LM3886 x 2 in parallel

[jmar]

I bought one of these modules off Ebay that is a prebuilt stereo amp with (2) lm3886 on board.

Looks like I can run these in parallel pretty easily with the addition of the output resistors. 1/10th ohm at 3w on each output and then into a 4 ohm speaker.

I've seen in some schematics as well, the components regarding the noninverting input being shown. Looks like a 20k feedback resistor and then a 1K and a 100uf electrolytic to ground.

Will I need to adjust those components for parallel operation?

I don't have the schematic on this amp and like I say, it's setup for isolated stereo operation. (I'll trace the PCB when I have time -- but thought I'd ask).

Also, it's calling for a dual 28vac transformer for stereo -- what would be the right voltage for parallel mode?

Thanks.

[Minion]

A 2x28V AC tansformer is too much voltage , that will give you +/-42v DC which is the absolute maximum the chip can take but if the Voltage in your area happens to fluctuate 10% it will bring your voltage over the limit , you will also dissapate more heat so you will need a Bigger heatsink ....

I would suggest a 2x24v AC transformer for 8 ohm loads and a 2X18v AC for 4ohm loads (each chip) ....

For going paralell , I would try to match the Gain set resistors on each chip to .01% , I would also match the Output resistors to 1% and maybe use a 5w instead of 3w as a hotter resistor will tend to drift in value ....


Good luck

[pacificblue]

Originally Posted by Minion

A 2x28V AC tansformer is too much voltage , that will give you +/-42v DC which is the absolute maximum the chip can take but if the Voltage in your area happens to fluctuate 10% it will bring your voltage over the limit , you will also dissapate more heat so you will need a Bigger heatsink ....

It is good to maintain a bit of headroom. On the other hand the IC withstands ±47 V at idle and the rails will sag under load. In AN-1192 the people from National chose 2x30 V transformers for 8 Ohm speakers. Of course they were big transformers, i. e. had only low regulation and heatsinking became an issue. 2x28 V AC should not pose a problem, as long as you keep the load at 8 or more Ohm for each IC, use big heatsinks and the non-isolated LM3886T package.

The isolated LM3886TF would indeed mean..

Originally Posted by Minion

a 2x24v AC transformer for 8 ohm loads and a 2X18v AC for 4ohm loads (each chip) ....

In parallel application each IC sees only half the load. A 4 Ohm speaker will appear as an 8 Ohm load for each IC. An 8 Ohm speaker will appear as a 16 Ohm load for each IC. So you can take advantage of the IC's voltage rating as long as you don't connect 2 Ohm speakers.

Originally Posted by Minion

For going paralell , I would try to match the Gain set resistors on each chip to .01% , I would also match the Output resistors to 1%

assuming that .01% should mean 0,1 % or better.

[jmar]

Ok --

So really running the LM3886 in parallel is really a no brainer.

4 or 8 ohm speakers are fine with a 24vac transformer and really I don't see any need to change the feedback resistors that they have on the board for normal stereo operation -- I see they are 1% -- I imagine they're matched well.

Just put the .1 ohm 5 watts in series with each output and that's it.

And parallel the inputs of course.

[pacificblue]

Originally Posted by jmar

I don't see any need to change the feedback resistors that they have on the board for normal stereo operation -- I see they are 1% -- I imagine they're matched well.

The datasheet specifies 0,1 %. If you think, the National engineers did not specify that correctly, explain why. Don't recommend to ignore that specification based on your imagination only.

[abraxalito]

To get reasonable current sharing between two paralleled devices, the gain setting resistors matter a whole lot more than the current sharing ones. Those on the outputs can be 5% without problems.

As I see it you're likely to run into poor sharing at low frequencies with those component values (100uF and 1k). This is because of the relatively poor tolerances of aluminium electrolytics (20% typically). A 20% change in the 100uF cap gives of the order of a 0.5% change in the gain at 20Hz, so your relatively expensive 0.1% resistors are rather wasted. The solution is simple - up the 100uF to 1000uF.

