Measuring DC?
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andrzejpw

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Well, I finished my meta with my dad a few weeks ago, and I'm already thinking about tweaking.
One question: on the website, it says that you can omit C1 if your source isn't outputting DC. How do I measure if my denon 370 outputs dc? Just set my multimeter and plug into the rca outs? How about the art dio? same thing?
 
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puppyslugg

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Connect the negative lead of the meter to the body (ground) of the rca jack and the positive lead to the center conductor of the jack. Set the meter to dc and if applicable, mv range. Same applies for the art/dio.

I think the denon should have a very low offset (less than 10mv?)because it is more than likely have a cap at the output. In which case you can remove C1. If you do remove C1, maybe you should put a label on your meta to remind you that the C1 is bypassed and not to connect unknown sources to your meta. Not necessarily to protect the meta, but your headphones. Wouldn't want to have an accident, would we?
 
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aeberbach

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Best to leave C1 in unless you have an oscilloscope. With a multimeter it will try to read DC but any AC present will probably cause it to show some reading, most likely an average of the AC present! It won't hurt the sound much and it can damage your phones.
If you had an oscilloscope then you would be looking for a waveform centred on 0 volts, and a more or less flat line on 0V when there was no sound coming out. Ideally what you would see on AC coupling would be the same as GND coupling with the input shorted.
 
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tangent

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You can test DC offset without any music, which makes aeberbach's point moot.

I suppose it's possible that in some circuits that the DC offset could shift when there's an AC signal present, and so to be safe I guess you should test that, too. To avoid what aeberbach is talking about, use a pure sine wave as your test signal. That way the AC swing in each direction is the same. I haven't tested this, but it seems to me that as long as the period of the wave is higher than a few Hz, it shouldn't show up on a DC meter.

Also, consider retesting after you jumper C1. This time, test for DC offset on the output of the amp when plugged into various sources. If the source has very little DC offset but the amp amplifies it (that's it's job, after all) to significant levels, you still have a problem. Also, the META42 will have its own DC offset, which can be significant depending on the amp's resistor configuration and the op-amp you're using. You need to test all of this in combination before you plug headphones into the result.
 
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KurtW

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Has anyone measured ANY CD player or DAC that had a DC component of more than a few harmless millivolts? None of mine do, including several Panasonic portables. I know its better to be safe than sorry, but I think the concern is largely overdone. I know the Corda HA-1 doesn't have any coupling caps, and I've never heard about one of those blowing up headphones. I think Jan said he's shipped over 400 of them. I've build three META42s and none of them had C1...maybe that's one reason why they sound so good! Note: I take no fiscal responsibility for for this advice, use it at your own risk.
 
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puppyslugg

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Quote:

Originally posted by KurtW
I've build three META42s and none of them had C1...maybe that's one reason why they sound so good!


KurtW,

You're speaking my kinda language! That's the same reason why I removed the input cap in my cha47. No cap is a good cap...if you don't really need it.


Mass produced audio equipment, I believe, most if not all have caps at the input/output. Caps are cheaper than a servo or matched components. But again, check offset, just in case.
 
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tangent

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I think that the CYA you're seeing here is because we've all seen people read stuff here and then try to apply it without sufficient understanding. As a knowledgeable builder, Kurt, it's all right for you to remove a capacitor without worrying. It's quite another to tell others that they can remove a particular cap without big loud disclaimers attached.
 
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andrzejpw

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Quote:

Originally posted by aeberbach
You do realize it is traditional to post a picture of the amp, right?


I know, I know.
I'll be borrowing a digicam in a day or so. . .
 
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KurtW

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tangent, at least you have brought up the issue of why you need (or may not need) C1. I don't remember reading about this issue with CMOY designs, it seemed the only question then was how big, not if it was needed.

I think if anyone does measure significant DC offset voltage in any equipment that might also be used by others, it would be nice to list it. My Panasonic models 510, 570 and 320 don't have any, so if anyone is using those they don't have to worry.
 
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