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Technical Questions - impedances and sensitivities

 

Silver Member
Username: Kbear

Canada

Post Number: 645
Registered: Dec-06
Hi folks, I've got one or two technical questions that I'd like to hear some feedback on.

I am comparing two integrated amps, basically trying to determine what might be an ideal match, re: amp and source. I found this article on the Decware website and part of it confuses me a bit.

http://www.decware.com/paper55.htm

Steve says that the input sensitivity of a preamp is how many volts it needs to come to full power. Most CD players put out 2 volts and most preamps require 1 volt. Okay, so one would think that in most cases then, you can put most any CD player with most any amp and get satisfactory results. But then he mentions that any volts above the input sensitivity will cause the amp to go into clipping. So then, is 2 volts too much? Amps usually only need 1 volt. Or, is this where the volume control comes into play? This is where he goes to next.

He mentions that when the volume knob is at 0 there is 0 volts. The ideal position of the knob is between 1/4 and 3/4 of the way up. I guess my question here is, if you have the knob half way up, are you using 1 volt from the CD player's output? And therefore, have you brought the preamp up to full power? Will anything more possibly lead to clipping? Steve then mentions that at this level the preamp is not adding any gain. I guess not, but then again if you turn the volume knob to 100% the preamp would be using the full 2 volts, and so should not add gain even at this level. When can a preamp add gain then? I always thought the volume knob is what controls gain. But if 100% on the knob equals using 100% of the source output then you can't ever really add gain. Either there must be another way or I'm just not getting something.

The talk about impedance is somewhat straightforward.

The components I'm comparing are as follows:

Amp 1: Input sensitivity = 125mV
Amp 2: Input sensitivity = 250mV

CDP 1: 2.4 volts
CDP 2: 2.0 volts
CDP 3: 2.0 volts

Well, the CD player outputs are pretty much bang on what is mentioned in the article. The input sensitivities are drastically lower though. I checked one other preamp to be sure and it's IS was 159mV. But I guess this means the CD players will have no issues driving these preamps.

CDP 1: Output impedance - 100 ohms
CDP 2: Output impedance - n/a
CDP 3: Output impedance - 596 ohms

Amp 1: Input impedance = 20,000 ohms
Amp 2: Input impedance = 14,000 ohms

So CDP 1 seems to have the better OI, though it's not like CDP 3's is too high. Amp 1 should be easier to drive than Amp 2, though Decware's article states that most Amps have an II of 50K ohms, so both of these are short of that and may be harder to drive than average. Does anyone think sonic benefits can be achieved in matching CDP 1 with Amp 1? I don't think there are any gross mismatches based on these figures, but is there a match that is likely to yield better performance than other possible matches? Thanks in advance.
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 14890
Registered: May-04
.

You have to keep in mind the specs are meant to reflect the performance of a component under (mostly) static conditions. To measure sensitivity and max output voltage, a sine wave (a single frequency) is used - typically at 1kHz. This has no relationship to music which is dynamic and broadband. Thus, when the output of a CD player is paired to the input of a pre amp (or pre amp to power amp) the "overload" spec is what you must consider. At what voltage does the front end of the input device begin to be overdriven.


Almost 30 years ago pairing the 2V max output of a common CD player with the overload point of a pre amp designed long before CD became a reality could be a problem and was often one of the factors in early "CD sound". The front of the pre amp was being driven into distortion by the excessively high output of the player. I am unaware of a manufacturer today who isn't building for the typical CD player's voltage output. The up side to this is most digital players supply sufficient voltage to make passive pre amps which have no gain a workable alternative to an active pre amp. If you are happy with only one source input, all you'll need is a way to control the output voltage of the player.


Next, you must consider that all CD's are not created equally and some have higher "average" levels than others while certain genres of music will have more difficult demands than other genres might. As with speakers bass is always the most difficult frequency range for any component to deal with due to the large voltage swings required for its accurate implementation. So, consider the CD as the source for the output voltage from the CD player before you consider the CD player as the source of output voltage to the pre amp. Since there is no real "average" output voltage from a CD as a result of disc variances, the waters of in/out specs becomes increasingly murky.


