I thought it was time we had a pro- Class D thread. There's plenty of threads about comparisons, or detractors of Class D.
That's fine, you don't have to like Class D amps, and if you don't please go participate on one of those threads.
For those of us who are very happy and excited about having musical, capable amps that we can afford to keep on 24/7 and don't require large spaces to put them in, this thread is for you.
Ralph, it appears to me that what underlies much of the disagreement
between you and Erik is that he is viewing the impedance of an output
coupling capacitor, if present, as contributing to and being part of the
component’s output impedance. While you are not, possibly because you
are considering "output impedance" as corresponding to "specified output
impedance," which as we all agree is often based on a mid-range
frequency such as 1 kHz. The capacitor’s impedance of course being
unlikely to be a major contributor to the 1 kHz output impedance in just
about any reasonable design.
Thanks Al! To be clear here, this was Off topic (we're working on our own class D circuit that is not based on any modules so you can draw your own conclusion about what my attitude about class D is); my main concern was was to try to express the idea that it was the output coupling cap in a given design that was determining the frequency response variation seen in some designs. Since there are tube preamps with a high output impedance that also do not have the rising impedance as seen (due to the fact that they have larger coupling caps), its hard to allow a generalization like 'high output impedance leads to frequency response errors' or the like. It doesn't have to was my point and I didn't have to look very far to find examples.
Put another way, its the rising impedance at low frequency, not the **overall** higher output impedance that causes the problem; I should have expressed it that way earlier!
@ricred1 I got my Bel Cantos through a dealer that made me an incredible deal. I have no skin in the game whatsoever, but if you're curious about the Ref600ms, send me a message and I can forward the dealer's info.
ricred1735 posts05-05-2017 5:43amI’m trying to get a hold of a pair of Bel Canto Ref 600s to compare to my Jeff Rowland 625 S2. May be time for a change. I’m curious to hear if they mate well with the Corus preamp.
I had been planning a change in my system for quite a while that just started coming together. I went from a system that paired a Cambridge 851N DAC and ARC DSi200 integrated amp to a system that pairs a PS Audio Directstream Junior with Bel Canto Ref600m amps. I've had the new DAC for a couple weeks and the amps just arrived yesterday. Still breaking them in, but so far so good. The jump in DAC SQ was major, as one would expect with a more than doubling of price. The first thing I noticed about the Bel Canto amps was greater bass authority. Besides that, I've got a lot of variables going on. We'll see. For now, I'm having a grand time.
As an aside, I work in the energy efficiency consulting industry and I like being able to leave my equipment on all the time without feeling like I'm wasting lots of energy.
I'm trying to get a hold of a pair of Bel Canto Ref 600s to compare to my Jeff Rowland 625 S2. May be time for a change. I'm curious to hear if they mate well with the Corus preamp.
I think Class D is as good as many Class A or AB designs. It may sound different, as some amps will sound different to each other regardless of the technology. Some may also deal with different speakers differently, just like Class A/AB designs.
I think there are some very affordable great sounding Class D amps right now. Please skim this thread for a number of good suggestions, and listen for yourself.
My Class D monoblocks sound as good as mega-buck class A amps to me.
Let’s say my hearing is impaired (for argument’s sake). Why would I buy anything more expensive I don’t hear an improvement for? :)
Given that this is Erik’s thread, I presume Guido and the others posting above won’t deem it inappropriate if I comment on the disagreement between Erik and Ralph. Apologies if that is an incorrect presumption.
Ralph, it appears to me that what underlies much of the disagreement between you and Erik is that he is viewing the impedance of an output coupling capacitor, if present, as contributing to and being part of the component’s output impedance. While you are not, possibly because you are considering "output impedance" as corresponding to "specified output impedance," which as we all agree is often based on a mid-range frequency such as 1 kHz. The capacitor’s impedance of course being unlikely to be a major contributor to the 1 kHz output impedance in just about any reasonable design.
