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Old 20th March 2005, 03:37 PM   #1
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Default BrianGT's LM3886 board, and hiss

I have a couple of Brians sweet LM3886 boards, but I am having an annoying hiss which I cannot diagnose. It is very slight, I have to have my ear 2-3 inches from the tweeter to hear.

Here are the step I have taken.

1) Reduced gain to 13.6 by replacing the 1K input resistor with a 1.75K this had no effect.

2) Removed and bypassed Ci, no effect.

3) Removed input decoupling cap, no effect.

4) did above two at the same time, no effect.

The only thing left that I can think of to do is to change the feedback resistor itself to a lower value, say 10K and then use a 500 ohm input resistor. Is that reasonable?

BTW, I have tried this with both batteries and Brian's snubber power supply, and there is no difference at all. No 60 or 120hz hum at all when using the PS.

My LM4780 amp is dead silet, no hum, no hiss. I wonder if this is a benefit/effect of Brians LM4780 board being a parallel design.

Any ideas? It sounds very good in every other respect. I just like perfection.
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Old 20th March 2005, 04:08 PM   #2
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Try to eliminate possible sources.

Short your input and remove input cable to rule out external noise source.

Disconnect one channel to rule out some interaction.

Maybe an output filter is needed. For inspiration you can see how I have done.

If you check the datasheet you will see that gain of 13 is near the limit for unstability. I think you should have less gain than 20.

Notice also that 2 inch from the tweeter is very close....
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Old 20th March 2005, 04:43 PM   #3
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Thanks Per-Anders,

I have already tried shorting/disconnecting the input, I forgot to mention that.

Do you have a link to your output filter?

Thanks.
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Old 20th March 2005, 05:25 PM   #4
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UPDATE:

Just for kicks and to satisfy my own curiosity I connected my two amps together in parallel just as the LM4780 PCN from Brian is.

The hiss was gone.

Does this mean that the two cancle out eachother in some way?

In any case, it makes my think the parallel LM4780 is close to the ideal chipamp.

This amo will end up in my nephew's bedroom, so I am not too concerned, I will rewire it as it was. It sounds great, but I wish there were a way to kill the hiss.
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Old 20th March 2005, 06:21 PM   #5
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I'll guess that you had series resistors when you did that? If yes, you needed an output filter.
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Old 20th March 2005, 06:36 PM   #6
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Quote:
I'll guess that you had series resistors when you did that?
Yes, .1 ohm dale, the same as what brian supplies for the LM4780.

Can you send me URL for an example of an output filter?
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Old 20th March 2005, 06:44 PM   #7
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What about components in the muting circuit, did you install just the 10k resistor (like it is with LM4780) or the additional cap as well, that I think Brian made space for?
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Old 20th March 2005, 06:49 PM   #8
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If parallelling the two amps solved it, it looks like an oscillation problem. That disappears when one loads the other. Did you use a zobel?

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Old 20th March 2005, 07:42 PM   #9
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Peter: Yes, I used the cap as well as the resistor for mute. Should I try removing the cap?

Janneman: Yes I used the zobel.
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Old 20th March 2005, 08:10 PM   #10
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Try without the cap, as 4780 circuit does not have this cap.
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