6 channel LME49830 power amp

[cuibono]

this is what the look like upright...

[cuibono]

alright, time for some more pictures. I've got all but one of the modules built and working. The last one is waiting for new fets (so far I've only killed one set!). I changed the layout some, so here are the boards. Right now, I consider these tested and done, although I might do some stability testing sometime.

This is the top, already mirrored. When printed, the board area should be 150mm by 66mm. Also, if you're making your own boards, don't forget to solder the top and bottom pads of any wire running on the top. There are a few vias that need a little piece of wire and soldering on the top an bottom - just look for any pad without a part going through it.

[cuibono]

here is the bottom. Be careful with the tiny traces under the LME chip.

[cuibono]

here is the top layer with silk. Notice the vias show up as orange pads

[cuibono]

Here is the bottom. There are a few parts here, including a zero ohm SMD resistor - one could just use a piece of wire here.

Notice I removed the mute switch and associated parts (including two headers), and the header for the bias voltage measurement (although there are still pads for attaching wires when you want to measure it).

Sorry, I didn't label any parts. I guess it will force anyone who wants to use this to trace it, which is probably a good idea.

[cuibono]

and another picture of the progress. notice the wire hanging off module number 2, it is for measuring the bias voltage. Also notice the tapped holes - very handy! (and yes, one of my taps broke during the process)

[cuibono]

What kind of measurements should I do? I'll probably do bandwidth and harmonic distortion, FFT the noise, and maybe see what it takes to make is oscillate. I'd like to figure out how much power and current is available with all channels driven. How should I do that?. I'm also a little concerned about driving loads less than 8 ohms, as the datasheet indicates it will be possible to draw too much power through the output devices, and move them out of their SOA. I'm wondering how unsafe it would be to drive 4 ohm woofers...

[AndrewT]

If the datasheet confirms that your 2pair cannot adequate drive 8ohm speakers then 4ohms must be worse.

Testing all channels at the same time uses a lot of resources.
Try testing each channel individually into a safe resistive load.
Find out how much the maximum output voltage drops from driving open circuit to driving an 8r0 load.
Find out how much the PSU voltage drops from quiescent to full power into an 8r0 load.
Find out how much ripple is on your supply rails when quiescent and at full power into 8r0.

What value of filters have you fitted to your inputs?
These are likely to define your power amp bandwidth. Testing should confirm this. If the bandwidth is narrower than the filters then something is a bit off.

[AndrewT]

Hi,
just checked 2pair of 240 on +-54.2Vdc (quiescent voltage) with +-30mF/channel.
8ohm 60degree phase angle gets to ~75% of the 10ms de-rated SOAR for Tc+55degC.
reducing the phase angle to 50degrees gets the output load line to about 50% of the 10ms de-rated SOAR. This is very approximately about 100% of the 100ms SOAR.
This seems to indicate that 2pair on +-54Vdc can drive a moderately reactive 8ohm speaker if you keep the heatsink below 45degC in worst case conditions.

An F2.5A fuse in each amplifier's supply rails should allow maximum power into any 8ohm load without nuisance blowing.

Do not expect reliability with severe reactance 4ohm speakers.

The model predicts 98.6Vpp at the output into 8r0 and 6.1Apk from a +-52.9Vdc loaded PSU.

You can also test into a 4r0 load and expect 91Vpk @ the output into 4r0 and 11.4Apk from +-52.1Vdc loaded PSU. You may need to increase the F2.5A to F4A to conduct this test. Do not let the heatsink warm up during either of these maximum power tests. Do them as near cold as the ambient conditions will allow.

[cuibono]

Andrew, thanks!

I was under the impression that the amplifier could drive an 8 ohm load with two pairs of output devices, thanks for confirming that. I'm a little disappointed though, because I would like to have a very reliable design. I guess no 4 ohm loads! Luckily, I'm using a fan on the heatsink, and not driving the speakers through crossovers (I'm doing XO actively in the computer).

Thanks for the fuse suggestion. I'll find some inline fuses I can splice into the cabling. Would you suggest fast blow types? Is this adequate protection for my speakers in the event of a short in a FET?