Regarding that terminological difference, I would have to agree with Erik, and consider "output impedance" to be whatever impedance is seen "looking back" into the output of a component, at a frequency that should be and hopefully is stated. Also, as he pointed out, measurements of **many** popular high quality components that have been presented over the years by John Atkinson in Stereophile, that have specified output impedances of perhaps a few hundred ohms, show impedance rises to the vicinity of 3 or 4K or even more at 20 Hz. Obviously the sonics of such a design would be compromised to some degree when driving a low impedance, such as 10K, or even 30K or more in some cases.
On the other hand, regarding this part of Erik’s statement that you quoted above:
I say high output impedance causes frequency response changes which vary based on the load. This is an irrefutable fact based on simple serial circuit analysis.
As I assume he would agree, high output impedance will not affect frequency response when driving a resistive load if the output impedance itself is resistive, and does not vary significantly at relevant frequencies. As I said in an earlier post, it is the **variation** in output impedance, as a function of frequency, that may be significant in that regard. Although of course under extreme circumstances high output impedance can in itself affect frequency response in the treble region, even if there is no variation as a function of frequency, due to its interaction with cable capacitance. Capacitance not being a resistive load, of course, by definition.
Hopefully this post will result in the divergent opinions expressed in the foregoing posts becoming less divergent.
Erik. Keep up the good work on this thread about class D amplifiers, Designers such as Atamasphere will beat a topic into the ground to prove their point. He isn't the first, won't be the last. Again, good work on your part. I have a Class D integrated hybrid, and couldn't be happier.
I have reached 100 hrs on my dragon s500 and not really much change.It's a little lean and bright,nothing horrible,compared to my 600m's.I won't fully evaluate it till past 1000hrs,I have it playing in my 2nd system for now.Time will tell.
Atma and all, let us please return this thread to its original topic. There are better places to discuss preamplifier design than this rare thread for class D amp enthusiasts.
I don’t "blame" preamps for changes in frequency response. I say high
output impedance causes frequency response changes which vary based on
the load. This is an irrefutable fact based on simple serial circuit
analysis. Anyone with a basic understanding of AC circuit analysis would
conclude the same.
And I showed the math for why that is not so: the first half of your quote here is false, the second half being based on the first is thus also false.
**Do the math**.
Its the coupling cap at the output, not the output impedance that governs the frequency response.
Example: I've seen ARC preamps with 20uf output coupling caps. If you put them on a 10K load, they will be as flat as they are on a 100K load in the audio passband. Yet the very same preamp according to ARC should not be asked to drive anything less than 30K.
Your claim to which I was objecting was that the higher output impedance of tube preamps leads to frequency response errors and the simple fact is this is not so- it depends more on the timing constant that may or may not be present at the output of the preamp in question. I showed the math. If you wish to refute this, then show the math.
I was taking a look at the NAD C390DD integrated. I like the concept of being able to change out the boards as upgrades become available. Anyone have experience with the sound quality of that model?
As we can see, if one is to point at a tube preamp and blame it for a change in frequency response,
I don’t "blame" preamps for changes in frequency response. I say high output impedance causes frequency response changes which vary based on the load. This is an irrefutable fact based on simple serial circuit analysis. Anyone with a basic understanding of AC circuit analysis would conclude the same.
It is also irrefutable that in general, tube preamps have higher output impedance than solid state. That is different than saying preamp X sucks, which is what you seem to have read.
the factor is not the output impedance (which is often only stated at 1KHz), its the coupling cap at the output. That is a bit different from ’output impedance’ and that is why I placed the correction.
A - I never limited myself to 1 kHz B - It makes no sense to talk about changes in frequency response if i was talking about a single point C - The coupling cap is a major if not THE major contributor to output impedance. However it is not the only issue. I never said it was. The factors that go into any devices output impedance is more complicated, but includes the coupling cap if any.
There is no logical way to make both of these statements true at the same time:
Tube preamps don't have a problem with output impedance
The problem with tube preamps is the impedance of the coupling cap
Since the first includes the second, you can't have both of these be true at the same time. Best,
The biggest issue with input/output impedance is the change in frequency response....(followed by supporting discussion with links to external sources)
You replied:
This is mostly misleading or outright false.
Now your latest posts seem to be supporting my argument. I’m not sure you even read what I wrote before you claimed it was a lie or misleading, but you launch into discussions about how Atmasphere preamps are different (which while informative does not make your case that I was wrong).
