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Old 22nd October 2009, 04:28 PM   #21
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Two solutions even beginners should be able to manage. Although it may take a bit of luck to find two pairs with 1 % match from only six resistors.

Most multi-meters measure better in the upper region of each measuring range. Bringing the expected voltage drop into that range should yield even better results. The usual recommendation is to use the upper third of each measuring range for exact data.
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Old 22nd October 2009, 04:52 PM   #22
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Quote:
Originally Posted by AndrewT View Post
A stereo pair of parallel chipamps requires a minimum of 4 output resistors.
Buy 6 (two go into stock) and series connect them using a 317 CCS passing 500mA. Set your DMM to 200.0mVdc and measure the voltage drop (~50.0mV) across each resistor.
Pick a pair that have as near as possible the same voltage drop. Now pick a second pair.
That will give you two pairs of near +-1% tolerance 0r1 resistors.
And the point of all that work towards getting such good tolerance output resistors is what exactly?
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Old 23rd October 2009, 05:47 AM   #23
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Default Spice analysis of the simplest AN-1192 parallel circuit

Here are some results from my analysis of the circuit given in figure 6 of AN-1192, page 8. In particular I was keen to explore how well the two amps share current at the extreme cases of all component tolerances giving the worst conditions, and at maximum output.

I ran a Spice simulation, using OP27's instead of LM3886s seeing, as far as I know, there are no official National Spice models of these amp chips. Because the OP27 model I used (provided as standard with my software package) has a current limit in keeping with the actual opamp capability, I scaled up the load impedances by a factor of 1000 so that 1mA in simulation became 1A in the real world. I didn't attempt to adjust the OP27 current limit to correspond to that on the LM3886. Unlike a real OP27, the Spice model appears to work happily at supplies of 35V I chose 0.1% gain resistors, 20% for the capacitor and 5% for the output sharing resistors, all at their extremes to make this a hideously worst case simulation.

Winding up the amplitude of the input source, it reached 1.55V peak just before the onset of clipping. The clipping voltage looked to be around 32V which is near enough to that on the LM3886 for a first stab. The high frequency (1kHz and above) sharing turned out about 3.4mA and 2.3mA, totalling 5.7mA into the load. Another, perhaps more helpful way of looking at this is to synthesise the equivalent resistive load each amplifier is seeing. In this condition, the higher gain amp (putting out 3.4mA) sees 6700ohms, the lower gain one, 9800 ohms (6.7 and 9.8 ohms in the real world). I consider this a reasonable match in the worst case for the higher frequencies even with 5% output sharing resistors.

In the real world this 5.7mA into the sim load corresponds to 5.7A and therefore, with a 4ohm load, 130WRMS. I doubt that a real pair of LM3886s would be able to achieve this continuously due to inadequate heatsinking, but in short bursts it might be quite feasible. The sharing resistors are dissipating 1.2W and 0.5W respectively under these conditions, so it looks as though 2W types will be adequate.

What happens at lower frequencies though is far from satisfactory. Due to the 20% capacitor tolerance, the gains deviate more strongly as the frequency drops from 1kHz, until at 20Hz, the currents in the sharing resistors become 5.9mA and 5mA. It seems at this point, with real LM3886s, the protection will certainly be activated - the amps are busy fighting each other. Put another way, the loads the two amps are now seeing are 3.8 ohms and 4.5 ohms - the benefits of adding a second paralleled amp have been completely lost. As the frequency descends still further, the amps will certainly go into total thermal shutdown - if they had unlimited current capability, a maximum output of 14A would be reached at the peak of 4Hz.

I'll go on to propose a fix for this problem in a subsequent post
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Old 23rd October 2009, 06:32 AM   #24
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I'm assuming one fix would be to lower Ri to about 100 ohms, and omit Ci totally.

