Confusion about LM3886....

[goldkenn]

As per data sheet, NS stated the chip deliver 68W into 4 ohms at +/-28V DC, but only 50W into 8 ohms at +/-35V DC, is that means the chip can't handle 4 ohms loading at +/-35V DC?

Or actually the chip could still work great at 4 ohms load, if using over-sized transformer and heatsink?

By the way, I remember I have read a post said, LM3886 sound best at +/-28V DC but harsh and dull at +/-35V DC, is that true? And what is the reasons?

[AndrewT]

Hi,
the chipamp has two main limitations that it must be designed to accomodate.
1.) power dissipation ability of the chip through that single interface with the heatsink.
2.) current limit set by the SOAR (spike protection) and temperature etc. This is quoted in the data sheet as varying between 7A and 11.5A, when the device is cold (Tc=25degC).

When these two are taken into account the chipamp is usually found to drive 8ohm speakers very well upto a maximum of about 60W (3.9Apk into resistive load).
It can also drive lower value reactive loads, but the supply voltage must be reduced to both lower the dissipation from the chip and also to lower the current demanded by the reactive speaker load.

Some builders design to the full 60W limit into 4ohm speakers. I reckon that is going to far, requiring 5.5Apk into a resistive load.

You can expect peak speaker currents to be at least double the peak current values delivered into a resistive load.
Using that information (if you agree with my contention) then 8ohm speakers could demand >=7.8Apk and 4ohm speaker requiring 11Apk when both are designed to be driven to same 60W maximum.

I reckon that chipamps need to be designed for half to two thirds of the maximum power (~30W to 45W) to get best performance. As word of caution, most designers/builders disagree with me.
Await alternative views before making your decision.

[goldkenn]

Sorry for my poor English which may can't really express what I confusing, but the 8 ohms 7.8A pk and 4 ohms 11A pk at 60W maximum is internally fixed and limited by the chip refer to working voltage, which if using another word, the chip are programmed? Or as I asked, I can squezze the full potential for both 8 ohms and 4 ohms, regardless of working voltage, by oversize transformer and heatshink? This is what I confuse.......

For example, if working at +/-35V DC, but I use a 300VA transformer for just one chip, and a huge heatshink which suppose for 100W pure Class A for just each channal or even use something could make the chip very cold for CPU, will LM3886 also work great at 4 ohms? Or still best with 8 ohms?

[Dxvideo]

Dear GoldKenn,

The IC has an internal protection system named SOAR. SOAR limites the output current at 8A in any case. It doesnt matter if you connect 4 or 8 ohm load.
Calcluations say; youre not in trouble with 32v supply, with 4 ohm load, but if you can solve the heatsinking. May be you must use an active cooling system in this case.

[AndrewT]

Hi,
a high current capability will allow a power amplifier to deliver full power.
It will not allow an amplifier to deliver more than it is capable of.

Chipamps work well into 8ohms. from any supply voltage provided the heatsink is adequate to keep the chip cool under all operating conditions.

If you want to run 4ohm speakers or 6ohm speakers or 4 to 8 ohm speakers then you must reduce the supply voltage to no more than described in the National datasheet.
However, I say that asking a chipamp to deliver 60W into 4ohms will not get the best out of the chipamp. I think one should design the supply voltage to deliver between 30W and 45W to get the chipamp to perform well. This helps to ensure that the internal limiting is not triggered at times of high current demand.

Look up the datasheet and you will find the cold amplifier current limit typical = 11.5A, minimum =7A.
Buy a chipamp and it will come with that limit preset between 7A and about 11.5A.
As the chip gets hotter the spike protection limits at ever less output current. Try to never let the spike operate into your normal speakers.

[goldkenn]

I think I get it..... so no matter the transformer VA and heatsink up to standard or oversize, if need to run the chip at 4 ohms, all I can do is lower the voltage, right?

And now comes the questions about the mute resistor; The value of the mute resistor base on the negative voltage as per the data sheet, typically will be 20K if working voltage at +/-35V DC, I know the calculation, but any recommandation of the an "universal value" of the mute resistor, which could shut down the mute function regardless of working voltage? if yes then I can simply rolling around with difference working voltage Vs difference loading imepdance, to test the best performance of LM3886.

[KSTR]

Hi,
10k is a good value the mute-R, it will safely un-mute the device for all power supply voltages (from +-10V to +-42V). You could also use a simple constant current source set at about 2mA.

Regards, Klaus

[goldkenn]

Thank you for remind me, I just too attention to the resistor value and forget about built a current source.