Lightspeed Attenuator - Best Preamp Ever?

George has pointed to exactly what the problem is. In order to produce proper bass, there can be no phase shift above 20Hz. Fletcher-Monson has nothing to do with it. If it were F/M at the heart of this, the same loss of bass phenomena would be heard by lowering the volume on any active line stage, yet that does not seem to happen. The loss of bass is unique to passives.

To eliminate phase shift at 20Hz requires a cutoff frequency of of 2Hz. IOW, the cutoff has to be about 1/10th the frequency to be amplified. So if the cutoff has risen to 5 Hz, effects will be heard at 50Hz- audible on most speakers. This low frequency phase shift is interpreted by the human ear as a loss of bass impact.

Note that the phenomena will not be had if the source has no output coupling cap. In such conditions there will be no low frequency pole and so no loss of bass energy or phase shift. But the vast majority of sources *do* have output coupling caps, and quite frequently, especially in digital gear, the output levels are so high that any amplifier will be driven into clipping by the DAC or CDP. So any passive used in this case will manifest the loss-of-bass problem.

Some sources that have output coupling caps (series caps) and are in the order of 10uf, and are usually bi-polar (2 x 20uf electros back to back), but these sound like cr-p, better to use plastics (polypropylene) but good ones of this size are expensive, and big. All these caps are there to stop dc offset, which if gets through can take out amps and or speakers.
In my view the best source output stages are direct coupled (no cap) this is harder for the designer, as usually they need to also to design a DC servo to keep the dc offset at a minimum 1-5mV, but when done right sounds the best as they are more transparent and have less colourations of a cap coupled output, and have none of the low frequency limitations.
Then there's transformer coupled output stages, don't even get me started on those, with high and low frequency limitations, ringing and well as current limitations.

Cheers George

George, does that mean that with some sources, Ralphs aregument is correct due to the coupling caps?

Ralph, what is your view of the argument that complexity required to add gain in a linestage does more harm than good when gain is not needed?

Wow did you guy's ever loss me... But it gives me something to learn about.

Tony

Pubul57, there are 4 functions of a line stage:

1) add any needed gain- not all sources are able to drive an amplifier to full output.

2) provide for input and volume control- this function is shared by most passive and TVC systems.

3) buffer the volume control from the output- this prevents the load from interacting with the volume control setting, and prevents the control from exercising a tonality.

4) control the interconnect cable- which is done by having a low output impedance which swamps (makes negligible) the capacitive, inductive, resistive and other aspects of the cable.

Of these four, the latter is least understood, even by the industry that makes active line stages. This issue though is so profound that it is arguably the most important beyond actual volume control.

Its been my experience that if the line section controls the interconnect cable, then it has a good chance of outperforming a passive control or TVC. The reason is that the interconnect cable will cease to be an important part of the system sound. I'm pretty sure just about anyone who has set up a system using single-ended cables is aware of how much difference the cables themselves can make.

With any passive volume control, the cables are paramount and must be kept short for best performance. OTOH if the line stage is designed properly then you can run cables of nearly any length and the difference between the most expensive and the least expensive will be hard to hear. This latter fact is one that most cable manufacturers would rather you not know.

IOW if you can hear differences in the cable between the line section/passive and the power amp, then the cable is not being controlled.

A barrier to performance in many preamps, particularly tube preamps, is the output coupling capacitor. It must be made large enough so that phase shift is not evident in the lower frequencies (no loss of bass, IOW) and it has to do this with a transistor amplifier since the manufacturer has no way of knowing what amp the preamp will be connected to. Since transistor amps have a lower input impedance (usually 1/10th that of tube amps) this forces the output coupling cap to be a rather large value.

There is no way you can make large coupling caps sound right- they introduce coloration out of inductance and other well-known artifacts of larger capacitors. IMO, direct-coupling is the way to go. This allows you to bypass a primary concern of most tube preamps. Once this is done, the circuit has only to be merely competent and it will outperform any passive or TVC made.

It comes as no surprise to me that passives and TVCs are as popular as they are. What this tells me is how poorly active line stages are at the functions I outlined above. But just because *some* are bad at it, does not mean that *all* are.