Thanks for the read
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Old 23rd October 2009, 06:40 AM   #25
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Off the top of my head, no that won't work very well. It will cause a significant DC offset at the output in the worst case. Since the datasheet spec for the input offset voltage is 10mV max, multiplying this by 21 we get 210mV. Now if the two amps have opposite offsets (sods law), that's going to give us 420mV across the sharing resistors. In series they're 0.2R so that's a worst case standing current of 2.1A between the two amps - not very pleasant.
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Old 23rd October 2009, 07:10 AM   #26
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Odd, I've always measured very low offset this way. I prefer to use a 1K Ri and a 47uF Ci for the most part, but that always nets about 4mV offset at the output. with very low values for the feedback network and no cap, I've rarely seen it cross 0.5mV.
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Old 23rd October 2009, 07:31 AM   #27
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Yeah, the typical offset is 1mV according to the datasheet. However even with that low offset voltage, the quiescent dissipation in two paralleled chips isn't insignificant.

If using 35V rails, a 2mV offset between two amp chips will result in 210mA - this has to come from the 35V supplies, so each chip will be dissipating 7.4W.

I was just checking out some other 3886 paralleled designs - it seems the popular Jeff Rowland 3 * LM3886 paralleled amp (plenty of clones on eBay) uses a multiturn pot to trim the offset and dispenses with the capacitor, as you suggest. But they're using 0.2R sharing resistors, not 0.1R, so are somewhat less sensitive to offset.

You're getting 0.5mV at the output with a DC gain of 21 ? That would mean your input offset is only 24uV! Where d'you get your instrumentation grade amp chips? How does this offset vary with time and temperature?

Last edited by abraxalito; 23rd October 2009 at 07:38 AM.
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Old 23rd October 2009, 08:11 AM   #28
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Quote:
Originally Posted by abraxalito View Post
And the point of all that work towards getting such good tolerance output resistors is what exactly?
is your intention to argue or to provide information?

Are my suggestions wrong?
If so, then tell us what is wrong.
If they are helpful, then move on and tell us what we need to do.
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Old 23rd October 2009, 08:17 AM   #29
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Originally Posted by abraxalito View Post
The high frequency (1kHz and above) sharing turned out about 3.4mA and 2.3mA,
Pity that the simulation does not predict how the different current, hence different power dissipation affects the individual temperatures of the ICs, and how those different temperatures affect the circuit's performance.

Quote:
Originally Posted by abraxalito View Post
at 20Hz, the currents in the sharing resistors become 5.9mA and 5mA.
At a first glance the relation 5,9:5=1,18 looks better than 3,4:2,3=1,48, though the question arises, why the current increases so much, when it should actually remain constant with resistive loads or fall slightly with a high-pass in place.


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Originally Posted by abraxalito View Post
It seems at this point, with real LM3886s, the protection will certainly be activated - the amps are busy fighting each other.
Are they? So the load does not pass 10,9 mA at 20 Hz? How much current flows actually from one IC to the other then?
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Old 23rd October 2009, 08:25 AM   #30
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Originally Posted by abraxalito View Post
..............In the real world this 5.7mA into the sim load corresponds to 5.7A and therefore, with a 4ohm load, 130WRMS. I doubt that a real pair of LM3886s would be able to achieve this continuously due to inadequate heatsinking,
for a pair of parallel chipamps to give a maximum (short to medium term) power output of 130W, the current into the load must be 5.7Arms when passing a sinewave.
The peak current will be ~8Apk.
This is 4Apk per chipamp if the loading is equalised adequately. Well inside the 7A limit for Tc=25degC specified in the datasheet.
Keep the chips cool and they will pass that 130W to the 4r0 load in the short to medium term (1ms to 1s).

But that is not the main problem that the paralleled chipamps need to address.
It's the high peak currents that a 4ohm speaker may demand on fast starting or fast stopping transients.
These currents can approach (and even exceed) three times what the nominal resistive load would predict. Yes, a 4ohm speaker driven to 32Vpk may demand upto around 23Apk, this would need three parallel chipamps running cold to get anywhere close to meeting that transient demand if any of the chips starts to limit @~7Apk.
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