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Can a receiver sense the impedance of headphones? - Page 2

post #16 of 26
1/31/13 at 6:40pm
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Quote:
Originally Posted by xnor View Post

Yeah I converted it to dB/V for you if you want to use the simpler Vout formula. dB/V does change with different impedances, because you need higher voltage for higher impedance to reach a milliwatt.

 

Ok, i'd ask for the formula, but this is turning into mathematic quicksand, so I'll just say thank you and keep 113 and 98 in my notes. 

Quote:
Originally Posted by xnor View Post

Btw, most receivers have a much higher output impedance than 10 ohms. Could be 100, 120, ... 470 ohms. Maybe even higher?!

 

That’s all I want to know now. Is there any way to figure that out without knowing receiver specs from Denon? I’m starting to finally understand the differences in ohm, voltage, and power, but now I'm extremely curious what the receiver's ohm output is. From a subjective sound quality perspective, the receiver actually kicks ass with headphones. I'm especially liking the Beyers on it. 

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post #17 of 26
1/31/13 at 6:57pm
Quote:
Originally Posted by Strangelove424 View Post

Ok, i'd ask for the formula, but this is turning into mathematic quicksand, so I'll just say thank you and keep 113 and 98 in my notes. 

[dB/V] = [dB/mW] + 10 * log10(1000/R)

 

 

Quote:
That’s all I want to know now. Is there any way to figure that out without knowing receiver specs from Denon? I’m starting to finally understand the differences in ohm, voltage, and power, but now I'm extremely curious what the receiver's ohm output is. From a subjective sound quality perspective, the receiver actually kicks ass with headphones. I'm especially liking the Beyers on it.

You could measure it with a voltmeter. You'll need some kind of adapter though.


Edited by xnor - 1/31/13 at 6:59pm
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post #18 of 26
1/31/13 at 8:24pm
Thread Starter 

I'm thinking of doing a DIY amp, and getting a voltmeter, but that wouldn't be for a while. Maybe I can ask around on the receiver threads to see if anyone has measuring equipment and a similar receiver. 

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post #19 of 26
1/31/13 at 11:53pm
Quote:
Originally Posted by Strangelove424 View Post

I'm thinking of doing a DIY amp, and getting a voltmeter, but that wouldn't be for a while. Maybe I can ask around on the receiver threads to see if anyone has measuring equipment and a similar receiver. 


IMO you should get a multimeter. It doesn't cost much, and can be handy sometimes.

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post #20 of 26
2/1/13 at 1:41pm
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Quote:
Originally Posted by proton007 View Post


IMO you should get a multimeter. It doesn't cost much, and can be handy sometimes.

I checked prices on multimeters and some of them are pretty cheap. Is there a downside to getting a $10 multimeter, are they any less accurate than the expensive ones? Also, do you know what kind of adapter I'd need to test a line out signal? If the meters all work with a +/- circuit, as Xnor mentioned, I think I need some kind of adapter. It could definitely be handy though. If I can get away with a cheap one, I'll probably do it. 

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post #21 of 26
2/1/13 at 2:15pm

adapter = TRS plug with some wire leads soldered on you can clamp the meter leads to. Give them something to separate them so they don't short. That's all. 

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post #22 of 26
2/1/13 at 2:27pm
Quote:
Originally Posted by Strangelove424 View Post

I checked prices on multimeters and some of them are pretty cheap. Is there a downside to getting a $10 multimeter, are they any less accurate than the expensive ones?

 

Some multimeters are very inaccurate accurate when measuring audio frequency AC voltage. Whatever model you buy, it is recommended to test the useful frequency range, if that information is not included in the specifications. Also, the accuracy of measuring the RMS voltage of anything other than sine waves may be poor, but that is perhaps less of a problem than a very limited frequency range.

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post #23 of 26
2/1/13 at 4:28pm

Yeah, just use a 50/60 Hz sine wave. That's what cheap multimeters can measure.

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post #24 of 26
2/1/13 at 4:38pm
Thread Starter 

Will the frequency range limitation be prohibitive when constructing an amp in the future? Can anyone suggest a good budget multimeter? And one last question... aside from shorting the jimmy-rigged TRS plug, is there a risk to damaging the amp by overloading something?

 

Edit: Also, with the TRS adapter, is headphone jack -> dual TRS ok or do I need headphone jack -> mono TRS to do accurate measurements?


Edited by Strangelove424 - 2/1/13 at 4:40pm
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post #25 of 26
2/2/13 at 8:00am

If you want to design your own amp a multimeter won't do. But to do simple checks whether you built an amp correctly a multimeter should be enough.

 

To do the impedance measurement, generate some 50-60 Hz sine wave and play it through the receiver.

 

Measure between GND and L or GND and R using the multimeter in 'V~' or 'AC V' mode (= Vnoload).

Repeat but now with a ~30 ohm resistor as load (= Vload).

 

Zout = (Rload * (Vnoload - Vload)) / Vload

 

example: Zout = (30 * (0.5 - 0.1)) / 0.1 = 120 ohms


Edited by xnor - 2/2/13 at 8:01am
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post #26 of 26
2/2/13 at 3:40pm
Thread Starter 

If I built an amp it would be from a kit, so I'd only be doing simple checks. Amazon seems to have a few (multimeters) with pretty good reviews for cheap. I'll remember those instructions when I test the receiver though. Thanks.

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