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  #1  
Old 10-15-2009, 11:38 PM
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Lightbulb How To Test Ouput of MP3 Players

How To Test Ouput of MP3 Players:
Thought people might find this handy.

What you'll need:
  1. 1 3.5mm cable (one you don't want to use for anything else after we're done with it)
  2. 2 16ohm resistors (preferably 1 Watt or more with 1% or lower tolerance) NOTE: You can use any resistance you would like to calculate the output at that load, most players spec sheets show the output at 16ohms.
  3. 1 Multimeter (AC Voltage and Resistance measurement required)
  4. 1 Pair of Wire Cutters
  5. Electrical Tape
  6. 1 Utility Knife

Lets get started:

First you will want to cut the 3.5mm cable to approximately 12 inches. Then using the utility knife carefully strip off the outer insulation about 1 inch. You will find 3 or 4 wires, 2 hot wires with insulation and either 1 or two ground wire(s). Using extreme care strip the insulation off of the two wires. They are very small and easy to cut through the wire, if you do cut it just cut the entire cable and start over(thats why we have 12inches of cable). If there are 2 ground(uninsulated) wires twist them together. Now take the resistors and twist one side of each and the ground(s) together. Next twist one hot wire to the opposite end of one resistor and repeat for the other hot wire. Now using the electrical tape, tape one hot wire to prevent it from shorting out the other hot wire or the ground. Align the uninsulated hot wire and the ground(s) so they will not touch. If you striped too much of the hot that is not taped that it could contact the ground easily you can apply a small amount of tape to prevent it shorting out. Don't completely tape the hot wire or you will not be able to connect the meter.

You are almost ready to measure the output of your player. Set your multimeter to OHMS and Connect the leads to the ground and one hot wire(I use alligator clips on my meter leads, but you can just touch the wires). Make note of the reading if you used 16ohm resistors it should be 16ohms +/-.

Now put the mp3 file I uploaded on your player and connect the 3.5mm jack. Set your multimeter to AC voltage(if it does not select the range for you set it to best display 0.01-1 volts) and connect the leads to the hot and ground wire like you did above. Start playing the mp3 file mentioned above and make note of the reading(after it levels off) on the multimeter(ie: The Zune HD had an output of 0.6 volts). After you get a reading stop the player and turn off the multimeter to save the batteries.

Now to the math YAY!

The equation we will be using is as follows:
P = (V^2) / R where P is power output in watts, V is voltage in volts, and R is resistance in ohms. Get your scientific calculator out or a piece of paper and square the voltage and then divide by the resistance. If we use the Zune HD as an example we would square 0.6 volts to get 0.36 then we divide by 16 ohms to get P=0.0225 watts. If you want to convert to milliwatts just multiply by 1000 and we get 22.5 mw.

Now I'm sure you're thinking thats a bit of work just to know what a spec sheet can tell you, but do we really believe everything we read? I hope you enjoyed reading this how to, I enjoyed doing it. I'm sorry I don't have any pictures at the time of posting, I left my camera at work and I know my cellphone won't do this justice. I will submit pictures this weekend. Feel free to ask questions.

On a side note you can use the same process to test the output of any amplifier, you will need a resistor to handle the output of the amplifier(ie don't use a resistor rated at 10watts for a 100watt amplifer).

-Cory
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  #2  
Old 10-15-2009, 11:47 PM
saratoga saratoga is offline
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What was the voltage out of your player without the resistor hooked up?
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Old 10-15-2009, 11:52 PM
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I never tried that because I hate to run an amp with no load but let me check....

Nothing 0.000. I would guess the amp won't output unless there is a load present.

Cory
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  #4  
Old 10-16-2009, 12:21 AM
saratoga saratoga is offline
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Quote:
Originally Posted by oppizzippo View Post
I never tried that because I hate to run an amp with no load but let me check....
Its ok to run an MP3 player without headphones plugged in. You won't damage it.

