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Help performing a volume match
post May 8 2009, 22:07
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I recently borrowed an Presonus HP4 (headphone amp) from a friend. Apparently it works quite well with high impedance headphones, but I wanted to check it with low impedance (25 ohms) headphones. I've found that some headphone jacks have a high out impedance and this alters the output frequency response somewhat.

I usually do a volume matched tests using an A/B box. I take the two pieces of equipment (say two portable music players) and play a 1khz tone. Then I use a multimeter in AC Volts mode and bring the volumes as close together as I can (usually a maximum difference of .002 volts). Then I plug both mp3 players into the A/B box, start the same song on each player, and switch between the two players to see if I can hear a difference.

I wanted to do a similar test on this HP4. I used a stereo Y splitter on my computer's sound card output. One end of the Y splitter went to the A/B box. The other end went into the HP4. Then I connected one of the headphone outs of the HP4 to the A/B box.

I volume matched with a multimeter using the procedure above. Even with the multimeter showing an identical voltage for both sources (sound card out and HP4 headphone out), the HP4 was much quieter than the headphone directly into the sound card. I had to nearly double the output voltage of the HP4 before it sounded like the volumes were identical.

What is going on? Did I make a mistake volume matching? Is this because the HP4 output impedance is higher (I'm assuming) than 0? I suppose if I knew the output impedance of the HP4 headphone out I could run a quick calculation and figure out if this explains the difference.

I then did a loaded RMAA test on the HP4 to see if that would uncover something. The bass frequencies had a serious hump and I'm almost embarrassed to post the other numbers. Let's just say I'm wondering if this thing is defective. But there was nothing to indicate why the volume is so much quieter than plugging the headphones directly into the sound card.

Anybody have any insight into this?

This post has been edited by odigg: May 8 2009, 22:13
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post May 9 2009, 17:40
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@odigg Are you doing the level matching loaded with your headphones? If not, try it.

QUOTE (Arnold B. Krueger @ May 9 2009, 17:41) *
The next problem you may encounter is that your multimeter itself may have less-than-idea frequency response.

Not important for level matching when the frequency in both measurements is identical.

This post has been edited by rpp3po: May 9 2009, 17:51
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