Please carefully note the difference in units in the two specs listed below (dBm vs dBu).
 
Here are the “specs” for a dbx 160A quoted from their website:

“Independent balanced and unbalance outputs can drive 600 loads to +24dBm simultaneously…”

Here are the “specs” for the Elysia xpressor 500 compressor (an api 500 series model that would fit in my lunchbox) quoted from their website:

“Maximum level:

Output  +21 dBu”

Can somebody comment on the significance of this value?
Is the dBu unit published on the Elysia webpage a typo or am I comparing apples and oranges?

I am researching modestly priced hardware compressors and the difference in output between rackmount and lunchbox units (i.e., api 500 series) was briefly mentioned in a thread on GS but was not fully addressed (at least in a way a non-EE could understand).  I was hoping somebody could offer some clarification and insight.

Dave,
dBm is a measurement of power level relative to 1 milliwatt.
dBu is a measurement of voltage relative to the voltage that would produce 1 milliwatt into a 600 ohm load (=.7746V).

So if we are always expecting a 600 ohm load then dBu is equivalent to dBm. Most often, you will not actually have a 600 ohm load (it will probably be a higher impedance). What that means is, if the specs you quoted are correct, the unit will still be able to output a maximum of +21 dBu, but the output power needed will be considerably less than the maximum the unit is cable of (+24 dBm).

I would guess the reason for the difference in the max dBm and max dBu values given is that they are saying that the output drivers are capable of producing a power level of +24 dBm, but as you increase the input signal level, something else internally will cause the unit to clip when the output reaches +21 dBu and above.

BTW, +21 dBu is plenty of output level to drive any DAW input.

Great question.

I think that some clarification may be helpful. With regards to the audio frequency range

0dBm = 1mw into a 600 ohm load which also = 0.7746vAc

0Bu = 0.776vAc unloaded... in other words it will be measured with a VOM that presents such a high impedance that there is no practical "load".

The "u" replaced the "m" because the 600ohm standard was abandoned as a standard. 600ohm connections still exists but there are also lots of other impedance levels that are used in audio connections.

A device that can produce 24dBm may be more robust than one that can only make 24dBu when there is no load.

Old time, actual, audio engineers will specify the dBm capability as a way of specifying the device's ability to produce adequate signal while connected to what, now a days, is considered a relatively high load at 600ohms.

Some gear that specs at 24dBu may only be able to hit that mark when it see less load. 10kOhm for example. That gear may, perhaps, only be able to achieve 22dBu when it is loaded by 600ohm.

The dBu spec lets the spec writers gloss over a few details which may make the spec seem more appropriate than it is.



That's off the top of my head... I hope I got it right.  ;-)


best regards,
mike
 
edit typo 1dBm has been corrected to say 0dBm

Back on the forum this morning and looked at this thread again. Just realized I misread the OP -- he was talking about 2 different compressors. Don't know how I missed that. So, the Elysia unit, which only quotes its output in dBu (+21 dBu), is not saying anything about the power handling capability of its output drivers based on the specs listed by the OP (although maybe Elysia might have other specs that state max load on output).

One refinement to what Mike said above "1dBm = 1mw into a 600 ohm load which also = 0.7746vAC" -- There is nothing about the unit dBm that includes or requires a 600 ohm load. However, here must be some load for power to be delivered-to in order to get a measureable amount of power. If the output is measured with it unconnected to anything else, then the output power in dBm would be -infinity, whereas the output in dBu would still presumably be 24 dBu max. In the case of the dbx 160A specs they did say that their 24 dBm spec was with a 600 ohm load and this is important information when specifying output power handling capability (ie it probably coudn't drive an 8 ohm load nor would you ever want to hook it up to an 8 ohm load). Seems like I remember dbx compressors being popular in the broadcast industry and it could be that input impedances to some of their gear may be as low as 600 ohm or 1Kohm, etc, so the dBm spec on the output of the compressor would be more important in that application.

I think it is correct to think that dBm does not have to be defined by a 600ohm load.

For example in radio engineering a dBm is referenced to another impedance. (you'll have to look number that up)

In Audio interconnections a dBm is almost universally referenced to a 600ohm load. That is a legacy from the telephone industry where it is a published standard. The term "line level" is taken from the telephone industry and used to refer to the telephone lines.

The need to define current capability was necessary because telephone lines can be many miles long.

As audio interconnection became more common for small facilities like studios there was a gradual shift away from the 600ohm standard because the higher impedance inputs and impedance bridges of 1:10 etc provide a wider range of useful frequency response. The new unit dBu was introduced after this transition to higher impedance inputs but the legacy is referenced through the fact that 0dBu = 0.7746vAC which coincidentally = 1mw into a 600 ohm load which also = 0.7746vAC = 0dBm. This made it easier for the new gear to be integrated with the old gear as the transition away from the 600ohm line level standard took place.

Now a days the term "line level" has been made more ambiguous. 40 years ago it was an defacto standard and it was a published standard.

The term dBu is not associated with a 600ohm load but it does pay homage to the legacy of the 600ohm standard as 0dBu and 0dbM are equal the same 0.7746vAC voltage.

I have to get back to work... most of this stuff should be easy to look up.

Gotta go.

best regards,
mike
 
 
edit typo 1dBm has been corrected to read 0dBm

 

Here's some follow up... I found a interesting quote on the web:

from: http://recording.org/diy-...mpedance-question.html


Bill Whitlock wrote:

"Thought you might be interested to know a little history behind the venerable "600 ohms". The first telephone lines that linked cities miles apart were actually existing telegraph lines. The wise engineers at Bell knew transmission line theory and realized that, even at audio frequencies, the lines were long enough to be true transmission lines (a pair of conductors behave as a transmission line when their physical length becomes more than about 1/10 wavelength at the highest frequency of interest). Therefore, they needed to know the characteristic impedance of the existing telegraph line pairs. They were typically #6 or #8 AWG wires spaced about 1 foot apart. If you do the calculations, an average value is about 600 ohms! To eliminate reflections (echoes), any transmission line must be driven from and loaded by a resistance equal to its characteristic impedance. So all telephone filters, transformers, etc. were designed for a 600 ohm system. And this hardware found its way into the first radio stations, and later into the first recording studios. Today, it is rarely necessary (or desirable) to "terminate" an audio cable unless it is a vintage passive filter or tube gear with transformers. Incidentally, audio cables begin to exhibit slight transmission line effects only when they are over about 4,000 feet long!"


best regards,
mike