# Coupling capacitor voltage rating?

Discussion in 'Electronic Design' started by Terry Pinnell, Feb 14, 2004.

1. ### Terry PinnellGuest

When you use a coupling capacitor to isolate the input and output of a
simple audio amp from DC, how do you decide what voltage rating is

Checking a couple of old projects, I found virtually no DC present
*across* the output capacitor, regardless of amplitude, frequency or
load. So could the rating be arbitrarily low, allowing me to use some
cheap surplus electrolytics I still have around?

2. ### John WoodgateGuest

I read in sci.electronics.design that Terry Pinnell <[email protected]
For best long-term reliability, if there is no DC present, you should
use a reversible electrolytic, or two ordinary ones back-to-back, maybe
with some DC bias applied to their junction. But in many cases, just a
single ordinary electrolytic is quite OK. I don't like to use anything
less than 6 V caps, though, in such applications.

If your caps are elderly, re-form them by connecting to a DC supply
equal to the marked working voltage and monitor the leakage current.

3. ### Frank BemelmanGuest

I would think that with a simple amplifier with a single supply,
a capacitor is needed, and that you would measure 1/2 the supply
voltage across the cap.

With a dual supply, no DC is measured, but you would not need
a cap then, a protection relay perhaps...

4. ### Fred BloggsGuest

Well if there is "virtually no" DC present then the simplest course is
to not use a capacitor at all.

5. ### Jim ThompsonGuest

What is the definition of "virtually no DC"? 1mV, 10mV, 100mV ?

...Jim Thompson

6. ### JamieGuest

if your using 2 Elctrolytics with back to back connections then the
voltage rating is the lowest value of the 2.

Thanks John.

8. ### Terry PinnellGuest

Not sure I follow. I'm using a capacitor (as usual with coupling
capacitors I thought) to *ensure* there's no DC...

See the example in my reply to Frank.

9. ### Terry PinnellGuest

Not sure I follow that, Frank. Maybe a specific example would help, so
here's one of the circuits I had in mind:
http://www.terrypin.dial.pipex.com/Images/ACFollowerOutputCap.gif

I applied 1V rms as input. Regardless of supply voltage, DC across C3
is a mere 270 mV.

10. ### Frank BemelmanGuest

Your op amp +input dc level is 0V, thanks to R4 and R3. Why is the
output at 270mV ? I expect that to be closer to 0V.

I would not expect an output capacitor in this situation, where
you have dual supplies. The same applies to power amplifiers, or
bridge amplifiers, like the ones in car-radios. No output cap.

With an amplifier that uses a single supply, the DC level from the
output transistors is 'placed' at about Vsupply/2. In that case
you need a cap between output stage and speaker.

11. ### Active8Guest

Well if there is "virtually no" DC present then the simplest course is
to not use a capacitor at all.[/QUOTE]

What is the definition of "virtually no DC"? 1mV, 10mV, 100mV ?

...Jim Thompson
[/QUOTE]
Yeah, and how would that affect the next stage is the real
question. The next stage might run ok, but it might put a DC level
on the speaker at the quiescent point.

12. ### John WoodgateGuest

I read in sci.electronics.design that Frank Bemelman
2004:
No only that, but eliminating R3, C4 and C5, and making R4 10 kohms to
earth would give more output swing. A 741 driving 1 kohm is not a very
happy bunny.

13. ### Frank BemelmanGuest

I was a bit puzzled about that C4 & C5 positive feedback. What
is it supposed to do? And isn't the 741 'verboten' these days?

;-)

14. ### John WoodgateGuest

I read in sci.electronics.design that Frank Bemelman
2004:
It bootstraps the 100 k resistor, making it look like maybe 10 Mohm at
low frequencies. I'd rather not go into what it does at 20 kHz!
Only by Bill Sloman, and he doesn't count. (;-)

15. ### N. ThorntonGuest

Hi

Firstly caps are rated by voltage, not dc component of voltage, so
what the dc component is isnt what matters. If you have a dual supply
amp on +/- 12v you need a 12v output cap, since the cap will see
nearly 12v at times.

Secondly output caps are used on dual supply amps when either
a) the V offset can not be guaranteed to stay satisfactorily low at
all times.
or b) as protection to the speaker in case of amp failure, which could
apply full supply V to the speaker.

Regards, NT

16. ### Philip A. MarshallGuest

for audio, NEVER have electrolytics in the signal path if you can
avoid it. about the only thing worse to use is tantalum.

17. ### BanGuest

That circuit is crap. The guy doesn't know anything about opamps. Here is a
more appropriate one:

+------+------------o
| | 9-30V
.-. |
220k| | | 100n
| | | ||
'-' +--||--GND
| | ||
|# | |\|
o-+---|#---+----|+\ #|
| |# | | >--+----#|---+--o
| 10u | +-|-/ | #| |
| | | |/| | 100u |
| | | | | |
.-. .-. +---)---+ .-.
| | 220k| | | | |
1M| | | | | 1/2 TL072 | |1M
'-' '-' | '-'
| | | |
=== === === ===
GND GND GND GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

ciao Ban

18. ### John WoodgateGuest

I read in sci.electronics.design that Philip A. Marshall
@4ax.com>) about 'Coupling capacitor voltage rating?', on Sun, 15 Feb
2004:
Indeed. All professional audio recording and broadcasting equipment uses
ONLY Leyden jars in the signal path.

19. ### Guest

Most caps rated at dc.
Many have only 1/4 (or less) rating at 1kHz.