Gain of line preamplifier

"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.

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.

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). It's pefectly legit for me to say that

Most of good preamps are direct coupled and than compare:

1: If using a low (<100ohms) source impedance (which most are).

2: Into a 10kohm passive pot with 1mt of low capacitance interconnect (which most good ones are).

3: Into a power amp with 47kohm or higher input impedance (which most are)

Did you ever check input impedance of Pass poweramps?
Do you think Pass poweramps are good or not?

Are you sure that 10kOhm passive volume control will be able to mute??

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.

Numerically 30...300Ohm output impedance which is substantially less variable vs. passive, but for some reason haven't been presented.

A common problem with passive controls (IOW variable output impedance) is a coloration at settings that are less than full output.

This can be reduced in some situations by using a control that is itself a lower value, like 10K.

You can't count on all sources having an output impedance of less than 100 ohms. Many phono preamps and vacuum tube CD players have output impedances that are higher than that. This can often limit the utility of lower value passive controls. The trick is really in the setup, if you get it right you can get excellent results.

IOW its not a guarantee that the passive will work in all situations- you have to be careful. In the case of controls with higher resistance values, the source impedance that results when the control is set to lower values is really a function of the interconnect cable preceding the control, the source preceding the control and of course the control itself.

This is of course true of a volume control installed inside a preamplifier, the difference is that there is literally no interconnect cable. This is important as the interconnect cables have some small capacitance per foot (which becomes a larger factor when higher impedances are involved); this is a non-issue in a dedicated preamp where such capacitances can be 1/100th or less as seen in an interconnect.

The high frequency rolloff thus produced is likely not in the audio passband unless the amplifier has a very high input impedance. The control's series resistance can mess with the Miller Effect of the input gain stage, independently of the rolloffs generated by the cables involved. Generally speaking, this will be more likely somewhere in the middle to upper 3rd of the control range where higher series resistance of the control combined with a high resistance to ground begins to interact with the input capacitance of the input gain stage of the amplifier.

A high frequency rolloff has phase shift effects manifesting as low as 1/10th the cutoff frequency according to the engineering rule of thumb. The cutoff frequency (-3db point) is defined as:

f= 1/ RxCx2Pi where R is resistance in Ohms, C is capacitance in Farads. To get a more meaningful calculation, it is useful to express C in microfarads, thus the resulting quotient 1,000,000 instead, the resulting f frequency is thus in Hz.

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.

At the points of mechanical contact (audio connectors) there is usually some sort of primitive diode issue to overcome due to slightly dissimilar metals used in the connectors. The higher the impedance driving such contacts the more audible their effects become. This will be expressed as an intermodulation, and is not present when the connections are hardwired.

This is why the choice of cable is so important when dealing with a passive control!

Now some of you may have noticed that there is a more complex situation then just the simple example I gave above! In addition to that particular rolloff, we have the interaction of the output coupling cap with the volume control, interconnect cables and amplifier input all in parallel. From the output coupling cap point of view, this is likely to be a negligible value, but from the amplifier input its another story.

The input capacitance of an amplifier is in parallel with the input, which is to say it acts as a rolloff factor. An input capacitance of 25pf is not uncommon and can be considerably higher in solid state amplifiers. This value is operating independently of the Miller Effect of the input stage (which also contributes to rolloff). The same formula applies. If you did the math, you will see that the rolloff at the output of the passive control is lower (if set at the same place in the prior example), in this case about 45KHz. Now artifacts are occurring down to 4.5KHz, on top of those going to 7KHz.

This makes things tricky. Obviously keeping the cable capacitance down is important- if the cables were 6 feet instead of 3 feet, the cutoff frequency would be cut in half, putting the phase shift artifacts an octave lower!

If the control is a higher value, the cutoff frequency is the example above is reduced and can actually be **inside** the audio passband! You can easily see that higher value controls are going to introduce a coloration no matter how good the parts are in the control, as the coloration is a phase shift introduced by the math and is incontrovertible.

Practically speaking, this limits all passive controls to no more than 100Kohms unless fidelity as defined in the traditional sense is not a goal.

Now it is a fact that phase shift in a simple signal like a sine wave is inaudible. This is not true if the phase shift covers a band of frequencies- the wider the band, the easier it is to hear. The human ear/brain system uses phase to establish where a sound is coming from, i.e. in contributes to soundstage perception. In addition, phase shift is interpreted by the ear as tonality- a simple 6db/octave rolloff at 50Khz will be heard as a darkness.

Some years back I had this demonstrated to me in spades, where I was trying to find out (for one of my dealers) why an old MFA preamp was so bright in the phono section. It turned out that there was a step in the EQ curve at 50KHz, resulting in a 6db per octave error in a frequency band that is often thought to be inaudible. But phase shift being what it is, the ear was easily able to detect the problem even though our hearing does not go that high!