[Sycraft]

So I broke out my PPA today after having it sit on the shelf for a long time because one of my coworkers wants to see it and run some test on it. I took the time to check it to make sure it was still in spec. All good, the outputs have 1.5mV or less DC offset, the biasing on the opamps is unchanged.

Well checking it got me to wondering about the DC offset of outputs on other things I could get my hands on like, say my soundcard in particular since that's probably one of the things that'll be used for testing. Both outputs read as above 30mV (30.4 in one case, 30.8 in the other). Ummm, is it just me or does that seem, well, high espically since this is a consumer product?

I'm pretty sure my reading is right, my metre was calibrate not too long ago.

Is that something I should be worried about driving my headphones with? I've been plugging them straight in since I don't care to buy/build an amp for work at this point. However it seems that if I did put an amp on it, I'd just be making things worse. Maybe I need to look at a non-DC amp for work.

 

[FallenAngel]

Uhm... As far as i know, you NEVER connect headphones straight to the Line-Out of a source, the power sent is rediculously more than what should be sent to headphones.

Also, what kind of soundcard is that?

 

[Sycraft]

Creative Labs Audigy 2. The port on it is allegedly rated for headphone usage, and works fine driving them.

 

[PinkFloyd]

30mV is not that high in the scheme of things. Anything < 50mV is "OK" for audio but, ideally, you're looking more toward < 5mV (preferably 0mV) Don't worry though, 30mV is not obscenely high and you're 'phones will be safe.

 

[threepointone]

hmm--on measuring voltage offset, what would it mean if i get some sort of insane randomly changing measurement from a source, goes from a few mV all the way up to a few hundred mV (directly from ground to left or right channel) but when i measure in parallel with a pair of cheapo earbuds, some reasonable (<5mV) offset is given? is this because some sources need a headphone load or something to work properly?

btw, i was testing the line-out of my iriver h340. the meter is a fluke 189 (i don't think it's been calibrated recently, though). the headphone out measured reasonable offset without any headphone load, around 1-2mV on one channel, 2-3mV on the other.

 

[Garbz]

Offset won't kill headphones very quickly till it gets really high. Even very low impednace loads (24ohms) have survived for me being pluged in and used at around 70mv for a while. And my hd580s have taken several hundred a few times already.

The offset needs to get very high before the coil starts cooking, but what offset does do is displace the driver, this ofcourse reduces the excursion limit when listening to music and if you have a high DC offset and start blasting with really loud music then there's a chance you could tear the diaphram.

 

[Sycraft]

I don't think tearing the driver is likely, I don't really care for blasting music, my ears are kinda sensitive. Plus these phones are allegedly rated to 500mw, and do 102dB per mw. According to my calculations 500mw * 102dB = way too damn loud for Sycraft. I doubt the soundcard could output it at anyrate.

I was just kinda supprised is all. I haven't used my PPA in ages, I've decomissioned my old setup and my new one doesn't have provisions for headphones yet. When I built it, DC offset concerned me since I had never used a DC coupled amp before. However it's offset is minimal, and the mixer that drove it had 0 offset as far as I could tell.

It was just suppirsing to me that consumer gear would have such a large offset. I figured it was more or less a non-issue with gear these days since true DC amps are rare. I've also debated using my PPA at work since I listen on headphones all the time there.

I don't know what the practical DC offset would be, I'd have to see how high I'd want to set my volume dial, but since it could in theory be as high as 300mv maybe I'm better off getting a non-DC amp.

Or maybe a better soundcard.

Or maybe both so I can use my PPA at home.

I'll have to bring a metre home and see how my gear here works.

 

[threepointone]

i just remembered that I have an audigy 2 too! tested dc offset, i got ~30mV per channel too. pretty nasty, especially if i had a cmoy at gain 6 without input caps, it'd go up to 180mV which is REALLY bad. My front headphone out, which seems to be attached to the soundcard internally, measures something like almost 0mV. I suspect they stuck in some input cap in the front headphone module, though (nasty electrolytics, in all likelihood). I've been using the back line out recently, though, since it seems like i screwed up something with the front module and shorted one of the USB connections with the headphone plug or something--i get all sorts of distortion and interference from it. hopefully my headphones don't get fried!

 

[threepointone]

double post

 

[amb]

Quote:

Originally Posted by Sycraft ... Or maybe a better soundcard.

IMHO that's the best solution. The Audigy 2 leaves much to be desired anyway.

 

[PinkFloyd]

OK. Assuming this is a soundcard (which it is) why not just fit some non polar output caps in series with the output... 470uF or thereabouts would do and that's you with 0.00mV per channel. Job done for a few pence and 10 minutes work.

Something like this:

 

[ECM]

i emailed grado about offset specs a few years ago. for my SR-125's (32 ohm) they said anything under 100mV was ok.

 

[thedoctor]

Quote:

Originally Posted by ECM i emailed grado about offset specs a few years ago. for my SR-125's (32 ohm) they said anything under 100mV was ok.

and i know i've read from somewhere that 20mV is the max safety limit for a low impedence (32 Ohm) cans.

 

[Garbz]

nah 20mV is the socially acceptable standard on this forum. Whenever there's a thread about this it's always keep it under 20mV. There's no real problem having an amp that sits permanently at twice that. It's just a design ideal really. Plus with most of the split supply opamp systems here offset is a sign of a wiring error. It's only really the gilmore amps where the offset comes into question, for some reason most of the other amps don't.

 

[Sycraft]

thedoctor: I'd believe headphone makers above all others, it's in their intrests not to blow out their phones. I'd guess the 20mV limit is more since people here are building amps, and thus it is in your intrests to make it properly.

PinkFloyd: Nope, the soundcard is the property of my employer. As such it is not going to be messed with in any fashion. Besides, if 30mV isn't a problem, there's no reason to modify it.

amb: True, but it is what I have and work isn't going to buy me a better one. I'm unsure at this point that I want to spend money on a system that's not mine. I'm just trying to decide how much money I'm willing to dump in to it.

I dunno, I'll think on my options. Mostly, I just wanted to make sure that the offset wasn't going to damage my headphones. Previously I had old Sonys I didn't care much about, but now I have some nice Audio Technicas I'd like to last.

I also have to decide what I want to do amp wise. I'm kinda tempted to go no amp at work and save some money, but I'm also thinking I might want to try an M3 at home, in which case I'd probably use the PPA for work. If I did, of course, I'd want to deal with the offset problem. I've also debated just getting a Firestone amp for work, and I think those have input caps on them.