The same issues apply to the volume control. It's much simpler to state absolutes when you are testing with sinewaves than it is to discuss real world conditions. The position of the volume control is dependent upon the voltage coming from the source and the input sensitivity of the power amp and the speakers. Low input requires more rotation and vice versa. High sensitivity downstream means less vc rotation. The only absolutes in a vc are its first and last stops, all else must be considered in relation to the entire system.


Regarding input sensitivity, it is once again meaured wtith a single sine wave. It's purpose is to inform you how much input voltage is required to have the component reach "full power" or, in simpler, more accurate terms which apply to both power and pre amp, full output voltage (since "watts" are typically consider to be a function of only voltage gain when measurements are taken on a test bench). The lower the sensitivity spec, the less input voltage is required for the component to output its specified full voltage. This is then played against the input sensitivity of the power amp and speakers and the aforementioned varible output voltage from the source to determine approximately where on the vc's rotation you will reach full output voltage. If the pre amp - in this case - has a very low input sensitivity spec and it is paired with a power amp of similarly low (the inverse of reason is the case here, "low" means the number is low not that the input sensitivity is "low" to the point it requires higher input voltage - got that?) input sensitivity, you might find the amp reaches full power at a minimal setting of the vc. This wouldn't allow for very slight adjustments to the vc without the overall volume of the system jumping too much for comfort. Ideally, you would want a system which provides the sensitivities and output voltages to have the vc mostly operate in its middle range. This doesn't take into account the "taper" of the vc which can place large amounts of gain in the first half of the range or can adjust in equal dB increments across its range. Taper is an issue I've dealt with on this forum and one that is a bit too large to fully describe in this thread at this time.



The key spec in most cases though remains the overload spec. You do not want any source to overdrive the front end of the downstream component.



Output impedance should always be kept as low as possible. That is a general rule which can sometimes be ignored to a certain extent. Tubes will almost always result in a higher input and output impedance when compared to solid state. If they do not, then it's quite possible the tubes are employing additional circuitry - buffers most likely - to bring the impedances down. Buffers are both good and bad but when they are done well, they are, for the most part, good things to include. The least reactive components tend to be those with low output impedances, strive for 600 Ohms or less output impedance from the pre amp. They allow you to drive long cables with minimal signal loss and to be "non-reactive" to the load impedance. Maintain at least a 10:1 ratio of output to input impedance specs between components and you should be fine. Less than that ratio and you will probably find yourself being limited by the components as far as system set up and you can easily have frequency aberations which would be the result of Ohm's Law taking effect.



The above considerations are one good reason whole systems comprised of a single manufacturer's components is always a good idea over mix and match if you are not familiar with how individual pieces from various manufacturers will work together. An all McIntosh, Jeff Rowland or Naim system should be compatible with each other component by the same manufacturer to eliminate potential headaches.

You're asking this question in the "integrated amplifier" section of the forum. Just as with single manufacturer systems, an integrated amp should have all of this worked out for the consumer. Unless the pre amp section has an unusually low overload spec or is completely passive, there shouldn't be any particular problems when it comes to pairing integrateds with other components.



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Silver Member
Username: Kbear

Canada

Post Number: 649
Registered: Dec-06
Thanks Jan. Amp 2 is my current amp, and it has a passive preamp stage. But I've partnered it with the matching Cd player. I would think any of the Cd players on this list would work though. As you said, it's a 10:1 ratio for input to output impedance. At 14,000 ohms for it's input impedance, that would mean matching it to a Cd player with an output impedance of not more than 1,400 ohms. How many Cd players have an output impedance of 1,400 ohms or more, and how many preamps an input impedance of 14,000 ohms or less? Probably not many, at least from the little I've seen, which would indicate that matching any given Cd player to an integrated should work almost all of the time. The Cd player with 596 ohms output impedance is the Rega Apollo, and Stereophile commented on the higher than usual figure, but said that ultimately it likely wouldn't be an issue for most amps.

So with this in mind, is there a way to identify likely mismatches by specs? I was thinking that a lot of what we hear is due to specs like these, and how one components specs match up with another's further downstream. But it seems that almost any Cd player can be paired with almost any integrated. You mentioned the overload spec, but also that probably all manufacturers nowadays take the output of the Cd player into account.

