What am I missing? Pre/power gain and input sensitivity.


I’ve read a few posts about power amps with lower gain needing a pre with additional gain, but no one seems to mention input sensitivity in those conversations. If my source outputs 2V and my power amp input sensitivity is only 1.2V then whether the power amp gain is 16dB or 26dB my pre amp is attenuating the signal and the amount of gain on the pre doesn’t matter at all. With a given set of speakers, to get the same SPL with 2 different amps they should just need to output the same voltage, regardless of how they get there.

Am I missing something?

cat_doorman
See, here’s the thing about the internet: zero editorial standards

Some people need to prove this every day.

Think of gain as a voltage multiplier. 26 dB of amp gain ~ 20x the input voltage.

16 dB ~ 6x the input voltage.

Of course, every device has a fixed output voltage, set by the voltage rails of the power supply. This is one of the amp watt limits.

To drive an amplifier with 16 dB gain to the same level, you must provide 20/6 more volts, or 3.33 x the voltage.

This will require your pre to put out more voltage than normal, and also more noise.

Another way to think about this is that 1 Watt into 8 Ohms is 2.83 Volts.

At 26 dB, the input required is 0.141 Volts to create a 1 watt signal.

At 16 dB the input required is 0.47 Volts to create a 1 watt signal.

Best,
E



Thanks for the confirmation, guys. No matter how much I read and think things are obvious, I start to doubt myself.

With standard digital outputs being so high it almost makes sense to use a lower gain amp so you don't need to attenuate the signal so much (all other things being equal).

I guess what I was missing is how different things are with analog. I didn't fully appreciate just how much gain MC phono stages need just to get some of those low output cartridges amplified to 300mV. So with an analog system of course you would need some gain in the preamp. While at typical listening levels you may not need it, it would be a shame if your system was limited to only what you absolutely needed.

I do know I have a big blind spot when it comes to vinyl. As a kid, once I got a Walkman, I switched to cassettes over 45s on a Fisher Price record player. I started collecting CDs in college and am now streaming more than I ever thought I would. Couldn't bear to listen to a cassette the last time I even bothered connecting my tape deck. My main objection to vinyl is the ticks, pops, and noise. I'm easily distracted. The nicer systems with well cared for records may sound incredible (haven't had the pleasure), but the price of entry to get to that level is quite a barrier (as well as not owning any records).

Right now I'm using a Schiit Saga+ (unity gain) as an inexpensive interim preamp while I explore some different power amp and speaker types. I think as long as I don't try any really inefficient speakers or go for face melting SPL I should be okay. For now. 
So how did we get into this situation, too much voltage from digital players and too much gain in preamps and amps, and how do we get out of it?
@cat_doorman It sounds like we are roughly the same age based on your walkman story.  I am also running all my music digitally, through my Marantz DAC and into a bottlehead tube preamp and then into Bob Latino VS-120, Zu Audio Omen DW speakers.  I have been experimenting with different preamps, a lot, even though my amp has an attenuator / preamp section.  I just don't find that I get the same "richness" out of my music without the additional preamp.  I have also been swapping in different tubes, and that make an enormous difference (12ax7, 12au7, 12at7).  I know you were more interested in gain, but I suggest strongly that you experiment with good tube preamps, if you like that sort of sound as much as I do.  I was using, and just sold a Rogue Audio 66 preamp, and that thing sounded magical with my setup.