RobL]

The Genelec 8030c is an analog active speaker with no dsp, and it measures great.
How they have done it in the s360 model , it might be a different story and they have probably used the help of dsp to optimize it, but the measurement of this monitor is better than for the 708p. The price is higher to.

S360 has more than twice the cabinet volume of 708p. The design constraints placed on the jbl engineers certainly impacted their options at mitigation.

 

[Tangband]

S360 has more than twice the cabinet volume of 708p. The design constraints placed on the jbl engineers certainly impacted their options at mitigation.

Thats true - one cant fool acoustic laws.

 

[dfuller]

Almost every speaker has a form of EQ, even the passive ones. There’s nothing wrong with EQ as long as there are no measurable issues. The issue we are seeing on this speaker is the port resonance is within the range of the woofer. No EQ can correct that as it’s excited by the woofer. The only way to reduce the resonance is not to activate the port. It means the woofer level at the port resonance must be reduced, which will create a dip in the woofer response.

Those resonances cam be and should reduced by geometric corrections. JBL must have other reasons for not doing it. Production cost or enclosure size comes to mind.

Oh don't misunderstand me, I have no inherent issue with EQ. This just isn't something an electrical filter can fix and has to be done elsewhere as you've noted.

What that fix is however I'm not sure. My first thought would be to make the ports intentionally lossy in the midrange, a la Neumann, or go for a different port design like a slot port that might have less chance of high level ringing like that. However that might have other unintended knock-on effects. This speaker has absolutely insane output capability for its size, and that may have been priority #1 that other things were traded off against.

 

[Toni Mas]

One could argue why having a passive crossover at all in this 708 speaker, if one will use a dsp anyway? A dsp can do the crossover to, with steeper slopes ( this design really needs it ) and feeding the dsp with a digital signal, thus avoiding any unessesary A/D conversion should be a thing that every true audiophile would love ?

Imo crossover point and slopes are far too critical choices to let end users alter them. I dont know any brand allowing this, and dsp only allow some eqs to correct response or tonal balance. High pass filtering to ease subwoofer integration is the only exception i know

 

[RobL]

Oh don't misunderstand me, I have no inherent issue with EQ. This just isn't something an electrical filter can fix and has to be done elsewhere as you've noted.

What that fix is however I'm not sure. My first thought would be to make the ports intentionally lossy in the midrange, a la Neumann, or go for a different port design like a slot port that might have less chance of high level ringing like that. However that might have other unintended knock-on effects. This speaker has absolutely insane output capability for its size, and that may have been priority #1 that other things were traded off against.

Absolutely, given it’s size, durability and output capability (three of the design objectives, I’m sure) what competion does it have that bests it?

 

[danbei]

One could argue why having a passive crossover at all in this 708 speaker, if one will use a dsp anyway? A dsp can do the crossover to, with steeper slopes ( this design really needs it ) and feeding the dsp with a digital signal, thus avoiding any unessesary A/D conversion should be a thing that every true audiophile would love ?

It could very well be that the passive crossover is bypassed when the "Bi-amplified" connectors are used, letting the DSP do the crossover work. Or that the DSP adds steeper filters on top of the passive filters. The passive crossover would be there to also enable use with one channel of amplification if needed. I'm not sure if this is what was done in the 708i though.

 

[GXAlan]

…and just so it doesnt get lost in the second part of the review, it would be well worth the time if @amirm, you could see if the standard EQ in any AVR or AVP you have lying around is “reasonably” good with the 708i. What if you can get 90% of the performance with dumb EQ and the fancy external processing just gets you the last 10%? It might make sense to go all-in for your stereo channels but rely on “good enough” for your rear and heights to save costs but still get the output SPL and directivity benefits of this design.

 

[ROOSKIE]

One could argue why having a passive crossover at all in this 708 speaker, if one will use a dsp anyway? A dsp can do the crossover to, with steeper slopes ( this design really needs it ) and feeding the dsp with a digital signal, thus avoiding any unessesary A/D conversion should be a thing that every true audiophile would love ?

Read my post #153.

