Dbm, DBu, DBv confusion...

Sorry for this dumb question  :-[
I have a mic pre accepting a maximum of +21 dbm
What it is in Dbu or Dbv ?
Thanks

www.sowter.co.uk/decibels.php

Thanks! but the input impedance of 5k ohm loses me

I am confused by those specs as well. OK, it has an input impedance of 5000 ohms. Typical mic impedance is around 200 , and you want to see at least 10 times that at the input of the mic preamp. 5K is well above that.

I assume the pre is capable of 60db of gain. Specs are unclear.

+21 dBm max on input would be something you would see at line level, not mic level. Mic level signals never get that hot. So I assume the wanted to say “+21dBu on output”.

dBm refers to the mic pre driving a 600 ohm load, which is pretty low. The mic pre will have greater headroom driving a bridging load like 5000 ohms or greater. Lower impedances are harder for preamps to drive that higher (matching vs bridging loads). So 600 ohm loading would be worst case condition for measuring headroom and distortion.

Maybe something was lost in translation on those mic pre specs?

thekid777 said:

Thanks! but the input impedance of 5k ohm loses me

Click to expand...

Me too....Hopefully someone can explain how the impedance does or doesn't affect the conversion from Dbm to DBu.....I think I have a small idea in regards to dBm is related to 1mW in 600 ohms impedance as  Dave P brought up  a bit starting at reply #156 here

https://groupdiy.com/index.php?topic=59681.140


But I'm still frightened.... :-[

AusTex64 said:

I assume the pre is capable of 60db of gain. Specs are unclear.

+21 dBu max on input would be something you would see at line level, not mic level. Mic level signals never get that hot. So I assume the wanted to say “+21dBu on output”.

Click to expand...

Exact! 60 db gain
The input can be switched from mic to line but goes to the same input transformer
Line 1 is mic input, Line 2 is line input

Maybe this is input who accept either line or mic input.
Consumer semipro audio interface? Probably things lost in translation you must be right about that.
60 db symmetry, maybe cmrr related.
5k input z isnt common but it exist on some mic preamp (ive seen focusrite unit which had that option iirc).

Dbm:dbu in value.
Dbm is the old unit used for broadcast and studio in the 50's/60's, it is indeed related to 1mw into 600ohms, some connections was done using power to drive next unit in the chain, this is related to historic phones line iirc. But many units was already using bridging input anyway...
Dbu doesn t have reference because of bridging input (low out Z into high input Z) so no real power involved, you transmit voltage in the chain.

Edit: no consumer interface! Is it a broadcast console?

KrIVIUM2323 said:

Edit: no consumer interface! Is it a broadcast console?

Click to expand...

correct!

This is an extremely old topic and probably covered here before.

0dBm =  1mW, a power specification. The conversions between different impedances is just math.

0dBu is a bit of a bastardized spec because decibels are supposed to only describe power ratios, but dBu is "same as" dBm ASSuming a 600 ohm termination (0dBu = 0.775V which is 1mW at 600 ohms).

In modern audio we deal far more in voltage than POWER so for most concerns dBu can be used in place of dBm interchangeably.

Your +21 dBm is probably easier to use just calling it +21dBu.

dBv is not a real thing, but there is a dBV which is like dBu but 0 dBV=1V  (around a 2dB difference)

Sorry there is lot of pedantry around decibels.  Modern usage has all but forgotten the "power only" constraint .

JR

Ok, that is why data are confusing.
Not a dedicated mic input neither line... in between both.
5k in z may seem low for line in and hi for mic in but in both case this is not a bad trade off, you don t ask too much for the preceeding stage in line in, in mic in some mic may like even better the higher input z than the typical 1,2k or 2,4k.

Hi John, so I should look at my converters output impedance which feed the console then
and be sure it is 600 ohms, make sense!
It seems dBu was invented by Allan Hurst who worked with R.Neve back in the days.
Here's a quote from him:
"I invented the dBu term - measuring volts . 0dBu = 0.775V rms, whatever the impedance.  Previously the industry used 0dBm where 0.775V is 1mW but only if dissipated in 600 ohms. In practice the various impedances in an audio chain  vary enormously, so 1mW is a different voltage depending on the impedance. Confusing. My idea is much easier to interpret."

Why should your converters have 600ohms out z?
They should be able to drive 5k load at +21dbu... which is not a too heavy load.
You are using bridging input.

John Roberts mentionned the 600 ohm termination so I thought it is what the input (connected to my converter) should see?

If we were in the 50's maybe...
Your converters have probably something like 10 to 100r so let s say 50r out z, multiply by 10 minimum for bridging condition,  input is speced at 5k. You are in a bridged connection condition, you transmit voltage, no need for( too much) power.

Hmm, my idea was too be sure what maximum level out of my converter I could put into the console.

21dbu.
The input impedance will define the current (power) you'll need from your converter to drive the input to specified voltage.
U:R×I
Try to calcul this with the value given in the tab at sowter link with R 600ohms and r 5000ohms. There is difference in current needs.

21dbu (rms) is 8,69volts so 12,28v peak
I=u/r
So for 600r input z : 12,28/600= 20,5ma

For 5000r input z: 12,28/5000=2,4ma

thekid777 said:

Hi John, so I should look at my converters output impedance which feed the console then
and be sure it is 600 ohms, make sense!
It seems dBu was invented by Allan Hurst who worked with R.Neve back in the days.
Here's a quote from him:
"I invented the dBu term - measuring volts . 0dBu = 0.775V rms, whatever the impedance.  Previously the industry used 0dBm where 0.775V is 1mW but only if dissipated in 600 ohms. In practice the various impedances in an audio chain  vary enormously, so 1mW is a different voltage depending on the impedance. Confusing. My idea is much easier to interpret."

Click to expand...

The lower-case “u” means “Unterminated.” This is why he says “Whatever the impedance,” because the impedance is irrelevant because our systems are doing voltage transfer,  not power.

Impedance isn’t strictly unimportant, but we are using “bridging” impedances, where the source Z << the load Z. In this set-up, if the load Z is ten times the source Z, things are good.  Remember you get a voltage divide effect from load and source impedance; bridging is meant to minimize that. If you were to match impedances, you’d lose half of your signal (6 dB) in that match.

thekid777 said:

Hmm, my idea was too be sure what maximum level out of my converter I could put into the console.

Click to expand...

The spec says 21 dBm, which we’ll take to mean dBu. And since 0 dBu = 0.775 Vrms, we can do some math and learn that +21 dBu = 8.7 Vrms = 24.6 Vpp.

What sets the maximum output level? The power-supply rails. 25Vpp is about what you can expect from op-amps powered by ±15 V rails.

What is the maximum output without clipping? That depends on input level and gain.  Remember input level times gain is output level, and the output can’t exceed the rails.

And since we’re having the discussion, remember most things are set up for a nominal input level of +4 dBu = 1.23 Vrms, so maximum output is really only a headroom thing. You don’t  normally set your preamp to output full tilt boogie.

Thanks guys! Real great to learn!

JohnRoberts said:

dBv is not a real thing, but there is a dBV which is like dBu but 0 dBV=1V  (around a 2dB difference)

Click to expand...

A little over 2.2dB offset. Show my work? OK, I will.

A) 0.775V over 1V gives a ratio.

B) Log 10 of that ratio is a hair over 1.1, so just double it in you head  for log 20 (voltage) would be a hair over 2.2 dB difference.

So there. ;D

Gene