Oomph or pressure?

And yet there are guys out there who don’t care for the result. “It sucks the life out of the music”, is a commonly heard refrain (really - I’m being serious here!). Maybe they are reacting psychologically to the need to turn the volume control up compared to an active preamp.

Precisely the complaint of this thread. Nelson goes on to say:

I suppose if I had to floor the accelerator to drive 55 mph, maybe I’d think the life was being sucked out of my driving. Then again, maybe I like 55. Nice and safe, good gas mileage…

Is impedance matching an issue? Passive volume controls do have to make a trade-off between input impedance and output impedance. If the input impedance is high, making the input to the volume control easy for the source to drive, then the output impedance is also high, possibly creating difficulty with the input impedance of the power amplifier. And vice versa: If your amplifier prefers low source impedance, then your signal source might have to look at low impedance in the volume control.

This suggests the possibility of using a high quality buffer in conjunction with a volume control. A buffer is still an active circuit using tubes or transistors, but it has no voltage gain – it only interposes itself to make a low impedance into a high impedance, or vice versa.

- essentially pointing out the same problems I did earlier in this thread.

Since all digital sources make way too much voltage to clip any amp made, a buffered control is a good idea, as it isolates the volume control from the output, and the buffer circuit helps to control the interconnect cable. Note that Nelson is careful to point out that this is an active circuit. We build similar circuits for customers that don't need the gain (but using tubes with a direct-coupled output).

is that normal for a passive?

It is a common complaint.

Passive systems, which include transformer volume controls, are a bit too simple to do their job perfectly. That is why you see so many threads about this topic.

If all the parameters surrounding the control are right, it will work pretty good. But getting them right can be a bit of a trick- it depends on the other gear in your system including the cables.

Transformer type controls are different from passive volume controls in that a transformer is used to do the job of volume control- you use different taps to affect the output voltage. The problem is that transformers have to be properly loaded in order to do their job of impedance transformation. The thing is, you're looking for voltage conversion, not so much impedance conversion, and the impedances on either side of the control (source and power amp) are not changing, so its job is made a lot more difficult!

When a transformer is not loaded correctly, it can ring (distort), it may not express its turns ratio correctly and may instead express some of its internal capacitance (which results in non-linear frequency response) and/or it can be rolled off if its load is below what is called 'critical damping'.

For this reason the transformer will need extra switch decks to switch in different value resistors as part of the loading scheme (the source and power amp being the other part of that, and as you might have guessed, its a bit of an uncontrolled variable as amplifier input impedances vary by about 10:1 depending on the amp).

The other part of this is that most people don't realize (including many designers) that the job of an active line stage has four aspects:
1) volume control
2) input switching
3) provide any needed gain
4) control the interconnect cable to minimize its effects on the sound of the system.

It is that last bit that is most widely misunderstood. Most passive systems don't provide any cable control until the volume is turned up (and then the impedance of the source is what is providing the control), which is why you hear a lack of 'oomph' at lower volumes.

IOW, it is quite possible that you can get less coloration with a well designed active line stage.