lm4780 2.1 volume control?

[mattjuk81]

I am hoping someone can help me (and I searched the forums and google to no avail) I am about to put together an lm4780 dual mono amp, but in the future I may decide to add an extra mono channel using one lm4780 to drive a subwoofer.

My question is how would I connect the rca signal to the extra chip (subwoofer mono chip) is it ok to split the signal coming into the amp, to a seperate volume pot for sub volume control, or would this mess up impedance etc...?

The signal coming into the amp would not be going through a pre amp but direct from a DAC.

[Mankysteve]

You'd take one channel spilt into two using a low pass filter on one line to provide the signal you need or the bass amp. You could even design the third amp to be active filter/amp.

[mattjuk81]

Originally Posted by Mankysteve

You'd take one channel spilt into two using a low pass filter on one line to provide the signal you need or the bass amp. You could even design the third amp to be active filter/amp.

If I take just one of the stereo channel, then surely if the lower bass signal happens to be on the other channel in a certain piece of music then I would not hear it ?

[pacificblue]

In theory yes. In practice everything below 100 Hz is usually the same on both channels anyway.

[mattjuk81]

Originally Posted by pacificblue

In theory yes. In practice everything below 100 Hz is usually the same on both channels anyway.

Ah thats ok then, but wouldn't splitting a signal into two, make one of the stereo channels slightly quieter?

[Mankysteve]

In theory your amp should have infinite input resistance so there shouldn't be any voltage drop. In reality you will loose a small amount of volume, how big or small this would be I'm not 100% never tried 2.1 my self so I don't really know.

I Can't think of any way of mixing both channels for a middle on with out loosing stereo ability.

Also in this case it's not true 2.1 but that's not really of importance.

[pacificblue]

Originally Posted by mattjuk81

Ah thats ok then, but wouldn't splitting a signal into two, make one of the stereo channels slightly quieter?

Yes, to such a small extent that you won't notice any difference.

Originally Posted by Mankysteve

I Can't think of any way of mixing both channels for a middle on with out loosing stereo ability.

You are probably refering to crosstalk through the mixer. It does usually not affect the imaging too much. If you want to calm your conscience, use a stereo low pass and put the mixer after that. That restricts the crosstalk to the region below the filter frequency. It takes a few more op amp channels than the usual setup with the mixer before a mono low pass.

[mattjuk81]

Could I not take both stereo signals and use 2 resistors, and take a signal from the middle of the resistors?

or use this --|SMC-1|AUDIO CONVERTER, STEREO/MONO | CPC

[Mankysteve]

You get cross talk between the two channels. say if you've gt a part of music where there nothing on the right channel and something on the left you'd get the left channel on the right.

[richie00boy]

You need a virtual earth summing op-amp circuit otherwise you will corrupt your stereo signals. Or take a look at the sub-woofer controller project on my website