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Old 15th August 2011, 07:00 PM   #11
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OK, but what speaker is it that needs more then 24 Apk in the LM3886's permitted operating voltage range?
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Old 15th August 2011, 11:10 PM   #12
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The 3886 is specified @ 7 to 11.5A maximum at 25degC.
National do not tell us how low that peak output current must be kept to avoid current limiting when Vce is high nor do they tell us how the current limiting is varied as the chipamp temperature changes from 25degC.

What if the limiter comes in at 5Apk when Tj ~100degC momentarily.
What if the limiter is also modulated to take account of Vce. Could that reduce the maximum peak current of a hot chipamp feeding a reactive speaker load to <<7A?

4 parallel chipamps will increase the total peak current into a reactive load on two counts taken from the previous suggested operation.
a.) the 4 way sharing reduces the demanded current.
b.) the chipamp temperature will be slightly lower due to less current demanded through any individual chipamp.

A cooler chipamp and less current demanded cannot make SQ worse and there is much in the app notes and datasheet that suggest it could increase SQ, particularly since the biggest problem of all chipamps is severely limited current capability.
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Old 16th August 2011, 03:58 AM   #13
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I expect we could get lower THD in the low end. Most chipamp has increasing THD due to thermal coupling. Reducing the load should minimize this problem.

We may also eliminate the protection circuit partially on below the peak current threshold. Has anyone investigated this?
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Old 16th August 2011, 05:45 AM   #14
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Default Three 3886 parallel board

This is the LM3888 three in parallel board I am playing with. It is purchased from TaoBao. The three 3886s are free samples from NS long time ago.
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Old 16th August 2011, 06:31 AM   #15
jcx is online now jcx  United States
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Linn seemed to think paralleling 3 chip amps was worth several US$K

About Linn power amps...

multiway loudspeakers with poorly designed crossovers can take large factors over the nameplate resistive load calculated current peaks

the thermal feedback distortion is easily eliminated by wrapping a good auality audio op amp around the chip amps in a multiloop composite - compensation is a little complesx but added loop gain can be 40-60 dB at the low thermal moduation frequencies

Last edited by jcx; 16th August 2011 at 06:39 AM.
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Old 16th August 2011, 07:53 AM   #16
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Quote:
Originally Posted by AndrewT View Post
National do not tell us...
Yes, they do in AN-898. The current limiter has a fixed threshold. A chip amp driven into that limiter will sound like any amp driven into a limiter.
As the IC is operating at its current limit, however the power transistors will heat up and the thermal protection will shut them off until temperatures are back to normal again. No music for a while and no THD at all.
Then there is the voltage limiter which also has a fixed threshold, but we cannot outsmart it by reducing the load current. If your speaker's reactance is too big, external flyback diodes may help to relieve the internal clamping circuitry.

Quote:
Originally Posted by AndrewT View Post
What if the limiter comes in at 5Apk when Tj ~100degC momentarily.
Then 5 A will heat the power transistor up to above the thermal threshold at 165 °C when the junction is at an average of 100 °C. In that case the heatsink is too small.
You can solve that by adding ICs to distribute the heat across more of them.
You can solve that by increasing the heatsink size or adding a fan.
Or you may ask yourself what you are doing that demands more than 15 A from your three-parallel-LM3886 amp, when it is already operating at an average junction temperature of 100 °C. THD may be the least of your problems in that situation.

Quote:
Originally Posted by AndrewT View Post
4 parallel chipamps will increase the total peak current...
And still my two questions remain.
Which speaker is it that will sound sufficiently better from four than from three parallel LM3886s to justify the additional investment?
Are four parallel LM3886s really cheaper than one LME498.. and a few transistors that give the same or an even better result?

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Originally Posted by panson_hk View Post
I expect we could get lower THD in the low end.
...where human hearing is less sensitive to THD?

Quote:
Originally Posted by panson_hk View Post
Most chipamp has increasing THD due to thermal coupling. Reducing the load should minimize this problem.
As should improved heatsinking and not using chip amps for loads they cannot reasonably operate on.
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Old 16th August 2011, 08:04 AM   #17
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Quote:
Originally Posted by pacificblue View Post
As should improved heatsinking and not using chip amps for loads they cannot reasonably operate on.
The former - probably not. Short term thermal time constants remain the same even when bigger heatsinks are employed. Its rather like saying a bigger battery capacity will improve the decoupling of some high speed logic - doesn't happen.

As for the latter, whose meaning of 'reasonably' ?
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Old 16th August 2011, 08:19 AM   #18
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by pacificblue View Post
Yes, they do in AN-898. The current limiter has a fixed threshold. A chip amp driven into that limiter will sound like any amp driven into a limiter.
That is not my interpretation of National's documents.
Quote:
SPiKe Protection is a “uniquely-smart” protection mechanism
that will adjust its output drive capability according to its
output operating conditions, thus safeguarding itself against
the most stringent power limiting conditions.
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Old 16th August 2011, 11:31 AM   #19
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Originally Posted by abraxalito View Post
The former - probably not. Short term thermal time constants remain the same even when bigger heatsinks are employed.
The delta T remains the same. Thanks to improved heatsinking the average temperature will be lower, so the peak temperature as a result of average temperature plus delta T must also be lower.

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Originally Posted by abraxalito View Post
As for the latter, whose meaning of 'reasonably' ?
The IC designer's meaning of reasonable. I quote from the datasheet.

Quote:
  • Component stereo
  • Compact stereo
  • Self-powered speakers
  • Surround-sound amplifiers
  • High-end stereo TVs
Nothing where to expect highly reactive, low-impedance loads driven at continuous max output power from max rail voltage while still expecting low THD.

Quote:
Originally Posted by AndrewT View Post
That is not my interpretation of National's documents.
I would put the stress in your quotation on "safeguarding itself against the most stringent power limiting conditions". Figures 8 and 9 of that document show that it is possible to dissipate 30 W for 100 ms at a case temperature of 125 °C or you can dissipate 40 W for 100 ms at a case temperature of 75 °C before SPiKe starts to operate. The latter corresponds nearly to the maximum output power into 8 Ohm from ±42 V rails.

Three LM3886s at Tc 75 °C will be able to dissipate the same 120 W as four LM3886s at Tc 125 °C before SPiKe starts to limit the output current. That would happen at more or less the maximum output power into 3 Ohm from ±42 V rails. And still no answer to my two questions.
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Old 16th August 2011, 12:06 PM   #20
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by pacificblue View Post
........... Figures 8 and 9 of that document show that it is possible to dissipate 30 W for 100 ms at a case temperature of 125 °C or you can dissipate 40 W for 100 ms at a case temperature of 75 °C
this seems to confirm that the protections/limiters are monitoring Tj and adjusting the limiting thresholds to suit the level and time of the transient.

That is why I arrived at "my interpretation". The protection levels vary with Tc and the higher the Tc, the lower the limiters are set.
On transients, I think that what I hear is the early onset of current limited transients, long before these transients ever approach voltage clipping. And this sounds worse if the chipamp is hotter. I try to keep my operating chipamps near cold and yet I still hear the effect as volume is turned up.

I chose to ignore your other two questions, because you know just as well as me that I have never listened to multi-chipamp topologies and thus can never have operational experience of a speaker model that meets your specification. In addition you also know I only work with 8ohm or higher impedance speakers. I don't own any 4ohm speakers.
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Last edited by AndrewT; 16th August 2011 at 12:10 PM.
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