Gain of line preamplifier

Czarivey you make a lot of sense. I agree with you, an active preamp with just the right gain to work. Most high gain preamps don't allow you to use the volume control at a higher level. An active preamp keeps the imp. constant is what you want. Yes passive preamps sound ideal on paper, but to me I have never liked one in my system and I have tried a few.

No one type of preamp is going to be ideal for every system. There are many variables to consider such as the type of source being used, gain of the amplification and output voltage or gain of the preamplifier.

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.

I, for one, believe providing such specs is important for the consumer to make better buying decisions.

We always show all the specs on our Musica Bella Audio preamplifiers and have always offered to customize to the personal requirements of any given customer's system.

"Variable output impedance is "heavy disadvantage" of any passive preamp even if you have substantially high input impedance of your amplifier."

Wrong Czarivey, ^you stated^ and the reason is.

A 10kohm passive can have a varying output impedance, at it's highest (worst) it's 2.5kohm at mid position.
Any amp/s that have input impedance 47kohm and especially higher will have absolutely no ill effect on the sound with 47kohm it's close to 1:20 impedance ratio. And if 100kohm input impedance the ratio is then 1:40 ratio. And if the volume is lower or higher than mid point, the ratio/s are even higher, still with out any effect.

The only thing that needs to be watched is interconnect cable capacitance that can cause a HF filter.
The HF filter caused by the interconnect capacitance and this high 2.5kohm output impedance. (BTW: same goes for many tube preamps as well as they can be that high or even higher)

If we look at a bad quality interconnect cable that has 200pf per foot of capacitance, that equates to say 600pf for a meter. This combined with the 2.5kohm of the passive pot will give you a -3db at 106khz! Well beyond our hearing.
All good quality interconnect that I have measured are below 100pf per foot, and this equates to -3db at 212khz! Up in bat hearing territory.

There's just some of the math on this without any voodoo.
I ask once again show the math without the voodoo to counter this if you can?

Cheers George

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

+1 on the importance of system synergy.

In addition to impedance related issues, it is also important to pay attention to system gain as well. You don't want to have too much or too little gain overall. Too little and obviously you aren't hitting the volumes you would like. Too much and you need to do too much attenuation. Not enough fine tuning in your volume control, and attenuator performance is usually at its worst at the low end of the spectrum.

IME, active and passive both have their strengths and weaknesses. Passives can be excellent for detail retrieval, but actives (again, IME) tend to have more dynamics and oomph. To get a preamp that had the detail I was hearing with passives and the dynamics of active linestages, I had to move to a much higher priced preamp. More tradeoffs need to be made the lower the price (though this isn't without exceptions).

But to come full circle: Yes, synergy. And your musical tastes and listening priorities also come into play here too.