[ablaze]

as above. using headphones straight from the source, without an amp. is 12mV acceptable? with say.. 50ohm cans?

 

[__redruM]

20mv is the limit, but I wouldn't connect $100+ headphones to it, especially anything ultra sensitive. What is it?

 

[PeterR]

The lower the offset the better obviously, but there's no set in stone limit where any phone will suddenly go poof if you exceed it. 12mv into 50 Ohms is equivalent to 0.003 mW, not likely to do damage even to very sensitive cans.

 

[aeriyn]

Er, this is on the guy's source. Whatever the gain of his amp, is going to muliply that DC offset by.

The simple solution to this is to put either output coupling capacitors on your source or put input capacitors on your amp, thus nullifying DC offset. You can't really get away with amping a source with 12mV of offset, especially if you need to set gain fairly high. Even with a gain of 3 to drive 50ohm cans, that'd still bump offset up to 36mV and that could possibly damage your headphones.

Oops, didn't see the not using an amp part. I wouldn't use good cans on it anyhow. I'm picky about offset, I don't give up until I get any of my DIY amps to have 2mV per channel or lower. I'd amp that source just so I could kill the DC offset with input caps.

 

[PeterR]

Quote:

Originally Posted by aeriyn Er, this is on the guy's source. Whatever the gain of his amp, is going to muliply that DC offset by.

He says he used no amp.
Quote:

Even with a gain of 3 to drive 50ohm cans, that'd still bump offset up to 36mV

That's right, assuming you turn the volume all the way up.

Quote:

Oops, didn't see the not using an amp part. I wouldn't use good cans on it anyhow. I'm picky about offset, I don't give up until I get any of my DIY amps to have 2mV per channel or lower. I'd amp that source just so I could kill the DC offset with input caps.

Oops, didn't notice you'd noticed

BTW, I've recently measured a quite popular commercial amp with 15mV-49mW offset (the latter figure is with the volume all the way up, so that's unlikely to happen in the real world). Never heard anybody complain about that one, and there seems to be no history of phones being damaged by it...

 

[__redruM]

Quote:

Originally Posted by PeterR BTW, I've recently measured a quite popular commercial amp with 15mV-49mW offset (the latter figure is with the volume all the way up, so that's unlikely to happen in the real world). Never heard anybody complain about that one, and there seems to be no history of phones being damaged by it...

What's the offset at the source? This is really setup dependant. You'd really have to measure the offset with a clean (0-1mw offset) source to know that the *anon* amp produces this much offset. And even then, you could have a defective *anon* amp.

 

[PeterR]

Quote:

Originally Posted by __redruM What's the offset at the source? This is really setup dependant. You'd really have to measure the offset with a clean (0-1mw offset) source to know that the *anon* amp produces this much offset. And even then, you could have a defective *anon* amp.

That's with inputs shorted to ground. The amp in question is DC coupled and uses an opamp with bipolar inputs in inverting configuration with the pot wired in as variable feedback resistor. There's no way this won't give you an offset that depends on source impedance and volume control setting, so no, there's no reason to assume a defect.

 

[__redruM]

So the pot is used to vari gain instead of input level. This would really multiply offset at the higher ranges. You'd almost have to put output caps on it.
Just because your earphones don't go pop all at once, doesn't mean damage isn't being done. My experience of DC offset is limited to what I've read, not actual experience. But when I plug my $500 headphones into something, I want a guaranty

 

[aeriyn]

Which is why I don't put the lid on any of my amps until I've got 2mV of offset or less plugged into a source I know is clean (my cambridge CDP, which either has output caps or is just well-made enough to have less offset than my meter can measure).

 

[ablaze]

hey guys,
thanks for the replies

I decided I'd like to get rid of the offset altogether
see what I did was remove the caps in green in this pic on my 0404 (they were stock 47uF 16v electrolytics I think)
http://brian.osupages.com/audio/pict.../0404mods2.jpg (thanks bg4533!)

from this thread
http://www4.head-fi.org/forums/showt...t=92725&page=3

my question now is what I should replace those caps with for the smallest hit on sound quality. polypropylene? 4 of those in the original position?

 

[silvervarg]

If I understod correctly those green caps was the output caps.
How could you have any offset with output caps in place?
Where you measuring the offset towards the correct ground point?

Replacing them with better caps could improve sound quality, but it should not affect the DC offset. since you want fairly large value caps on the output I suggest that you use a low impedance electrolytic parallelled with a small value of a plastic cap. Due to size restrictions you probably need to choose a plastic cap that will fit rather than using one that is the best.

 

[breez]

edit: And of course an amp isn't used here so this can be disregarded. First I read and understood that no amp is used, but then all the amp talk made me forget

One could put a capacitor in series after the resistor which goes from -input to gnd. DC gain is then at unity and offset will not be multiplied. Of course I don't know what affects the SQ most, this cap to gnd from feedback circuit, an output cap or an input cap. Also an electrolytic may be needed in this application depending on the feedback resistor values...