[bead]

I am worried about testing my M3 using my on-board PC audio output as a source. I don't really have any other sources to try it on and I haven't built a DAC yet, but I still want to test it out. I have no "crappy" headphones to test the M3 out on first before I plug in my HD595s.
 
I read a little about being careful that the sources doesn't have too high of a DC offset as the M3 will multiply that. What kind of test setup can I do to be sure that whatever source I do end up testing the HD595s with wont fry them?
 
Thanks.

 

[wakibaki]

If you have a multimeter you can simply measure the DC offset of your source. Play a track and measure the DC voltage between the right output and ground and the left output and ground. You can access these by plugging a PC audio cable into the output. I mean a cable with 3.5mm stereo jacks at both ends. The sleeve (shaft) is ground the ring is right and the tip is left. Make sure you are using the DC milllivolt range. The reading should be no more than 20 millivolts, although personally I would like to see 2mV or less.
 
Connect the source to the amplifier and measure the output in a similar fashion. If the DC on the output is 20mV or less the phones will be safe from destruction.
 
It's just possible that the offset will be exaggerated with no load connected, I'm not familiar with the M3 circuit. If it's worryingly high post again before connecting the phones.
 
w 

 

[qusp]

just put some caps inline between the soundcard and the m3 to be sure there is no dc

 

[bead]

what value of caps?

 

[m1abrams]

Quote:

what value of caps?

Well lets do the math.  If your MMM is a standard build with a 50K pot then your input impedance is ~47ohms.  You want the cap to filter out frequencies below say 10Hz.
So solve for C
freq = 1/(2*pie*R*C)
 

 

[bead]

33nF?

 

[DingoSmuggler]

You somehow got out by a factor of 10 there, 33nF will have a corner frequency (-3dB) of ~100Hz
330nF would be for 10Hz
 
imo 1uF is a good value input capacitors for a ~50k-ohm input - easy size to get, corner frequency about 2Hz