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Originally Posted by oppizzippo View Post
Nothing 0.000. I would guess the amp won't output unless there is a load present.
Could you try it with a larger load resistance then? Either 1k or the line in jack on a PC would work fine. I'm curious what the open circuit voltage is.
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  #5  
Old 10-16-2009, 08:30 AM
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Quote:
Originally Posted by saratoga View Post
...
Could you try it with a larger load resistance then? Either 1k or the line in jack on a PC would work fine. I'm curious what the open circuit voltage is.
I'll test that tonight.

Cory
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  #6  
Old 10-16-2009, 09:55 PM
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Originally Posted by saratoga View Post
Could you try it with a larger load resistance then? Either 1k or the line in jack on a PC would work fine. I'm curious what the open circuit voltage is.
With my Bose 2.0 Speakers the load was 8.4kohms the voltage was 0.625volts.

Cory
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  #7  
Old 10-21-2009, 02:03 PM
NewGuy NewGuy is offline
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Hmm, interesting. let's go in the opposite direction and calculate the dB level that is delivered to our ears:
if Pmax is my player's maximum output, and r is the headphone's resistance,
Vout=sqrt(Pmax*r)
with my player, Meizu M6, Pmax is 16mW and my phone's resistance is 16 ohm, so in my case, at maximum volume, Vout is roughly 0.5V.

now, according to phone's specs, their sensitivity is 121dB/V, so at max volume 118 dB of sound are delivered (-3dB is half the SPL value) - It doesn't really make sense because it's too damn loud... and at 10/40 volume - which is probably a quarter of the output, you get a whopping 112 dB of sound pressure. This can't be right, right?
anyway, where did I make a mistake?
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Old 10-21-2009, 03:48 PM
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Quote:
Originally Posted by NewGuy View Post
Hmm, interesting. let's go in the opposite direction and calculate the dB level that is delivered to our ears:
if Pmax is my player's maximum output, and r is the headphone's resistance,
Vout=sqrt(Pmax*r)
with my player, Meizu M6, Pmax is 16mW and my phone's resistance is 16 ohm, so in my case, at maximum volume, Vout is roughly 0.5V.

now, according to phone's specs, their sensitivity is 121dB/V, so at max volume 118 dB of sound are delivered (-3dB is half the SPL value) - It doesn't really make sense because it's too damn loud... and at 10/40 volume - which is probably a quarter of the output, you get a whopping 112 dB of sound pressure. This can't be right, right?
anyway, where did I make a mistake?
If the phone's specs say 121dB per volt and your max volt out is half a volt wouldn't max dB be half of 121 dB aka 60db?
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  #9  
Old 10-21-2009, 08:03 PM
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Quote:
Originally Posted by TomCat39 View Post
If the phone's specs say 121dB per volt and your max volt out is half a volt wouldn't max dB be half of 121 dB aka 60db?
That is how I understand it. I could be wrong tho.

Cory
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  #10  
Old 10-21-2009, 08:20 PM
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Originally Posted by oppizzippo View Post
That is how I understand it. I could be wrong tho.

Cory
Hey Cory, you seem to know a bit about electronics. Is there any good tricks to find impedance without an impedance meter?

Can it be done with 2 multimeters? From what I can remember, an impedance meter connects both in parallel and in series to read impedance (current and volt reading?)

I know impedance meters used to be quite expensive so I'm sort of wondering if there is an alternate way to get the measurement, so one doesn't have to purely go by what they read on the spec sheet.
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Old 10-21-2009, 08:22 PM
ninjackn ninjackn is offline
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Quote:
Originally Posted by NewGuy View Post
now, according to phone's specs, their sensitivity is 121dB/V, so at max volume 118 dB of sound are delivered (-3dB is half the SPL value) - It doesn't really make sense because it's too damn loud... and at 10/40 volume - which is probably a quarter of the output, you get a whopping 112 dB of sound pressure. This can't be right, right?
anyway, where did I make a mistake?
Your units don't work out at all!

Your head phones are 121dB_SPL/V. The measured voltage is 0.5V. You converted 0.5V to -3dBV and added them together. dBV isn't compadible with dB_SPL/V.

