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Old 25th October 2002, 04:01 PM   #1
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Default What are the correct output line levels for unbalanced & balanced?

Im working on the analog end of a audio DA board. What are the correct output line levels? I created a CD which had a simple 60 Hz sine wave for measurement with a standard volt meter. The sine wave went from 32767 to 32767 in amplitude.

Here are some measurements I took:
2.14Vac = Harman / Kardon HD200 CDP
1.92Vac = Sony 209ES CDP
0.93Vac = Sound Blaster Live
0.65Vac = (Line out) Portable Panasonic SL-S360 CDP


You can see my confusion here. What should the voltage be for 10dbm unbalanced out and +4dbu unbalanced out?

What should the voltage between the + & - signals be for 10dbm balanced out and +4dbu balanced out?

Brian.
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Old 25th October 2002, 05:18 PM   #2
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Default Correct levels

0dbm is 0.775 volts ( for 1mW into 600 ohms)
-10dbm is =0.316x0.775 = 0.245 volts.
+4dbm is 1.228 volts.

There is no standard output on most machines. I believe that the Red Book spec for CD's is 2 volts. I have never checked this and cannot be sure. I do not see many machines with 2 volts output. Most are between 1 and 2 volts. Tubed outputs may be higher depending on how they are implemented.

If you keep within 1 and 2 volts you would be following the general norm. If you keep the signal high you can avoid a buffer or gain stage before the power amp .
Cheers.
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Old 25th October 2002, 05:59 PM   #3
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Default clarification

I agree with ashok's post, but here is a little more clarification:

First of all, you have your levels backwards. They are usually specified as +4dBm and -10dBu, not the other way 'round as you have them. These are often called "Pro" and "Consumer" levels, respectively.

dBm is a power ratio relative to 1mW, whereas dBu (or dBv as it is sometimes written) is a voltage ratio relative to 0.775Vrms. (There is also dBV, which is a voltage ratio relative to 1Vrms.)

With a 600 Ohm load, 0dBm = 0dBu = 0.775Vrms. dBu is more relevant to consumer equipment, since the load is never as low as 600 Ohms. -10dBu (0.25V) is supposed to be a normal line level for consumer equipment. I'm not sure what the peak level is supposed to be, or if there is a standard, but 2V is probably more than enough. The exact value isn't critical, as long as it is not high enough to overload the preamp or whatever it is feeding.
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Old 25th October 2002, 06:15 PM   #4
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Default Re: What are the correct output line levels for unbalanced & balanced?

Quote:
Originally posted by Brian Guralnick
You can see my confusion here. What should the voltage be for 10dbm unbalanced out and +4dbu unbalanced out?

What should the voltage between the + & - signals be for 10dbm balanced out and +4dbu balanced out?
Forget about all the dBm and dBu stuff. With digital, it's dB Full Scale or dB FS. 0dB FS for a Red Book CD player is 2 volts RMS. 2 volts RMS from hot to ground on an unbalanced output and 2 volts RMS between the + and - lines on a balanced output.

se
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Old 25th October 2002, 07:27 PM   #5
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Default Re: Re: What are the correct output line levels for unbalanced & balanced?

Quote:
Originally posted by Steve Eddy


Forget about all the dBm and dBu stuff. With digital, it's dB Full Scale or dB FS. 0dB FS for a Red Book CD player is 2 volts RMS. 2 volts RMS from hot to ground on an unbalanced output and 2 volts RMS between the + and - lines on a balanced output.

se
OK, I can live with the 2 volt RMS.

Now, what should the ouput impedance be?

For example, in the balanced output reference design for Crystal's CS43122 DAC, they use 100 Ohm resistors in series on the +&- outputs. Is this correct. Is it the same for unbalanced outputs?
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