Gain question

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Regnad

Gain question
« on: 26 May 2012, 03:49 pm »
The NC400s sound great driving my very (81dB) inefficient speakers but I need to set the preamp 9dB higher to get the same volume as with my 275 watt tube amps.   I have looked at the gain info for both amps and I hope someone can help me understand the relationship.

Ncore says voltage gain of 25.8dB for full power (400W, 4ohms)
Conrad-Johnson says 0.5v to rated power (275w, 4ohms)

Thanks for any enlightenment!

roscoeiii

Re: Gain question
« Reply #1 on: 26 May 2012, 03:57 pm »
The first spec is your gain in dB (The figure that makes the most sense to me the non-technically inclined).

The second seems to be the amp's sensitivity.

Below is a response to a question that I posted about amp sensitivity & its relation to gain on Audiogon. Almarg who responded is someone I regard as extremely knowledgeable.

From Almarg on Audiogon:

   Roscoeiii: Am I understanding input sensitivity correctly, as follows?: The higher the input sensitivity (say 2V to reach full power, as opposed to 1V to reach full power), the more travel I will have in my preamp volume control. So someone without sufficient fine tuning in volume control or who wants to move out of the 8-10 range and more into the 10-1 o'clock range should try to find an amp with higher input sensitivity.

    Do I have this right?

Almarg: Yes, provided that "full power" is the same in both cases.

But just to confuse the issue further, your statement would be more precise if it referred to a higher input sensitivity NUMBER, rather than to a higher input sensitivity. A higher input sensitivity number, properly speaking, corresponds to lower sensitivity. In other words, a more sensitive amp is one that requires less input voltage to reach full power.

 Roscoeiii:   I am not clear on if input sensitivity can give an idea of the amount of gain in dB (and what other specs might be needed to calculate dB gain from input sensitivity).

Almarg: If gain is not explicitly specified, and it often is not, it can be calculated to a reasonable approximation from the specified sensitivity and the specified maximum power capability.

As you realize, gain is the ratio of output voltage to input voltage, expressed in db.

The ratio of two voltages, V1 and V2, are converted to db based on the formula (20 x logarithm(V1/V2)).

The rated maximum output power into 8 ohms can be converted into voltage based on the equation P = (Vsquared)/R, where P is power in watts, V is volts, and R is resistance in ohms.

So for example in the case of an amplifier rated at 200 watts into 8 ohms and having 2 volt sensitivity, if we represent the output voltage corresponding to the 200 watts into 8 ohms as "Vout" and the 2 volt sensitivity as "Vin," we have:

200 = ((Vout)squared)/8

From which it can be calculated that Vout = 40 volts

Therefore the gain in db is

20 x log(Vout/Vin) = 20 x log(40/2) = 26 db

That is an approximation, as I indicated, in part because it does not reflect margin that may be built into the maximum power specification. But it will generally be a reasonable approximation for most purposes.

Regards,
-- Al

Regnad

Re: Gain question
« Reply #2 on: 27 May 2012, 06:06 pm »
Al, thanks!

This shows that the CJ amp has 36dB gain vs the 26dB of the NC400.   Explains the 9dB preamp boost needed.   Now I want to know the pro/cons of such different gains.

Thanks again.

roscoeiii

Re: Gain question
« Reply #3 on: 27 May 2012, 06:12 pm »
Wow. 36 dB gain is pretty high.

As long as you don' t run out of headroom, there are a number of advantages to lower gain. Namely, lower noise (all things being equal) & preamp control advantages. The preamp advantages are that you get a more fine grained volume control, since each step on the volume control is smaller. You are also able to use a preamp's attenuator in a more ideal range. Attenuators performance is almost always at its worst at the low end of the preamp range (say below 9 o'clock in a standard circular attenuator).

Regnad

Re: Gain question
« Reply #4 on: 28 May 2012, 04:54 pm »
Thanks,  having 81dB speakers probably helps with noise...

roscoeiii

Re: Gain question
« Reply #5 on: 28 May 2012, 05:04 pm »
Yes, 81 dB would explain it.

FYI, I found the below article & accompanying thread on gain structure to be very educational:

http://www.diyaudio.com/forums/diyaudio-com-articles/186018-what-gain-structure.html