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April 2019

 

Bought a pair of JBL 306P active monitor s.

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Although an entry level JBL product, it still has the "JBL Professional" badge on it (suspicious claim).

 

Built from 5/8" MDF without any internal bracing.

 

Front baffle is injection formed plastic, and rear baffle is thin aluminum plate holding the crossover and the Class D plate amplifier (STA350BW chip based)

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Notice the thin 1/2" foam like acoustic absorbent. More absorbent will be added for sure. JBL foam isn't enough.

Obviously, DATS reveal what was predictable. JBL selected the driver from the "economy shelf".

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The tweeter QTS is quite high at Q=1.1 and the horn loading has the FS falling to 1500 Hz. Not bad, but some Scan Speak tweeters have 900 Hz FS, not quite the same thing.

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Not much headroom, but good enough for the JBL engineer squeezing all they can from that 6"+1" style bookshelf.

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The woofer has an issue at 250 Hz. I suspect this could be because it was measured inside the cabinet, with the rear cover removed. 

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The woofer FS is a tad high, but at 50 Hz on a small 6" bookshelf speaker, it’s acceptable. 

Inductance is a bit high as well, but since it's an active speaker with some DSP added, it should not matter much.

 

We'll see how it does.

stay tuned, more to come.

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I unmounted both speakers from the baffle and measured them again.

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The tweeter FS increased a bit to 1550 Hz. All is fine.

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The woofer issue at 250 Hz is gone, meaning the cabinet created the anomaly, and from online research ~240 Hz is where the cabinet resonates like crazy.

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Cabinet stiffening modifications are coming.

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I knew that once I received the speaker it would fail the "knock test" miserably.

 

These are the drawbacks of online shopping, when everybody (not audiophile) raves about products which have a low threshold of quality. 

Front baffle damping

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Used expanding foam. (~3/4 of a can for both baffle)

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The foam expanded a little too much, but cutting the excess off was easy enough. No mess and some weight was added. 

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Resulting baffle is sturdier and heavier.

Heavy speakers are always something one should look for.

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Sonics may change very little from this time-consuming mod, but I just had to do it anyway.

 

Also, I knowingly reduced the cabinet’s internal volume a bit, which shouldn’t be too much of an issue. 

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Input board:

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JRC2115, low end opamp in the signal path (performing XLR to RCA transformation and activates the gain switch)

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Not a very good opamp here, but I'll leave in place as they’re designed for low voltage operation and the input board works on a single +7.5Vdc. Using a big league opamp at such low voltage may not yield any sonic benefits, as they are designed for higher voltage operation.

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All caps will be changed.

  • C104, C108, 22uf input capacitors (polarized) will remain as it’s a single supply system. I’ll use Panasonic FM of 47uf caps instead

  • C203, 4.7uf output capacitor. A 47uf Panasonic FM cap will be used here. 

  • C108, 100uf PSU capacitor. A Panasonic FM, 470uf cap will be used.

  • C101, 100uf, capacitor. I’m not too sure about this one. It interacts with the gain switch and may or may not pass signal. I won't take a chance, and I’ll replace it with a Panasonic FM of same value, as I buy Panasonic FM’s by the hundreds.

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Input capacitor swap or even bypass will be the first step a DIY’er looks for a mod, as it always yields quantitative sonic benefits.

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Opamp upgrades should be considered too but remember that this is a budget product and using a $50 OPA627 may be not be cost effective.

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Main board:

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Few important caps to be changed here:

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  • C209, "FILT+" to CS5341 chip, as per datasheet, this cap will be increased from 1uf to 100uf Panasonic FM

  • C215, "VL" cap to CS5341, will be increased to 22uf Panasonic FM

  • C206, "digital power" to CS5341, will be increased to 100uf Panasonic FM

  • C216, "VA power" to CS5341, will be increased to 47uf Panasonic FM

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  • C256, "VCAP" to STM8S00, as per datasheet, quality ~2uf film cap will be used here to replace the cheap electrolytic used by JBL.

