VRMS & Gain Calculation

[gregorio]

High Gain = 16.5db, for example. I don't know which of the 50 that is.

Yep, that can be an issue and is sometimes cleverly employed to mislead. For example noise floor is usually given in dB but often it’s not clear that it’s dB(A), which is a weighted scale that will appear to be significantly better than an unweighted scale. For example, which has the lowest noise floor, a DAC with a noise floor of -123dB or one with a noise floor of -120dB? It’s impossible to know for sure but if the -123dB is dBA, then probably the other one!

In your case it’s probably fair to assume it’s dBV, although in the case of an amp, it might be dBW.

G

 

[Elegiac]

Yep, that can be an issue and is sometimes cleverly employed to mislead. For example noise floor is usually given in dB but often it’s not clear that it’s dB(A), which is a weighted scale that will appear to be significantly better than an unweighted scale. For example, which has the lowest noise floor, a DAC with a noise floor of -123dB or one with a noise floor of -120dB? It’s impossible to know for sure but if the -123dB is dBA, then probably the other one!

In your case it’s probably fair to assume it’s dBV, although in the case of an amp, it might be dBW.

G

For practical purposes, so I should still be fine working it out like this:

2.1*10^(-0.35/20) ....yeah? I mean, it's dBV spl after all.

Figures give the high gain peak to peak voltage as 37vpp, and that's 13 vrms, which is what... 22 dBV. Which isn't 16.5 db given for high gain

 

[VNandor]

For practical purposes, so I should still be fine working it out like this:

2.1*10^(-0.35/20) ....yeah? I mean, it's dBV spl after all.

Figures give the high gain peak to peak voltage as 37vpp, and that's 13 vrms, which is what... 22 dBV. Which isn't 16.5 db given for high gain

Could you walk us through what you are actually trying to calculate? I don't think many people can make sense of the above. I certainly can't.

 

[Elegiac]

Could you walk us through what you are actually trying to calculate? I don't think many people can make sense of the above. I certainly can't.

Alright, let's make this simple then.
I'm applying this: 2.2*10^(16.5/20)
To this:

Spoiler

 

[VNandor]

Okay, I think this chart makes sense but it took me a bit to recognize what's what in it. Now, what do you want to know?

Alright.
If I input 2.1vrms into a device which then applies a gain of 0.96, or a loss of -0.35 db, to the signal, would I then be outputting about 1.8 vrms?
Trying to nut this out, and this is the closest I feel I've come so far. Any advice would be much appreciated.

This device isn't going to apply a gain of 0.96. What gain setting do you want to use? Your input's level is going to be 2.1Vrms that much is clear to me. Maybe you want to know how much you have to attenuate your 2.1Vrms signal to match the 2Vrms input sensitivity? Or maybe you want to know the level change at the output in terms of voltage if you apply a gain of 0.96 (same as -0.35dB) at the input?

 

[Elegiac]

Okay, I think this chart makes sense but it took me a bit to recognize what's what in it. Now, what do you want to know?


This device isn't going to apply a gain of 0.96. What gain setting do you want to use? Your input's level is going to be 2.1Vrms that much is clear to me. Maybe you want to know how much you have to attenuate your 2.1Vrms signal to match the 2Vrms input sensitivity?

Ah, sorry no no. Past that bit now, I think. Gregorio said that there was no way to know what "16.5 db" for high gain was referring to, it could be dBV or even dBW, whereas I was just treating it as a catch-all figure that I could use to figure out voltage output. And that little equation thing ( 2.2*10^(16.5/20) ), was how I was doing it. So I just assumed that if I put 2.2 vrms in I'd get 14 volts out on high gain.

 

[danadam]

In your case it’s probably fair to assume it’s dBV, although in the case of an amp, it might be dBW.

Isn't gain just a ratio of output to input? In which case "dB" (without any suffix) is the proper unit?