Your comments about the volume control being too sensitive makes me think that may be an issue with Amp 1. It's input sensitivity (pre amp) is 125mV, and it's power amp figure is 782mV. From what I can tell these are relatively low. Perhaps the 20K ohms input impedance (for the pre amp) isn't that high though, so that might have some mitigating effect. But I wonder if, given the input sensitivities, matching it with the Apollo might be a good match, with it's higher output impedance? Or perhaps to speakers that are not too sensitive (unlike Klipsch for example). Or perhaps it's more complicated than that?
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 14892
Registered: May-04
.

I think we've discussed this before. A nominal impedance spec is stated at one frequency unless otherwise noted. Unless your passive pre amp employs a buffer stage before the vc, the actual input impedance of your passive pre amp is the impedance of the vc at any setting. Therefore, the input impedance of your pre amp is constantly changing as the position of the vc changes. Your 14kOhms would be a number that applies only to one spot on the vc and only at one frequency, that's what this sounds like to me. An input buffer is employed to maintain a constant input impedance and is a common item in most active components.

No matter what arrangement your passive pre amp uses, buffer or no buffer, since the vc is the only device the CD player "drives" it is virtually impossible to overdrive a volume control. Active circuits are easily overdriven but a passive system makes for the cleanest signal path in that respect.


The output impedance of the Apollo is not of major concern since it is unlikely the player will be located more than about one meter from the pre amp. Since most active pre amps have some buffer stages at the front of the input stages and that most pre amps can muster around 45-50kOhms input impedance to begin with, the somewhat high output impedance of the Apollo is unlikely to be of consequence to most users. With a passive pre amp, however, the best advice is an audition before a purchase. And that's really the best advice overall since most static measurements do not in any way reflect the conditions presented by music playback. With few exceptions, there aren't the truly oddball components that will not be compatible with almost everything else built within the last decade.


It's good that you looked but what you found probably doesn't provide sufficient evidence to actually base a decision solely upon numbers. Give a listen. With a passive pre amp stage, if the music doesn't sound compressed at various volume settings and does not sound rolled off on top or flabby on the bottom, then your numbers will probably align without problems.


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Silver Member
Username: Kbear

Canada

Post Number: 650
Registered: Dec-06
Yes, we've touched on this before, I think with respect to interconnect options for the amp, as Stereophile warned about that. I was looking into these specs a bit more now though, as they relate to component matching. But what you are saying is that they are only worth a cursory look to make sure there are no serious anomalies. Specs are static and don't reflect real world musical playback, so listening a must. Makes sense, thanks for the clarification.
 

Gold Member
Username: Magfan

USA

Post Number: 1193
Registered: Oct-07
specs are advisory.....
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 14893
Registered: May-04
.

Specs are incomplete. They should be viewed as a road sign which reads, "------" up ahead".


"------"?! What the f*** is "------"?!!! Is that good or is that bad?


I've asked this question of the objectivists who insist all components that measure alike will also sound alike. What spec tells you that when all specs are incomplete? How far down into the minutia of measurements must we go before we can be satisfied that "all" specs are indentical? They always ignore the question.


The point is we have a very gross overview of a component given the typical specs shown by any manufacturer. Even the measurements taken by JA at Stereophile are meant only as road signs warning of potential problems. For the most part it is assumed that any component which measures "this" in this frequency or at this voltage output will tend to also measure "that" at another point. That's a very broad assumption in many cases.

Audio has for decades employed what as known as "weighting" systems. They were generically created by the Japanese and those selling magazines where the Japanese manufacturers advertised back in the 1950-60's. Some weighting systems existed before that time but the Japanese raised the bar to incredible heights when it came to excluding information. Take signal to noise ratio for example, any component can be "weighted" to indicate some aspects of the raw data have been excluded, there are at least three commonly used weighting techniques for signal to noise measurements for any given component - a tape deck having a different weighting system than would a CD player or a pre amp and so on. Occasionally the weighting method has some commone sense but most often it is in place merely to make the numbers look better for a specific component. For the most part - again - most of these weighting schemes do not need to be mentioned in specifications or even in measurements. JA knows what he is leaving out and as long as he is consistent with all of his measurements all of his similar measurements can be held against each other - but not necessarily against another lab's work. And, to be fair, JA has made it known in earlier statements what he measures and how and why he measures such stuff. He just doesn't mention it every time and a new reader could be somewhat misled by his numbers should they not take the time to investigate what is being done in JA's work.


