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Help performing a volume match
odigg
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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Arnold B. Kruege...
post May 9 2009, 16:41
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QUOTE (odigg @ May 8 2009, 17:07) *
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.


The first problem I see is that you are matching levels using a multimeter, but using music as your signal source.

I don't know how to do that accurately, and I've only been doing ABX tests for about 30 years! ;-)

Music bounces around way too much to get reliable indications in my experience.

Your next move should be create files with test tones using an audio editor, or find any of the many examples of these that are already on the web.

The two primary frequencies for test tones a 400 Hz and 1,000 Hz. They should be recorded at levels that are sufficient to easily get a good stable reading on the multimeter. This is your basic level-set.

The secondary test tones are for the purpose of picking out serious frequency response problems. On the low end, I'd recommend 20, 50, 100, and 200 Hz. On the high end, I'd recommend 5 KHz, 10 KHz and 15 KHz.

The next problem you may encounter is that your multimeter itself may have less-than-idea frequency response. Usually a cheap multimeter's response is good from 50 to 500 or 1,000 Hz, but above 1,000 Hz anything may go. 10 dB down at 10 KHz is not the least bit unusual.

The best solution is of course to spend the big bucks on your multimeter if you have this problem. However, you can work with what you have as long as you have a large enough indication to see +/- 0.1 dB variations (+/- 1%). The work around is to see what indicaitons you get in an environment where you can presume flat response, write those down, and use them for your reference. From 50 Hz to 15 KHz most modern on-board audio interfaces are good enough, but you should confirm that with a Rightmark test, which you have indicated some familiarity with. Common audio low-mid range audio production audio interfaces like the M-Audio AP 2496 and 24192 are decent reference sources.

Here's some quick rules of thumb - +/- 1% is about 0.1 dB, and that is always good enough matching. 5% is 0.5 dB which is questionable but is OK at the frequency extremes - 20 Hz and 20 KHz. 10% or more is probably undesirable for a good clean ABX, except maybe at 15 Khz or above and 50 Hz or below. The gold standard is +/- 1% or 0.1 dB.


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rpp3po
post May 9 2009, 17:35
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QUOTE (Arnold B. Krueger @ May 9 2009, 17:41) *
QUOTE (odigg @ May 8 2009, 17:07) *
I take the two pieces of equipment (say two portable music players) and play a 1khz tone.

The first problem I see is that you are matching levels using a multimeter, but using music as your signal source.


Obviously, no.
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rpp3po
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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Arnold B. Kruege...
post May 10 2009, 00:41
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QUOTE (rpp3po @ May 9 2009, 12:35) *
QUOTE (Arnold B. Krueger @ May 9 2009, 17:41) *
QUOTE (odigg @ May 8 2009, 17:07) *
I take the two pieces of equipment (say two portable music players) and play a 1khz tone.

The first problem I see is that you are matching levels using a multimeter, but using music as your signal source.


Obviously, no.


Right - posting too early in the morning. ;-)
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Arnold B. Kruege...
post May 10 2009, 00:44
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QUOTE (rpp3po @ May 9 2009, 12:40) *
@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.


I made that point, but this time it must be too early in the morning for you!

BTW, non-flat response *is* an issue if it makes the meter indication so small that the meter you are using has insufficient resolution, because it rolls off so much. I made that point, in the post you're cricitzing, you missed that, too.
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odigg
post May 11 2009, 16:29
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QUOTE (rpp3po @ May 9 2009, 12:40) *
@odigg Are you doing the level matching loaded with your headphones? If not, try it.


I read this and immediately gave myself a whack on the head. How did I not think of that? It worked brilliantly and I was able to volume match to .001v.

The HP4 is boosting the bass (verified by RMAA) a bit. My onboard sound is rolling off the bass a bit (verified by RMAA). Using an A/B switch I could tell the bass was slightly fuller on the PreSonus. I tried to make this blinded to a minor degree by unpluging wires, closing my eyes, dropping them on the table, switching them around, and plugging them back in without looking at the setup to figure out which wire went where.

I could easily pick out the HP4, but it bass difference isn't drastic. If I had to physically move the headphone from my sound card to the PreSonus to A/B I wonder if I would have even picked it out. The PreSonus does seem to be distorting (or enhancing the audibility of the existing distortion) some bass notes on very bassy songs.

I didn't sit there for hours to tease out the differences. I've done so many listening tests and amps always sounds mostly the same so I'm a little tired of doing these.

Thanks for the help. It is greatly appreciated.

This post has been edited by odigg: May 11 2009, 16:30
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scud80
post Apr 21 2012, 05:33
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QUOTE (rpp3po @ May 9 2009, 08:40) *
@odigg Are you doing the level matching loaded with your headphones? If not, try it.


resurrecting this thread because i'm having a similar issue. i don't understand what the above statement means. could somebody perhaps explain it a little better for me?

i have very little experience with electronics (just got my first multimeter in the mail yesterday), but i'm very interested in being able to level-match my audio gear for comparisons.
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pdq
post Apr 21 2012, 12:50
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QUOTE (scud80 @ Apr 21 2012, 00:33) *
QUOTE (rpp3po @ May 9 2009, 08:40) *
@odigg Are you doing the level matching loaded with your headphones? If not, try it.


resurrecting this thread because i'm having a similar issue. i don't understand what the above statement means. could somebody perhaps explain it a little better for me?

i have very little experience with electronics (just got my first multimeter in the mail yesterday), but i'm very interested in being able to level-match my audio gear for comparisons.

The output impedance of a headphone amplifier is often not near zero ohms. This means that the output voltage when loaded by the headphones is different than unloaded. Since the impedance of the headphones can vary with frequency, the frequency response can also be different loaded vs. unloaded. Also, since the impedance of the left and right headphones may not match exactly, level matching loaded may not be the same as unloaded.

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