Edit - I've just had a look at the AN-1192 and it seems they didn't get this one proof-read by Bob Pease. 0.1% resistors are most certainly not required on the + inputs to the amp chips. 1% output resistors are also overkill.

[pacificblue]

Originally Posted by abraxalito

To get reasonable current sharing between two paralleled devices, the gain setting resistors matter a whole lot more than the current sharing ones. Those on the outputs can be 5% without problems.

Originally Posted by abraxalito

A 20% change in the 100uF cap gives of the order of a 0.5% change in the gain at 20Hz, so your relatively expensive 0.1% resistors are rather wasted. The solution is simple - up the 100uF to 1000uF.

Edit - I've just had a look at the AN-1192 and it seems they didn't get this one proof-read by Bob Pease. 0.1% resistors are most certainly not required on the + inputs to the amp chips. 1% output resistors are also overkill.

More like 2 % at 20 Hz, but the resistors are still not wasted. There is only little musical content around 20 Hz. It is much more important to get the rest of the audible spectrum equally shared.

The DC offset should also be similar on both ICs. That is why the blocking resistors are also 0,1 % types. Looks like overkill, but maybe Bob Pease did proof-read and he knows something about the ICs we don't.

1000 µF in the gain loop makes little sense, if the DC blocking cap on the non-inverting input is not adjusted accordingly. And even then it is not really a practical value.

The 1 % output resistors must be judged with the power dissipation in mind. Paralleling chipamps only makes sense, if you want to take advantage of the amplifier's power limits. The nearer you get to the limits, the smaller an imbalance can be tolerated. If the rails stay well clear off the LM3886's rating, you can use bigger tolerance. With a 2x28 V transformer 1 % seems more recommendable, while 5 % is certainly sufficient for a 2x18 V transformer. But then again, with such a low voltage there is no need for paralleling.

[abraxalito]

Originally Posted by pacificblue

More like 2 % at 20 Hz

Do show your calculations. I'm guessing they went something like this :

A 100uF cap has an impedance at 20Hz of about -j80 ohms.

At the tolerance extreme, this goes up to -j100 ohms.

This difference of j20 ohms is indeed j2% of the 1k resistor. So is that how you reached your answer?

but the resistors are still not wasted. There is only little musical content around 20 Hz.

It matters not one iota that the content at 20Hz be 'musical'. If this is being fed from a turntable, the rumble content won't be insignificant and will certainly extend well below 20Hz. If the OP listens to well recorded organ music on CD, there will be significant musical content. As an engineer I prefer not to legislate for the OP's musical tastes or choices of source. Besides, there will still be a diminishing gain error at frequencies higher than 20Hz. With your (miscalculated) 2% figure, there's still an error of 0.2% at 200Hz - are you going to say there's no musical content there too?

It is much more important to get the rest of the audible spectrum equally shared.

Do please show your reasoning.

The DC offset should also be similar on both ICs.

Have you looked at the LM3886 datasheet to understand what causes an offset at the outputs?

That is why the blocking resistors are also 0,1 % types.

Do please show your reasoning, bearing in mind that the input bias current for this chip shows up at 1uA maximum, whereas the offset voltage can go up to 10mV worst case.

Looks like overkill, but maybe Bob Pease did proof-read and he knows something about the ICs we don't.

An interesting hypothesis - what do you base it on? If he did indeed proof-read this AN, he's let various other howlers through along with these, some unrelated to the device characteristics.

1000 µF in the gain loop makes little sense, if the DC blocking cap on the non-inverting input is not adjusted accordingly.

I'm looking at fig 6 (page 8) and I see only one DC blocking cap, a 1uF mylar. Since that's in circuit for both paralleled amps, please explain how its going to cause a sharing problem.

And even then it is not really a practical value.

Do please explain why you think 1000uF is impractical.

The 1 % output resistors must be judged with the power dissipation in mind.

