LM3875 at 60 volts?

[AndrewT]

Yes Leglandu,
That diagram is very informative and useful.
But first a gripe with National.
They state on the diagram 4 ohms (Greek symbol omega).
But they actually mean 4r0 pure resistance as the dummy load. similarly for 6r0 and 8r0.

They do not imply by the use of "ohms" that these are test results for driving reactive speakers.

Let's look at the RLoad=4r0 plot.
It shows a maximum of 55W into 4r0 @ <=0.03% distortion+noise and that this is available when +-33.5Vdc is at the Power Input pins.
55W1 into 4r0 is equivalent to 21Vpk & 5.24Apk at the output PIN.
We can see from other diagrams and data that the 25degC 10ms pulse typical output limit is 6Apk and that the clipping voltage is ~4.5V for 8r0 at +-35Vdc supply
The clipping voltage for 4r0 is bound to be much higher than ~4.5V due to the doubled current being delivered. I wonder why they don't show 4r0 clipping voltage as they do for the 3886?
the difference between +-33.5Vdc supply PIN voltage and 21Vpk output voltage @ 5.24Apk is 12.5V. That cannot be the 4r0 clipping voltage. which at worst would be <=~9V

This diagram is showing a protected limit even though it states 0.03% D+N and that is confirmed by the continued low output current still available at even higher supply PIN voltages. The output has been limited at well below maximum output for a 0.03% D+N limit.

You can do similar for 6r0 Rload.
You can do even more by examining the 8r0 performance for which extra data is provided. (75.5W into 8r0 and the equivalent 34.7Vpk @ 4.34Apk when +-40Vdc is at the Power PINs and 0.03% D+N, 5.5V clipping limit @ +-40Vdc)

[Leglandu]

Originally Posted by AndrewT

... It shows a maximum of 55W into 4r0 @ <=0.03% distortion+noise and that this is available when +-33.5Vdc is at the Power Input pins. ...

Sorry but concerning the 4 Ohms load, I see 55W when +- is 28Vdc, not 33.5... if you are still referring to the above diagram, so not sure to perfectly understand all of your demonstration.

Now, I feel confused about their diagram : how can this chip provide up to 66W in 6 ohms and ~77W at 8 Ohms for 0.03% THD+N ?

I thought it was about the max power available before clipping with the falling curves to show when the max power of the amp collapsed and that's all

But perhaps I was not so wrong as clipping seems to arrive brutally, per their "THD+N vs Output Power" diagram below ?

[Leglandu]

Oups, seems that I repeat what you already explained AndrewT, sorry, my English is poor so I often have understanding problems...

[AndrewT]

my oops !!

I have gone back and looked and see I picked out the voltage for the 6r0 load and the power for the 4r0 load.

You are correct to not simply believe everything I type.
It is always worth reading, thinking, deciding which if any parts to read as believable and which are nonsense and which, if any, are fact.

[danielwritesbac]

This project needs a voltage regulator board added. Decibel Dungeon has some examples. Building a Gainclone chip amp with a regulated power supply (PSU). Pedja Rogic Audio Pages - Chip Based Power Amp i.e. Gainclone - Regulated Supplies

[sgrossklass]

Originally Posted by nige838

I went ahead and hooked it up, it ran for awhile and the chips didn't seem to get too hot, except one channel was around 10X louder than the other.

Probably some contact problems left in the preamp section. Clean all switches and pots in the signal path with mild contact cleaner that possibly has a sealing function (vaseline spray will do for the latter if need be). I guess you have checked your feedback resistors?

BTW, I'd look into swapping the main filter caps. Those are likely to be rather small in a 16 watter, and they're like 30 years old and not likely to be premium quality anyway. (I hope you have already replaced remaining "small potatoes" 'lytics with good-quality new ones.) I'd look for mechanically-fitting 6800µ to 8200µ, 35V or 50 V jobs.

[danielwritesbac]

Originally Posted by sgrossklass

I'd look for mechanically-fitting 6800µ to 8200µ, 35V or 50 V jobs.

I wouldn't look for the 35v for a 30v rail because they won't last a long time.
Use the 50v or 63v caps.