LM3875 and LM3886

[soundNERD]

Are the LM3886 and LM3875 interchangeable. I know the pins are different and the 3875 doesn't have the mute function, but can I use the same schematic for them each? The picture is attached and the site it is from is http://david.lewander.com/projects/amp/amp.htm

Thanks, Mike

[soundNERD]

Here is the picture:

[JoeBob]

Any circuit one works in, the other should as well (except of course, when driving lower loads, and the like. but I meant generally). They both had a + and a - input, an output and 2 rails, anything else is just fancy gimmicks (mostly), like the mute.

When I use the 3886, I always leave the mute off.

[macboy]

The most important difference between the LM3886 and 3875 (or 3876) is that the 3886 is optimised for 4 ohm loads, and the 3875/6 are optimised for 8 ohm loads. If you look at the spec sheets, you will see that the 3886 can deliver up to 68 W into a 4 ohm load, but only 50 W into an 8 ohm load. The 3875/6 on the other hand, delivers up to 45 W into 4 ohms, but 60 W into 8 ohms.

This is important when considering what device to use.

The load is not simply the impedance of the speaker; it depends also on the topology of the amplifier you choose. The amp schematic that you showed to us uses a bridged topology. For a bridged amp, the impedance of the load that each of the amplifiers will see is 1/2 of the speaker impedance. For a parallel amp, the load is 2x of the speaker impedance. So, if you use a bridged amp for an 8 ohm speaker, each amp effectively sees a 4 ohm speaker. Generally, you can't (or shouldn't) do bridged into 4 ohms, because each amp sees a 2 ohm load, which is too low.

Note also that if you are planning to use more than one speaker, you have to adjust the load impedance in that case too. If you run two (of the same) speakers in parallel, then the effective speaker impedance is 1/2 of the impedance of each speaker. Two speakers in parallel make one with effictively 2x the impedance of each separately.

So if you are using the circuit shown and want to use a 8 ohm speaker, then use the 3886, not the 3875. The 3875 will definitely work, but it will not deliver quite as much power.

You cannot use a 4 ohm speaker with a bridged amp such as the one shown. Instead, you should use two 3875's in parallel (see other posts in the forum for more info). They will work together to drive the speaker and you'll get up to 120 W out of the amp.