question about this schematic ?

[AndrewT]

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?

[Leon08]

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.

[RobertE]

Originally Posted by AndrewT

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.

[RobertE]

Originally Posted by Leon08

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 ?

[Leon08]

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.

[macboy]

Originally Posted by RobertE

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. F_c=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.

[Leon08]

But why he needs this filter at all? An amplifier has already an iput filter.

[RobertE]

Originally Posted by Leon08

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.

[cliffforrest]

Originally Posted by Leon08

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.

[RobertE]

Originally Posted by cliffforrest

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???