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From: miso on 20 Feb 2010 23:34
On Feb 20, 1:07 pm, Nemo <z...(a)nospam.nospam.nospam.nospam.co.uk>
wrote:
> I am trying to evaluate whether to use tantalum versus OS-CON aluminium
> electrolytics as the output decouplers for some linear postregulators
> after a (relatively low noise) switcher. This is for a high gain
> amplifier that will be operating down to audio frequencies. I've come
> across references to aluminium electrolytics being noisy, but no firm
> details (kind of odd as there are lots of details about ceramics'
> problems under bias, wet vs dry tantalums etc). There's also some
> references to "low noise" aluminium electrolytics for audio work, but I
> don't know whether to take them seriously as audiophools believe all
> kinds of weird stuff. So, can anyone advise if aluminium electrolytics -
> specifically low ESR solid electrolyte types - have some kind of noise
> problem? I know tants are NOT microphonic but I've not come across any
> info about electrolytics and microphony one way or another.
>
> Assuming they do not, I favour them over tants because their ESR is
> lower, I get the impression that tants' ESR is poorer at low
> frequencies, and I can get electrolytics at higher voltages (I like to
> run caps at about double their rated voltage under the impression this
> improves their reliability). The load on these linear regs will be
> fairly constant, so ripple current will be low.
>
> Thank you,
> --
> Nemo

But the cap isn't in a gain path, so I don't see this being an issue.
I think you would need a combination of gain and high impedance to get
a cap to sing.

I'm partial to oscons.
From: Robert Baer on 21 Feb 2010 03:35
Nemo wrote:
> I am trying to evaluate whether to use tantalum versus OS-CON aluminium
> electrolytics as the output decouplers for some linear postregulators
> after a (relatively low noise) switcher. This is for a high gain
> amplifier that will be operating down to audio frequencies. I've come
> across references to aluminium electrolytics being noisy, but no firm
> details (kind of odd as there are lots of details about ceramics'
> problems under bias, wet vs dry tantalums etc). There's also some
> references to "low noise" aluminium electrolytics for audio work, but I
> don't know whether to take them seriously as audiophools believe all
> kinds of weird stuff. So, can anyone advise if aluminium electrolytics -
> specifically low ESR solid electrolyte types - have some kind of noise
> problem? I know tants are NOT microphonic but I've not come across any
> info about electrolytics and microphony one way or another.
>
> Assuming they do not, I favour them over tants because their ESR is
> lower, I get the impression that tants' ESR is poorer at low
> frequencies, and I can get electrolytics at higher voltages (I like to
> run caps at about double their rated voltage under the impression this
> improves their reliability). The load on these linear regs will be
> fairly constant, so ripple current will be low.
>
> Thank you,
boy, you run a tant like that and STAND BACK; do not tell anybody at
Homeland "Security"...
From: Nemo on 21 Feb 2010 06:46
I said...

>(I like to run caps at about double their rated voltage under the
>impression this improves their reliability).

Ahh... meant I to write half their rated voltage. Well, it shows you
were paying attention! 8)

Thank you for the advice. OS-CONs it is!
--
Nemo
From: Fred Bartoli on 21 Feb 2010 10:38
John Larkin a �crit :
> On Sat, 20 Feb 2010 21:07:48 +0000, Nemo
> <zzz(a)nospam.nospam.nospam.nospam.co.uk> wrote:
>
>> I am trying to evaluate whether to use tantalum versus OS-CON aluminium
>> electrolytics as the output decouplers for some linear postregulators
>> after a (relatively low noise) switcher. This is for a high gain
>> amplifier that will be operating down to audio frequencies. I've come
>> across references to aluminium electrolytics being noisy, but no firm
>> details (kind of odd as there are lots of details about ceramics'
>> problems under bias, wet vs dry tantalums etc). There's also some
>> references to "low noise" aluminium electrolytics for audio work, but I
>> don't know whether to take them seriously as audiophools believe all
>> kinds of weird stuff. So, can anyone advise if aluminium electrolytics -
>> specifically low ESR solid electrolyte types - have some kind of noise
>> problem? I know tants are NOT microphonic but I've not come across any
>> info about electrolytics and microphony one way or another.
>>
>> Assuming they do not, I favour them over tants because their ESR is
>> lower, I get the impression that tants' ESR is poorer at low
>> frequencies, and I can get electrolytics at higher voltages (I like to
>> run caps at about double their rated voltage under the impression this
>> improves their reliability). The load on these linear regs will be
>> fairly constant, so ripple current will be low.
>>
>> Thank you,
>
> I've never encountered noise from electrolytics. Considering that
> self-discharge time constants are typically days or weeks, I can't
> imagine a serious noise mechanism. Charge one up and see if it makes
> noise.
>

Oh, sure there is: that's leakage current instabilities.
But I've found it significant only once, when doing my 200pV/rtHz
preamplifier where it had to go down to 0.1Hz
I had to resort to a bunch of bulky 2.2uF/250V polypro film caps.

BTW, caps microphony is only significant in "high impedance" circuits.
More precisely, the cut off frequency is given by the circuit loading
resistance x cap value.

--
Thanks,
Fred.
From: John Larkin on 21 Feb 2010 13:16
On Sun, 21 Feb 2010 00:35:41 -0800, Robert Baer
<robertbaer(a)localnet.com> wrote:

>Nemo wrote:
>> I am trying to evaluate whether to use tantalum versus OS-CON aluminium
>> electrolytics as the output decouplers for some linear postregulators
>> after a (relatively low noise) switcher. This is for a high gain
>> amplifier that will be operating down to audio frequencies. I've come
>> across references to aluminium electrolytics being noisy, but no firm
>> details (kind of odd as there are lots of details about ceramics'
>> problems under bias, wet vs dry tantalums etc). There's also some
>> references to "low noise" aluminium electrolytics for audio work, but I
>> don't know whether to take them seriously as audiophools believe all
>> kinds of weird stuff. So, can anyone advise if aluminium electrolytics -
>> specifically low ESR solid electrolyte types - have some kind of noise
>> problem? I know tants are NOT microphonic but I've not come across any
>> info about electrolytics and microphony one way or another.
>>
>> Assuming they do not, I favour them over tants because their ESR is
>> lower, I get the impression that tants' ESR is poorer at low
>> frequencies, and I can get electrolytics at higher voltages (I like to
>> run caps at about double their rated voltage under the impression this
>> improves their reliability). The load on these linear regs will be
>> fairly constant, so ripple current will be low.
>>
>> Thank you,
> boy, you run a tant like that and STAND BACK; do not tell anybody at
>Homeland "Security"...

dV/dT kills tantalums. If you use, say, an LM317 type reg, and bypass
the adj pin to ground (which cuts noise, too) that can limit output
slew to a safe value. If you use a reg with a slow-start option, that
can protect tantalums.

I've never blown up a tantalum that was run at 1/3 rated voltage. If
the gear has to run at, say, -20C or so, that's worth doing.

The polymer caps have such low ESR that some regs will get unstable
with them as loads, especially switchers.

John

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