What the other people said is correct, 121dB_SPL/V *0.5V = 60dB_SPL.
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  #12  
Old 10-21-2009, 08:30 PM
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Quote:
Originally Posted by TomCat39 View Post
Hey Cory, you seem to know a bit about electronics. Is there any good tricks to find impedance without an impedance meter?

Can it be done with 2 multimeters? From what I can remember, an impedance meter connects both in parallel and in series to read impedance (current and volt reading?)

I know impedance meters used to be quite expensive so I'm sort of wondering if there is an alternate way to get the measurement, so one doesn't have to purely go by what they read on the spec sheet.
If you want to calculate impedance like you see on headphone spec sheets ie: Impedance @ 1kHz: 300 (Sennheiser HD650) you just need to play 1kHz and measure the resistance across the output. You will need to make a cable that would allow you to get at the wires while you have your headphones plugged in. I would use a headphone extension cable and just strip off the insulation. You can make a 1Khz tone with Audacity and save it as a Wav(best) or an Ogg or mp3.

EDIT: I should note that headphones/speakers have different Impedance at different frequencies. That is why Impedance is measured at 1kHz as a standard point of reference.

Cory
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Last edited by oppizzippo; 10-21-2009 at 08:45 PM.
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  #13  
Old 10-21-2009, 10:55 PM
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Quote:
Originally Posted by oppizzippo View Post
If you want to calculate impedance like you see on headphone spec sheets ie: Impedance @ 1kHz: 300 (Sennheiser HD650) you just need to play 1kHz and measure the resistance across the output. You will need to make a cable that would allow you to get at the wires while you have your headphones plugged in. I would use a headphone extension cable and just strip off the insulation. You can make a 1Khz tone with Audacity and save it as a Wav(best) or an Ogg or mp3.

EDIT: I should note that headphones/speakers have different Impedance at different frequencies. That is why Impedance is measured at 1kHz as a standard point of reference.

Cory
Ahhhh okay, I didn't clue in about measuring at specific frequencies. I knew impedance changes with frequency due to inductance and AC relationship.

I didn't know that 1Khz was the standard for measuring on audio equipment.

I guess my old teach from many moon ago was discussing impedance meters for other equipment such as say a TV, or appliances etc. So I was thinking in that mind set when contemplating impedance of headphones, which just wouldn't work. Since the operating parameters change constantly when in use.

Thank you, I see now, finding the impedance of headphones is far easier than say an appliance. I remember he said the impedance meter is needed because just using a multimeter to read the impedance with the appliance in operation, changes the whole circuit and gives you an incorrect reading. I guess, with headphones, the change is negligible?
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Old 10-21-2009, 11:37 PM
NewGuy NewGuy is offline
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Originally Posted by ninjackn View Post
Your units don't work out at all!

Your head phones are 121dB_SPL/V. The measured voltage is 0.5V. You converted 0.5V to -3dBV and added them together. dBV isn't compadible with dB_SPL/V.

What the other people said is correct, 121dB_SPL/V *0.5V = 60dB_SPL.
All right. My mistake. However, if so, then at full volume, i'm only exposed to 60dB, which is much lower than the safe level (which is considered 80 dB).
So all the warnings about the hazards in blasting music in full power are bullshit?
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Old 10-21-2009, 11:46 PM
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All right. My mistake. However, if so, then at full volume, i'm only exposed to 60dB, which is much lower than the safe level (which is considered 80 dB).
So all the warnings about the hazards in blasting music in full power are bullshit?
The warnings are generalizations. Each headphone will be different. Try taking a decent pair of Sony earbuds and cranking the volume to max and see if your ears don't ring and possibly hurt when trying to listen to them. Especially with say Metallica or some other high pitched metal.

You headphones on your rig happens to provide more than adequate volume yet safe enough to not be worried of hearing damage. If you want more volume from your headphones, get a headphone amp. But then you may have to be concerned about hearing damage if it pumps them too much.