So I challenge your "statement of fact" as having anything which directly refutes my statement with evidence.
A statement of fact is not an attack. You are taking this personally.
Here’s the work:
What, exactly, are you claiming is a result of the choice of coupling cap? Distortion or output impedance?
Here’s a formula for calculating cutoff frequency: F=1,000,000/CxRx 2Pi
Normally you see this formula with a 1 instead of a million; I used the latter so that f is in Hz (-3db point), R is in ohms and C is in uF.
The coupling cap at the output of a preamp, in concert with the input impedance of the amplifier used determines the cutoff frequency.
example: a solid state preamp has a 10uf output coupling cap. The input impedance of the amp is 10K.
1.59Hz=1,000,000/10K x 10uf x 2Pi
We can see from this example that if a tube preamp has a 10uf coupling cap that it too will have a cutoff of 1.59Hz into the same amp.
This means there will be no appreciable phase shift at 20Hz so bass impact will be unimpaired, since the cutoff is 1/10th the lowest frequency to be played. A cutoff at 20Hz will mean that phase shift exists up to about 200Hz. The phase shift will cause the system to sound lean.
Many tube preamps **do** have such large coupling caps unless the designer has not done their homework (or has figured out that the larger the coupling cap, the more coloration it imposes, and so has elected to limit the capacitor size so as to get greater transparency). As a manufacturer you can’t forecast to what amps the preamp will be paired.
The size of the coupling cap will not affect the output impedance unless one is able to graph the impedance curve; if rising at lower frequencies the culprit will be the output coupling cap and otherwise not the output impedance of the preamp.
Now how much **distortion** the preamp makes can be affected quite a lot by the load that it drives. That is likely the more powerful argument for being careful about what preamp drives what power amp. Tube preamps often have very low distortion; in most cases its a good idea to have them drive a higher impedance so as to take advantage of that fact. Our preamps again are an exception- they regard 10K as an effortless load.
Please note:
This supports, not undermines, my statement.
yes, this is evidence of how this was simply a statement of fact and not an attack.
The biggest issue with input/output impedance is the change in frequency response...
I was very specific in what I was talking about. I did not say "the biggest issue with tube preamp sound quality." I said the issue with "input/output impedance."
As we can see, if one is to point at a tube preamp and blame it for a change in frequency response, the factor is not the output impedance (which is often only stated at 1KHz), its the coupling cap at the output. That is a bit different from ’output impedance’ and that is why I placed the correction.
The biggest issue with input/output impedance is the change in frequency response...
I was very specific in what I was talking about. I did not say "the biggest issue with tube preamp sound quality." I said the issue with "input/output impedance."
If you see another issue with input/output impedance to be more important, please show your work.
My prior post was a correction and not an attack. I was merely pointing out where the problems were and stated why, basic engineering principles included. More are below.
You called my statement:
mostly misleading or outright false.
That’s not a correction. If you had said "I feel distortion is a bigger issue" then it’s a matter of opinion as to which is more audible. If you want to say I wrote falsely or in a way to mislead back it up.
You then state:
This has more to do with the choice of coupling capacitor at the output of the preamp than it does the output impedance!
What, exactly, are you claiming is a result of the choice of coupling cap? Distortion or output impedance?
The general rule of thumb is a 10:1 impedance difference between the two; as long as you hit that margin with the amp you have in mind its likely no worries.
I'm sure you make fine products, but would you then please explain the
variance in all the tube preamps in the Stereophile section if not due
to output impedance issues?
My prior post was a correction and not an attack. I was merely pointing out where the problems were and stated why, basic engineering principles included. More are below.
We won't allow Stereophile to review our products (we don't agree with their editorial policy which seems to be tied to their advertising and I know this from direct experience) which is one example of why if you limit yourself to their pages, you won't get the full picture.
This has more to do with the choice of coupling capacitor at the output of the preamp than it does the output impedance!
Please note that this phenomena has to do with solid state just as much as tubes.
