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Old 7th December 2003, 07:39 PM   #1
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Default Capless signal path?

My latest amp uses no caps in the signal path, it's pretty much a DC amplifier. It doesn't use ground as a signal ground so I can't use more than one channel per power supply. Is there any real advantage to not using caps?
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Old 7th December 2003, 07:55 PM   #2
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nope, you are just adding a risk...
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Old 7th December 2003, 09:07 PM   #3
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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
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Old 7th December 2003, 11:38 PM   #4
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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.
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Old 7th December 2003, 11:49 PM   #5
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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.
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Old 8th December 2003, 12:08 AM   #6
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Default Who Stole The Bass?

Hi,

Quote:
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,
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Old 8th December 2003, 12:14 AM   #7
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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 (US or Euro ones? Pick your choose).
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Old 8th December 2003, 12:18 AM   #8
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Hi,

I guess driving a Volvo with two sets of safety belts per occupant does make it a safer car??

Cheers,
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Old 8th December 2003, 12:18 AM   #9
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I had my drivers burned twice ($500 loss each time), and it was never because the input cap wasn't there.
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Old 8th December 2003, 12:24 AM   #10
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Quote:
Originally posted by Peter Daniel
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.
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