capacitor coupled vs. direct coupled ouput

[hugobross]

repairing an old amp with just a single transfo, and caps at the output to prevent DC at the speaker connectors, I considered to use such capacitors at the outputs of my amps (between output and centre tap of symmetric psu).

I don't like relais anyway, neither complex DC protection circuits. Those caps are easy to use and reliable and will protect my speakers if necessary.

Do you think there are any disadvantages involved with this use?
I know the caps must be large to pass the lower frequencies. Why isn't this method used anymore (costs, distortion)?

Thanks,

HB.

[halojoy]

There was a time, I remember, when the big thing about an new amplifier was the words "Direct DC coupled".
Now most amps use it. It is no big thing.

But soundwise it is not so bad to use Caps, as one might think, especially since
there has been some development in quality for Big Caps.
Most people would not notice any change in sound by using caps.

I ask:
Why the fear of using a cap in Amp output,
if you use them in Loudspeaker input
filter anyway?

[hugobross]

Thanks,

does it matter what type of capacitor is used?
(of course the voltage and the capacitance must be correct)


HB.

[Tube_Dude]

hei Hugo!

For the capacitors in the output do not introduce significant distortion they need to be correctely biased...in a amplifier with dual rails (+-) this condition os not meet.

You coud use two back to back but their ESR will add

Resuming : the ouput with a capacitor only make sense in a single rail power amplifier...of corse in my humble opinion

Regards

Jorge Santos

[hugobross]

Originally posted by Tube_Dude


Resuming : the ouput with a capacitor only make sense in a single rail power amplifier...of corse in my humble opinion

yes I know, if everything runs smoothly. I was thinking to add them to prevent damage to speakers when something goes wrong inside the amp I'm just too lazy to make some DC protection, and caps are so easy to add

HB

[tiroth]

Well, you figure you need a cap of at least 4700uF/50V for the average amp, but then consider the ripple current requirements and it becomes more difficult.

Of course, large electrolytic in the signal path is considered heresy. ^_^ Seriously, though, there is ample evidence of the electrolytics causing problems as coupling caps. This isn't like a hovland vs wima difference.

I am working on some tweeter amps which will be capacitor coupled. Setting the pole 2 octaves below the rated bandwidth still allows for reasonably sized polypropylene cap.

[Tube_Dude]

hei Hugo

And why not use a simple fuse ,theirs drawbacks (fuse verso capacitor) are very well balanced...

And the fuse is even simpler!

Cheers

Jorge Santos

[tiroth]

A correctly sized fuse will display nuisance-blowing, and even then it will take many tens or hundreds of milliseconds to blow. A really good DC detection circuit generally interrupts the current in ~25ms.

DC protection is hard, which is why many people throw up their hands and say they are not dealing with it. ^_^

[peranders]

Originally posted by Tube_Dude
You coud use two back to back but their ESR will add

In this case in it's totally unneccessary since you HAVE DC all the time across the cap.

Also to remove the capacitor is not an easy task. The bias point at the output stage is not at the signal ground, nor half the voltage. Forget it (for now)!.

[Tube_Dude]

Hei Paranders


I only talk about the back to back electrolytics because in the case of the first post in the trhead the power supply is symetric...Hugobross said I considered to use such capacitors at the outputs of my amps between output and centre tap of symmetric psu).

Of course only in such case the back to back make sense!

regards

Jorge Santos