^^ You are correct about that. Redbook has most DACs too much voltage for most amps made.
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Well, it seems to me the gain issue ( voltage ) has just be proven by
kosst's last post, taken from Nelson Pass himself. Take care of the
impedance issue, using buffers ( 0 gain ) and the source can drive most
power amps quite well. Enjoy ! MrD. This is certainly true- if your only source is digital. If other sources are involved (such as a tuner, tape or phono) then it won't be the case. We've built buffer-only preamps for some of our customers that are digital only, but at least one of those shipped his preamp back after about 5 years and had us install the active gain circuitry as well, despite all his sources being digital. If you have more than one source, occasionally you run into the issue that not all digital sources actually conform to RedBook output standards. These sources will need additional gain. The thing is, if you do your design homework, the distortion and bandwidth limits of the active line stage can be much better than those of the source itself and if you do your engineering homework, its possible to insure that the kinds of distortion it does make are of the type to which the ear isn't particularly sensitive. |
And Al, a source that gives out 2v is always going to give it out, even if you shunt half of it to ground because you don’t need it, because you have more gain to make it back up in the active preamp.
So the distortion from the source "whatever it is remains the same", it does not change because you’ve decided not to use half of it and shunted the other half to ground.
If anything shunting more of it to ground "could" make it distort more. Not all volume controls shunt signal to ground. The traditional volume control does not shunt any at all. But there are shunt-style controls and they have been around for a while. They rely on a resistor (usually of significant value) to be always in series with the source (meaning that the resistor affects the sound if not carefully chosen), so that when the volume control is shunting the resulting output, the source will not distort or be bandwidth limited (IOW not significantly loaded). Now if the shunting element is the same value as the series element, the signal loss will be 6db, as the the output voltage will be cut in half (at full volume). With most passive systems this sort of loss is unacceptable- so the shunt portion of the circuit (the actual control) is often much higher than the series element; as much as 10 times higher. The question then is - what is the value of the composite control? For example George has stated quite often that his controls are "10K" (10,000 ohms). If its a shunt-style system as one might infer from the above quote (since many controls don’t shunt the signal at all), then is he stating that the series element is 10K? That would allow for the minimum value that the source (DAC or CDP) would be 10K. This would imply then that the *actual* control is considerably higher. At any rate, the statement in the quote above is problematic. The designer of a passive control I would **assume** would not want to give an active preamp an edge over his product by loading the source in such a way as to increase its distortion! If this point is ignored by some passive designers, it may explain why good active line stages can easily outperform passive devices in some cases. FWIW, no *active* preamp loads a source in the way George describes; the volume control (if at the input of the preamp, which is a good place to put it) is chosen to be a value such that no source of any type will be challenged by it!! Again, this is just common sense engineering (much like the built-in headroom that all competent preamps and sources have), and I hope that this is obvious to anyone reading these words. |
Hmm. George, I'm not worried about it- I simply acknowledged that you were correct yet you're still going off. If anyone seems in damage control mode, its you right now. Just calm down.
You'd have to imagine that I'm well aware of what our amps can and can't do; our amps don't use feedback, so speaker choice is important. But the flip side of that has consistently been that if the speaker requires the amp to have feedback, it also has no chance of sounding like real music (I can outline exactly why easily enough). So I just vet each sale to make sure the customer is going to have a success. Works pretty well- that's why we're one of the older manufacturers in high end audio- 42 years in about 10 days.
I don't remember which customer lent us the Lightspeed, but we had it for a week. As passives go, it was one of the better ones we checked out. If you are on a budget and have only high level sources, its not a bad way to go. IOW like any passive it can do well but not state of the art; plain and simple. That's why I'm not worried; passives are just a little too simple to do everything correctly (the better your system gets, the easier this is to hear). Cable control and buffering the source to prevent distortion are two example of why and I can name more.
Now that's not an insult, its just a statement of fact. State of the art nearly always costs more and that shouldn't bother you.
BTW, your comment about headroom in preamplifier and source design is still false and no amount of remonstration on your part will change that. I chose not to continue that bit but you insist on attacks. Just calm down and it will be OK.
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If the passive has a 10K impedance maximum George's statement is true.
Apparently not all Lightspeed controls have such a low impedance; we had one here that was much higher. So the model you get can have a big effect. Also, 300pf is a low value even for a 3 foot interconnect (A 6 foot cable will double that capacitance), although low capacitance cables are available that are much lower. Its my recommendation that such cables (such as are used for phono cartridges) be used unless a lower value of passive control (such as 10K) is employed. Keep in mind though that not all sources are happy driving a 10K load (which may well be in parallel with the input of a solid state amp, resulting in an even lower impedance) so its best also to check with the manufacturer of the source component to insure that it can drive such a load successfully. |
Hence the statement I quoted above appears to me to be incorrect, and
George’s statement that "interconnects are the determining factor here"
is likely to be correct. @almarg This seems to me a bit of a catch-22, as the very high impedance of the LDR at certain settings pretty well means that any cable is going to impose a bandwidth limit so in effect you could never have full bandwidth. With a 3 foot high quality cable (which would have 300pf) in such cases you would have a roll-off that is very real world. Another issue is that a high impedance thus imposed could have negative HF bandwidth effects due to Miller capacitance in the amplifier. The latter is one topic that rarely gets mention in these passive/active debates but its very real. |
If a power amp has an input sensitivity of say 1v input to make it reach
full wattage output (eg: clipping) There is no need for the preceding
stage (whether it be a preamp or direct from the source) to put out any
more than say 2v so long as it's clean.