The conditions of testing abound in audio, most everything is meant to fudge the numbers. When you read JA's comments concerning any particular component's measured performance, you also must keep in mind JA does have biases just as do the rest of us. Atkinson is, IMO, very convinced of the superiority of solid state and multi-driver speaker systems. If you don't take his biases into account, then you've only read, "------" u- -he--".


I would say my experience has been this is even more obviously true when you are reading white papers from manufacturers. They tend to wear their biases on their sleeves while deciding what is relevant to their case and what is not. As I've said many times here, everything in audio is a trade off, I know of nothing where I can give you one thing and not take away two others. Those two might not be important to you and they might not really be important to most people, but you should be aware they exist before you go off thinking all is lolipops and candy and what you've chosen only has up sides and no down.



I would take your "Steve says ... " as a roadsign warning of "------ up ahead".




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Gold Member
Username: Magfan

USA

Post Number: 1194
Registered: Oct-07
Rule #1 is to compare like-to-like. With various weighting schemes and indeed the dozen or so methods of measuring power, that is out the window and therefore almost meaningless.
Like-to-like is important but not done all the time.
Some weighting schemes are done not to make something look better, but to compensate for human hearing. To give equal weight to all frequencies would do an injustice to whatever is being measured. People are more sensitive to mid and higher frequencies, but fairly insensitive to lows. Why give them the same importance when reporting data?

Now, why do 2 pieces which measure the same not sound the same? Is it just possible that not everything is being measured? Is it possible to measure everything? As a thought problem, I'd hope you could measure everything. As a physics problem, I KNOW you can't. At some point the measurement interferes with the equipment being measured. Uncertainty is what a physicist would call it. It is possible, even, for measurements to interfere with one another. The link, below, is about physics, but I personally believe this can be expanded to include many other phenom. Including hifi.

And, since specs will never be complete, I'll stick to 'advisory' and caveat emptor still applies.

http://en.wikipedia.org/wiki/Uncertainty_principle
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 14895
Registered: May-04
.

"Some weighting schemes are done not to make something look better, but to compensate for human hearing. To give equal weight to all frequencies would do an injustice to whatever is being measured. People are more sensitive to mid and higher frequencies, but fairly insensitive to lows. Why give them the same importance when reporting data?"


That's certainly true, but how many people know that? How many people know which weighting systems exclude which information? If no weighting system is identified in measurements or specs, what is the average consumer going to assume? Advisory? OK, but which is the better advisory device, "Don't play with matches", or, "Don't throw gasoline on an open flame"? At some point advisory crosses over into "------" up ahead" when information is excluded.


"Now, why do 2 pieces which measure the same not sound the same? Is it just possible that not everything is being measured? Is it possible to measure everything? As a thought problem, I'd hope you could measure everything. As a physics problem, I KNOW you can't."


And that's certainly my point to the objectivists, which is why I assume they willfully ignore the question. Not too long ago ST reviewed an amplifier in Stereophile (no measurements were taken by JA) and interviewed the designer. ST had given the amp glowing review and the designer stated this was due to his ensuring perfect squarewave performance not from in to out but along every circuit and in every stage of the amplifier. Now, if that amp spec'd at 150 watts @ 0.001% THD, would it sound identical to another amp producing 150 watts @ 0.001% THD that did not fare well in innerstage squarewave performance? No one would ever measure innerstage sqaurewave performance without being told to look at that measurement for a good reason.



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Gold Member
Username: Magfan

USA

Post Number: 1198
Registered: Oct-07
Knowledge is power.

And YES, totally agree. The measurement guys can use whatever data to justify anything.

You can't simply skip the specs....or measurable stuff. And, boy, trying to explain measure weighting to someone is *&^%@# impossible without drawing or math. In this ADD age of 12 minute attention spans (average time between commercial interruptions) it is really tough. No instant answers.

Try to explain Uncertainty or Chaos to a real determinist (Numbers Are Reality) type and there you go.

Frankly, I had much better luck playing with gas and matches.
I used to make my own BBQ starter Napalm out of unleaded and Enzyme laundry detergent.

