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Measuring DC offset
in your headphone amplifier
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| Zero DC is ideal , some is tolerable
and too much is bad news. No decent amp should have any more than
20 mV of DC offset at its output. If the amp has a big DC offset
on its output it will cause a shift in the position of the headphone
diaphragms which will degrade the quality of the sound and will prevent the diaphragm from working optimally. If the DC is very high it may even fry the coil, causing expensive damage to your favourite headphones. It's always worth checking the DC offset after building a kit amp and you go about it as follows: |
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1: Arm yourself with a good multimeter and set it to the 200mV setting on the DC range:
2: Turn your amp on with nothing connected to it (no 'phones or interconnects etc.) 3: probe between Sleeve and tip (ground & Right) on the headphone socket as shown. The reading will tell you if there's any DC offset present and, if so, how much:
The reading should ideally be 0.00mV but seldom is.... anything below 20mV is perfectly acceptable (closer to 0.00mV ideally). Once you are happy that the offset is within limits then repeat the test but this time with the source connected....... play a CD and then hit the pause button... now measure the offset with the CD player connected and disc on pause. If both measurements are pretty close to 0.00mV (say something like 3.74mV) then you're good to go! Another thing..... turn the volume control up and make sure the offset remains the same at all levels (it will if it's a FET opamp) If you cannot get the DC below say 20mV then it will be necessary to fit DC blocking output caps in series with the left and right outputs. Something like a 470uF non polar capacitor connected in series between the amps output and headphone socket will block the DC. Repeat the same for the left channel by probing between Sleeve
and ring (ground & left) as shown:
The reading between channels will invariably be different ie: L = 3.74mV and R= -0.05mV nothing to worry about here, perfectly acceptable. When probing connect the black lead to COM on your multimeter and the red lead to V. Always probe the sleeve (ground) with the black lead. Most FET opamps like OPA627BP generally measure around 0.07mV (or thereabouts) and some measure a perfect 0.00mV and will not require DC blocking output caps. LM6171 will measure something crazy like 800mV hence the need for DC blocking output caps or trimpots. The AD-8065 I have just inserted into the WNA measures 0.01mV (left channel) and 2.06mV (right channel) so the offset is VERY low and well within limits.... that's not to say that all 8065's will measure the same and you'd have pick through a few of them to select two that measured 0.00mV.
Basically, anything under 20mV is totally acceptable. 20mV to 50mV is borderline and anything greater than 50mV is a definite NO NO.
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