lowest load for line level equipment to not harm the output stage?

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I would like to know what would be a typical minimum load for line level equipment. I am considering a switching application that will shunt the signal to ground through a small resistor (e.g. 47R) when in the "OFF" state. This low of a load is almost a short compared to a nominal 10k load and might even be less than the output impedance of the source. I assume that the output stage of line level source equipment should not be loaded down so heavily since the current draw will be relatively high. On the other hand, as long as equipment would not be severely stressed or damaged, this would only happen for the "OFF" state of the switch and the on state would have resistance on the order of the usual 10k Ohms.

So, how much loading (e.g. what is the minimum resistance) should I use in place of the 47R if I want to be in a "generally acceptable" range for loading of line level equipment?
 
That's part of the advice I am l looking for... what is typical?

I understand that it all depends on the output device capabilities in terms of current, ability to survive a short, etc. I am looking for "best" or at least "safe" practice type advice. I could make it 680R, or 1k, 2k, 4k, but increasing the shunt resistance decreases the attenuation of the "switch".

Also, keep in mind that the low impedance loading is only present when the switch is off and as long as the device can survive unharmed how it behaves in this state (e.g. high distortion, frequency response deviations, etc.) is not important.
 
Input impedances are usually in the range of 10k-100k, but most decent quality opamps are specified at being able to drive a 600 ohm load. This is usually a minimum tolerable load whilst also giving good performance. Stereophile typically test line outs down to 600 ohms just to see how robust they are. This doesn't represent the norm in terms of what loads the source is likely to see, but some equipment does use lower than average input impedances. So it is nice to know your kit is capable of driving it.

Most modern opamps are short circuit protected, some will specify a duration that they can survive with a short on the output too.

If the performance is not critical in this low impedance state and if the opamp is capable, I think I'd go with 600 ohms.
 
It depends if the noise is dominated by the stage preceding. If it is then the attenuation will cut down the signal and noise by the same degree.

For example if the stage preceding has a SnR of 110dB and the stage after has an equivalent noise floor of 120dB, then even with 3dB attenuation you're still going to get 110dB SnR in the second stage. If the second stage had a noise floor of 100dB, without attenuation you're looking at 100dB SnR, with attenuation this will then fall to 97dB. Minus a tiny bit of noise summing.

What is it you are building?
 
What is it you are building?

I'm building an analog input selector switch that is connected to an ADC having a 10k Ohm impedance.

The circuit is shown in the attachment, representing one input connected to the 10k load of the ADC input. I have about 20 of the LDRs that I can use to select from different inputs or pairs of inputs. This configuration saves one LDR per channel compared to other circuits that I have seen, and is completely passive.
 

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  • LDR_passive_input_selector.png
    LDR_passive_input_selector.png
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If VG1 is the source, then Rin & Cin won't give you a LPF.

Based on what it "appears" you are wanting to do, hows about this

Oops, I stuck that in there as a way to try and use Rin for that purpose but never did a sim of the transfer characteristics. You are correct, it doesn't do anything as I have drawn it. Moving "Rin" to the ACD side of the LDRs and putting the cap after Rin as you have shown gives the proper behavior. Thanks for pointing that out.

Anyway, more specific to my application, the LP filter might not be at all necessary since the ACD has a steep anti-aliasing filter with >65dB rejection above the Nyquist frequency. On the other hand it might be good to reject RF signals and the like, so I could drop the capacitance down to 100-220 pF.
 
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With the circuit i posted, you will acheive what you want :)

OFF = + 5Meg to the source = no low loading.

ON = Correct LPF implimentation.

Sure, tinker with the R & C values for the LPF.

In future it would be helpful & quicker if, initially you post a fuller description & screenie etc ;)

All the best with your project
 
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