[Kal525]

I just decided to measure the DC offset while powering down my meta42 because i was hearing a small thump in my headphones as i was powering it down. I have a SPDT switch hooked up in series with the power supply. The max DC offset which i measured was about .732 volts with the average being from .5 - .63 volts. Does anyone know if this is normal or if it has the potential to harm my headphones? Thanks.

 

[CaptBubba]

Is that with or without headphones plugged in?

If you are worried about it, just be sure to unplug your headphones before you turn the amp off. If you are really worried about it you could put in an output capacitor, but that still would put some DC through your phones at turn off (and do bad things to the sound, but that's another issue).

 

[Kal525]

That's with the headphones unplugged. I get only about .001 Volts while in operation so the value i got when turning it off kind of scared me. I'm not able to detect any offset on power up as i only have a digital multimeter, but i do not hear any pops on power up. If this range is normal on power down than it's fine by me, i just want to make sure that it's not doing bad things. Another output capacitor would probably do more harm than good.

 

[morsel]

That .7V offset does not sound normal or good, Kal. Bubba, an output capacitor would sonically ruin the META42.

Try batteries and let us know if the problem goes away. It could be that you are using a bad power supply or have it hooked up incorrectly. For example, If you are using a dual supply you need to switch both rails. If you are using a single supply it has to be floating, that is, neither rail can be tied to AC ground. Or you might have a problem inside the amp itself. Anyway, do the test with batteries, and also with nothing connected to the input.

 

[Kal525]

The numbers you see above are with just the power supply hooked up, no inputs and no outputs.....i just did some searching and found this post written by tangent:

http://www4.head-fi.org/forums/showt...ight=dc+offset

Quote:

I decided to follow my own advice: I powered up an AD8620 in a META42 (v1) with a DSO attached and I see a -1V (approx.) pop on startup. This won't hurt anything -- that's about the normal excursion for most headphones.

If 1 V is normal on startup, than shouldn't .7 V be harmless on shutdown? It only lasts for a split second.

 

[CaptBubba]

Quote:

Originally posted by morsel Bubba, an output capacitor would sonically ruin the META42.

Notice the phrase about doing bad things to the sound. I think you are exaggerating the effect it would have, but I don't think it matters because you don't need one anyway.

My amp does -1V on shutdown just long enough to register on a DMM if I have no headphones plugged in. With headphones in (V6) it goes to -.4V and only registers half the time. This doesn't worry me as this is only 3mW/ch, while the V6 say they can handle 1W. Similarly, with HD580s, a -1V swing at turnoff is about 3.3mW/ch.

 

[Kal525]

I have HD600 headphones, do you think what i'm seeing will harm them or shows instability at all in my amp?

 

[morsel]

Perhaps the voltage swing is not a big deal. I didn't realize it was so brief.

On the other hand, I am vehemently against output capacitors. They seriously impede deep bass and are bad news loadwise. Yuck!

 

[Kal525]

So than i shouldn't worry about it right? It's normal. I think i'll unplug my headphones when turning it off anyway. I just want to make sure that this momentary DC offset is normal.

 

[Subsonic]

Hmm...that's strange my meta42 also, is giving out DC voltages. I didn't notice it before so I think it must be recent. Is this what happens when everything gets "burned in"? Isn't C1 suppose to filter out the DC voltages? When I turn the amp on I get roughly .250v and when I turn it off I get .350v (on average). I can hear the thump in my headphones.

Specs:
Amp: AD8620 (class A)
Output Buffers: 8x EL2002
Ground Buffer: 1x EL2002
C1: 0.22 ìF 50V
C2: 4x 470/25v Elna Cerafines
C4: 6.8uF/50v Wima
Power Supply: Elpac 24v

 

[Kal525]

I think that the purpose of C1 is more to block DC offset while the amp is in use, giving you a steady signal without DC offset. My guess is that shutting the unit off or turning it on results in spikes which are too great for the C1 caps to filter completely, but if you didn't have these caps in i think the the offset we would be measuring would be even greater, there is probably a way to connect the power supply to elliminate these spikes on shut down and startup, but it would probably involve getting a different switch.

 

[puppyslugg]

C1 is to block DC offset from the source. If you know your source has a low DC offset, then C1 can be eliminated.

 

[aos]

It's a normal thing to have variable voltage on the output (resulting in thump) when turning an amp on or off. Amplifier is in transitional state at that time and the result is voltage variation until it stabilizes. Depending on magnitude it may be a mildly annoying thump or a powerful one. Since it's usually mild and lasts less than a second, it isn't going to hurt. On powerful amps there is usually a relay that doesn't let headphones/speakers be connected until a few seconds pass and the system is stable, and also disconnect it immediately when the power is turned off to avoid the same effect upon shutdown.

Some amplifier architectures are less sensitive to this than others.

 

[ppl]

This is normal with the Phones unpluged. I also measure about 0.7 to 1.2 volts upon powering up almost any solid state amp. Output blocking caps will not help eitther since thay also will give sutch a pop untill thay are charged up. I have not had any Headphone damage from leaving my Sony MDR-7506/V6 pluged in all the time during power on and off. yes there is a thump on power up and power down. If yoy think this mightt damage yoyr phones then don't plug them in untill applying power to the Amp first. (Dosent most operating manuals say somthing like this "unplug and turn off this device prior to making any connections")