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Old 3rd November 2010, 03:57 PM   #11
AndrewT is offline AndrewT  Scotland
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Hi,
what does the description say that this chip can do?

I think the switches on the 3 inputs are there for testing only.

I think the chip has 3inputs and 1output, does the datasheet say how the 3inputs are switched?
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Old 3rd November 2010, 04:08 PM   #12
Leon08 is offline Leon08  Germany
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By wording I would think not, but I don't understand it
What do you not understand?
That's very easy.The current always goes the way of the smallest resistence.So if put a 1k resistor to ground then some current, not all, will go through him to ground, but if you put a 1R resistor to ground then almost all current will go to ground and it will be like a short.
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Old 3rd November 2010, 04:57 PM   #13
RobertE is offline RobertE  United States
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Quote:
Originally Posted by AndrewT View Post
Hi,
what does the description say that this chip can do?

I think the switches on the 3 inputs are there for testing only.

I think the chip has 3inputs and 1output, does the datasheet say how the 3inputs are switched?
This all works fine in my controller. The switches are voltage high/low binary. The ic is 58k ohm input and 10 ohm output.
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Old 3rd November 2010, 04:59 PM   #14
RobertE is offline RobertE  United States
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Originally Posted by Leon08 View Post
What do you not understand?
That's very easy.The current always goes the way of the smallest resistence.So if put a 1k resistor to ground then some current, not all, will go through him to ground, but if you put a 1R resistor to ground then almost all current will go to ground and it will be like a short.
I meant why not put a 1M ohm, so all the current goes to the amp? Is it just because the capacitor would be too small ?
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Old 3rd November 2010, 05:55 PM   #15
Leon08 is offline Leon08  Germany
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Maybe because of capacitor or some other reason.I don't know for which purpose they intended this filter.That all depends on the purpose of your circuit.
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Old 3rd November 2010, 06:01 PM   #16
macboy is offline macboy  Canada
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Originally Posted by RobertE View Post
I meant why not put a 1M ohm, so all the current goes to the amp? Is it just because the capacitor would be too small ?
Then only a very small current will flow through the resistor. This could be less than the leakage current through the capacitor, or less than the leakage current of the input pin of the amplifier (or opamp) following this circuit. That can and will cause a DC offset which, if you have no other DC blocking downstream, can blow your speakers, overheat your amp, etc.

Besides the possiblility of a steady state DC offset, you would likely get "thumps" when switching inputs, etc. This due to the extremely low cutoff frequency of the high-pass filter formed by the R and C. Fc=1/(2*pi*R*C). Even with the values suggested (220uF and 1kOhm) you get a transient lasting about 1/2 second before falling to 10% of the initial value. With a 1 Meg resistor (and 220 uF), it would last 1000 times as long... about 8 minutes. I'm not kidding.

Any value is a compromise as already pointed out.

Last edited by macboy; 3rd November 2010 at 06:04 PM.
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Old 3rd November 2010, 06:06 PM   #17
Leon08 is offline Leon08  Germany
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But why he needs this filter at all? An amplifier has already an iput filter.
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Old 3rd November 2010, 07:29 PM   #18
RobertE is offline RobertE  United States
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Originally Posted by Leon08 View Post
But why he needs this filter at all? An amplifier has already an iput filter.
That's kind of my question... Wouldn't an amp always have dc blocking caps??? Why wouldn't it?

I've tried the circuit with and without it, and I don't see a huge difference. I think without it some dc is getting processed by the amp because at high volumes the bass handling seems a lot worse.
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Old 3rd November 2010, 08:09 PM   #19
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Originally Posted by Leon08 View Post
But why he needs this filter at all? An amplifier has already an iput filter.
It is NOT a fliter!

Th cap is a DC block, the R is a ground level restorer.

Always needed if it is NOT known what is plugged into it! If you can control it and there is an input cap then not needed.
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Old 3rd November 2010, 08:23 PM   #20
RobertE is offline RobertE  United States
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Originally Posted by cliffforrest View Post
It is NOT a fliter!

Th cap is a DC block, the R is a ground level restorer.

Always needed if it is NOT known what is plugged into it! If you can control it and there is an input cap then not needed.
So why wouldn't all amps just have dc blocking caps on the inputs???
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