I don't think it is hard to see why a model with a complex crossover design that uses both active and passive components makes sense in some installs/use cases.

Just being able to use 1 channel of amplification vs 2 is more than enough for some.

Additionally this speaker doesn't need an AC connection. It can be used well away from AC power.

I think people who are criticising this speaker are really just saying the resonances that occur around 900Hz is due to poor design and could have been solved by better design to start with, because as it stands DSP can correct those resonances to a large extent but can't correct them perfectly - better results could have been achieved by a correct design in the first place before DSP was applied. Some people think that this compromise or oversight is not acceptable in such a high tier product.

Yes, I am aware of what a number of people think. My viewpoint is that there is no 'before DSP was applied'.
The speaker was always intended to intergate DSP.
There is no 'before'.
It only appears that way because we can measure it before connecting the entire intended product.
The use of a complex ' crossover' using both passive and active(DSP) capability is seemingly very confusing for a lot of folks.

 

[jhaider]

The other benefit of this design is ability to put as much amplification as you need. Running out of power is one of the top reasons active speakers run out of dynamics. They have to limit cost and heat generation when the amps are built into the speaker.

One interesting thing about power - from my discussions the engineers responsible for 7-series did not think bi-amping conferred much advantage for these speakers. In fact, they recommended a single channel with more power over active biamping! Maybe the company line has subsequently changed, given the engineering staff turnover post-Samsung acquisition and the release of the biamped P-models. I do not know as I've had no reason to ask. But that's what the people responsible for these speakers were saying 4-5 years ago.

At first I was puzzled, but after some reflection that made some sense to me as to these speakers. Going back to first principles, biamping has two benefits. First, you can control a drive-unit's individual performance, including outside its nominal passband. Second, your power goes to voicecoils rather than resistors and other lossy passive components. But, if your drivers have a steep enough passive crossover and reasonable out-of-band behavior (or you address cone breakup via passive notch filters per Purifi's guidance) the first advantage isn't so compelling. As for the second - there are definite noise floor advantages to having passive filters in front of a sensitive compression driver, as well has having more power available to the woofer. Admittedly, I have not tried them biamped. But it will be interesting to see how the final performance of the single-amped 708i implementation differs from biamped 708P, even if this particular 708i is not tested in biamped mode.

 

[Rick Sykora]

Yes, you put the BR port area and woofer Sd (logarithmically) in relation to each other and thus correct the different levels. But since Amir does not make "real" nearfield measurements anyway (the mic distance is too large for that), the measurements are only guidelines.

Amir always performs the measurements the same way, different speakers can be compared as long as the ratio of BR port area to woofer Sd is roughly comparable.

What you should pay attention to is the sound pressure level distance of the intended Helmholtz resonance (BR port tuning) and the resonances with the highest sound level and if the baffle step correction is included or not.

Here is the BR-port near-field measurement of a 2-way bookshelf speaker with 8'' woofer. In yellow is the frequency response without baffle step correction (BSC), in green with.
View attachment 253841
I didn't bother to damp and optimize the resonances of the BR-Port (simply a pipe without optimization). Since Q of the "loudest" resonance is high (the resonance is therefore relatively narrow), the design is just fine (mediocre).
Between the desired Helmholtz resonance and the undesired BR port resonances is about 11dB sound pressure difference without BSC. Good designs achieve considerably more difference (>15dB). This ensures that the BR port resonances hardly play a role in the frequency response of the speaker.

Now take another look at Amir's BR port measurement of the JBL 708i. There, the sound pressure difference without BSC is just about 2-3dB. This is not a good result for a speaker in this price range.
View attachment 253843



It is good that this has been pointed out. We can discuss the audibility or inaudibility for a long time (as for example with HD measurements).
But the fact is that a well-designed speaker in this price range does not have to show such severe BR port resonances (similar to how a certain SINAD is expected with hifi devices from a certain price range onwards).