Believe me, Jan, that if my Square Wave and Waterfall plots, not to mention my Smith Chart are not in order, I can't sleep.
 

Platinum Member
Username: Nuck

Post Number: 15039
Registered: Dec-04
Ears do not have a flat response to input, so what's the point?
Flat power and flat speakers still sound dead, but ears liven things up a bit.
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 14898
Registered: May-04
.


"Believe me, Jan, that if my Square Wave and Waterfall plots, not to mention my Smith Chart are not in order, I can't sleep."



I once knew someone who couldn't sleep without his beloved Smith Chart. But, then, it later became known that Smith was sleeping with everyone.





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Platinum Member
Username: Nuck

Post Number: 15041
Registered: Dec-04


Smith said I was special....
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 14902
Registered: May-04
.

Oh, Nuck, Smith said everyone was "special".



Everyone learned not to trust him.








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Platinum Member
Username: Nuck

Post Number: 15053
Registered: Dec-04
Smith owes me 50$, dammit!
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 14906
Registered: May-04
.

No one who knew him would loan Smith money ...



uh ...




ahhhhhhhh ...







Oh, Nuck!





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Platinum Member
Username: Nuck

Post Number: 15055
Registered: Dec-04
Dagnabbit, Muskie! Old toon.
 

Gold Member
Username: Dmitchell

Ottawa, Ontario Canada

Post Number: 3959
Registered: Feb-07
I had to google Smith Chart.
 

Silver Member
Username: Kbear

Canada

Post Number: 663
Registered: Dec-06
Jan's comments about the volume control being a little too sensitive is definitely an issue with my Audiolab amp. Whereas with the Exposure a change in volume resulted in an every so small increase, if I take two steps up on the Audiolab by mistake the volume jumps. One step isn't too bad. I can live with it because I like the amp in all other respects...just have to be a little more careful with the volume.
 

Gold Member
Username: Magfan

USA

Post Number: 1223
Registered: Oct-07
What do you think, David? A single chart representing impedance and phase data with one line, with the possibility of adding frequency markers. This is really easy to read, once you get used to it.
 

Gold Member
Username: Dmitchell

Ottawa, Ontario Canada

Post Number: 3962
Registered: Feb-07
I'll let you know Leo in about 6 years, when I figure it out.
 

Silver Member
Username: Kbear

Canada

Post Number: 666
Registered: Dec-06
I remember that chart from another thread. I didn't spend a lot of time with it, but to me it wasn't as easy to grasp as the traditional graph. Maybe it's got something to do with the fact that we learn about graphs starting in elementary school, and use them pretty much up until University. So when you see one, and understand what the two variables are, they are very easy to understand.
 

Gold Member
Username: Magfan

USA

Post Number: 1225
Registered: Oct-07
I have been 'pushing' this chart for a while. A single line/circle representing both impedance and reactance of a speaker.

http://www.hometheaterhifi.com/images/stories/feature_reviews/technical_articles /2008-01-impedance-smith-chart.jpg

I like 'em because the horizontal line is 0 reactance=pure resistive. L / R on this line is pure resistivity. Up or down....represents capacitive and inductive reactance. So, from a pure theory standpoint, the closer a speaker remains to the horizontal line, the better. Too far Left on this line is low impedance. So if a curly q is too far from the center line and far to the left, that is the 'bad' zone. The same amount of excursion from the center line while being further to the left (higher impedance) becomes less and less 'bad'.

Once you get accustomed to the presentation, you may like it better than the double graph with the multiple scales that Stereophile uses. At least they actually go to the trouble to measure stuff.
 

Gold Member
Username: Magfan

USA

Post Number: 1226
Registered: Oct-07
The relationship of this data and sound quality is MUCH less clear. Some speakers with 'weird' loads sound fine, but ruthlessly weed out amps with poor ability to drive highly reactive loads.

Other speakers with more benign loads sound awful.
So, all the usual rules apply when matching speakers and amps.

The speakers I am most familiar with, Magnepan, get bad rapped as a 'tough load'. This is generally NOT the case. Low impedance true, but they never get into even the 3s. Phase angles remain mello for the most part. The bad news is poor sensitivity, across the entire lineup.
I don't know about the 20.1s, but they have a push/pull magnet arrangement which may help in this regard.
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