Capless signal path?

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I disagree with the prior post on sound quality, but agree on risk.

What you should do if you are going capless is to make measurements to ensure that the output resides at the right DC level before the cap ..... If not, you can't go this way without making modifications.

Petter
 
Well after all the attention given to passive crossovers and why they are bad (phase shift) I thought I would leave caps out of my amp altogether. I have built a working amp without caps but because ground references are different on every channel, I need a separate power supply for each channel. Sound quality is the reason I did this in the first place so if there's no benefit to excluding caps, I may as well add them. The thing that I was thinking about is that most devices (computers, cd players, dvd players) have a dc blocking cap on their output. If I had a cap on my amp's input, it would just be a part of the CD player's DC blocking cap (2 caps in series) so my efforts seem to be in vain.
 
It all depends on you. If you hear the difference the coupling caps make, you may try to avoid them. If you hear no difference, it's better to leave them in a circuit. After all the good drivers are expensive.;)

PS: I hear the difference the coupling caps make ( especially the cheap ones) and if possible, I'd rather avoid them.
 
diyAudio Senior Member
Joined 2002
Who Stole The Bass?

Hi,

The thing that I was thinking about is that most devices (computers, cd players, dvd players) have a dc blocking cap on their output. If I had a cap on my amp's input, it would just be a part of the CD player's DC blocking cap (2 caps in series) so my efforts seem to be in vain.

Yes, most PCs, CDPs, DVDs do have DC blocking caps at their output. Thank God for that...

So if you're confident with that there's no need for a cap at the inputstage of the preamp or amplifier unless you have a conventional volumecontrol there and need to shield it from DC on its wiper.

Now, have you considered what happens if two coupling caps are put in series?

Is that amp you talk about DC coupled throughout because you leave out a coupling cap at the input?

Are you sure the PS caps aren't in the signal path?

Knowing where the DC goes is one thing, understanding where the AC goes is the key I'm afraid.

Cheers,;)
 
Bear in mind that while you may be able to control the DC
offset generated by an amplifier so no output cap is needed
(provided you have failure protection circuitry, or take the
risk of failure), you cannot control what goes into the amplifier.
What if something goes wrong earlier in the signal chain so
you get several volts of DC into the amplifier? If you can still
regulate the output offset to a reasonable value, then fine,
otherwise it seem wise to have an input coupling cap, which
anyway is usually not of higher value than you can use a good
film cap.

Then, for those who prefer to risk burning their speakers to
avoid a coupling cap, fine, just be aware of what might happen
if a DC servo earlier in the chain goes weirdo.

Edit: Just saw Franks parallel post. Since output caps must
usually be of much larger value than input caps, making it
hard to avoid lytics in many cases, it might, perhaps, be more
sensible to use an input blocking cap, and remove the output
one in previous equipment, provided the design of that equipment
allows this. Just my :2c: (US or Euro ones? Pick your choose).
 
Peter Daniel said:
I had my drivers burned twice ($500 loss each time), and it was never because the input cap wasn't there.

I believe you, but if you leave it out and the preceeding
piece of equipment is also DC-coupled at the output, you
have two pieces of equipment that can fail so they might
burn the speakers. Is is all about statistics.
 
Christer said:


I believe you, but if you leave it out and the preceeding
piece of equipment is also DC-coupled at the output, you
have two pieces of equipment that can fail so they might
burn the speakers. Is is all about statistics.

It is true. And since my amp is not protected by a cap, I always check the source for DC offset, before I connect it to the amp. Let's face it, if your woofer is $500 to replace, you simply have to do it. And I got 4 of them;)
 
diyAudio Senior Member
Joined 2002
Hi,

Is is all about statistics.

If we have two caps in series the second one isn't biased at all as it doesn't carry any DC.

Assuming the one in front of it can fail than, statistically spoken the one behind it can fail just the same.

And since we have two caps in series the total value is cut in halve and for a give F/C we'd need twice their size to maintain the same F/C as before.
Now, I wonder how that's going to help the sound if you know that bigger caps are inherently slower to release their charge?

Cheers,;)
 
diyAudio Senior Member
Joined 2002
Hi,

Since output caps must usually be of much larger value than input caps, making it hard to avoid lytics in many cases, it might, perhaps, be more sensible to use an input blocking cap, and remove the output one in previous equipment, provided the design of that equipment allows this. Just my (US or Euro ones? Pick your choose).

I'll go for the Euros today, thank you.

But it seems you don't get the point, Christer.

The input and output cap in an amplifier do have totally different functions; leaving the ouput cap of the amp out at the risk of speaker killing DC is silly and I never count on DC servos to save my speakers.
Having two caps in series, one at the output of the source and one at the input of the amp is plain redundance and also has an impact on bass performance of the both of them as they're in series.

The Volvo analogy should have triggered that already, or so I'd hoped.

Cheers,;)
 
There are no caps ANYWHERE on the signal path, not input, not output. As long as you apply 1VDC on its input, you'll get 50VDC on its output. What I was asking is that a cd player with a coupling cap will cause a 90 degree phase shift no matter what. If I add a capacitor to my amp's input, it will effectively join itself to the cd player's cap, so it won't add any more phase shift, correct? My amp doesn't use output caps so there's no need for that. I should be able to use electrolytics on the input because they'll always have a positive voltage across them. Should I opt for a different type of cap? Input impedance is high enough so that I won't need a high value, 10mF or so should be fine.
 
Peter Daniel said:


It is true. And since my amp is not protected by a cap, I always check the source for DC offset, before I connect it to the amp. Let's face it, if your woofer is $500 to replace, you simply have to do it. And I got 4 of them;)

Sure, but the previous equipment may still fail after you have
measured it. You do get an increased probability of failure
the more such DC-coupled equipment to chain up.
 
fdegrove said:
Hi,



If we have two caps in series the second one isn't biased at all as it doesn't carry any DC.

Assuming the one in front of it can fail than, statistically spoken the one behind it can fail just the same.

And since we have two caps in series the total value is cut in halve and for a give F/C we'd need twice their size to maintain the same F/C as before.
Now, I wonder how that's going to help the sound if you know that bigger caps are inherently slower to release their charge?

Cheers,;)

I have a feeling you must have misunderstood me. I didin't
advocate two caps in series. I said, get rid of output caps and
use input caps instead. since you are picky about cap sound
I am sure you prefer a cap small enough that you can get
a good polypropylene one.
 
fdegrove said:
The input and output cap in an amplifier do have totally different functions; leaving the ouput cap of the amp out at the risk of speaker killing DC is silly and I never count on DC servos to save my speakers.

I did say that you should have proper protection against DC
on the output if you leave out the output cap and care about
your speakers. If you don't like relays, maybe a crowbar might
still be satisfactory to you. If you don't like the sound of caps,
especially big lytics, I cannot see how you can stand a power
amp output cap, but maybe they are small enough to be
film type since you
go for those glowy things. Don't know enough about that stuff.
 
diyAudio Senior Member
Joined 2002
Hi,

What I was asking is that a cd player with a coupling cap will cause a 90 degree phase shift no matter what.

And where did you read that?

If I add a capacitor to my amp's input, it will effectively join itself to the cd player's cap, so it won't add any more phase shift, correct?

Phase shift won't change but it's not a shift, more like a group delay which is NOT the same.

Add the same value cap as the one on the output of the source and tell me you don't hear the difference.

While you're at it justify the use of that cap in the first place for I just don't see the logic.

Any decent CDP, DVD and what have you should have no DC at the output whatsoever if it has to hit the market.

Cheers,;)
 
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