'Flat earth' usually has something to do with a lack of math coupled with ignorance. Here are the issues of what a passive has to face:
With any passive volume control the control element is a series element with the source. The more you have to turn the volume down, the greater the source/volume control impedance becomes.
Now most digital products make about 4 volts output, so you do have to knock the signal down in order to use it.
So if the source impedance is 1K, to listen to something you might have the control turned down so only 200mV is coming out of the source. With an amplifier with 25 db of gain (most amps have 30) this will result in about a watt and a half from the amp (nice for low level listening). But this might mean also that the source impedance is now well over 10 or even 50 times higher, depending on the control!
You can easily see here that the cable between the control and the amp has suddenly gotten really critical.
One way to get around this problem is to keep the value of the PVC fairly low- 10K is about the limit though, and a lot of sources will be unhappy with that, as the higher the volume setting, the more the control is in parallel with the input of the amp. If the amp also has a 10K input impedance your source might be asked to drive 5K at full volume. This could kill the bass in many systems.
This problem is solved in an active preamp because the volume control is buffered from the load of the interconnect cable and the power amplifier input by some sort of active circuit.
A passive of course has no such circuit, so now the increased output impedance of the volume control and source exacts a price upon the sound, and this will be that the capacitance of the cable is able to roll off high frequencies at a lower frequency (introducing phase shift, which messes with the soundstage presentation), and if the source has an output coupling cap, we will see a low frequency loss as well.
(Resistors and caps are often the elements of filters. To roll off highs a resistance is often followed by a cap to ground; the example of the PVC turned down above is an example. If you want to roll off bass, put a resistance in series with a coupling cap.)
Note here that *it does not matter the quality* of the control! What matters far more is the *value* of the control. This is not a quality issue so much as a math issue. Quality is something you can mess with but the math is pretty hard to get around, sort of like trying to break the laws of physics.
So when you have a passive control you have to be very careful- for example if the amplifier input impedance is high you will have greater high frequency rolloff due to the cable. If the amplifier input impedance is low (10K for example) bass will be a problem at all positions of the control except full volume.
The only real solution is to place the passive control at the input of the amp. This could be really inconvenient (especially with monoblocks), unless the setup was also remote-controlled. Otherwise the use of a passive is always going to be a compromise. Anyone who has a successful setup will probably also have had to pay special attention to the interconnect cable at the very least, unless they had a lot of good luck, good advice or both. One thing is certain, PVCs are not a universal panacea!
As I pointed out in a post above, one of the functions of a preamp is to control the interconnect cable. This is done by having a very low output impedance. When you *raise* the output impedance, as all PVCs must do, instead of controlling the cable, you exacerbate its effects due to the math involved. |
The volume control in the MP-1 is a custom-built switch with gold contacts and dual wipers to insure contact performance. It has 4 decks so we can control each phase (inverted or non-inverted) as well as both channels. |
A preamp has four functions:
1) control gain
2) provide any needed gain
3) provide input switching
4) control the interconnect cable
Of these functions, most digital sources provide one, which is to control gain. Unfortunately, Redbook specs call for the digital device to put out 4 volts, which is far too much for any power amplifier, so right away the signal has to get knocked down (and on the way, degraded) in order to be useful.
The problem is that many volume controls degrade the sound, and if they are not buffered (as in passive controls) the math often works against you, causing loss of bass if the control is anything less than full on, and often a lack of dynamic punch. This is caused by the resulting output impedance of the control in series with the source being so high that the interconnect becomes of sort of tone control that is interjected.
Now you could throw a lot of money at that problem by looking for just the right cable and many audiophiles do. But the simple fact is that the passive control is a big compromise, not based on the quality of the control so much as the approach: the control and interconnect effectively interact with the source impedance to behave as a filter.
One way around this is to have a PVC with a lower resistance value overall, but this can be hard for some sources to drive.
A buffered volume control does not have this problem. You will find that most preamps are far less susceptible to the cable than passive controls are as a result.
FWIW, the industry would be better off if the output of digital devices was limited to about 2 volts. One really does have to wonder what the authors of the Redbook were thinking!
Due to the variability of competence in line sections of preamps, you will find that the issue of passive (and TVC) volume controls VS active line stages will be an on-going debate. There are line stages that can so completely control the interconnect cable that you can no longer hear the differences between them. IMO/IME this is when you really start getting to transparency, as the filter I mentioned earlier is removed. |
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The loss of transparency introduced by the preamp needs to be counterbalanced by other sonic benefits. Actually if the preamp is working right, most people experience an increase in transparency, not a loss. I have already mentioned one reason why; an additional one is that many digital controls don't work that well. So the digital device works better if run at full volume, run through an active line section and the preamp's volume control is used instead. |
Wadia is still very much around. Owned by the same company that owns ARC. They are a good example of why an analog preamp is handy- their volume control is an example of what I was talking about earlier.
