Preamp gain vs. poweramp sensitivity

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

Paul, FWIW, I don't believe your amp will see the output impedence of your source. It will see the output impedence of your (passive) line stage. Does the 'passive' line stage you are considering have a constant output impedence, or does the output impedence change with the position of the volume control. Some do, some don't and it can make a difference (but with and input impedence of 180K it might not). It could also affect you choice of IC's and the lengths you could use (short is always good with passives).

Lastly, I observed a lot of folks going to passives with a lot of initial satisfaction over what appears to be a gain in detail, clarity, etc, but concurrently I've seen quite a few change back (including me) feeling that they lost some dynamics with the passive. Try before you buy if you can.

Most passives do vary OI, you're right, with 10K being the top end. Into 180K, I think that'd still be fine.

I have tried a PLS on my system with the previous amps, which have only half the input impedance, and are less sensitive, and it seemed very good to me, including the dynamics.

Active preamps just seem like so much overkill when you've got a strong source and good impedance on both ends.

I agree with the responses about the passive. While your setup is certainly a great textbook match for a passive, some will still prefer an active stage. The good news is you can get a used passive and sell it at no loss if you don't like it. Personally, I've always like one with the right setup. There is usually a Placette or 2 for sale here and they retain their value very well. Give it a try.