Since these pics are useful for comparison, I wanted to point out that not all measured port resonances may not be solely due to the port resonating. They could be due to driver or other cabinet parts resonating too. There can also be more than one frequency at which a port resonates. The first pic above seems a pipe resonance from a simple cylindrical port. Whereas we know the JBL shown does not have a simple cylindrical port, more than just near field measurements are needed to diagnose the source(s) of the resonances shown. It may be a more complex port resonance but what is shown could also be some speaker part(s) resonating too.

As Dr. Toole points out, higher Q resonances like the speaker in the first pic are not as audible as broader ones. Unfortunately, the JBL resonances(s) from 500-1000 Hz are rather broad and likely more audible. As @ctrl and others have mentioned, this is more obvious as it makes a rather ugly showing in the FR measures too.

Before we condemn the JBL's designer(s), need to consider whether some of the issues shown here might be other defective parts, manufacturing problems or induced by other shipping/handling that has happened along the way to get to Amir.

 

[ROOSKIE]

Yep...
I thought I showed in post#141 why a level correction is advantageous. There is the following graph of the near field measurements of BR port (pink) and woofer (yellow) and the level corrected port frequency response (cyan) of a 2-way BR speaker:
View attachment 254165

Howdy, I showed this type of chart earlier Post #135 and it was largely ignored & to be fair with this speaker even with some large adjustment, the JBL speakers port leakages(pipe resonace) will have influence on the frequency response, so the question is ---------->

Oh don't misunderstand me, I have no inherent issue with EQ. This just isn't something an electrical filter can fix and has to be done elsewhere as you've noted.

-------------->

I think the term 'Pipe resonace' is really throwing people.
It is not (to my understanding) what it appears to me that many think it is.
It is linear output, it is behaving like an additional transducer in the area where the 'pipe resonance' is.
It might help to think of it as 'pipe leakage' even though it is a resonance.
All of the sound a transducer makes is a resonance.
The port is transducing in the midrange.
When 750hrz is passed to the speaker via content, the signal is being reproduced in the room via air movements by both woofer and leakage from the port.

If the pipe resonates(again think of 'leaking' in order to picture the effect) at 750hrz enough to add 4db of output there above the woofer output level you simply reduce the 750hrz signal by 4db. This can be achieved either through passive (very expensive and wasteful in a design that will use a known active signal input) or active methods.

Again the midrange output of the port is linear.
Measure this speaker at different SPL levels and you will get essentially the same frequency response.
You can therefore through PEQ/DSP/Passive elements neutralize the added output/air movement of the port in the midrange.

Some design compromise is always a factor. Some reasons have been discussed why a port maybe allowed to leak more strongly than some other design. None-the-less, if the port is going to transduce/leak/resonate audibly in the midrange then one can see how it makes sense to place it right next to the woofer to minimize interaction issues and then treat the resulting frequency response accordingly using the solutions available to the design.

Post DSP/PEQ/passive solution, the port in this design may still rough up the area a bit when viewed at 1/20,1/24,1/48 octave or whatever pixel peeing level you use due to possible minor comb filtering and minor phase issues.

The frequency response is essentially corrected easily.
If you fear other issues.
Look at the waterfall plots and the distortion plots.
IMD plots would likely help here.
So one would have to look at what measurements show the port response is an issue in the final EQ'd form?

A comparison test would be hard or impossible to orchestrate.
Though ask yourself this.

***In a blind test with this design, would someone be able to tell if some midrange sound is coming from the port and not only the woofer once properly combined to a flat response level?
If you think YES, then answer the question=WHY? ***

My personal answer right now is NO, it will not be audible.
I have not seen a reason demonstrated for such a thing as of yet.

 

[Rick Sykora]

Howdy, I showed this type of chart earlier Post #135 and it was largely ignored & to be fair with this speaker even with some large adjustment, the JBL speakers port leakages(pipe resonace) will have influence on the frequency response, so the question is ---------->


-------------->

I think the term 'Pipe resonace' is really throwing people.
It is not (to my understanding) what it appears to me that many think it is.
It is linear output, it is behaving like an additional transducer in the area where the 'pipe resonance' is.
It might help to think of it as 'pipe leakage' even though it is a resonance.
All of the sound a transducer makes is a resonance.
The port is transducing in the midrange.
When 750hrz is passed to the speaker via content, the signal is being reproduced in the room via air movements by both woofer and leakage from the port.

