DC Blocking Capacitors

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dome406

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Joined 2004
#1
I built a Audisector gaiclone a while ago and am very pleased, however I have always been abit worried about plugging things in for the first time and about whether they output DC.

I know you can use capacitors to block DC but I am not sure which type to use, can you use any type availble? because I have some 1500uf and some 10uf electrolytic panasonic ones left (polarised) which I want to use or, would I have to buy a special type? e.g. non polarised or not electrolytic and if you can use polarised which way do you put them.

Also I know they may effect the sound.

Thanks Very Much,
 
#2
Hi Dome,

You could probably connect up two of the 10uF electros back to back (ie in series with either +ve to +ve or -ve to -ve). that will give you around 5uf. This is a bit bigger than I have seen generally used but shouldn't be a problem.

Ideally you should use something like a 2.2uF polypropylene cap. This is non-polarised, but has the disadvantage of being physically bigger and much more expensive 🙂

Tony.
 
#4
apart from price??? 😉

I've only used generic brand polypropylene crossover caps personally, so I can't comment on whether I think there is a difference, between audio grade and standard polyprops. I will say that I thought that they were an improvement over the bipolar electrolitics that they replaced though 🙂

Tony.
 
#5
dome406 said:
Thanks for the fast reply, I will probably get some polpropylene. components keep getting harder to get and more expensive in the uk 🙁 which is a shame.

Is there a difference between audio grade and normal?
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As you probably know, in audio, when you get to a certain quality stage you also enter the law of diminishing returns for any new improvement.

There was an article in Audio Electronics some time ago where they tested several polypropylene caps, from cheap to very expensive, and differences (if any) seemed to be very, very small.

One thing that is important, though, is what type of wire are the terminals made of. If they are non-magnetic then I don't think there will be much of a difference between the caps.


Carlos
 
#6
carlmart said:
There was an article in Audio Electronics some time ago where they tested several polypropylene caps, from cheap to very expensive, and differences (if any) seemed to be very, very small.

One thing that is important, though, is what type of wire are the terminals made of. If they are non-magnetic then I don't think there will be much of a difference between the caps.
Click to expand...

Depending on your system, the difference can be also very big.

If you have Panasonic FC 10u left, use those for coupling. Don't use them back to back, just a single cap. If you'll pleased with the sound, leave it like that.

Otherwise, I would suggest BG N 4.7u, which IMO are better for coupling than most pricey exotic brands. They are also cheap, at $3.25/pc.
 
#7
Why not use polyester (mylar) caps? They're also non-polar as the polypropylene, cheaper and quality wise, they (theoretically) should be between polypropylene and aluminium electrolytics.

In case you really decide for polypropylene, the cheapest I've seen are here: www.reichelt.de, search for "vis folie"
they seem to be come from Visaton (though I don't know if they just buy them from someone else)

Cheers,
Dominique
 
#8
polyester is a good low cost option, I'd say definitely better than cheap electros, though Peter feels that the BP black gates are better than even the polys, and at the price I'd say they are a bargain... (if unlike me you can source them locally).

I just realised that my input caps I repaced with polyproylene weren't electros, they were polyester green caps, and I did notice an improvement. don't think it was psycoacoustic but you never know 😉

Tony.
 
#10
Sory to revive an old thread, but my question falls right in line with this topic.

I also built an Audiosector kit, but its a LM4780 in stereo configuration. All parts are stock and per the kit.

Recent modifications to my PS1 lead me to trust it a little less, until it proves it self. Also I like switching between iPODs and have noticed some sound quality differences between models.

I have read on here that many suggest putting a small cap across NFB resistor. I believe on the AS LM4780 that is R5 and R6. What values should I start with? 1uF?

Also many suggest placing capacitors across the input resistors (R0 and R4), would 1uF also be a good starting point? I also see 2.2uF thrown around.
 
#16
The resistors involved are R2 and R5 for 1 channel (and R1/R3 for the other so i won't repeat for those).

Capacitor -
Parallel to R2 = reduce gain of high frequencies
In series with R2 = increases gain of low frequencies*
*Not used for audio amplifiers because DC gain = open-loop gain
Parallel to R5 = increase gain of high frequencies
In series with R5 = reduces gain of low frequencies - DC gain = 1

At any rate, putting any non-linear components in series or parallel in this place alters frequency response. Sound difference, yes. Accurate reproduction, hmm...
 
#17
Trooper46 said:
in this thread NFB caps are discussed.
http://www.diyaudio.com/forums/chip-amps/182706-my-new-3886-amp-seems-light-highs-3.html

in post 27 the author states that after installing a NFB capacitor his DC offset was reduced. Maybe I am misinterpreting some of the content of that thread.
Click to expand...

Yes, but that cap is not across NFB resistor but in series with gain setting resistor (R3/R5=680R). The value should be also much higher than 1uF (22uF at least)

This cap will prevent amplifier DC gain, which means whatever DC offset from a source appears at amp's input, it won't be amplified.

If you install input coupling cap, as I suggested earlier, the feedback cap is not really needed, unless chip by itself produces excessive offset.
 
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