The future of preamps

12-13-13: Audiolabyrinth
@ Atmasphere, Hi, your name would not happen to be Ralph would it?, Are you in relation to the atmas-phere audio company?

LOL! Yes, Keith, meet Ralph Karsten, Founder and Owner of Atma-Sphere music systems.

Audiolabyrinth, Krell states that the gain of their amps is only about 25db. FWIW that is quite unusual (I had assumed their gain to be a more typical 30db), so in a high power amplifier you would indeed need more than 4 volts to full output. The spec on your amp is 3.58 volts for full output, so your amp probably makes right around 500 watts, assuming a similar gain structure.

The industry standard is 1 volt FWIW; that standard is really eroded at this point. However, most audiophiles do not push their amps to full output all the time- you still have to knock down the signal from a digital source if you want to use it. So a preamp is still useful.

The specs indicated in the manual for the Krell 700cx are 3.58 volt sensitivity (for its balanced input), 26.4 db gain, 700/1400/2800 watts into 8/4/2 ohms respectively.

26.4 db is a voltage gain of 20.89, based on db = 20log(Vout/Vin).
20.89 x 3.58 = 74.79 volts.
(74.79 squared)/8 = 699 watts into 8 ohms, corresponding almost exactly to the 700 watt rating.
Double that into 4 ohms (= 1398 watts), and double it again into 2 ohms (= 2796 watts). The results again correspond almost exactly to the rated power capabilities.

Ralph, I think you were right in the first place!

Concerning Keith's reference to "3.58 volts min," that is probably the result of a miscommunication. Sensitivity specs are usually neither min nor max, they are simply the (approximate) input voltage required to drive an amp to its rated maximum output power. And based on these calculations, that certainly appears to be the case here.

Regards,
-- Al

An additional point: The 700cx is spec'd as being capable of putting out a maximum output voltage of 84 volts RMS.

Based on the amp's voltage gain of 20.89x (see my previous post), that output voltage would be produced by an input voltage of:

84/20.89 = 4.02 volts.

Therefore if someone indicated to Keith that the amp can handle an input of 18 volts without clipping either what that person said was misunderstood, or that person misunderstood the question, or that person didn't know what he was talking about.

Regards,
-- Al

Keith- Meet Al, our resident technical analyst and voice of reason on all things electrical/quantitative. He's not a manufacturer or designer. He's kind of like our Spock but with a love of music ,a keen dispassionate "eye" for techno-babble, and a willingness to help all of use non-EE types understand some of the basic science, math, and engineering behind things audio.