+4/-10dBu operating levels

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James K

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Hi,
I'm pretty new to DIY audio. I'm having trouble understanding the +4/-10 dBu operating levels. Basically, I don't really know what they mean. I know that they lead to a difference in level if they're combined incorrectly but I don't understand how that relates to the circuit. How to I make a circuit operate at either level and how can I install a switch/jumper to change between them?

For example: I want to build this preamp sometime soon http://www.jensen-transformers.com/as/as017.pdf . What would the operating level of this be? What is it determined by and how could I alter it?

As I said, I'm pretty new to building this stuff so any help would be much appreciated.

Cheers
James
 
The simple explanation is that a +4dBu level is approximately 12dB louder than a -10dBv level or about 4x as much voltage gain. So if you want a +4dBu output, it has to have 4x more gain then the circuit that is giving you the -10dBv output.

There is some history re: dBV, dBm, and dBu, and what they are referenced to, but again, in a *practical* sense, this refers to what 0VU means on your machine:


0dBV = 1V
-10dBV = 316mV

0dBu = 775mV
+4dBu = 1.233V

so at -10dBv level, 0VU = .316 volts at +4dBu 0VU = 1.223 volts.

You can go back and forth w/ active circuitry or passive circuitry (ie a 1:4 or 4:1 transformer.) +4 signals tend to be more often balanced (XLR) and referred to as "pro" levels, -10 tend to be unbalanced (typically phone or rca) and referred to as "consumer" level.

Also, e.g. the Jensen circuit has a gain of 10 to 445 in voltage, but all it does is take whatever microphone output you have, e.g. 50mV and gives you 20 to 53dB (10 to 445x voltage) gain, it isn't an issue of -10dBv or +4dBu. That is the operating level of whatever you are connecting to, e.g. your console.

The Jensen website has a good discussion of balanced and unbalanced signals and there are other threads here that discuss different operating levels. But again, don't conflate the two.
 
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Hi,
thanks for the reply. So basically, a +4dBu output just outputs at a louder level than a -10dBu output and a +4dBu input requires a louder input signal? And the values relate to the signal voltage referenced to 0VU?

Also, what is the advantage, if any, of the +4dBu operating level?

Cheers
 
"Louder" was probably not the best way to describe it. The signal is 12dB "hotter" or 4x as much voltage. But yes, a +4dBu input would require a higher level signal, conversely a -10dBv input would not always appreciate the hotter input. Sometimes this doesn't matter depending on how much gain the +4dBu input has, but I've had problems with e.g. a Dolby 362 not accepting a -10dBv nominal output, it just didn't have enough gain to properly calibrate.

To iterate: -10dBv level: 0VU = .316 volts
at +4dBu level: 0VU = 1.223 volts

I suppose the advantage of the higher operating level is theoretically is more headroom and so a better signal to noise ratio, but that is going to depend on other factors as well. Mixing and matching between the two may cause problems, especially if most of your gear is -10dBv, because you are adding a gain stage and/or attenuating stage.

As an aside, I have a Sony DAT deck that has a +4 balanced input, and it goes straight to an attenuator which drops it to -10 and sends it out over a RCA cable to the actual DAT A/D side of the box. (Sony PCM-2500), I'm not using it right now, but plan to just by pass the balanced input and use the RCA.
 
Cheers, I think I understand now. So if I wanted to build, say, an EQ with unity gain to operate at +4dBu, how would I vary the circuit from if it was -10dBu? Would it just involve calibrating the meters to read 0VU at the appropriate voltage? Could I then make it switchable to -10dBu by boosting the input by 12dB and attenuating the output by the same?

Thanks
 
James K said:
Cheers, I think I understand now. So if I wanted to build, say, an EQ with unity gain to operate at +4dBu, how would I vary the circuit from if it was -10dBu? Would it just involve calibrating the meters to read 0VU at the appropriate voltage? Could I then make it switchable to -10dBu by boosting the input by 12dB and attenuating the output by the same?

Thanks
Click to expand...

1. Why do you need the +4dBu level?

2. to answer your specific question you'd need a 4x gain stage at teh front end to bring the level up, and then a similar attenuation stage to reduce by four the output, both can be either active circuits or transformers. Nothing says you can't have it "switchable" depending on your design goals. Are you interfacing w/ -10dBv gear? If so, there really isn't any advantage to operating a specific piece of gear such as the EQ you mention above at the higher level, if everything else is at the lower level.

FYI the Otari MX5050BII series tape recorders have (unbalanced) XLR inputs, switchable between either -10dBv or +4dBu and a lot of better sound cards are software selectable between the two.

But you're not calibrating any meters. In the Otari example above, the unit is selected to operate at one of two levels. 0VU is still 0VU, it is just expecting and giving either a higher or lower voltage at its I/
 
Thanks, I understand that now :)
At the moment all my studio gear operates at -10dBv but in the future I hope to upgrade to stuff that will work at +4dBu. I just wanted to make sure that any gear I build will work at that level.

Cheers
 
Thanks, I understand that now :)
At the moment all my studio gear operates at -10dBv but in the future I hope to upgrade to stuff that will work at +4dBu. I just wanted to make sure that any gear I build will work at that level.

Cheers
 
I have only a few pieces of gear that operates at +4dBu a couple of Dolby units. A lot of gear will be OK at either level, and some switchable like the Otari and some soundcards. FWIW, I am currently running the Otari at +4dBu using the balanced amp on my board to the Otari ins and using an Ebtech transformer on the outs, to a dedicated two track return. I'm only doing that at the moment because of a lack of custom made cables and patch bay. Once I have those I'll probably use it unbalanced at -10dBv unless I use Dolby with it.

There are various line level shifters:

Active: Product: LA-40mkIII | TASCAM

Passive: http://www.ebtechaudio.com/lls-2man.pdf

The manual for hte ebtech also explains some of hte issues w/ impedance if you use passive.
 
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