Yup.
If you are running a passive volume control, it does not matter how neutral the control is (and the Lightspeed is one of the more neutral ones we have seen), the problem is the interaction of the cables, the source impedance and the input impedance of the amplifier interacting with the resistance of the volume control setting.
That's a lot of variables, and what you can draw from it is that the higher the volume setting on the passive, the more the source impedance is what determines the sound. At full volume you get the least interaction- its the source impedance vs the cables at that point.
IOW, as you reduce the volume, **no matter how good the passive is**, you will loose bass and impact. This is why a buffer between the volume control and the interconnect cable is so important; as you reduce the volume setting the bass and impact is not affected. |
Pubul57, it could well be that there could be some advantage to that, if your speakers have higher efficiency and your sources have plenty of voltage. |
Yes, Al, George seems to have missed my point entirely.
If you think about what happens when you install a resistance after a capacitor, then you have a start at what is happening with any passive system. The cap has a variable impedance vs frequency, which changes depending on the time constants in the system that it is part of. At some low frequency the impedance will be seen to increase. This is how capacitors are used to roll off low frequencies in equalizers and how the low frequency poles in active electronics are set up.
When you add to that impedance, you are changing the timing constant. This is simple math. In effect you are increasing the output impedance of the source. When that happens, you get a bass rolloff when that resulting composite source interacts with the input impedance of the amp.
The thing to note here is that none of this has anything to do with the *quality* of the passive control. It can be the best out there (and the Lightspeed is certainly on the short list in that regard) and this will still happen because these effects arise out of simple physical laws **not the quality of the control**. |
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. |
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. |
Having done some audition in this regard (we 'switched', if you will pardon the pun, to our custom Shallco part about 15 years ago as a result of some of these auditions), the difference in contacts that George is describing above is clearly audible.
Its my opinion that the volume control is what shoots many preamps down (especially preamps with remote control) before they can even get off the runway. One of the consequences is that the majority of line stage technologies are in a deplorable state- I don't fault anyone for thinking that a passive might be better. |
Digital volume controls have always been an irritant. As far as I am concerned, they are a failed concept. Given the brilliance of the Lightspeed system, it would seem a natural in a DAC or CDP. |
Clio09, I had the opportunity to compare the Lightspeed against one of our own preamps and I have to say it was the most neutral passive I have heard.
Like any passive I have heard, it had less bass impact than the preamp, which as I have already mentioned is mathematically unavoidable with a passive if you have a coupling capacitor at the output of the source (in this case a DAC). I contest the idea that *all* switches will have issues relative to a light-activated device (my concern would be the linearity of the light activated device...); obviously in practice both can be quite good.
FWIW the Shallco uses gold contacts, with a double-spring-loaded wiper.
However the preamp in this comparison had not only the Shallco switch (custom-built) but a stage of gain, a set of coupling caps and then a direct-coupled vacuum-tube buffer. It was also driving 24 feet of cable, where the passive was driving 3 feet. The two were gain-matched to avoid Fletcher-Munson errors.
The big difference was in the bass as I mentioned. If you go with the idea that the preamp was hampered by its active circuits, then the idea that its volume control is audibly inferior falls apart.
Its my contention that one of the biggest failings of many tube line stages is the coupling cap found at their outputs, so we found a simple way to get rid of it in out designs. Apparently, that is a bigger deal than I had thought. |
Jult52, you have no worries, in fact this is more of an ideal situation (so long as you can get the amp to make satisfying power)- you *want* the amp to have less sensitivity than the maximum level of your source, as this allows you to run the passive volume control at higher levels. The math works more in your favor in this case- you are less likely to loose bass and impact since you will not be running the volume control at a lower level.
When the amp has higher sensitivity, so that your source makes more voltage than you need, then the math works against you; with any passive as you turn the control down more, the more bass and impact is reduced. |
I can think of one: If a remote were built into the product, the actual volume control system (as a module) could plug directly into the input of the amplifier (with separate left and right hand channels) rather than being in a box that needs an interconnect at its output.
In this way, the interconnect would play a lesser role in the resulting sound as it would only be used at the input and not at the output. IOW you would then be able to get around the traditional problem that affect all passives- the inability to drive a cable. In this system the remote would be mandatory, but it would be easy to build balance and mute functions into the system. |
George, I appreciate that you want to keep it simple but you already have that product. If something was created pretty much like you and I have described, it would be better, but it would not be as simple. But that's OK- there would still be a market. |
Clio09 a low output impedance will mean that the preamp can drive a load of less than 1000 ohms without loss of bandwidth, voltage or increase in distortion. You don't need negative feedback to do that. In the old days, tube circuits did that with an output transformer. You can imagine, being an OTL manufacturer, that we use a different technique (which is patented). If your DAC uses coupling caps, they would be fairly close to the output of the unit, but there is no set rule on that. You might ask with the manufacturer. Except still I do not believe anyone has yet built the transparent active preamp, as they all sound so different, more so I believe than the sound of different interconnects. George and I feel very much the same way in this matter, which is to say that many line stages do seem to fall well short of the ideal. But at the same time it is apparent that George has not heard *all* the line stages out there. |
