Author Topic: How to lower noise of active speakers?  (Read 5916 times)

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Offline David Hess

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Re: How to lower noise of active speakers?
« Reply #25 on: June 06, 2016, 03:17:44 pm »
R1 or R4 is just wrong.  They should both be the same value and if they are different, common mode noise at the input is going to end up at the output.  I would use 1% metal film resistors for R1 through R4 and 1% capacitors for C1 and C2 to raise the common mode rejection.

The impedance levels are all reasonable for the TL084.  I do not think noise from the TL084 is a factor here.

As a test, I would short the inputs to the LM3886s (I assume there are two) and see if the noise remains.  If it does, then the source is the LM3886 circuits.  I am suspicious of the LM3886; the datasheet does not list input referred voltage and current noise but its input bias current makes me suspect it is intended to operate with low impedance sources like an LM833.  The low input impedance used for the noise specification supports this as do the circuit examples.
 

Online edavid

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Re: How to lower noise of active speakers?
« Reply #26 on: June 06, 2016, 03:59:47 pm »
It looks like there is no DC bias path for U1A  :-//
 

Online Kleinstein

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Re: How to lower noise of active speakers?
« Reply #27 on: June 06, 2016, 05:41:09 pm »
The capacitor behind the pot should not be there - this would also avoid the problem of missing DC path for U1A.

Having one buffer OP for both filter sections is ok, there is only minimal interaction between the filters - even than its a question if it's better or worse to have worse filters at one OP.

Having the same value for R1/R4 would not change much with the gain, only a factor of 2/3.

Different input impedance is possible if the signal source is a one sided source - going to ground with the one side, while the other input sees the output impedance of the source. Anyway if needed adjustment would be better done with R2,R3. There is no really need for 1% precision für C1,C2 ,  5% or 10% should be good enough. Currently R1/R4 are 50% off.

Having amplification and than attenuation again makes some sense, to get a higher signal level and thus less noise influence. The trouble could be too much gain behind the filters.
 

Offline Funkensteyn

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Re: How to lower noise of active speakers?
« Reply #28 on: June 07, 2016, 09:45:16 am »
There was an typo in the labeling again, the buffer and lowpass were both labeled UA, sorry about that.

It looks like there is no DC bias path for U1A  :-//

That's UB now, just that we are on the same page. Assuming an input bias current of 10nA the drift would be 10nA/10uF=1mV/s=60mV/min, right? I never noticed any malfuntion even after hours of operation.

Different input impedance is possible if the signal source is a one sided source - going to ground with the one side, while the other input sees the output impedance of the source.
Sure, didn't think about single-ended input.

Later today I have some time to poke around and will report back if shortening the lm3886s will improve on the noise situation.

Thanks for all your help, I already learned quite a lot!
 

Offline macboy

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Re: How to lower noise of active speakers?
« Reply #29 on: June 07, 2016, 04:14:23 pm »
Good job reversing the schematics.

One way to determine the filter response is to simulate the circuit using e.g. LTSpice, then run an AC analysis. This will plot the gain/phase over a specified frequency range.

You didn't include the power amplifier section in the schematics. Did you have a chance to at least determine the gain of the LM3886?
 


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