IPB

Welcome Guest ( Log In | Register )

Help performing a volume match
odigg
post May 8 2009, 22:07
Post #1





Group: Members
Posts: 629
Joined: 25-July 08
From: USA
Member No.: 56264



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
Go to the top of the page
+Quote Post
 
Start new topic
Replies
Arnold B. Kruege...
post May 9 2009, 16:41
Post #2





Group: Members
Posts: 3700
Joined: 29-October 08
From: USA, 48236
Member No.: 61311



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.


Go to the top of the page
+Quote Post
rpp3po
post May 9 2009, 17:35
Post #3





Group: Developer
Posts: 1126
Joined: 11-February 03
From: Germany
Member No.: 4961



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.
Go to the top of the page
+Quote Post

Posts in this topic


Reply to this topicStart new topic
1 User(s) are reading this topic (1 Guests and 0 Anonymous Users)
0 Members:

 



RSS Lo-Fi Version Time is now: 30th August 2014 - 00:44