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Capacitor with large over voltage
Capacitor with large over voltage
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Old 25th May 2019, 08:48 AM   #1
mchambin is online now mchambin  France
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Default Capacitor with large over voltage

Is there be a good reason to use a voltage 10 times more than needed ?
I saw two huge 33F 450V caps in mic preamps for phantom power isolation.
I wonder why such large, pricy caps where 48V only is needed.
I am puzzled because this is from a designer who does know about high quality mic preamps.
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Old 25th May 2019, 08:50 AM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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"I am puzzled because this is from a designer who does know about high quality mic preamps".
I have my doubts!
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Old 25th May 2019, 09:29 AM   #3
mchambin is online now mchambin  France
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Quote:
Originally Posted by JonSnell Electronic View Post
"I am puzzled because this is from a designer who does know about high quality mic preamps".
I have my doubts!
May be. There was two design flaws and the peak led could not work at all.
However the PCB was very good, the preamp a best at low noise.
Well, I'll use 63V phantom caps and forget about this.
I wonder too, about a common mode choke at the inputs. I have doubts about its actual effectiveness and only find those made for mains power.
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Old 25th May 2019, 10:45 AM   #4
JMFahey is offline JMFahey  Argentina
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Quote:
There was two design flaws and the peak led could not work at all
Now youve piqued my curiosity, what would be those flaws?
Is it a ready made product or a kit you assemble?
Or just a schematic and layout and you are on your own?

Is it a commercial Pro audio product used at recording Studios or an Audiophile product?

Inquiring minds want to know.
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Old 25th May 2019, 12:14 PM   #5
mchambin is online now mchambin  France
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This was about four mic preamps boards and steppers from a small company in Australia specialized in pro audio gears.
The design is a clone of "Cohen double balanced mic preamp", same as MonteGeneroso from Samuel Groner. A well proven very good design that is easy to find and well documented on the web.
The peak lead part was not populated, I understood why, later, it was a wacky design that could not ever work.
First flaw: The trimming to adjust balancing was way too sensitive. I had to change resistor values.
Second flaw: In the second stage that simply does A - B and B - A, from the first stage, with two op-amps and height equal resitors, two of those were of wrong value at some wacky trimmers to achieve perfect balancing. A trimmer joke that could not ever do its purpose. However, no big deal, it could work well without such a super duper balancing. I left as is.
I like so much, this design, that I will remake it, with:
_No fancy AC and DC balancing at the second stage.
_Servo to replace the trimmer that balances the first stage and to accept no so well matched transistor pairs.
_Vu meter driver.
_Correct peak lead.
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Old 25th May 2019, 12:22 PM   #6
mchambin is online now mchambin  France
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The wacky peak led.
The intention was: Red blinking when too hot. Steady green when mic open.
This with a tricky use of a JFET. Could not work because of loose charateristics of JFET. Well another of these designs that has worked once, on the corner of a kitchen table..
The four instances of this contraption were acting in four different ways....
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Old 27th May 2019, 01:08 PM   #7
kodabmx is online now kodabmx  Canada
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Capacitor with large over voltage
I've used higher voltage parts than needed because they were surplus and affordable...
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Old 27th May 2019, 01:24 PM   #8
starkeyg is offline starkeyg
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Hello,

Well, according to testing done by Cecil Bateman (believe linear audio web site has his articles posted), the higher the voltage rating of a electrolytic, the less distortion induced.

By making a blocking cap much bigger than needed, with a higher voltage rating (and ideally bi-polar) you can knock the distortion down to to an amount that is quite had to even measure.
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Old 27th May 2019, 08:06 PM   #9
Mark Tillotson is offline Mark Tillotson
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While that is true, it matters when the ac signal voltage across the capacitor is large, for a phantom power circuit the capacitor ac voltage is tiny (much less than the microphone output signal), only the dc component is large, so I think its just cargo-cult circuit design.


Where signal voltages are large across a capacitor its a known issue (line level filter circuits for one).
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Old 29th May 2019, 07:10 PM   #10
mchambin is online now mchambin  France
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Quote:
Originally Posted by Mark Tillotson View Post
While that is true, it matters when the ac signal voltage across the capacitor is large, for a phantom power circuit the capacitor ac voltage is tiny (much less than the microphone output signal), only the dc component is large, so I think its just cargo-cult circuit design.


Where signal voltages are large across a capacitor its a known issue (line level filter circuits for one).
I do not think the "tiny ac voltage" is a valid point, because a tiny distortion can result in a substantail distortion ratio.
Nevertheless, I plan to use polypropylen film caps, despite high cost and large size when over 10F.
May be, electrolythic caps are acceptable for phantom isolation based on actual mic distortion and ac voltage across the cap. I am interested to know more about this.
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