Also, they say you can get hearing damage without actually feeling pain when listening....
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Old 10-22-2009, 09:29 AM
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Quote:
Originally Posted by TomCat39 View Post
Ahhhh okay, I didn't clue in about measuring at specific frequencies. I knew impedance changes with frequency due to inductance and AC relationship.

I didn't know that 1Khz was the standard for measuring on audio equipment.

I guess my old teach from many moon ago was discussing impedance meters for other equipment such as say a TV, or appliances etc. So I was thinking in that mind set when contemplating impedance of headphones, which just wouldn't work. Since the operating parameters change constantly when in use.

Thank you, I see now, finding the impedance of headphones is far easier than say an appliance. I remember he said the impedance meter is needed because just using a multimeter to read the impedance with the appliance in operation, changes the whole circuit and gives you an incorrect reading. I guess, with headphones, the change is negligible?
Using a standard Multimeter should be close enough for our purposes. I believe you need a special meter when sending a specific frequency is not possible like in audio equipment.

Cory
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Old 10-22-2009, 09:03 PM
saratoga saratoga is offline
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Quote:
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Your units don't work out at all!
dB are always unitless, so hes ok.

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Originally Posted by ninjackn View Post
Your head phones are 121dB_SPL/V. The measured voltage is 0.5V. You converted 0.5V to -3dBV and added them together. dBV isn't compadible with dB_SPL/V.
No he is correct. Well closer to correct. Halving voltage subtracts, rather then divides. However since we're talking about amplitude, it subtracts 6dB rather then 3dB (3dB is for power).

So 121-6 = 115dB.

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What the other people said is correct, 121dB_SPL/V *0.5V = 60dB_SPL.
60dB would have been 1000 times lower volume.
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Old 10-22-2009, 09:06 PM
saratoga saratoga is offline
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now, according to phone's specs, their sensitivity is 121dB/V, so at max volume 118 dB of sound are delivered (-3dB is half the SPL value) - It doesn't really make sense because it's too damn loud...
You always believe the specs? They're often made up.

Quote:
Originally Posted by NewGuy View Post
and at 10/40 volume - which is probably a quarter of the output,
The volume dial is exponential. So 10/40 is probably thousands of times less power then 40/40.

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you get a whopping 112 dB of sound pressure. This can't be right, right?
anyway, where did I make a mistake?
Everyone on this page should look up the wiki entry on dB. Log scales aren't the same as linear scales!
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Old 10-22-2009, 09:11 PM
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Using a standard Multimeter should be close enough for our purposes. I believe you need a special meter when sending a specific frequency is not possible like in audio equipment.

Cory
I did a quick test with my cheep Craftsman multimeter and my super expensive(read nice) Fluke 87 multimeter and found the Craftsman multimeter easier to use because it was less reactive to the changes making it much easier to read. If you bump the headphones it will cause the meter to go crazy. I retested all readings from above and found them to be acceptably accurate.

Cory
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  #20  
Old 10-22-2009, 09:12 PM
saratoga saratoga is offline
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I guess my old teach from many moon ago was discussing impedance meters for other equipment such as say a TV, or appliances etc. So I was thinking in that mind set when contemplating impedance of headphones, which just wouldn't work. Since the operating parameters change constantly when in use.
Well they change as the device does different things, but you can measure how much power a TV uses just the same as headphones. It'll just be different as you change the brightness or whatever.

Quote:
Originally Posted by TomCat39 View Post
Thank you, I see now, finding the impedance of headphones is far easier than say an appliance. I remember he said the impedance meter is needed because just using a multimeter to read the impedance with the appliance in operation, changes the whole circuit and gives you an incorrect reading. I guess, with headphones, the change is negligible?
No thats not true. A multimeter will work fine with an appliance. It doesn't change the working of the system, since in voltage mode the impedance is nearly infinite (draws no current) and in current mode the impedance is nearly zero (drops no voltage).

Of course if you're asking this online I suggest that you ask someone experienced with electronics to supervise you before you try it. AC voltage can be quite dangerous.
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