DC blocking caps ?


2010-09-09 3:11 am
Does it matter which direction they're hooked up0?

I know the schematics show the polarization in certain directions, but for small DC does it matter? Any difference between input vs. output caps?

Testing with my multimeter seems to show that it doesn't matter.
Hi Robert, even for small voltages like 5 Volts, reversing the polarity of a capacitor in a circuit can lead to self destruction. Some polar capacitor may have a small endurance to reverse polarity but I would not count on it for long term stability.

If you want to use the capacitor to block DC from the audio signal, a polarised cap should not be used for sake of audio quality, but there is no destruction if you use it for a .5 V audio signal.

Please describe more the DC voltages, the shematic and the type of capacitor you would like to use !


2010-09-09 3:11 am
These are on audio lines, so the DC is quite low, but almost all of the schematics I've reviewed show polarized DC blocking caps on the audio lines. Some of the DC blocking ones are 10u and I am not even sure I've seen non-polarized in these ??? Many I've seen are .1 or .01 and these I've found in non-polarized ceramic disc.

Why do you not recommend polarized caps for audio quality?
Film capacitors are usually recommended for performance, there is plenty of literature on this all over internet / in books, they are never polarised. Electrolytic capacitors are polarised unless specified otherwise. Paper dielectric is supposed to sound best, followed by metalized paper, metalized polypropylene, propylene, and electrolytic. For some people, or some audio systems, it makes no difference. So take this with a grain of salt.
Consider connecting two in series with the positive ends conected together. obviously each of the 2 has to be 2X the size of a single cap for the correct lower cutoff frequency. putting a film cap in parallel will help as well.

I learned (from Jim Williams at Linear Tech in one of his ap-notes) that when you are in the noise measurement biz you want the capacitor with extremely low leakage. A good wet-slug tantalum will set you back about as much as a vintage Bordeaux.

In any event, required reading is the article on capacitor selection on Walt Jung's website.
People can sometimes get too excited about eliminating electrolytic capacitors from audio circuits. Provided they don't carry much audio current, have DC bias and do not form the dominant LF pole, then in most places they don't do too much harm. A good circuit with electrolytics in appropriate places will usually sound better than a poor circuit with more exotic components. Deliberately avoiding electrolytics may force you to use complicated circuits which can bring their own problems.


2010-09-09 3:11 am
Thanks for all the great info ! This is a car audio device, so in most cases external noise is more a factor than anything else. I am just trying to understand the basic circuit design, and components.

The puzzling thing was the circuit showed polarized dc blocking caps on the audio inputs and I installed them backwards and it didn't seem to affect anything.
Elna Silmics (as mentioned above) are reasonably priced and sound good too.

I recently refurbished a B&K ST-140 and used Silmics with good results. There is a 47uF capacitor on the input and the original was a little no-name Taiwan electrolytic. It could have been my imagination, but replacing it with a Silmic resulted in a cleaner, less muffled sound.

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...for DC blocking? It seems people use ceramic...

Ceramic caps in audio are usually recommended for supply bypassing only (i.e. between supply rails and ground). They are not normally recommended in the signal path.

What are the best "small" cap type to use for DC blocking? ...the circuit calls for 10u (and polarized).

DC blocking caps in the signal path do not need to be polarised. The schematic will show a polarised cap because for 10uF a polarised electrolytic is cheapest and smallest.

Film caps are available in 10uF, and will give better sound and be more reliable in the long term, if you can fit them on your board. AFAIK, polyester/mylar are physically the smallest.
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the input of an amp often has a low voltage offset.
A polarised input cap should be aligned to match that input offset.

Electrolytics are often specified as suitable for +-1Vdc without damage or degradation.
Audio signals can easily exceed that 1Vpk threshold. If one then adds on the input offset you can have upto 3Vpk across an input cap, (and 2Vpk in the other direction). This demands it is aligned to put the least stress on the electrolytic.
Normal audio signals do not add to the peak voltage across a capacitor, unless the capacitor is blocking significant amounts of lower frequencies. Remember, any signal voltage developed across a coupling capacitor is not seen by whatever comes next. If you had a 6V signal sitting on a 1V DC level then you could pass it with a 3V limit coupling capacitor (provided there is no sub-LF present). However, in practice you need to take account of start-up level shifts which may be much larger (and slower) than audio signals.


2003-02-17 7:38 pm
how do these threads get so many posts without earlier mention of Bateman's "Capacitor Sound" article's recommendation of biPolar Al Electors with full thickness oxide grown on both foils being the best measuring if you have to use Electrolytics - higher V rating also improved distortion