Paralleling LM4780 chips?

[Bazukaz]

Hum may be caused by poor PCB layout(especially grounding).
What schematic are you using ? PCB layout ? Could you post it ?

[jackinnj]

Originally posted by Peter Daniel
In combining 2 parallel LM4780 chips (4 x LM3886), it may be a good idea to increase output resistors to 0.22R or even a bit more.

Why?

Probably a better idea to match the gain resistors until the offset is nulled out.

the R*I^2 losses get pretty big when you double or quadruple the resistance values. why burn power when you can listen to it?

[Peter Daniel]

I was using 0.22R resisitors myself and didn't notice any drawbacks.

What do you mean by matching gain resistors to null the offset?

[jackinnj]

should have said "matching gain and input resistors" to null the offset.

[Dominick22]

Bazukaz,

I bought 4 LM4780 chips complete with kits from Peter Daniel at audiosector.com.

I reccomend them 2-fold.
1st, I have built 1 amp already using his kits and I am more than pleased.

Second, as you can see, Peter is more than willing to offer his expertise...even if some of the things he says are above my skill level!!

So, go to audiosector.com and click on LM4780 Dual mono kit.
That is what I am working with.

TO EVERYONE: Thank you all for the replies although I am having some trouble understanding some of the terminology.

Maybe I will post some pics of my project in development!?

Dominick

[Bazukaz]

Well , the kit looks good.It has support of paralell configuration,so values should be already matched and you should not need to do anything.
My advice(already mentioned) : add a resistor between your amp and speakers.you don't need large - ~0.5-1 ohm.It should be non-inductive.
Plug in and watch if your amp smokes

Now about instabilities : imagine one amp starts oscillate , at a frequency of few MHZ or so. Another will then try to compensate.
So both amps will be stressed in parralel config.Adding two more amps in paralell may even worsen the problem ...

[Dominick22]

Do I put tha .5/1 ohm reisitor on both the negative and positive leads or just one of them?

And which one would be the safer to try first?
Dominick

[ChristianThomas]

Hi

You're driving electrostatics which get quite capacitative at HF. This could easily be the cause of instability, especially if layouty isn't ideal.

The most usual solution for this is an inductor on the output - Ie. after your R22. Get a 2W 2R2 resistors and wrap 8 to 12 turns of 0.7mm enamelled wire around it so the two are in parallel. This should just about eliminate the idea of the speaker being the source of your trouble - and you should be able to use just two chips in parallel.

Hope this helps

Christian

[Bazukaz]

You just put 0.5 ohms resistor (non-inductive , 5 or more watts preferred) in series with loadspeakers.

Another way is to use L-R filter.
for this , you need a coil of around 1uH(15 turns on 1.5 cm diamter with 1.5 mm enamel wire).The inductor is then connected in paralell with 10 ohm 2 watt resistor).The filter is connected in series with loadspeaker.

[Dominick22]

UPDATE:

I want to thank everyone who is following this thread.

After some trial and error testing, I can now control the speaker volume with the pot however the audio signal is masked greatly by static. I can here music behind the static.

Also, BOTH chips are now heating up very quickly...even with a sink attached.

Baz, I tried using the resistor tip at the output and it only seemed to completely block the entire signal. Everything was reduced in violume wholly. It did not improve the sound.

I was looking at the original chip schematics from National and I saw the ci1 and ci2 for parallel config. I am unsure were to place this in the circuit??? Should I literally just create a lead off of 1 of the 4 V- pins??? Should I do 1,2, or all 4 V- pins???

I am reaching for ideas here. Maybe I shouldnt jump from a pin at all???

At least we are making progress.

Dominick