The old specs:
There was one rectangular "racetrack" full-range driver per channel in this 2.0 system 40mm x 100mm or 1 1/2" x 4". There were places for tweeters to go in and online tear-downs had tweeters, but mine came without any and there was nowhere to plug them in and no crossovers, although a lot of empty room in the driver compartments.

• SPL: 81 dB
• Bass Reflex (but there were no ports for this or passive resonators)
• Impedance: 4 Ohms (actually this was measured off-state resistance but quoted as impedance)
• Unshielded.
• 25 + 25 W Amp.
• 14.5V SMPS at about 5A.

My thinking:

The first watt of power is the most important. I felt like LG did something amazing with their DSP and enclosure to manage to get the range they did out of these little speakers. I think the excursion was pretty high. I wanted to test my theory for myself and others. I play guitar a bit and close to a lot of musicians, so I kinda lamented the sound experience with TVs. Like we gave up on a century of speaker development to have these sleek bars. They sure look good and can be good if you get really expensive ones with a ton of engineering in them. But fighting against physics is hard. So my idea was to use some bookshelf speakers with the amplifier only of the sound bar and have the niceties of bluetooth when I'm lazy and optical in etc for permanent connections.

So I only need 1W of power to get 81 dB of sound right at 1m? Didn't feel like it at all. But I went with the stated numbers. 25W per channel of output power suggests about 14 dB of gain. This should bring up the volume to 95dB. Listening from 10 feet away drops this by say 9 dB but there was 3 dB of room gain, so a final 89dB. This is what I think I was hearing so I thought I could keep running with these numbers. So I just had to build it out into a nice chassis and choose some speakers to go with it was my hope.

New (to me) speakers:
Since this is a test setup, I wanted to try it on some speakers I'm not too attached to. I gave away my Panasonic SA-PM25 speakers years before, but they were pretty ok. Nvm. I found some cheap old Sony SS-CMD373 speakers x2 on eBay for the test. They were not damaged and the resistance matched the stated impedances. I know impedance and resistance are not the same thing.

Each one is/has:

• 5 1/4" Woofer
• 1" Tweeter
• Impedance: 6 ohms
• SPL: 91dB is my estimate based on comparing 3 driver specs of the same type and averaging. Crude, but not a bad starting guess. The Panasonic speakers I gave away had 86dB and were 1" smaller drivers. For the purpose of calculating I guess we could estimate it to be 88dB at the low end.
• Frequency response about 40 Hz - 20kHz comparing it by ear to other speakers I have. Maybe goes a little lower. My phone app SPL meter shows activity around 30Hz but low amplitude.

Calculations:
So, for these new speakers with the LG Amp, the first watt should get me say 88dB at 1 m.
The gain of the amp should be 14dB as before at full volume, but scaled due to impedance difference.
Since the impedance scaling is a factor of 4 ohms / 6 ohms, the power into 6 ohms would now be 25 x 4/6 = 16.7W.
This is worth 12dB.
So at full volume at 1m distance, the volume should be 100dB.
Making adjustments for the 3m listening distance and room gain brings this to 94dB.

Did it work?
Sure it makes a sound. The bass was ridiculously high at low volume on the remote. I had to eq -12 dB on the bass band of 60-100Hz on my android eq. That and add about 4 dB to each of the other bands brought it close to balanced. So now I could turn up the volume. The LG remote has a range of 40 button presses from min to max. At about level 16 up from 0 the amp was going into an undocumented protection procedure and flashing a red light. Power cycling and reconnecting would solve it. This is where I started scratching my head. What was going on?

It feels like I'm not even getting past the first 1W of power!


Diagnostic:

• I tried one speaker at a time with similar results.
• I tried 8 ohm speakers which could be turned up without hanging the amp. But low max volume.I checked and resoldered the cables.
• I added an aluminium heatsink to the DSP and a big one to the amp chip. Measured that most of the circuit board was 26 degrees C in a 24 degree C room (79 F in a 75 F room). The amp chip got to 56 C or 133 F.
• The system is passively cooled, with a lot of air vents and a steel case. The heatsink itself reached 25 C at the tips, so was more than enough and I used thermal paste and it was screwed down firmly to the chip.
• The voltage buck converter didn't get hotter than the PCB.

So I am absolutely surprised why some kind of protection circuit is kicking in at 14 steps out of 40 on volume.

What could be happening?
Ohms law suggests that the current would be lower with a higher impedance.
With the original 4 ohm load per channel, I = 14.5 V / 4 ohms = 3.6A equivalent to 52W which is double the rated capacity. These speakers never clipped at max volume.
With the new speakers at 6 ohms per channel, I = 14.5 V / 6 ohms = 2.4 A equivalent to 34W which is also higher than the amp capacity.
The buck converter chip AP6503 has a 3.5A current limiter at which point it drops to 70% output to protect the amp chip. This seems to make sense. So it should never stop at 2.4 A with the new speakers. At worst the output should drop to 70%.
The amp chip is TAS5733L. Its a 4 channel digital input amp 10W per channel at 10% THD. LG combined two channels to make one left, and the remaining two to make right. This made it 20 W per channel. Then they upped the PSU voltage to 14.5 to get it to 25W which is accepted practice on the datasheet, up to a max of 16 V.
This chip locks out if the voltage sags to 5.4V. There is a decent input supply capacitor on the 14.5 V supply, and an unpopulated spot for another. I doubt the voltage could sag that much when the speakers are limiting power consumption to less than the original amount.
There is an Automatic Gain Limiter function I don't really know about.




Why am I hitting any limits at all? Is there something about load impedance I am missing after a week of obsessively reading about this?

I look forward to your ideas. Sometimes it's something obvious. Sometimes not.



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