If the pipe resonates(again think of 'leaking' in order to picture the effect) at 750hrz enough to add 4db of output there above the woofer output level you simply reduce the 750hrz signal by 4db. This can be achieved either through passive (very expensive and wasteful in a design that will use a known active signal input) or active methods.

Again the midrange output of the port is linear.
Measure this speaker at different SPL levels and you will get essentially the same frequency response.
You can therefore through PEQ/DSP/Passive elements neutralize the added output/air movement of the port in the midrange.

Some design compromise is always a factor. Some reasons have been discussed why a port maybe allowed to leak more strongly than some other design. None-the-less, if the port is going to transduce/leak/resonate audibly in the midrange then one can see how it makes sense to place it right next to the woofer to minimize interaction issues and then treat the resulting frequency response accordingly using the solutions available to the design.

Post DSP/PEQ/passive solution, the port in this design may still rough up the area a bit when viewed at 1/20,1/24,1/48 octave or whatever pixel peeing level you use due to possible minor comb filtering and minor phase issues.

The frequency response is essentially corrected easily.
If you fear other issues.
Look at the waterfall plots and the distortion plots.

Here is the distortion plot...

It shows a major rise in distortion in the same range as the "port" resonance(s). You might argue audibility but cannot claim this speaker is linear in this frequency range.

 

[Adi777]

I know this thread is about the JBL 708i, but someone mentioned the JBL SCL-4 and I have a question. Will the SCL-4 be audibly better with anechoic data than without it? Will it be audible at all? Isn't it rather pointless to bother about it?

 

[ROOSKIE]

Here is the distortion plot...

It shows a major rise in distortion in the same range as the "port" resonance(s). You might argue audibility but cannot claim this speaker is linear in this frequency range.

Howdy,
This rise in HD has nothing (frankly at all in anyway) to do with what I said about the Port output in the midrange being linear.

If the port leakage was non-linear then it would leak different amounts at different SPL levels from the SPL levels the speakers is playing at.

You follow me?
Since it is linear turning it down 4db via some method turns it down 4db at all output levels. You can use PEQ to neutralize the extra output of the speaker.

The whole post I made describes this in some detail which makes what you said here seem strange to me.

As far as a major rise in distortion, I am hard pressed to see a rise from approx 0.5% THD at 86db1m to approx 0.9% THD @96db1m in a tiny region much of an issue.

The 708p by the way has higher HD levels there.

 

[Rick Sykora]

Howdy,
This rise in HD has nothing (frankly at all in anyway) to do with what I said about the Port output in the midrange being linear.

If the port leakage was non-linear then it would leak different amounts at different SPL levels from the SPL levels the speakers is playing at.

You follow me?
Since it is linear turning it down 4db via some method turns it down 4db at all output levels. You can use PEQ to neutralize the extra output of the speaker.

The whole post I made describes this in some detail which makes what you said here seem strange to me.

As far as a major rise in distortion, I am hard pressed to see a rise from approx 0.5% THD at 86db1m to approx 0.9% THD @96db1m in a tiny region much of an issue.

The 708p by the way has higher HD levels there.

While I may have picked on you (sorry), not looking to discuss details further based on this limited speaker review. As my earlier post suggests, I think too much time is being spent trying to explain some of Amir's measurements without enough specific data.

If we had a speaker being diagnosed under more controlled conditions, I think we all might have a more productive discussion with regards to understanding the resonances shown here. Since Amir seems to prefer medical analogies, this resonance discussion is akin to seeing a patient with multiple symptoms, doing some general tests and trying to diagnose the underlying ailment(s). What is really needed is more specific and targeted testing in my opinion. Since this is not in Amir's plans for this JBL (or most other speakers), I see little point spending time guessing at a diagnosis remotely with the measurements provided.

 

[beatelund]

I found that missing 4mm allen key you've been searching for:

View attachment 253883

It must have been the JBL engineer who forgot that when the logo was put in place