Gain of line preamplifier

10-24-14: Czarivey
Manufacturers instead of gain would often provide you input sensitivity and max output voltage. 2 ways can be worked around this problem:
1. Contacting manufacturer about gain info
2. Use following formula to calculate gain:
a) Max_Voltage * 0.707 = RMS Voltage
b) 20Log(RMS Voltage/Input Voltage)

I'm not sure this is correct. For preamps and line stages, when input sensitivity is specified it usually represents (with the volume control set at max) the input voltage required to drive the output to an indicated level, such as 1 volt, that is representative of a realistic operating condition. Not the input voltage required to drive the output to its specified maximum. The specified maximum is the maximum output voltage the preamp or line stage is capable of providing without clipping or gross distortion, and would usually and hopefully be a far higher voltage than would ever occur under normal operating conditions.

You may be thinking of power amps, where the specified input sensitivity usually represents the input voltage required to drive the amp to its rated maximum output power.

Also, I believe that maximum output voltage specs for preamps and line stages are usually expressed in rms terms, not peak, so the 0.707 factor would usually not be applicable.

Regards,
-- Al

P.S: An example of what I am describing would be these specs for the Parasound JC2:

Input Sensitivity:
200 mV for 1 V output
Total Gain: 14 dB
Maximum Output: 8 V

Note that 20log(1 volt/200 mV) = 20log(1/0.2) = 14 dB, corresponding to the indicated gain.

Regards,
-- Al

Ralph, thanks for your characteristically informative, knowledgeable, and detailed input. It all makes perfect sense to me.

One small nit, though:

A typical cable might have 15pf per foot. With a 3 foot cable and a 100K volume control set halfway across the scale, this results in a rolloff (-3db point) at 70KHz, meaning that phase shift artifact is going all the way down to 7KHz.

I think you are assuming that the halfway setting would result in a 50K output impedance. But provided that the output impedance of the source component is small in comparison, and assuming that the two ends of the pot are in shunt rather than in series with the signal source, it seems to me that the impedance seen looking back into the output of the pot would be close to 25K at that setting, and the 3 db point would be around 140 kHz.

10-28-14: Czarivey
Almarg, what year/century is it now?? I thought that today we mostly use direct coupled outputs unless it's a tube preamp(many already use direct-coupling as well).

Note that my comment to which you were responding specifically referred to tube preamps. I think it is safe to say that a substantial majority of tube preamps utilize coupling capacitors at their outputs (Ralph's designs being notable exceptions). And as can be seen in the measurements John Atkinson usually provides in conjunction with reviews in Stereophile, it is not uncommon for the output impedance of tube preamps having capacitively coupled outputs to rise from a few hundred ohms at mid and high frequencies to 3K or 4K or even more at 20 Hz.

10-28-14: Czarivey
Unity gain solid-state preamps such as McCormack, Wyred will kick S to all passives ez and swapping A/B is the best way to hear and know. It's as easy and similar as to swapping Chevy Aveo with BMW Z4 roadster.

***In properly configured system*** I think that a better comparison would be between the Bimmer and a Porsche Boxster or Cayman. I have never taken sides in the never-ending active vs. passive controversies because it seems clear to me, based on what I perceive to be an overwhelming preponderance of the anecdotal evidence, that either approach can work well in a well chosen and properly configured system.

Regards,
-- Al

I'll second George's point that the variation of passive preamp output impedance as a function of volume control setting doesn't matter, as long as the worst case (highest) output impedance at any volume setting is suitable for the application.

I'll add that what can often be an issue with **active** preamps or sources is variation of output impedance as a function of **frequency.** Especially if the design utilizes a coupling capacitor at its output, as most tube preamps do, which commonly results in a large rise in output impedance in the bottom octave. Again, however, as long as the worst case (highest) output impedance at any audibly significant frequency is suitable for the application, that won't matter.

One minor clarification to the example George provided: The 2.5K figure for the impedance seen at the output of a 10K pot set to its mid-point will be increased slightly as a consequence of the output impedance of the component driving the pot. But if that component has a low output impedance (as it should, if it is to be used with a 10K passive), that addition will be essentially negligible.

10-27-14: Response34
We all have our views on passive vs active and this subject will be debated till the end of time but one thing we will never get away from is system synergy and choosing components that work together properly for "our" personal system requirements.

Well said!

Regards,
-- Al