How to Measure dBu and dB?

[Phrazemaster]

And for that matter, THD?

I'm getting a pair of B16s and I noticed in the assembly I need to be able to measure these quantities...I'm sorry...I googled these and am having trouble understanding how to measure them.

I have a Fluke 17B didgitial multimeter; I read somewhere that 1dB=.775v but is this a straight conversion?

Anyone know how to do these measurements? I also have a DAW (Sonar) but don't know how to use it for any of this...

Thanks,

Mike

[Phrazemaster]

Surely I'm not the only one confused with this complex sounding measurement?

Would anyone please shed light on now to do these measurements, what equipment do I need? A special multimeter, another gadget? Or just a calculation?

In the spirit of giving and having a forum, please share.

Thanks,

Mike

[DPLight]

dB with no reference value (3rd letter) is only a ratio between 2 levels and is not a level as such. dBu can be easily calculated from a voltage level.

0 dBm is a voltage it take to achieve 1 milliwatt in a 600 ohm resistor. Which is .775 volts. At some point in history the 600 ohm stopped being the standard working impedance, so dBu is basically the same but gets rid of the 600 ohm load thing and is just a referenced to a voltage of 0.775.

dBu = 20LOG(Volts/.775)

I think this is correct but please google dB calculations and verify.

[Phrazemaster]

That gives me a place to start; I appreciate this. I will post back my adventures when I get to this point in the build (kits are on their way as we speak!).

Just my $.02 but it would be great to have this in the setup doc, as it's not a "standard" type of DMM measurement (at least for those not as experienced as I). Not dinging you Tim; you've done a simply marvelous job and it must be exhausting to have to field such amateurish questions all the time.

I'm really pleased with your kits, build quality, and the sound is simply superb on my J99 (the singer I work with has sung in major recording studios and her jaw hit the floor when she heard herself tracked through it!).

Thanks again,

Mike

[PJD3]

Hello Phrazemaster. I just finished a B16 build and am also searching for a way to understand and go about measuring and making good adjustments on the B16. I have access in work to nice Fluke Scopes and meters, and also good waveform generators. I'll be interested in following up on your progress towards a successful adjustment of your B16. I've had great success in building my pre's,(2 C84's and an N72, plus am building the D11, too). I've measured many things over the years, at work and at home but, THD+N sure hasn't been one of them! I'm so looking forward to having my compressor up and running, confident that its been adjusted for peak performance. Good luck and I hope you'll continue to post your progress! Thanks!

Your fellow B16 builder,
Phil Donovan

[Phrazemaster]

Thanks Phil!

My B16s are still in the mail to me! So it will be a week or two before I have these built...

I'll post what I find out; in the meantime maybe Tim could be persuaded to make a little setup doc explaining this (hint, hint!).

I'm sure we will not be the first builders to be posing these kind of obvious q's, and this will actually prevent an avalanche of support emails to him...

[Phrazemaster]

I'm inching my way towards understanding the final B16 setup info.

Here's a couple links that have been helpful:

http://www.soundonsound.com/sos/1994_ar ... ibels.html

http://www.allaboutcircuits.com/vol_3/chpt_1/5.html

I still don't understand what it means to "Adjust the signal
generator output level to -10dBu" as the setup doc says. I believe it will mean to set the sig gen output to a certain AC voltage level, but what that is, is still not clear to me.

Will post further upon further understanding.

[Phrazemaster]

Found an online calculator:

http://www.sengpielaudio.com/calculator-db-volt.htm

Plugging -10dBu into it (step 35 of setup doc), we find this results in a voltage of: 0.245volts (RMS).

That's something I can use! Now to get hold of a meter that measures RMS...

Tim you've given us a maze in the final setup of this compressor, but I'm giving myself an education and getting the cheese.

I'm still on the trail of measuring THD+N; so far have found some software that does it (this is not intuitive tho!):

http://audio.rightmark.org/index_new.shtml

Tim would this work? If you want to throw out any hints, I wouldn't mind at all! What do you use to measure THD+N? Yes, Google is my friend; yes, I've been visiting her often and much.

Mike

Mike

[Phrazemaster]

For when you get *really* serious about your DIY'ing audio gear:

http://www.nti-audio.com/Home/Products/ ... fault.aspx

[PJD3]

Thanks for sending this along phrazemaster, I've been already lokking at all this to make more sense out ot it. Great stuff and I feel inclined to get to know it. Its time!

Thank you,

Phil

[Phrazemaster]

You're most welcome Phil! I'm really a newbie, even though I've done a few kits etc.

That online calculator looks to be the answer for at least getting the right readings on your multimeter. But you need a "true rms" meter from what I'm understanding. I don't have one! I'm looking into them as we speak; I think I'm going to get the Fluke 87V as it seems the best all-around for electronics work.

It's alsop $300+! But you get what you pay for!

Please post any other tidbits you may find, and thanks for sharing. My B16's arrived a couple days ago and are currently cooling off in the box, getting ready for their build with papa. lol.

[tpryan]

This isn't nearly as hard as you're making it out to be. Look for some free audio analysis software for your computer or plugins for your DAW. Exact absolute levels are not important, levels relative to 0 dBFS are fine once you know what that is for your interface. For THD, any FFT plugin should work. Just adjust R7 for the smallest 2nd and 3rd harmonics you can. Or you can rent an HP 8903B, which is what we use for quick bench measurements.

[Phrazemaster]

Thank-you Tim, this helps.

Anything is easy once you know how to do it - I'm sure I am making it more difficult than it needs to be, but I'm new at it all and there is no information already in-place that I have found- I'm thinking and googling it all together. While I have strong math and physics skills, I'm not an EE.

That's why including how to do this correctly should be in the setup doc, IMO (if it's "so easy"

Don't take it wrong please; your kits are superb!!

Thanks for the insights.

Mike

[PJD3]

Yeah, I'm sure to be guilty for making too much of it.
I would like to hear what you think about this. An engineer where I work told me I could look at the output sine wave on my shop Flukescope (we also have many models of scopes) and see the harmonics being generated under the sinewave. Then it would a matter of adjusting the trim pot for the lowest amplitude of second and third harmonics. He said that I could then look at the noise (which may appear as a staticy cloud) and find the "sweet spot" for the best compromise of noise and THD. Does that sound like a feasible approach, too? Thats something thats readily available to me and the boss told me to go for it. I have had trouble getting computer based things working for me! Scaredy cat me.
Thanks for your feedback Tim, Some of us need a little kick in the ass of confidence. I know I do.

Thank you,
PHil

[DPLight]

Lower harmonics would be very hard to see on a standard Oscope display riding on the fundamental sine wave. Low even order harmonics will make a sine wave start to turn into a ramp or triangular waveshape. Low odd order harmonics will make a sine wave start to turn into a square wave. I don't think you'd start to notice a sine wave riding on a fundamental until you get up around the 5th harmonic. An spectrum analyzer app or plug in is the way to go.