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Old 5th September 2002, 11:55 AM   #1
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Default Output Resistor Question

I have a amp based on the LM3886, and the output resistors are specified to be 10 Ohm with an inductor (wire spinned around it).

However, I used a 30 Ohm resistor and no inductance/solenoid or what you call it. Will this affect the sound, besides limitng the power output?

/Chrisian V
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Old 5th September 2002, 02:05 PM   #2
Duo is offline Duo  Canada
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Where on the output is this resistor used? from ouput to ground? or what?
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Old 5th September 2002, 02:17 PM   #3
audioPT is offline audioPT  Portugal
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Well, that resistor and the inductance are used to avoid the amplifier oscilation and to hold some capacitive loads.
If you want, assuming all of it is well assembled, simply don't use that resistor and the inductance.
Using that 30 Ohm resistor (in series with the output, right?) maybe some fire occurs (or just smoke)
The sound maybe become unaltered, but the power is only that is dissipated by the 30 Ohm resistor. Is this is one of 5W, and the LM3886 delivers, I think, more than 60W, the result is ... PUFFF!!!
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Old 5th September 2002, 04:58 PM   #4
sam9 is offline sam9  United States
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Default Output inductor/resistor

This sounds like a Marchand kit. I'm not sure about the best value for the resistor, but the coil can be important. The IC amp like most (all?) discrete amps can operate at frequencies well beyond human hearing. It is possible for it to be generating output in these inaudible ranges. Hopefully, these are filtered out before they reach the output stage, but if not they will be treated just like any other signal.

The inductor serves as a final filter to block signals beyond the audible range. If not present, there are two bad things that might happen A) curent is consumed amplyfying what you cannot hear which may limit performance in the audible range and B) the high frequency signal may be sufficient to damage your tweeters. AND this happens with out any audible indication that you are aware of until something fails.

In sum, I suggest you leave the inductor in unless you KNOW there are other measures in place to prevent the amp from passing high (~20k+) frequencies.

If I haven't described this correctly, I welcome any enlightenment offered.
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Old 5th September 2002, 08:01 PM   #5
Duo is offline Duo  Canada
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I did not ask you what the resistor is for(I already knew that) I just want to know if it's in parallel with the output or in series or what... You normally don't use an inductor and a resistor at the same time anyway. Usually it's a resistor and capacitor, or an inductor alone in series with the speaker, but not both topologies. I hope this clarifies my question. Thanks
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Old 5th September 2002, 09:07 PM   #6
joensd is offline joensd  Germany
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i own a lm3886 amp which has the same circuit.
10Ohm resistor paralleled by 0.7uH coil in series with the output.
itīs the same circuit in all of the kits iīve seen.
and itīs the circuit from the datasheet of the lm3886
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Old 5th September 2002, 09:13 PM   #7
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I say try the amp without it. Just use an RC network to
ground (.1 uF in series with 5 ohm)

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Old 6th September 2002, 11:41 AM   #8
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Duo,

It's in series with the load. also check this thread: question about input stage of this amp

HB.
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Old 6th September 2002, 04:55 PM   #9
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Christian:

I read the replies and don't see the answer to your question.

Yes, besides limiting the output power the resistor, whether 10 or 30 ohm, will change the frequency response of the speaker. The greatest effect will be in the bass but there could be large variations across the band depending mostly on the design of the crossover.

The inductor is there to act as a short circuit across the resistor at audio frequencies. At higher frequencies the inductor acts as less of a short and the resistance in line with the speaker increases.

It is my suggestion that you use the inductor and 10 ohm resistor. If you don't have an inductor you can make your own by winding about 20 turns of #14 magnet wire around a 3/8" wooden dowel. After removing the dowel you may place the resistor inside of the inductor if you wish.
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Old 6th September 2002, 06:18 PM   #10
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Hi,

I removed the resistor, and naturally it plays louder, maybe somewhat different.
My friend suggested that the inductor was for protection of the circuit. Apparently, the speaker-cables can act as antenneas and if a current should flow towards the chip, this inductor would stop it.

I don't know if this is true, but it works well without it.

Best Regards,
Christian
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