Newsgroups: rec.audio.car

>In article <>, kirk@hpcc01.corp.hp.com (Kirk Lindstrom) writes:
>> OOPS!
>> I did it again and forgot power is 10log(P1/P2) while voltage gain and
>> spectrum analyzer displays are 20Log(V1/V2).  Reason is 
>>   Power=Voltage*current=V*I=V^2/R
>
>Right so far. 
>
>> Thus if talking power gain where the formula is dbW=10log(P1/P2) and you
>> want to convert to voltage gain, then db=10log[V1^2/V2^2]=20log[V1/V2].
>
>I'm not sure what you're trying to say here.  Maybe you're saying correct
>things but I'm misreading you.  In which case, please take this as a correction
>for the benefit of the other people who misread you the same way:
>
OK.  I was saving a few steps:
  db=10Log(P1/P2)=10Log[(V1^2/R)/(V2^2/R)]=10*2Log[V1/V2]=20Log(V1/V2)

>In general, gain is gain, and dB are dB.  There is no such concept as
>"converting from dB[power] to dB[voltage]", and no need to express decibels
>as "dBW" or "dBV" as opposed to just "dB". 
>
I don't agree.  Read on and see if you agree with what I write.

>Consider an amplifier driving a load R with a 1-volt RMS waveform.  
>Now suppose we increase the output to 10 volts RMS.  How many dB is that?
>(note, all "log(x)" = log{base 10}(x))
>
>	From volts:  gain = 20*log(10/1) 
>                          = 20*1 = 20 dB
>
>	From watts:  gain = 10*log((10*10/R)/(1*1/R))
>                          = 10*log(100/1)
>                          = 10*2 = 20 dB
>
>In other words, db[power] = db[voltage], provided that the load impedance is
>the same for the two measurements.  
>
For clarity, we (myself and many I work with) usally specify what units
we are working in since db is a relative measurement.  I believe many use
db and dbW (and dbV) interchangealby in/with the MKS system since W=1. 
Correct?

In other words, since db is relative we must state or assume with what
it is relative to.  For example, page 64 of Dec. 92 Stereophyle shows
a McIntosh 1000 Watt power amp.  The meter on the amp has two 
calibrations for a single scale labeled:

         WATTS        .01   .1    1   10   100  1000
                  |----|----|-----|----|----|-----|-|=|
      DECIBELS   -60  -50  -40   -30  -20  -10    0  +6

Here McIntosh is saying that 0db for their amp is full power and has
the potential for +6db peaks.  (I wonder how much this amp costs?)

>In audio work, one usually does not measure amplifier gain by measuring watts 
>in vs. watts out, because the load impedance *won't* be the same for the two 
>measurements.  Audio amps are typically voltage amplifiers, with only the
>final stage providing current to a low-impedance load.  
>
>Now, if you have dB and want to convert back to power ratio or voltage ratio,
>then, yes, you need to apply the proper divisor.  For any gain g expressed in
>decibels:
>
I think this is where my problem lies since the measurement I did at home
was with an output power meter and the ones at work are with spectrum
analizers.  (The problem is, I don't think I have the distortion levels
correct for the spec's I list in the graph mainly since THD is the sum
of all the distortion products.  One thing I did do right is show that
LOTS of power that you don't want is generated when an amp is overdriven.)

>Note that it doesn't matter whether the original dB figure was obtained from
>power or voltage measurements. 
>
As long as E2 and E1 as well as P2 and P1 must be in the same unit system.

>For the example problem, if you look at a spectrum analyzer display and see
>that the 180 Hz output is 6 dB below the 60 Hz, that indeed means that the
>analyzer is measuring voltages with a 2:1 difference... or that, when this 
>signal is passed through a power amp and applied to a speaker, the power of
>the 60 Hz component will be 4x that of the 180 Hz component (assuming that 
>the speaker impedance is the same at both frequencies, which isn't usually
>likely for this particular pair of freqs!  Oh well...). 
>
With HP analyzers, I get REALLY lazy and use the cursor which gives power
(dbm) and voltage (uV) for 50 Ohm systems.  8-)

>	--- Jamie Hanrahan, Kernel Mode Consulting, San Diego CA

Thanks for the info./clarification.  I'm trying to write a FAQ type thing
to explain power and distortion a bit.  It definatly needs more work.
I do believe that dbV or dbW for the the y-axix labels needs to be shown
so that we know that 0db=1V or 1 Watt.

Kirk out
=> "We are what we pretend to be."  -  Kurt Vonnegut Jr.
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| Kirk Lindstrom - OCD Product R & D | Hewlett-Packard Co.  M/S: 91UA |
|   Engineer/Scientist, Hardware     |                                |
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Newsgroups: rec.audio.car