[sdiy] mixer

[harry]

Hi Antti...

I'm not sure I agree. The source resistance from the driving stage will indeed
be the parallel combinaton of the output resistance of that stage and the input
resistance of the next.  So from that point of view I agree.

But as far as the input is concerned... they are not in parallel. If the source
resistance
is much lower than the input resistor... the input resistor dominates.

Look at current flow into the summing junction.  Make the input a voltage source
if you like (zero resistance). The input current is dominated (limited) only by the

input resistor....   No matter what happens... the only way to get more current
into the summing junction is to lower the input resistor.

Add the source impedance and they are more in series, not parallel (looking
from the summing amp)

The self-noise of the resistor is proportional to resistance, so the bigger we make
the
resistor, the worse the noise gets.

The trade is to use the lowest input resistor we can stand, that does not interfere
with
the driving stage.

H^) harry

see.. that was non political, yes ???

Antti Huovilainen wrote:

> On Thu, 1 Nov 2001, Glen wrote:
>
> > Isn't that excessively high? Remember, the higher the input resistance value,
> > the higher the background noise will be. All the line-level equipment that I
>
> Not quite. The higher the source resistance looking at the amp is, the
> higher the noise. Source resistance consists of driving stage output
> impedance in parallel with the mixer input resistance. So if the source
> has low impedance, the input impedance looking at teh mixer op-amp is
> dominated by that. So if the input resistance is 47k and the driving
> impedance is, say 1k, then the total resistance is about 0.98kohm ~ 1kohm.
> The rule of thumb is to have input impedance >= 10 x driving stage output
> impedance.
>
> Antti
>
> "If you don't eat yer meat, you can't have any pudding!
>  How can you have any pudding if you don't eat yer meat!?"
>    - Roger Waters

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