Here, I have no grasp of what you're trying to communicate. The tolerance of a resistor and its dissipation are orthogonal quantities.

Paralleling chipamps only makes sense, if you want to take advantage of the amplifier's power limits. The nearer you get to the limits, the smaller an imbalance can be tolerated. If the rails stay well clear off the LM3886's rating, you can use bigger tolerance.

If we already know the load then I might agree with you. But we don't - loudspeakers do vary. But you're missing the point here - the effects of the gain errors swamp the effects of having the sharing resistors closely matched. Do the math.

With a 2x28 V transformer 1 % seems more recommendable, while 5 % is certainly sufficient for a 2x18 V transformer. But then again, with such a low voltage there is no need for paralleling.

Depends what the load impedance presented by the loudspeaker is. Also it depends how low distortion the OP wants - looking at the datasheet, its clear that lower impedance loads make the distortion higher. So paralleling amps has benefits beyond higher powers.

[pacificblue]

Wow, what a lecture!

Originally Posted by abraxalito

Do show your calculations. I'm guessing they went something like this :

100 µF ±20 % leads to worst case values of
80 µF at 20 Hz = j99,472 Ohm
120 µF at 20 Hz = j66,314 Ohm

Worst case for 0,1 % resistors is
Ci1 = j66,314 Ohm, Ri1 = 999 Ohm, -> Xi1 = 1001,199 Ohm, Rf1 = 20020 Ohm -> a = 20,996
Ci2 = j99,472 Ohm, Ri2 = 1001 Ohm, -> Xi2 = 1005,930 Ohm, Rf2 = 19980 Ohm -> a = 20,862
a average = 20,929, a error = ±0,32 %

Worst case for 1 % resistors is
Ci1 = j66,314 Ohm, Ri1 = 990 Ohm, -> Xi1 = 992,219 Ohm, Rf1 = 20020 Ohm -> a = 21,358
Ci2 = j99,472 Ohm, Ri2 = 1010 Ohm -> Xi2 = 1014,887 Ohm, Rf2 = 19800 Ohm -> a = 20,510
a average = 20,934, a error = ±2,03 %

Originally Posted by abraxalito

there's still an error of 0.2% at 200Hz - are you going to say there's no musical content there too?

I am not getting you there. You think 0,2 % is a severe error, but the resistors need not be closely matched. Isn't that a contradiction?

Originally Posted by abraxalito

Do please show your reasoning.

Your previous answer shows that you already followed my reasoning. I won't answer to nit-picking.

Originally Posted by abraxalito

Have you looked at the LM3886 datasheet to understand what causes an offset at the outputs?

Should I? Are the mechanisms different from the ones in other amplifiers?

Originally Posted by abraxalito

Do please show your reasoning, bearing in mind that the input bias current for this chip shows up at 1uA maximum, whereas the offset voltage can go up to 10mV worst case.

You should probably ask the designer or the datasheet author. I can only guess that they tried to match Ri to Rb in an attempt to improve CMRR and thus decrease DC offset.

Originally Posted by abraxalito

An interesting hypothesis - what do you base it on?

What do you base yours upon that he didn't? Infallibility, because his name is well-known?

Originally Posted by abraxalito

I'm looking at fig 6 (page 8) and I see only one DC blocking cap, a 1uF mylar. Since that's in circuit for both paralleled amps, please explain how its going to cause a sharing problem.

I don't know, how it is going to cause a sharing problem, and I didn't write that it does.

It makes little sense to filter the non-inverting input at 3,38 Hz and the inverting input at 0,16 Hz. Those two filters should have similar corner frequencies and add up to a reasonable quality factor. More than half an octave of separation between them should be an exception.

Originally Posted by abraxalito

Do please explain why you think 1000uF is impractical.

It has no advantages that justify its size and cost. You will find few amplifiers with a 1 mF capacitor in the inverting leg, if any. Are you prepared to accept the accumulated experience of the majority of amplifier designer's as proof?