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  • C265, C272, output to 5Vdc and 3.3Vdc positive LDO voltage regulator, as per datasheet, Panasonic FM of 220uf to be used here. Input ones (C274, C268) will be changed to 100uf as well.)

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  • C223, C925, power supply for main amp module, given it's a SMPS, using big uF value doesn't yield benefit, as a low ESR capacitor matters more. I’ll see what I have in stock for those two and upgrade them.

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Under main board:

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I've decided to leave the stock 1000uf there. The STA350BW class D chip datasheet only calls for 1000uf.

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Moreover, being fed by a SMPS, putting 200,000uf reservoir capacitor isn’t really useful, as the cap is recharged ~400,000 times per second compared to 120 times per second (for 60 Hz North American main) for a conventional linear power supply. 

 

This 3,333-fold increase in frequency reduces the capacitor requirement size considerably.

Also, SMPS have active feedback and output voltages, which never sag when used within their capacity. A small MKP bypass should suffice.

 

Capacitor ESR is the name of the game in SMPS. I've decided to add a 5.1uf MKT Wima on the chip side and add a 100uf Panasonic FM on the SMPS side (in parallel to the stock 470uf) 

Internal cabinet bracing:

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I URGE YOU TO DO THIS! IF YOU ARE TO DO ONE MOD, THIS ONE IS THE MOST IMPORTANT...

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I may have gone a bit overkill, but I wanted to make sure to eliminate the ~240 Hz mode.

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As you can see, several braces were added while leaving enough room for the flared port to pass.

Internal cabinet stuffing:

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I re-used all of the stock 1/2" foam stuffing but placed it differently. (away from the wall as far as possible.)

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Added Roxul SAFE n’ SOUND 2" thick, 7" x 7" piece directly behind the bass driver.

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Arranged top stuffing as per picture, to leave plenty of room for the vent to operate without restriction.

 

Bass reflex stuffing is always tricky, between adding too much stuffing and not enough.

 

  • Always watch for mid-range frequency leaking out from the reflex port.

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Hiss Measurement:

 

Yes, those monitors hiss a bit.

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Yellow: is the reference background noise at my listening position, with the JBL turned OFF, but TV and the rest of the system ON (energy seen at 120 Hz and 240 Hz is the buzzing transformer from the other power supply in the house)

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Red: is the background noise with JBL ON. Barely any difference, at my 1.5 meter listening position. The hiss from the JBL is basically inaudible 

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Cyan: The hiss from woofer measured @ ~1cm from woofer dust cap with all systems ON. To show the hiss frequency.

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Red: Is the hiss from tweeter measured @ ~1cm from tweeter dome (inside the horn) with all systems ON.

Just to show the noise frequency

Measurement:

 

Yes, those monitors have built in DSP’s and it shows :)

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Overall, they’re very impressive, given they are $300 CAD each.

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Distortion is a tad high but, $300 for a speaker box, driver and built in amplifier? What else do you expect? A basic Dayton Audio MDF cabinet sells for ~$150. You couldn’t build anything that good for less money. Impossible.

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Bass was falling like a stone at ~70 Hz. The 24" measurement is mostly due to the speaker being in the center of room with no wall re-reinforcement.

 

Near the rear wall, at my listening position, they managed ~50 Hz flat. DSP will be used below 200 Hz to tune the sound to my liking, while gaining more low frequency output.

Measurement at listening position:

 

Left channel shown, right is almost identical.

DSP added to correct for my room.

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DSP shown here is dictated by my room. Your setting will vary, so copying my setting will be useless.

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Boosted a bit the low output at 44 Hz

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Added a steep filter at 33 Hz to prevent the 6" woofer trying too hard at very low frequency, when it just can't do it anyway.

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A small notch was added to correct the 160 Hz and 110 Hz summation mode.

 

As usual, DSP can't do anything for the null. They are room mode cancellation and only acoustic treatments will solve this

The JBL 306P as they look on my computer set-up

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Foam speaker stands reduce vibration to the TV and table. They help to gives better overall sound

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The speaker vibrates as its on the light side. I'll add metal plates to weight to them down. The bonus is that the plates will increase structural rigidity.