 

[VNandor]

Ah, sorry no no. Past that bit now, I think. Gregorio said that there was no way to know what "16.5 db" for high gain was referring to, it could be dBV or even dBW, whereas I was just treating it as a catch-all figure that I could use to figure out voltage output. And that little equation thing ( 2.2*10^(16.5/20) ), was how I was doing it. So I just assumed that if I put 2.2 vrms in I'd get 14 volts out on high gain.

In this case the gain refers to voltage gain. It isn't really a shorthand for dBV or dBu or dBSPL anything like that. In this case, it simply gives you the ratio between the output and the input voltage in dB. This distinction is important because you can't just plug in 16.5 into a "dBV" calculator and expect to get the correct output level.

Note that if you take the input sensitivity for the given L/M/H gain, apply the gain to it, you get the L/M/H output levels.
So take the low gain for example: The input sensitivity is 25Vrms. Apply the -14.0dB gain. You get 4.99Vrms. The output is 14Vpp which is 4.95Vrms so they are quite close.

You really shouldn't put 2Vrms while using the high gain because according to the specs, if your input level is greater than 2Vrms then your output is going to get clipped.
But, if somehow the signal wouldn't get clipped, using the high gain, the output would be at 14.7Vrms (around 41.6Vpp).

 

[Elegiac]

In this case the gain refers to voltage gain. It isn't really a shorthand for dBV or dBu or dBSPL anything like that. In this case, it simply gives you the ratio between the output and the input voltage in dB. This distinction is important because you can't just plug in 16.5 into a "dBV" calculator and expect to get the correct output level.

Note that if you take the input sensitivity for the given L/M/H gain, apply the gain to it, you get the L/M/H output levels.
So take the low gain for example: The input sensitivity is 25Vrms. Apply the -14.0dB gain. You get 4.99Vrms. The output is 14Vpp which is 4.95Vrms so they are quite close.

You really shouldn't put 2Vrms while using the high gain because according to the specs, if your input level is greater than 2Vrms then your output is going to get clipped.
But, if somehow the signal wouldn't get clipped, using the high gain, the output would be at 14.7Vrms (around 41.6Vpp).

Thank you! That's something to watch out for (clipping) but general consensus seems to be (and my own experience of feeding 2.3 volts into a voltage gain of 15.5) is that you can go a bit over the recommendation and be okay. We'll see.

 

[gregorio]

Isn't gain just a ratio of output to input? In which case "dB" (without any suffix) is the proper unit?

Sort of. As @VNandor stated, we’re not really dealing with dBV because dBV is the ratio to the reference of 0dBV which is 1v, rather than a ratio just between some arbitrary output and input level.

However, if we’re going to do calculations or relate it to a linear scale then we have to broadly know what sort of dB scale we’re dealing with, a root-power dB scale, such as voltage or SPL, or a power scale such as Watts, because a change in power by a factor of 10 equals a 10dB change in level while a change in amplitude (root-power) by a factor of 10 equals a 20dB change in level.

So technically, just writing “dB” is confusing, incorrect and shouldn’t be used, one should really use “dBr” (dB Relative) which is simply a relative difference to something else, which needs to be defined so we know if we’re dealing a power or root-power. In practice we often just use “dB” but it will lead to confusion unless the context is obvious.

G

 

[castleofargh]

The overwhelming majority of amps are voltage amp and the gain is voltage gain. Unless clearly specified otherwise, that the story. For the remaining cases(special designs and no clear specs) you need to take the guys responsible for publishing those specs and force them to walk barefoot on Legos for 4 hours.

 

[gregorio]

The overwhelming majority of amps are voltage amp and the gain is voltage gain.

Agreed, which is why I said it’s fair to assume we’re dealing with dB of the voltage (root-power) variety. Although an important metric for amps is power output and gain, so there is the potential for confusion. Regardless though, they should be forced to walk barefoot on Lego for 4 hours anyway!

G