Audiolabyrinth, I can tell you with great confidence that you have your numbers wrong. Krell amps can be driven to full output by any preamp made, so they have at least 30 db of gain. That means that they need less than 2 volts. I bet though that their *preamps* can make 18 volts (we can make 32 volts with ours) before clipping; this is done to keep the preamp totally unstressed while doing its job.
Mapman, the reason not to own a digital preamp is simply that you will have to sell the unit to upgrade. So far all digital products have had a short shelf life as digital continues to evolve. Analog is a bit more stable design-wise.
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Audiolabyrinth, Krell states that the gain of their amps is only about 25db. FWIW that is quite unusual (I had assumed their gain to be a more typical 30db), so in a high power amplifier you would indeed need more than 4 volts to full output. The spec on your amp is 3.58 volts for full output, so your amp probably makes right around 500 watts, assuming a similar gain structure.
The industry standard is 1 volt FWIW; that standard is really eroded at this point. However, most audiophiles do not push their amps to full output all the time- you still have to knock down the signal from a digital source if you want to use it. So a preamp is still useful. |
another situation is that I cannot use typical tube pre-amps or sources direct to amp, or I would have to activate the coupling capacitors that are not active inside the amp to protect is from tube unstableness! Audiolabyrinth, best I can make out, this is some form of myth that solid state people seem to transmit. Its not actually true. In the case of our preamp, its output is direct-coupled and servo-controlled. We've had people using it with all manner of solid state amps over the last 20 years; doesn't seem to be a problem. FWIW our MP-1 was the first balanced line preamp made. It can drive the Krell just fine. |
I'm a bit confounded why it would be implicitly, and quite arrogantly assumed I'm not using my ears when stating the above; I most certainly base my findings on actual listening impressions Phusis, I would guess that the reason is you have had a spat of bad luck, as your experience is one of the minority. A lot does depend on the preamp though and they are not all created equal! Some simply act as filters in the system. If you have been working with such preamps I would not doubt that you reasonably arrived at your conclusion. |
Audiolabyrinth, despite what you quoted in the manual, that idea that somehow tube preamps have DC at their output is a myth! Most tube preamps have a coupling capacitor at their output and so cannot produce DC.
I suspect that is something concocted to sell solid state preamps.
Any tube preamp manufacturer has to contend with the fact that their preamp is going to get used with a solid state amp at some point. There are many threads on this forum to that effect. Many transistor amps have direct-coupled inputs, so any designer knows that if you have DC there it will cause problems for the amp and possibly the speaker too. But fortunately its not something that is a particular concern, as coupling capacitors are very effective and will prevent DC from being a problem at the output of the preamp.
The only thing you have to be careful about (which is true of solid state preamps as well!), is to turn the preamp on first and wait for it to stabilize before turning on the amp. |
Audiolabyrinth, I think I need to clarify that. I think there are maybe 3-4 tube preamps with direct-coupled outputs (Berning made a hybrid called the TF-10 years ago, there was the Messenger and we make two of them. Pretty sure Krell was not talking about us... I think we would have heard about that.
Anyway, the rest of the tube preamps made have either an output coupling cap or they have an output transformer. Either way they won't be making any DC! That is actually a greater danger with a solid state preamp, although most of those have output coupling caps too. |
Audiolabyrinth, it uses a servo that corrects the DC. The servo also lets you know if there is a problem it can't correct. Its simple, and very reliable.
The actual circuit does not want to make large amounts of DC in the first place so the servo only has to make small changes. We have a patent on how this is done. There are several advantages- part of the reliability comes from the fact that the circuit can't put out large amounts of DC even if it were damaged, so it does not need a protection relay or complicated housekeeping circuits to make it work. Its reliability comes directly from its simplicity. |
if his source has volume control" it will be the most transparent, dynamic, and least coloured way of getting the sound of the CDP/DAC to the poweramps.
