[divie23]

Anyone?!

I've measured offset on my amp which is 0.9mV and 1.4mV for each channel. The offset on my source is 0.6mV per channel. Should I be worried considering I omitted an input cap on my DIY amp?

Cheers!

 

[Arzela]

Not at all, those are very low figures.
Concern sets in if it's over 10 mv.
If your driving high impedance phones, even 25 mv is ok.

 

[300_baud]

Normally, for a source, you shouldn't have over 2.5 mv coming out, or otherwise the opamps in an amplifier cannot compensate for it, and may amplify the dc offset (assuming you don't output caps).

 

[Tomo]

Divie -

It is fine. I can even live up to 50mV or so. It isn't much of a big deal unless you are feeding the output to a power amplifier (speaker).

T

 

[300_baud]

Actually, from what i recall from diyaudio or audioasylum, 100 mv into speakers is okay, whereas 100 mv into 99% of headphones is trouble. It was something due to the fact that the drivers in headphones are smaller and more susceptable to heat damage.

 

[divie23]

thanks guys. its been a few yrs since i last posted but glad to see the place is as friendly as ever!

 

[was ist los?]

So how do i measure dc offset?

 

[Glassman]

Quote:

Originally posted by was ist los? So how do i measure dc offset?

Keep quiet - don't play music - and measure voltage between output left channel to ground and also right channel to ground.. if it's under <fill in yourself> mV it's OKay

I think 10mV couldn't hurt anything..

 

[jeffreyj]

When does dc offset become a problem? Well, try putting Ohm's law to work, folks, specifically, P(mW)=(V^2/R)*1000

Measure the DC resistance of your favorite headphones.

Measure the offset voltage from left and right outputs to ground.

Plug those numbers into the equation above to get the amount of power in milliwatts the offset will put through your headphones.

For example, 100mV of offset into 32 ohm headphones equals 0.31mW - too much! This is only 5dB below 1mW, the power level at which the nominal sensitity for headphones is measured.

100mV into 600 ohm headphones is 0.0167mW, which seems insignificant but it's still only ~18dB below 1mW.

added the text below:

Because power is proportional to the square of the applied voltage, even a modest reduction in offset voltage pays off nicely. Reducing the exorbitant offset voltage I used in the above examples down to a much more reasonable, but still not so good, 20mV, would give these results:

20mV into 32R = 0.0125mW/-19dB
20mV into 600R = 0.000667mW/-32dB

But as far as what voltage will start to heat the voice coils up appreciably, well, that's a different story. I doubt that even 100mV of offset would result in noticeable heating, but it will certainly result in distortion from the reduction in driver excursion!