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'[EE] DC Block Capacitor'
2008\04\19@013644 by

I have a circuit that calls for a 1uF capacitor to block any potential
DC on an audio line. I don't happen to have any of those handy at the
moment. While I'm waiting for my order to come in, what would happen
if I used something I do have, such as .1uF capacitors? I have a ton
of them.

I guess I'm looking for a rule of thumb in how DC block caps are even
chosen. It would seem that the larger the capacitance the longer it
takes to reach equilibrium and block DC. Am  I wrong?

Josh
--
A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.

On Sat, 2008-04-19 at 01:36 -0400, Josh Koffman wrote:
> I have a circuit that calls for a 1uF capacitor to block any potential
> DC on an audio line. I don't happen to have any of those handy at the
> moment. While I'm waiting for my order to come in, what would happen
> if I used something I do have, such as .1uF capacitors? I have a ton
> of them.
>
> I guess I'm looking for a rule of thumb in how DC block caps are even
> chosen. It would seem that the larger the capacitance the longer it
> takes to reach equilibrium and block DC. Am  I wrong?

To AC an ideal cap is actually a frequency dependent resistor, with a
value that decreases as the frequency increases.

So, the "DC blocking cap" actually forms a high pass filter. The -3db
cutoff is related to the value of the cap, and the value of the the load
impedance.

Generally the value of those caps is pretty arbitrary, as long as the
cap you choose results in a -3db point below the frequencies you want
you're good to go.

For example, at 100Hz:
www.cvs1.uklinux.net/cgi-bin/calculators/cap_imp.cgi
1uF ~= 1.59 kohm
0.1uF ~= 15.9 kohm

These numbers aren't that small, but if your load impedance is >> then
these values you're good.

Personally, I'd just sim the difference to make sure you won't degrade
performance.

TTYL
On Sat, Apr 19, 2008 at 12:36 AM, Josh Koffman <joshybeargmail.com> wrote:
> I have a circuit that calls for a 1uF capacitor to block any potential
>  DC on an audio line. I don't happen to have any of those handy at the
>  moment. While I'm waiting for my order to come in, what would happen
>  if I used something I do have, such as .1uF capacitors? I have a ton
>  of them.
>
>  I guess I'm looking for a rule of thumb in how DC block caps are even
>  chosen. It would seem that the larger the capacitance the longer it
>  takes to reach equilibrium and block DC. Am  I wrong?
>
>  Josh
>  --

You can use the smaller capacitor.  You will lose some of the bass,
but the circuit will still work fine.

Regards,
Mark
markrages@gmail
--
Mark Rages, Engineer
Midwest Telecine LLC
markragesmidwesttelecine.com
Thanks for the replies guys...I think I get it now!

Josh
--
A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
Josh,

Or, if you've got space, you could connect 10 x 0.1uF caps in parallel.

But Basically it just comes down to what the load impedance is as to
whether  you need 1uF or if 0.1uF is adequate.

RP

On 19/04/2008, Josh Koffman <joshybeargmail.com> wrote:
> Thanks for the replies guys...I think I get it now!
>
>
>  Josh
>  --
>  A common mistake that people make when trying to design something
>  completely foolproof is to underestimate the ingenuity of complete
>  fools.
>  --
Herbert Graf wrote:

> 1uF ~= 1.59 kohm
> 0.1uF ~= 15.9 kohm
>
> These numbers aren't that small, but if your load impedance is >> then
> these values you're good.

I once used 0.1 in my youth projects, had been instructed so :)
When HiFi became more important, I noticed 1uF and above being used, and
I guessed it was due to more "respect" for strong bass frequencies.

--
Ciao, Dario
> Or, if you've got space, you could connect 10 x 0.1uF caps
> in parallel.
>
> But Basically it just comes down to what the load
> impedance is as to
> whether  you need 1uF or if 0.1uF is adequate.

OR, if this is a "just for now" fix, a larger value will
usually do no harm. 10 uF, 33 uF, 100 uF.

These will (almost certainly) be electrolytic whereas the
original may have been a bipolar (polarity unimportant) cap.
Alumin(i)um electrolytics have generally inferior
performance* for audio decoupling compared to Mylar or
whatever bipolar 'plastic' capacitors. In many cases you
won't notice.