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A 1400 Hz crossover, 4th order, maintains speaker composure at higher SPL. Had that been a simple 1st order tweeter as per lots of cheap passive designs, the tweeter would have produced smoke at high SPL.

 

Well Done JBL

Toilet paper mod :)

 

Below resonance, the response of the cone of a sealed speaker is still under control, due to the restraining force of the air in the box. While the cone of a ported speaker is not restrained i.e. it is no longer under precise control.

 

Lowering an enclosure tuning frequency does bring some benefit from sealed (more control) without causing too much loss in bass response like  a real sealed system would do.

 

Our factory 6" plastic cone speaker has a box tuning frequency of 46 Hz. The toilet paper roll modification lowers the tuning frequency to 42 Hz.

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Toilet paper roll was used as per picture.

LOVE DIY test.

Subjective result is a tad better too :) You should try it

DIY Metal plate addition mod.

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Stock 306P are not very heavy. I've decided to cut four metal plates to add to the top and bottom, to strengthen the chassis and make it heavier at the same time.

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8.5"x 8.75"x 1/4" thick, screwed and glued (screw to be removed once glue cures, and the hole will be re-filled as well. Picture shown isn't final)

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I weighted them on my precision scale, and they came in at five pounds per plate, ~10 pounds of weight per speaker was added. Quite a lot considering the stock JBL weight of 13 pounds.

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Playing loud, the speaker barely vibrates from  my unscientific "hand feel test". The table on which they stand, they remains much more still. This is a great improvement even if I can't measure it due to lack of equipment to measure vibration. Subjective listening test is a strong positive too.

Blocking the port:

 

(aka DIY Variovent)

​Even with the toilet paper port extender to lower the tuning frequency, those tiny bookshelves do sound like bass reflex. BR is BR and will always sound like BR.

 

Going sealed would require too much LF boost, and the power amplifier could run out of power and clip. Not the best idea.

Aperiodic tuning would my next best bet.

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Enter the polyester partial port fill. It restricts air flow to almost nothing. Just a tad, to let the bass driver breathe. Port still gives output… albeit very little.

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Sound is much better. Different DSP settings are required, as well as more low frequency compensation. But overall, the polyester will stay.

Interestingly enough, the stuffing doesn’t kill the peak we see at 1.1 kHz. I was expecting to attenuate it more, but it seems it doesn't remove more than 5 dB’s. 

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With the polyester inserted, the box is tuned as aperiodic. It sounds much tighter than the bass reflex configuration but does require more low frequency boost via DSP. I may have to measure the driver to make sure the relatively small built-in 56-watt amplifier doesn't clip. I don’t expect those 6" bass drivers to be much more than 87dB efficient, and with 6 dB of boost at 42 Hz, the power is doubled, and excursion of the driver is quadrupled...

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I’ve got to keep physics in mind here, and not ask the DSP to give more signal than the 306P can handle.

Ferrite mods:

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https://www.gearslutz.com/board/low-end-theory/895365-jbl-lsr305-modification-ferrite-cores.html

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As per this thread, this mod on post #4 had me perplexed and curious. I've tried it of course. :)

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I must concur that subjectively, it improves the sound.

Sound is fuller, more natural and less edgy.

The hiss remains the same, with no improvement on that aspect at all.

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I honestly can't explain why it sounds better. It just does.

I have theories but that’s all they are… theories.

 

HIGHLY RECOMMENDED.

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Ferrite mods: BE CAREFUL.

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CAREFUL FOR THE FERRITE MOD, THOSE FORK TERMINAL ON THE PCB ARE NOT REGULAR CONNECTOR, SEE PICTURE ON LEFT, TO REMOVE THEM, ONE NEED TO PUSH THE TAB IN THE MIDDLE OF THE TERMINAL TO UNLOCK THEM FROM THE PCB.

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PIN TO PUSH

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