Having the ability to put out
30v instead of the 2v, and said to sound better because of it!, is a
total furphy by the one who said it.
George's confirmation of his earlier comments leaves no doubt that he's clueless on this topic! Any competent engineer knows this. Any linear circuit has distortion that increases with output. If the circuit only has to do 2 volts but can make much more, its distortion can thus be kept extremely low- to the point that its difficult to measure. This is why many active preamps have such low distortion, because they are operated well below their limits. This is just good engineering. |
Many amplifiers overloaded with 2 volts ? Are we talking about power
amplifiers being overloaded, or preamplifier sections being overloaded ?
30 volts output ? Al did answer this, I hope its now clear. Simply: With the sources feeding them, preamps active or passive giving
out UP TO a clean 2v to an amp that only needs 1v in to give it’s rated
output before clipping, there NO USE at all for those same preamps
active or passive to be able to give 10v 20v or 30v output.
George's statement here is clearly false; it doesn't appear that he's trying to mislead so much as he seems to have no idea of what he's talking about. As I said before, active preamps are often built to produce far more output than is asked of them so that distortion at their normal operating range of less than 2 volts will be devoid of distortion. Most sources like CD players have a similar overload margin built in to them as well for the same reason, despite the maximum output being the Redbook spec. The amount of overload margin used in a vinyl cutting system is even higher, to insure that its impossible to overload the cutter or the cutter amps. IOW, the inclusion of overload margin in audio products is a common practice among competent designers. |
2 volts might be alright; try 4 volts on for size :)
If you have 2 volts though, many amplifiers will be overloaded.
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@mrdecibel Overload margin.
Our preamp can make over 30V into 10Kohms without clipping. Its nice to know that no matter what signal gets thrown at it, its distortion will be difficult to measure with the best test equipment even when driving the power amp to overload.
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Correct MrD, as Nelson Pass says, today we have way to much gain (voltage) with the sources we have. He's not the only one to say that- its been a beef of mine for years. Whoever came up with the Redbook specs at Phillips and Sony blew it on this one, since any amplifier made will be clipped by a source that supports the Redbook spec. IMO it would be better if they dropped the output voltage, even by a little bit, since the less you have to knock down the signal to make it useful for an amplifier the better. |
@tweak1 FWIW dept: our MP-3 preamp can be ordered as a balanced tube buffer with no active gain stage.
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Second, there is no such thing as an amplifier being overpowered for any
set of speakers. Period. Musical transient peak demands typically can
require 100’s of watts to the speakers. I doubt your class D amp of 125W
RMS is up to muster. Actually there is, but if the speakers are only 94 db and the amp only 125 watts, the amp isn't too much power by any means! If a class D amp, I would want more power just to make sure I never got near clipping, a fundamental difference between tubes and solid state (including class D). Too much power in most cases means that the amplifier is operating in a portion of its low power region where its distortion is actually higher than at higher power levels (typically this is about 5-7% of full power with many push-pull amps). The only amps that I know of that don't have this property are SETs and the OTLs that we make, where the distortion linearly reduces to unmeasurable as the power is decreased. Its that first watt that's the most important!Regarding critical listening, my amps are on all day and a good portion of the night as well- and I can't say that all that is spent in critical listening. But I've come to expect a certain level of competence out of the system and if its not doing it, my listening gets pretty critical real quick! So tubes all the way (at least for now- we have a class D amp we're brewing up; its got a pretty high bar to meet). |
Because 90% of systems are a great match for passives, and are the most
dynamic /transparent way of getting the source to the amp except for
going direct if you can do it without "bit stripping". While the first half of this is false, I do agree that bit stripping is not a good way to do a volume control. You don't want to lose resolution when turning down the volume. @tweak1 Sorry, I misread your initial post. |
Its an active (buffered) control. Shouldn't this thread have a different title? "What could be better? Hardly any noise or distortion added by these
simple passive parts. No feedback, no worrying about what type of
capacitors – just musical perfection. 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."
George always leaves out the rest of Nelson's words, as if they don't exist. Here they are, as you can see Nelson does not think passives are the last word by any means:
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.
If you put a buffer in front of a volume control, the control’s low
impedance looks like high impedance. If you put a buffer after a volume
control, it makes the output impedance much lower. You can put buffers
before and after a volume control if you want.
The thing here is to try to make a buffer that is very neutral. Given
the simple task, it’s pretty easy to construct simple buffers with very
low distortion and noise and very wide bandwidth, all without negative
feedback.
There are lots of different possibilities for buffers, but we are going to pick my favorite:
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