Comparing preamp gain specs: dBV vs. dB

[RogerPease]

This is probably a little nerdy so I apologize in advance. This might
only be for the engineers in the audience. :-P

Executive summary: How do you compare a dBV spec to dB?

Background: Sometime ago I installed a K&K pickup in my CEO 7 and I
encountered the famous low rumble that can happen. At around 120 Hz
in my case. I quickly discovered the joy of high pass filters. :-) Taking it
further I've been learning a lot lately from reading here about EQ-ing and
impedance matches. Many thanks to all on the forum for that!

[As an aside, I tried the experiment of adding 1 megohm resistor from
signal to ground on a 1/4" TS cable to try for a better match to the
10 megohm inout on my Baggs Venue. It might have reduced the bass
slightly but it might have been my imagination, too. If there was a
difference it was well within what the Venue could compensate for.]

All this triggered me to spreadsheet the published specs for a few
acoustic preamps. Mostly I was curious about what freqs were covered
and the amount of boost or cut available. But a discrepancy on the
preamps' max gain to line out appeared.

The Baggs Venue & Session claim a max gain of +26 dB. The Grace Felix
& Alix claim 21 dB. However, the Fishman Platinum Pro & Stage preamps
list their max output gain as +6 dbV.

Now +6dBV looks like a low number compared to the others. How do you
compare dB and dBV? I read (well, mostly read) the wikipedia entry for
decibel. There has to be reference in order to measure gain. I get that dBV
is gain relative to 1 volt. What are the other gain figures measured relative
to? How do you compare a dBV spec to dB?

Thanks, _Roger

[jonfields45]

I believe dBV are specifically referenced to 1 volt RMS. So with a one volt RMS input, a maximum gain of 6 dBV would yield an output signal of 1*10^(6/20), or 2 volts RMS. Thus a more common usage might be to say -10 dBV meaning 0.316 volts (1*10^(-10/20)) RMS as a way to specify an appropriate input signal, for example.

My most recent experience with a Fishman ToneDEQ was it was not capable of significant signal gain. My mixer has line inputs associated with mic inputs and they have plenty of gain, but to use the ToneDEQ with the line only inputs required all the various ToneDEQ volume controls near max.

I think for your purposes you can consider dB and dBV the same. In the old days of expensive active components, good input/output headroom and low output impedance could not be assumed so referencing to a specific voltage (dBV), or specifying dBu (I think it was dB unloaded) used to make sense.

[sdelsolray]

The main difference between dBV and dB is:

1) dBV (and dBu, dBFS, etc.) each have a different set reference point built in to the designation. Thus, from the spec +12 dBV you can determine the precise voltage of the signal.

2) dB does not have a set reference point. Thus, you cannot determine the precise voltage of the signal from the spec +12 dB...unless you add your own reference point first.

[RogerPease]

Many thanks for your thoughts, gents!

I guess the Fishman Platinum products just don't add a whole lot of gain.
They do have a 10 dB boost so it's possible to get a pretty good bump.

My Baggs Venue claims max 26 dB plus 9 dB of boost for a total of 35 dB.
Keeping it in perspective since it's only one spec out of many important
ones and we all know how malleable manufacturer specs can be... :-)

Again, thanks! _Roger

[agedhorse]

dBV and dBu represent an absolute level relative to either 1V or 1mW into 600 ohms.

dB represents the DIFFERENCE in level

A maximum gain of 20dB is defined as (output = 10x the input) (no absolute level)

A level of 0dBV would be 1 Volt

A level of 0dBu would be 0.775 Volt

A level of +20dBV would be 10 Volt

A level of +20dBu would be 7.75Volt

Note that expressing power in terms of dB uses a different ratio value due to a squared factor in the power equation.