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Old 16th October 2015, 06:04 AM   #1
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Default Output coupling capacitor value/model recommendations?

Hey there

I am building a new output stage for a cd player...i.e going direct from the dac to cap to pre...

my preamp is direct coupled so i will need coupling on my cd out to block DC...



considering that my preamps input if 50k, what would my appropriate choice for capacitor value be?...and should i use merely a cap or a resistor along with it?

cheers

J
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Old 16th October 2015, 08:35 AM   #2
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No resistor needed. The cap and input impedance form a high pass filter, ie a frequency-dependent voltage divider. The formula is C = 1/2πfR.
So, for example, for a 10Hz low frequency cutoff, C = 1/2*π*10*50k = 320nF.
A good 1uF polypropylene cap would work. Others may have a particular favorite.
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Old 16th October 2015, 05:18 PM   #3
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Great to know, thank you!


On similar note......i am on a hunt for a good "neutral/transparent" cap, (suitable for my line level application) and seem to be finding mixed reviews everywhere........

What would you personally recommend (beneath the "teflon" price range hehe) ?
And why is it acceptable to use a much higher rated voltage, (i.e 250v-600v) for line level?
would not a drastically higher v rating than the intended (low 1.5-2v) signal, reap alternate values/frequency/cutoff?

j
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Old 17th October 2015, 12:31 AM   #4
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The 1uF polypropylene is my personal recommendation.
Most caps of these types (polypropylene, C0G, polyester, etc.) are found with voltage ratings >50V. A lot of the difference is simply due to the dielectric properties. How thin would a 16V teflon cap dielectric need to be? The large difference in signal voltage won't matter for the AC coupling you're dealing with. It might with very high-precision filter and timing circuits, but I'm not certain of that so it's a big maybe. The electrolytic story is a bit different; the dielectric is less stable.
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Old 17th October 2015, 05:09 AM   #5
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Quote:
Originally Posted by sofaspud View Post
How thin would a 16V teflon cap dielectric need to be?
While watching the ballgame, tried to use my foggy clues to find an answer. Came up with ~0.25 micrometers.
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Old 17th October 2015, 06:12 AM   #6
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A 1M resistor from output to ground gives a discharge path to prevent damage if someone plugs the cable in with the power on. It has no effect in normal operation
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Old 17th October 2015, 10:56 AM   #7
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Quote:
Originally Posted by Fidelogos
And why is it acceptable to use a much higher rated voltage, (i.e 250v-600v) for line level?
In electronics it is almost always acceptable to use a component with a higher rating than strictly necessary. In this case the relevant voltage is not the signal voltage but the likely DC voltage to be blocked.

Quote:
would not a drastically higher v rating than the intended (low 1.5-2v) signal, reap alternate values/frequency/cutoff?
Only if the capacitor is seriously nonlinear, in which case you might not wish to use it for audio. The cutoff frequency is fixed by the capacitor value (and circuit resistance); it has nothing whatsoever to do with the capacitor voltage rating.

Choose your LF cutoff frequency - not too high, not too low. Calculate what cap value you need, then round up to the next normally available value. Determine what DC voltage rating you need, then find one which exceeds this. Choose a dielectric - for a coupling cap of the correct value almost any film cap willl do. Don't spend too much money. Add a ground leak resistor, as davidsrb suggests. Job done!
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Old 17th October 2015, 03:51 PM   #8
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Fantastic guys!!

Thanks
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Old 17th October 2015, 04:11 PM   #9
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Quote:
Originally Posted by DF96 View Post
In electronics it is almost always acceptable to use a component with a higher rating than strictly necessary. In this case the relevant voltage is not the signal voltage but the likely DC voltage to be blocked.


Only if the capacitor is seriously nonlinear, in which case you might not wish to use it for audio. The cutoff frequency is fixed by the capacitor value (and circuit resistance); it has nothing whatsoever to do with the capacitor voltage rating.

Choose your LF cutoff frequency - not too high, not too low. Calculate what cap value you need, then round up to the next normally available value. Determine what DC voltage rating you need, then find one which exceeds this. Choose a dielectric - for a coupling cap of the correct value almost any film cap willl do. Don't spend too much money. Add a ground leak resistor, as davidsrb suggests. Job done!

Thanks---One more inquirey

(hopefully)

So lets say i use a cap and choose to use a "ground leak resistor as well"-- i suppose the same 1/2nfr formula applies? i.e 1/2xPIxCapxRESxPREAMP= my cutoff?
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Old 17th October 2015, 04:36 PM   #10
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Yes, same formula. That is why the 1Meg resistor was suggested. It is much higher in value than the 50k impedance, so will have little effect on circuit formulas or operation.
A circuit rule of thumb is when a value is 10x or more another value, one of the values can be ignored, e.g. for parallel resistors the larger value can be ignored; for parallel capacitors the smaller value can be ignored.
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