Input sensitivity question

[jacojbass]

ok, I am sorry if this has been asked, but I need to clarify something. My power amp say 1.4 vrms for full power at 8 ohms, is the vrms for full power at 4 ohms different? My Manual does not specify.

[Downunderwonder]

I don't think the input "knows" what the output is doing. It just wants 1.4V

[Calaverasgrande]

people overthink this input sensitivity thing. If your preamp is a solid unit with a decent line out that is all that matters. It gets much more iffy when using DJ gear into power amps, as those are all over the map for output level. I used to bring line level pads with me when renting gear to DJ's. As long as your amp/preamp has a balanced output (XLR or 3 conductor phone plug) you should be cool.

[jacojbass]

I appreciate the input, but none of this answer my question...

[billfitzmaurice]

Originally Posted by jacojbass

My power amp say 1.4 vrms for full power at 8 ohms, is the vrms for full power at 4 ohms different?

No.

My Manual does not specify.

Because the input sensitivity and load impedance are not related. It's like the engine in your car. Does its horsepower rating vary with the number of passengers?

[Bob Lee (QSC)]

I agree with Calaverasgrande that people overthink "input sensitivity" specs.

The gain doesn't change with a 4Ω load versus 8Ω, but the amp probably clips at a somewhat lower output voltage. Therefore, the "input sensitivity" spec would be that much lower. Does it matter? Really, no. An answer to the question won't even be useful if you don't first understand "input sensitivity."

The amp doesn't "want" 1.4V. That's just what it takes at maximum gain to put out "rated power" into an 8Ω load. Let's say into an 8Ω load the amp clips at 56V RMS (i.e., 392 W into 8Ω) and its maximum gain is 40×. Divide 56 V by 40, and that's the "input sensitivity": 1.4V. Let's say that same amp clips at 50V RMS into a 4Ω load (i.e., 625 W into 4Ω). 50V/40 = 1.25 V; that would be the "input sensitivity" with a 4Ω load. Keep in mind that "input sensitivity" specs only apply to full gain, and if you're always running your amp at full gain unnecessarily, you're leaving a sizeable chunk of signal-to-noise ratio on the table.

Unless your preamp is really weak, it can probably put out a lot more than the power amp's "input sensitivity" spec, so don't worry about it. It's not as useful as knowing the gain.

[jacojbass]

thanks you Bill, and Bob. That is what I was looking for. For what it is worth my preamp put's out 0db and drives it, but i am looking into getting a mod to boost the preamp to the 1.4 vrms output. This is why I asked ion the first place.

[Bob Lee (QSC)]

Do you know what preamp you have?

[jacojbass]

sans amp paradriver

[Bob Lee (QSC)]

The Para Driver DI owner's manual is vague about its output capability. "0 dB" isn't a signal level; "0 dBu" or "0 dBV" or something like that would be, and it also doesn't say if that's a nominal level or a maximum.

I'm guessing that with a 9V DC supply, the circuitry could potentially be capable of putting out up to about 2.8V RMS, or +11 dBu—or about 5.6V RMS or +17 dBu if the balanced output uses a symmetrical drive circuit.

I think you should call Lloyd at Tech21 and find out what the Para Driver DI can do, unmodded or modded.

[jacojbass]

Sweet Thanks again Bob.