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Preamp Output Capacitor Voltage Rating?
Preamp Output Capacitor Voltage Rating?
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Old 1st August 2019, 09:13 PM   #1
cal3713 is offline cal3713
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Default Preamp Output Capacitor Voltage Rating?

I often see large voltage ratings on the capacitors used at the outputs in preamps, for example 600V or 650V... however the actual voltage coming out of the caps is minuscule. Similarly, I regularly see 100V caps labelled for "loudspeaker use", but it seems like these should be usable at a number of places inside the circuit, like on a preamp's output capacitors.

Is there something going on I don't know about, or is it totally fine to use 100V capacitors in this location?

Obviously I'm circuit dumb, so I mostly just feel like I'm doing adult legos or raw part swapping when I'm working on this stuff.

Thanks for the education.
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Old 1st August 2019, 09:16 PM   #2
Dagwood is offline Dagwood  New Zealand
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I've often wondered the same so will look forward to the replies.

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Old 1st August 2019, 09:28 PM   #3
MarsBravo is online now MarsBravo  Netherlands
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A capacitor should have at least a voltage rating that is higher than the actual dc voltage across it. Even better: the maximum power supply voltage, so it can never break in the event of electronic mishap.
One might prefer exotic capacitors for their sound (or lack of it!), having by accident or purpose surprising or even crazy values.
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Old 1st August 2019, 09:45 PM   #4
MarcelvdG is offline MarcelvdG  Netherlands
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For a valve preamplifier, it could be that the capacitor indeed needs to block a few hundreds of volts of DC bias voltage. For reasons of reliability, it is then good to choose a capacitor working voltage that's comfortably above the actual voltage. Even in this case, 600 V is quite a lot.

For a solid-state preamplifier, the required blocking voltage will usually be below 20 V, or maybe up to 52 V higher for professional equipment when you take into account the risk that someone might accidentally connect the output to a microphone input with phantom supply. 100 V is then more than enough.

When you want to use polypropylene output capacitors because of their extremely low distortion and wide acceptance in the audiophile community, you will find that although types rated for only 160 V or 250 V exist, most have a voltage rating of at least 400 V. That's simply because the thinnest polypropylene foils that can be easily manufactured are already thick enough to handle 400 V.
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Old 2nd August 2019, 03:24 AM   #5
PRR is offline PRR  United States
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Quote:
Originally Posted by cal3713 View Post
I often see large voltage ratings on the capacitors used at the outputs in preamps, for example 600V or 650V... however the actual voltage coming out of the caps is minuscule.....
Why does my kitchen oven have 500 degree insulation, when the outside of the oven is barely warm?

Obviously there is 400-500 degrees inside the oven, which you do NOT want to come out. Likewise the inside of a vacuum-tube amplifier has 250V operating and 450V turn-on surge, which you do NOT want to get out.

Using "speaker caps" for amplifiers raises other issues.
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Old 2nd August 2019, 03:37 AM   #6
cal3713 is offline cal3713
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Thanks for the input folks. I was really hoping that you all would talk me into putting my 100v Jupiter Copper foils on the outputs of a 6sn7 preamp (the VTA sp14). Glad I came to ask before giving it a shot...
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Old 2nd August 2019, 03:39 AM   #7
dotneck335 is offline dotneck335  United States
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Quote:
Originally Posted by MarcelvdG View Post
For a valve preamplifier, it could be that the capacitor indeed needs to block a few hundreds of volts of DC bias voltage. For reasons of reliability, it is then good to choose a capacitor working voltage that's comfortably above the actual voltage. Even in this case, 600 V is quite a lot.
When you want to use polypropylene output capacitors because of their extremely low distortion and wide acceptance in the audiophile community, you will find that although types rated for only 160 V or 250 V exist, most have a voltage rating of at least 400 V. That's simply because the thinnest polypropylene foils that can be easily manufactured are already thick enough to handle 400 V.
That is right on. Also keep in mind that higher voltage electrolytics will generally have a lower ESR than lower voltage ones.
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