JBL LSR305 Teardown and Analysis

2 June 2015

The JBL LSR305 has a space age looking waveguide that indeed does provide subjectively omnidirectional tweeter pressure, while the wide flared port has practically no wind noise. The resulting sound lacks a bit of detail, but is otherwise outstanding at its $130 price point. Surprisingly, the speaker does not offer over-power protection or any power saving capabilities, both of which could have been implemented using the existing chips. There is also a peculiar bug, which might be destructive. I am particularly interested in the design because of the switched mode power supply, digital filtering and class D power amplification.

Here are the guts, let's dive in!

The power supply takes the right half of the board and provides fixed 19V output to the power amplifier and 8V to the rest of the digital logic (later stepped down to 3.3V and 5V). The top left quadrant has the STA350BW DSP + class D power amp chip and the output LC lowpass filters, while on the lower left there is the STM8S00 MCU, CS5341 ADC and the shared clock generator.


The heart of the board is the STA350BW (on the right above). It has 2 bridged 50W power amplifiers (hence the single rail power supply) and 4 second-order digital filters per channel, implementing the cross-over. The datasheet claims 0.09% THD+N at 1W, going to around 0.5% above 10W. The chip has no attached heatsink and certainly not required - mine idles at 40 °C, and heats up to ~60 °C during operation. The LSR308 uses the same PCB, but has a heatsink given higher operating voltage and power output.

The STM8 MCU (left) has 2 functions:

  1. Load up the filter coefficients and initialize the rest of the DSP.
  2. Read the trim switches and the gain potentiometer at the back and update the DSP config appropriately.
Thus the speaker is completely software controlled.

The drivers are an 8 Ohm tweeter and a 4 Ohm woofer. Replacing them with resistors and overpowering the system shows its 2 main weaknesses - there is no overpower protection, the amplifier just starts clipping. Even worse, the cross-overs seem to have a fixed point overflow bug and at high levels not only a sine wave is strangely modulated, but also a strong bass might cause output through the tweeter.

JBL claims 43 Hz - 24 kHz response (at unspecified attenuation). Here is my in-house frequency plot: good flatness, except for the minor smiley shape.


Frequency response. The notch at 70 Hz is a room anti-node.

Idle power consumption off the wall is just 4W. Class B monitors typically consume ~20W.

Despite its weaknesses, the JBL LSR305 is a beautiful and well sounding speaker. Also don't forget that you do not need an expensive balanced audio source to drive them - with a $2 RCA to mono-jack adapter (which shorts the negative input to ground), you can easily drive a pair of the speakers from an onboard laptop/PC soundcard, phone, tablet, etc.

Here is my review of the accompanying sub - LSR310s.


Comments for JBL LSR305 Teardown and Analysis

Kevon on June 5, 2015

"The drivers are an 8 Ohm tweeter and a 4 Ohm woofer. Replacing them with resistors and overpowering the system shows its 2 main weaknesses - there is no overpower protection, the amplifier just starts clipping. Even worse, the cross-overs seem to have a fixed point overflow bug and at high levels not only a sine wave is strangely modulated, but also a strong bass might cause output through the tweeter" So How do we fix the shortcomings and enhance the speaker?


Rouslan on June 5, 2015

The fix would be to make the MCU monitor the signal, like I did in my previous post. JBL's board already has the programmer port, but we don't have the original program code to modify. Anyway, the current design might just be fine - perhaps they used high wattage drivers able to withstand all the amp can feed them.


Vladimir on December 20, 2015

Hi Rouslan. You mention you got modulated sine wave at high levels with the JBL LSR305. Did you try the +4dBu input sensitivity setting? This is recommended by JBL if you are getting any distortion. Best to use a +4dBu balanced output audio interface (DAC) and have the JBLs also set at +4dBu, which is pro audio standard. Thank you for the photos of the inside. Cheers


Pelle Jaspers on September 18, 2016

who does this mean that it isnt a great idea to play bass guitar through them or is the chance small that i hurt them.


lucas on December 14, 2016

I would love to know why one of my pair suddenly stopped working. It doesnt power on. The 19V are achieved by a switching source?? I heard a lot of people whera having this problem too.


Rob on December 23, 2016

Hi Rouslan, seems that my original comment was overwritten with my reply to your question. Maybe your blog only allows one comment per email address? Anyway, in case this comment over-writes my prior one, I'll put it here too! ------------- You asked about where I got the speakers and I said: >The hiss on mine is definitely audible from within a couple feet. > And, I have them on my desk, so they are normally that close. :) >I bought mine recently in the U.S. from Sweetwater.


Rouslan on December 23, 2016

Hi Rob, mine certainly hiss from the tweeter, but not as loud as I've seen others describe it. Perhaps audible from within a couple feet in a quiet room. I had bought 3 when they originally came out and a 4th just recently, they are identical. Which country did you buy yours in?


Shail on February 7, 2017

Hey Rouslan! Thanks for putting this together. The STA350W Documentations says "Dual-band DRC dynamically equalizes the system to provide speaker linear frequency response regardless of output power level. This feature independently processes the two bands, controlling dynamically the output power level in each band and so providing better sound clarity. " Do you think this feature has been implemented to make the actual speakers "behave" more Linearly?


Tim on February 27, 2017

I do have a pair of LSR305, so what will be the easier ways in modifying and upgrading the amp board? Thanks in advance!


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