Of course, the ultimate indicator is a graph of the output impedance vs. frequency. If you see it rising as it approaches 20Hz, this **might** indicate a loss of bass impact depending on the input impedance of the amp. The general rule of thumb is a 10:1 impedance difference between the two; as long as you hit that margin with the amp you have in mind its likely no worries.
The output impedance curve of our balanced preamps looks the same as their frequency response curves; we cut them off at 1Hz. So regardless of the load its driving, the preamp will have flat response from 1Hz to over 200KHz. Ours are not the only tube preamps with direct-coupled outputs that have ever been made- as a result you can't just assume that if it has tubes that it will have troubles making bass into a solid state amp with a 10K input impedance or the like. Generalities are often misleading that way.
Perhaps you feel your preamps are less susceptible than most, which I could believe. I did not make my statements absolute. If you’d like to submit FR charts at different loads like Stereophile has and at different volume settings this would prove your preamps outstanding exemplars.
I have to say that for you to call my statement in its entirety misleading or false when anyone can pull up page after page of examples in Stereophile measurements of tube preamps showing variance in their frequency response based on difference in the impedance they are driving is pretty gutsy.
Anyone can also compare solid state preamps to this and see that in general they perform better by this measure.
So I hope that maybe you misread the generalities and nuances I was making and re-think your reply or produce engineering principles and data which makes all the other data and the basic electric principle of impedance in series and voltage division a quack science.
The biggest issue with input/output impedance is the change in frequency
response. This is especially bad with tube pres as they usually have a
high output impedance. Driving a low impedance input amp can affect the
overall response and deviate from ideal. With purist tube pre's even the
volume control setting can affect things because they lack additional
buffer stages that would prevent this.
This is mostly misleading or outright false.
Output impedance **might** affect frequency response (in some cases, certainly not if the output is direct-coupled) but also affects distortion.
Tube preamps do not necessarily have high output impedance. That depends a lot on the design of the circuit! If it employs feedback, its output impedance will be fairly low and driving a 10K impedance should be no worries- check with the manufacturer (our preamps don't employ feedback and 600 ohms or less is no worries)! In a purist tube preamp, the volume control won't affect things (other than volume) despite there being no buffer; its all about how well the design is executed!
As an example, we make two tube preamps that have balanced outputs (and were the first in the high end audio world with such) and the load that they can drive makes no difference- the frequency response is flat, owing to a direct-coupled output. They also support driving 600 ohms as they were intended to support the old school balanced standard. Yet because they lack loop negative feedback, their output impedance is relatively high (compared to most solid state preamps), yet their distortion is very low driving any amplifier made with a balanced input. I've used our MP-1 to drive loudspeakers directly; how many solid state preamps can do that?
Whew. Thanks Todd, Erik and Al. That is a lot to digest but I think I understand it a bit more now. This does a lot to explain the basis for such terms "system synergy" and "system compatibility".
And now, back to our regularly scheduled topic of Class D amps are Dandy!
In the situation Autre described in his post dated 4-29-2017 it is very unlikely that impedance compatibility issues were present. The Rogue Sphinx has a very high input impedance (more than 100K according to Stereophile's measurements). And although I don't know what the input impedance of the
Peachtree Nova 125 SE
is, it is most likely much higher than the output impedance of the solid state Onix CD player which was driving it.
Impedance incompatibilities between line-level source components, such as CD players, and integrated amplifiers such as those, or between preamps and power amps, are likeliest to arise when a tube-based output stage is driving a solid state input stage.
Regarding the 10x rule of thumb guideline which Todd alluded to, I would state it as follows:
The input impedance of the amp (or other component that is receiving a line-level input signal) should be at least 10 times the output impedance of the preamp or line-level source component that is driving it, at the frequency within the audible range for which that output
impedance is highest. Which in the case of preamps or source components having capacitively
coupled outputs (such as the majority of tube preamps) will usually be
at 20 Hz. And the output impedance at that frequency will often be far
higher than the specified output impedance (which is usually based on a
mid-range frequency such as 1 kHz), because the impedance of a capacitor rises as frequency decreases.
That doesn’t mean that there
will necessarily be a problem if the guideline is not met. It depends on
how the output impedance **varies** as a function of frequency. What it
means is that there **won’t** be an impedance compatibility problem if
the guideline **is** met.