Originally Posted by abraxalito

Here, I have no grasp of what you're trying to communicate. The tolerance of a resistor and its dissipation are orthogonal quantities.

Yes, sorry, that sentence can be misleading. I was refering to the power dissipation in the chipamp, not the resistors. Based on my assumption that paralleling is used to make the most of the ICs' power output, they (the ICs) would be used next to their limits.

Originally Posted by abraxalito

the effects of the gain errors swamp the effects of having the sharing resistors closely matched. Do the math.

I also refer to those resistors as load-sharing resistors out of habit. The truth is probably more that those resistors act as buffers between the IC outputs, so that any imbalances can be converted into heat in them, instead of the ICs' output stages. Close matching them is necessary, when you take the ICs to their voltage limits as I explained before.

Originally Posted by abraxalito

Depends what the load impedance presented by the loudspeaker is.

Originally Posted by jmar

a 4 ohm speaker

[abraxalito]

Originally Posted by pacificblue

Wow, what a lecture!

Not really, since a lecture is generally unidirectional. Here I'm interacting with your remarks so its more of a tutorial. But I agree its rather long - that's because you've made so many mistakes. So here's what I'll do - I'll just deal with one group of your mistakes at a time to keep the post to a manageable length. So for now, I'll just deal with the calculations for the gain matching of the two amps.

100 µF ±20 % leads to worst case values of
80 µF at 20 Hz = j99,472 Ohm
120 µF at 20 Hz = j66,314 Ohm

No disagreement there.

Worst case for 0,1 % resistors is
Ci1 = j66,314 Ohm, Ri1 = 999 Ohm, -> Xi1 = 1001,199 Ohm, Rf1 = 20020 Ohm -> a = 20,996
Ci2 = j99,472 Ohm, Ri2 = 1001 Ohm, -> Xi2 = 1005,930 Ohm, Rf2 = 19980 Ohm -> a = 20,862
a average = 20,929, a error = ±0,32 %

By which you show that the capacitor tolerance on the 100uF has moved the gain matching error well beyond that for the resistor tolerance. Which was my point. If you do this again but use 1000uF you'll find the error reduces considerably. To save you the trouble, I'll do it myself :

C1a = j6.6 ohm, R1 = 999 ohm; X1 = 999.02 ohm, Rf1 = 20020 ohm; a = 21.04
C1a = j9.9 ohm, R1 = 1001 ohm; X1 = 1001.05 ohm, Rf1 = 19980 ohm; a = 20.96
average a = 21, error in a ±0.04 giving ±0.2%

This is the same (to the number of significant figures in the calculation) as having no capacitor at all, so the value of the relatively expensive 0.1% resistors hasn't been thrown away.

Worst case for 1 % resistors is
Ci1 = j66,314 Ohm, Ri1 = 990 Ohm, -> Xi1 = 992,219 Ohm, Rf1 = 20020 Ohm -> a = 21,358
Ci2 = j99,472 Ohm, Ri2 = 1010 Ohm -> Xi2 = 1014,887 Ohm, Rf2 = 19800 Ohm -> a = 20,510
a average = 20,934, a error = ±2,03 %

You offered your 2% figure to correct my initial figure of 0.2%. Now I see your comment of 'more like 2%' is based on gain setting resistors with 1% tolerance. However, I've never suggested the gain setting resistors be 1% types - in my judgement they should indeed be 0.1%. I agree that with 1% resistors, the match is unaffected by capacitor tolerance. So it seems you misunderstood my original comment and took it out of context - that context was already accepting the need for 0.1% resistor match in the gain setting resistors. Actually my 0.2% was based on conservative hand waving, I accept your more accurate figure above of 0.32% - which only makes my thesis stronger and yours weaker.

I am not getting you there. You think 0,2 % is a severe error, but the resistors need not be closely matched. Isn't that a contradiction?

Please point out where I've said that the gain setting resistors need not be closely matched? As I've set out above, they should indeed be matched to 0.1% but that degree of match is rather wasted with a too small value of capacitor.