Preamp gain vs. poweramp sensitivity


Hello,

Just purchased an Art Audio Carissa which has input sens. of 600mV (very high). At first, I was thinking that I would want a high-gain pre because the Carissa puts out only 16W/ch and could benefit from some preamp gain.

However, I believe upon considering the sensitivity that a high-gain pre is NOT desired because:

1) I would need to open the VC only a bit before the Carissa was driven to full power.

2) For the above reason, the high-gain is wasted anyway because the pre wouldn't be able to put out what it's capable of without overdriving the Carissa.

Do I have this straight? The relation of gain to voltage is not clear to me. The preamp is specified as outputting 2V yet, with a standard 2V source, that is actually unity gain! So what exactly does "20db of gain" in a preamp mean?

Thanks for clarification on this question.

Paul
paulfolbrecht
Paul
Good question.You can look at my questions I just had with swapping out andf old Denon for a buddy with Maggie 3.6's whose input sensitivity must have ben really hot because now new higher powered Bryston (which meets THX therefore 2MV standars for input sensitivity) requires much more of the volume dial (which I told was in uitself not a bad thing in that pre's will wrork beter if they get turned up and don't just use 7'oclock to say 10 o'clock).But freind I had was freaked that he went from alteral bi-map 150 stereos (thuis a totasl of 225 watts totalk output) to a the Bryston which benches near 300 wpc.But he had to put his Mac 712 pre at 12 o'clock high to get what he got at 9 0'clock from the Denons.Bryston very helpful eplaining unity gin to me and I'd have to know what the Mac and Deono's were set at to understand it.But still it was just a difference that through him.Harmonics ,well everything sounded beter with Bryston and when turned up to 3'clock ones reeas began to bleed.But alwaysd the neuraoitic he says new CD's sopund great but his Blue Note CD's purchased in 80's sound like they were recorded in Rudy Van gelders parents living room (hold they were actaully recorded in his parents living room) and now sound like like crap.Tried to explan to hoim that iuncrease of quality from medum current mid fi to a true audiofile amp would no longer allow his amp to cover up the crap.Now his Magnepans SOUND like superior speakes whereas before I got depressed everytime we played music on his system and it sounded so bad.I thought "Why is everybody saying Magnepans are maybe the best bang for the buck in Hi end unless you needd massive volume or foundation shaking bass".Now I know.
But your question about the 20 db of gain is in intruiging to me as well.
G'luck
chazzbo
what exactly does "20db of gain" in a preamp mean
The pre amplifies the input ~100 times. Assuming this is voltage gain, of course.
As to the 2V in 2V out, remember the pre has a volume control. So it attenuates the output as required.
Check the rated output impedance of the pre & compare with the input of the amp.
Gain and sensitivity are actually 2 ways of talking about the same thing. One can be calculated from the other.

Sensitivity tells you how much input voltage it takes to drive the amp to full power, 600 mV is actually on the low side as many amps take 1.5 (1500 mV) volts or so to drive them to full power.

Gain tells you how much a device multiplies the input voltage. It is expressed either as a ratio of output to input (Vout divided by Vin) or using a log scale of dB.

gain in dB = 20 log (Vout/Vin)

20dB of voltage gain is actually a factor of 10, not 100.

A preamp with 20dB of voltage gain will take a .2 V input signal and multiply it times 10 to make it 2 V if the volume is all the way up.

Your amp takes 600 mV to produce 16 watts.

It takes about 11.3V out of an amp to produce 16W into 8 ohms. (power = voltage squared divided by ohms)

Your amp has a gain of 11.3 divided by .6 = 18.8 or expressed in dB = 20 log 18.8 = 25.5dB

The confusion with the 2V output ratings is because sources and preamps are rated differently. The 2V from a source is the maximum level it can produce. The 2V spec from the preamp was with a specified input level with the volume all the way up and not the maximum that it can produce, which will usually be at least 10V and usually more.

If the source did hit a peak of 2 V and the volume was all the way up with a 20dB (times 10) preamp then it would try to put out 20V. Since your amp only needs .6V for full volume it would clip. For this reason most line level preamps are actually used with their volume controls less than maximum and are actually attenuating (reducing) the level of the signal. It's not as bad as it seems since the average output level of the source is much, mush less than the maximum it is capable of.
Herman,

Thanks so much for that great explanation! Exactly what I was looking for.

I have to ask you now - would you suggest a passive linestage with this setup? My source's output imp. is 300ohm and the amp's is a very high 180K - this seems like a good case for a passive pre to me. Agree?

In other words, I am now concluding that ANY active pre, much less a high-gain one, makes little sense here. Impedance matching would be the only possible reason for an active preamp and that would not be an issue at all, certainly not with a passive pre with a typical 10K output impedance.

What do you think?

Thanks again.

Paul