As well as having capacitance a capacitor will have some
equivalent series resistance and an inductive component.
Also some "leakage". Aluminium electrolytics tend to allow
you to choose all 3 to be worse compared to many
alternatives :-) are usually cheaper per capacitance.

Russell McMahon

Josh,

The capacitance is calculated based on the lowest frequency you want to
pass.
Since capacitive reactance is inversely proportional to frequency, the lower
the
frequency you want to pass, the higher the capacitance.

As to whether a .1 uF cap would work, the answer is yes, kind of.  It will
work,
but you won't get the full range of frequencies that you would with the 1 uF
cap.
This is because the capacitive reactance is too high at the lower
frequencies.

Try it and you'll see what I mean.

Regards,   Jim

{Original Message removed}
Apptech wrote:
{Quote hidden}

Good coverage.

The other disadvantage of aluminum electrolytics is that, with time, the
electrolyte
evaporates and the cap value declines until it no longer can perform its
job.Stopping
this by encapsulating (in a plastic coating) only partially works (but
WILL lengthen
the life of the cap this way).

So if you are designing a product that needs to operate for a long time,
use solid electrolytics.
I prefer tantalum, but some folks like other stuff. But there are minor
gotchas with tantalums,
so go figure.

--Bob Axtell
> The other disadvantage of aluminum electrolytics is that, with time,
> the electrolyte evaporates and the cap value declines until it no
> longer can perform its job

I recently overhauled a nice mid-80s stereo amp which had some
intermittent problems caused by bad electros, mostly in the pre-amp.
Many were well dodgy and had to be replaced

Long ago, I worked for Muzak. LOADS of amplifiers in various places.
I found out two big things:

1: Amplifiers and electronics in general, like to be left on.

2: Electrolytic caps are a major source if not THE major source of
failures for equipment that uses them.  (assuming it was well designed
in the first place.)
On Sat, 19 Apr 2008 19:23:21 -0400, David VanHorn wrote:
> Long ago, I worked for Muzak. LOADS of amplifiers in various places. I
> found out two big things:
>
> 1: Amplifiers and electronics in general, like to be left on.
>
> 2: Electrolytic caps are a major source if not THE major source of
> failures for equipment that uses them.  (assuming it was well designed in
> the first place.)

Absolutely agrees with my experience over the years as well.

3: The "cleaner" the power source, the longer a given device will last (in
general).

I have a friend who is a PCB designer and not very PC savvy. I tend to
build computer systems for him at the same time I build for myself. We end
up with identical systems and it's easier for me to support common hardware
and software components for both of us.

We both have identical APC SmartUPS UPS supplies for our systems. His
systems have hardware die about twice as often as mine. What's the
difference? His AC Power has many more "disturbances" than mine -- we put
an AC line monitor on his incoming power service and recorded as many as
100+ disturbances (severe spikes, dips, etc.) a day. Mine sees a few a day

We now buy 2x of all the system critical components when we build for him
so we have replacement parts for repair a few years down the road. This has
paid dividends several times now. He also has problems with other consumer
electronics dying that I don't have.

Matt Pobursky
Maximum Performance Systems

A few notes.

the formula for the -3db point of the RC high-pass filter you're
creating is:  F= 1 / 2*pi*R*C, with C in farads.  R is resistance to
ground, so this would be the source impedance in parallel with the
resistance "seen" by the cap, normally a resistor of some value to
ground to set the overall input impedance of the circuit.  Check your
horowitz and hill...

If the circuit is a mic preamp for condenser mics, you'll need at least
a 50V rated cap to block the 48V phantom power.  63V rated preferred.
If the phantom power is on/off switchable, then use a bipolar cap.
Mouser carries nichicon bipolars, such as 10uF 50V: 647-UWP1H100MCL  (I
use it in a design).  There are others there and at digikey.

According to people whose ears / hifi audio opinions I trust, tantalums
sound terrible, ceramics can cause noise due to a minor piezoelectric
effect they have, and bipolars sound better than polarized
electrolytics.  Of course, as mentioned in this thread, any sort of
plastic-based cap is preferred over all of these.  However, cost and
physical size, particularly if they have to be 50V rated, makes them
prohibitive in many situations.

J

Josh Koffman wrote:
{Quote hidden}

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