[ozshadow]

Why exactly are they never used other than in the power supplies and as DC offsetting?

 

[PinkFloyd]

Quote:

Originally Posted by ozshadow Why exactly are they never used other than in the power supplies and as DC offsetting?

Almost every commercial headphone amp has input & output caps what do you mean?

 

[ozshadow]

I never see them on the portable designs here.

 

[00940]

They're not necessary with split supplies. Why would we use them ?

 

[ATAT]

Heh, i Guess it comes down to why use em..

with a single-ended design (like hte szerkers) the output cap is a necessity because dc offset = bias > 1/2 signal strength.

While with a split supply / push pull type topology you have nearly zero offset (if you're careful) so there's no real "need" for an output cap.

The reason you dont see em on portable designs is because electrolyts take up space. Honestly, why add em if you don't need em?

 

[gsteinb88]

Quote:

Originally Posted by ATAT Heh, i Guess it comes down to why use em.. with a single-ended design (like hte szerkers) the output cap is a necessity because dc offset = bias > 1/2 signal strength. While with a split supply / push pull type topology you have nearly zero offset (if you're careful) so there's no real "need" for an output cap. The reason you dont see em on portable designs is because electrolyts take up space. Honestly, why add em if you don't need em?

All true, and even with the highest end cap, its going to end up affecting the signal. Caps in signal path=bad.
-g

 

[Garbz]

Jump on the headwize library and have a look at all the amps. Many of them do still have output caps. These amps are the ones which don't have split supplies and float the output at some DC level, normally PSU voltage /2.

Split designs don't need them true, and an output cap would be safer yes. The reason we'd rather use input caps instead of output caps to kill offset despite every amp having a small offset anyway is that input caps can be a far lower value, and thus a different grade. If you used an output film cap like 0.1uf (which I get away with just fine in my phono stage) in a headphone amp, then there'd be no bass curtosy of the interaction between the headphones and the capacitor.

As an asside capacitors can fail and when an output electrolytic fails the headphones get the full force of the DC that's applied

 

[Rescue Toaster]

Quote:

Originally Posted by ozshadow Why exactly are they never used other than in the power supplies and as DC offsetting?

The capacitors are merely hiding in the power supply. They're still there all right.

 

[DaKi][er]

Quote:

Originally Posted by ozshadow Why exactly are they never used other than in the power supplies and as DC offsetting?

ok. I pose the question - Where do you think they should be used, and why?

 

[theexec]

errr.. i dunno -_-a they should only be used in power supplies to regulate power?

and should not be used in signal path

that seems to be what i learnt here

oh.. i forgot cross overs in speakers..

 

[ATAT]

Some uses of caps i can think of..

Power supply ripple reduction..

Storage of charge for high current draw..

Kill DC offset (input / output caps)

Decrease frequency response (bandwith stabilization caps..)
(also caps on power pins of opamps..)

Bass boost

Also, I have absolutely no qualms putting caps in the signal path.. if its necessary to stabilize the feedback loop..

 

[00940]

On the other hand, if you really want to use output caps, those are good for their price :

But they won't fit a mint tin

 

[Garbz]

Caps have a simple function. They store charge, and their voltage can't change instantly.

This gives rise to all sorts of uses, some mentioned above. Their ability to store charge means that when fed a ripple voltage, the output voltage will have a smaller ripple. Great in PSUs. It also means they can surpress / cause ringing. Produce ocillations. Increase voltage using a "flying capacitor" or "charge pump" design. Near opamps they store enough of a charge to supply the amp with very sharp transients which would otherwise interact with the circuit and produce nasty effects like voltage drops on the power supply.

Because the voltage is a function of change in currents, at high frequencies they present a low impedance, and at low frequencies a high impedance. At DC they present an open circuit which is what they are used for as DC blocking caps allowing AC to flow, and also allows us to use them in filters to block bass, pass treble etc.

The fact the relationship between voltage and current is an integral also gives rise to a property that phase offsets current and voltage by -90degrees, which can be used to nullify the +90deg phase offset in an inductor and increase the powerfactor for tranmission systems.

That's all I could come up with in my drugged state

 

[guzzler]

If you're maths is ok, you could try reading through this:

http://www.mellowparenting.demon.co....er/compimp.htm

There's a mistake in the the last line of the Vout equation derivation. Replace the 2(pi)f by the square of that and it's fine

 

[PinkFloyd]

Quote:

Originally Posted by DaKi][er ok. I pose the question - Where do you think they should be used, and why?

They should "definitely" be used in commercial amps as they offer a degree of safety against DC destroying your headphones (belt and braces) and the manufacturer doesn't want his amps frying your precious 'phones so he fits DC nulling output caps between the output and head socket. He also fits DC blocking input caps just incase there is the odd customer out there who has tons of DC present on his source output. Usually fitted as a belt and braces "safety" device and generally cheaper than fitting a DC servo or expensive fet opamps.