Georgelofi, this is not really true, unless that source also has the ability to control the cable. If it does not a preamp may well be more transparent, more dynamic, wider bandwidth, etc. despite being an extra gain stage. As you well know, many sources have compromised volume controls where the compromise allows for remote control. In fact this very reason is why many manufacturers do quite well selling line stages. You install the preamp, turn the volume control on the source all the way up, and use the volume control in the preamp *because it sounds better*: more bandwidth, resolution, transparency, etc. Now a manufacturer of CDPs and the like could fix this problem but so far very few of them even recognize that its even possible to control the cable, let alone how to do it, so for the foreseeable future there will be a market for active line stages for precisely the kind of product you mentioned. |
You put a White Paper out on this Atmasphere and you'll be shot down in flames. Mathematically how is the Nuforce not going to control the interconnects to the Belles?? You flat earthers are just too much!!! Actually an article on this subject sounds like a good idea as the math seems to be poorly understood, apparently by even members of the industry; thanks for the suggestion. The rule of thumb here is: if you can hear differences between interconnect cables then the source is not controlling the cable. This has been understood for the last 60 years. The history of balanced line operation goes back to the phone company, and its success was immediately taken up by the recording and broadcast industry and resulted in what was called 'hifi'. Some folks here might recall a series of letters that Stereophile published back in the 90s in their letters to the editor column; these letters were written by several different audio engineers. In the letters, these engineers stated their astonishment at how audiophiles gave credence to the idea that cables make a difference in the sound. Now anyone in high end audio understands that cables *do* make a difference, the question is, why would an audio engineer think that they didn't?? The answer is, engineers are used to working with professional audio gear, which supports the balanced line standard. High end audio equipment for the most part does not. So its my surmise that these engineers did not think about that when they sent these letters in (basically describing the high end audio cable industry as charlatans). It was apples and oranges. However the laws of physics have a way of hanging around and are still very much with us today. I have already described the math of why a passive control can't control a cable and why if you are going to have success with one, why you will have to put some time into auditioning cables to go with it, and often spending a lot of money on those cables, with only limited success being the result. I will go into that in more depth in the article. |
Almarg said quote- "The 32 volts could be 1000 volts and you still would not have a problem". Yes I would!, I do not want a volume control to fully drive my amp on a clock value starting at 6am going to 3pm being the preamps full out-put, and making my amp fully driven at 8am!!!, That is not what I want to do! Audiolabyrinth, it sounds from this post that you do not understand the difference between output capability and gain. The two are different. In this case, the MP-1 can make about 32 volts. But it needs a lot of input to do that, as it does not have all that much gain and no source I can think of has that sort of output. Normally, it would be making much much less than that, not even a volt most of the time (signal and volume control dependent). So output level is what it can achieve, gain is how much signal it will need to get there. A lot of gain, not much signal, not much gain and it will need a lot of input. In your case, it would work out great as your amps have low gain also. Essentially, it would be impossible to cause the preamp to distort on your system. In fact its distortion would be so low that it would be very difficult to measure. Its that latter aspect which is why you want the high output ability- the more it has, the less distortion it is going to make when driving an amplifier. |
I'm saying a preamp that has much lower output will play lower levels of volume that I like, most source I use are 2.5 volts output to 5 volts output, you said, times that by 5, thats to much volume, I like to turn the volume control to full setting and still have a little head room to the amps power with no distortions or clipping If the volume control is working properly in the preamp the fact that it is set high or low will not affect the sound other than the volume itself. So you have no worries in that regard. What works in your favor is your amplifiers have relatively low gain, helpful if your digital sources have high output. |
Audiolabyrinth, no preamp manufacturer would get very far if their preamp had DC at its output! Any preamp manufacturer has to deal with the fact that their preamp will be used with a transistor amp that is direct-coupled from input to output.
A DC level at the output can cause all sorts of problems, and not just for transistors. If a tube amplifier has a direct-coupled input (our amps do and many others do as well), a DC level at the input would cause the input circuit to make distortion and could adversely affect gain.
So it is simply something that preamps don't do! That is why I said earlier that Krell was promoting a myth.
Preamps, *all preamps*, do not have DC at their output. It does not matter tube or transistor.
So the Krell person you spoke to who is looking for 'documentation' need look no further than this post. No manufacturer in their right mind would make a preamp that puts out DC. It is literally that simple. |
You have rights to be cautious Audiolabyrinth about protecting your expensive poweramps, I agree get it in writing.
A tube pre if it's output is capacitor or transformer coupled gives some safeguard to precious large power dc coupled poweramps.
But if a tube pre is direct coupled and you get a bad tube or something goes haywire then your dc coupled power amps are in grave danger, even if the pre has output relays which can switch too late.
If something goes wrong, the dc offset in a direct coupled tube preamp premap can be up to 100 x larger that a transistor pre. Your poweramp may survive a dc coupled transitor pre going bad, but not a dc coupled tube pre. Audiolabyrinth, George has no idea of what he is talking about here. I don't know of any tube preamp that is capable of doing the things he describes above, and that's after decades of making tube preamps with direct-coupled outputs. IMO he is trolling. To put it more clearly, if the MP-1 did develop a fault, it could not make any high voltages as George suggests. We've been making the preamp since 1989 and FWIW it does not have a need for protection relays. It was designed to be fail-safe. To go 25 years like that should tell you something. The preamp uses a patented circuit similar to what is used in our power amps. One of its advantages is that it is so reliable that it does need protection circuitry. I hope that you understand that in order to stay in business, any manufacturer has to offer products that will not cause headaches down the road :) I have already said why you will not find any manufacturer that offers 'documentation' regarding DC at their outputs. Put bluntly, the idea is preposterous. IOW, George is wrong, this is not something you need to worry about. Its simply not an issue. |
Charles1dad, it appears you are correct based on the post just above. |