If Stereophile has reviewed the preamp or source component, the measurements section of the review will usually indicate the output impedance at 20 Hz as well as at other frequencies. But if only a nominal impedance can be determined, such as a manufacturer's specification that is presumably at a mid-range frequency, to be safe I would suggest a ratio of 50x or preferably even 75x.
One of the spec’s commonly sited is S/N (signal to noise) or related THD+n (Total Harmonic Distortion + noise). The problem with Signal to Noise is that it is often cited at full power, and not at a more reasonable amount, like 1 watt.
So, a 1000 watt amplifier can claim 10 dB more S/N than say a 100 Watt amplifier that sounds exactly as noisy. The full power S/N ratio becomes useless for comparing amps and how quiet they are. What I wish reviewers and manufacturers did more consistently is rate the S/N at 1 watt or 10 watts. Something small so we can compare what we’ll actually hear.
Distortion is also something that tends to go down as power goes up until the limit of the amp is reached. This is why these figures can be misleading. This is something stereophile does a little better, in showing graphs of THD vs. power output. Now we can at least look at charts to compare noise and distortion as more usable power outputs.
The other thing, I think that the amplifier community is having a hard time keeping it’s prices up to make boutique manufacturing possible. This makes it necessary to have big differentiation. For instance, having a 1,400 watt amplifier is cool and all, but in my living room, at my listening levels does it mean my sound is better than my 250 W amps?
I don’t really know all the answers to these questions at all. I’m just sharing where I start to scratch my head. :)
The biggest issue with input/output impedance is the change in frequency response. This is especially bad with tube pres as they usually have a high output impedance. Driving a low impedance input amp can affect the overall response and deviate from ideal. With purist tube pre's even the volume control setting can affect things because they lack additional buffer stages that would prevent this.
Good ways to see this is to look through the Stereophile archives and look at tube preamps. In the measurements section you will see the frequency response driving different simulated amps. Here is a link:
@autre I won't pretend to be smarter than I am.. I don't understand why it matters either, but preamps have a characteristic output impedance and amps have a characteristic input impedance. If memory serves me right, the amp input imp should be 10x higher (or more) than the preamp output imp. Having them too close can apparently cause the system to sound off or cause an increase in noise.
Anyone with more detailed knowledge, PLEASE addend or add!
I'm playing catch-up on the posts but thanks for the recommendation on the Wtred gear Scott. That STI has been on my short list as well, most likely the v1 since the used prices sre mire in my budget. I just haven't been lucky enough to snag one on the forum yet....
Todd, thanks for the reply. Please pardon my ignorance but I'm not sure what you mean by different input impedance. I'm still a bit of a rookie at the terms and what they mean towards the resultant sound.
If I may throw in my thoughts on the sound quality of class D amps, I can sum it up in 3 words: I'm a fan!
Class D skeptic. I was once, the latest from Merrill Audio, is excellent The Thor mono blocks at $4800 are very musical and best in class under $7k IMO.
@samac, yes all class D amps that have been in my system took over 1K hours to break-in and stabilize. But this has also been my experience with class A/B amps, with CD players, with two tube-based line stages, and with the Rowland Aeris DAC..
On the other hand, I have not tried in my system every amp on the market today, so I will not categorically state that all amps require over 1000 hours to break in...
I love the idea of a class D website, but to what purpose? Reviewing, comparing, contrasting? Or all of that plus sales?
It sounds like fun, but also like a lot of work.. though getting a steady stream of amps to review could lessen the pain.
I’m curious now about the sound of lower power vs higher power amps at low volume. I may have to try a little Dayton audio class D one evening, though it may be meaningless to compare a $60 amp to a $1000 amp and expect any sort of meaningful conclusions.
You know, I was just thinking of something. It would be really fun to marry a Nelson Pass First watt Class A amp to Class D voltage rails via transformers.
I'm glad you mentioned break-in hours again. I'm at 300 hours with my amp and it continues to settle in and get better.
Do you feel the lengthy break-in associated with class-D runs across the board? As I mentioned above I am running an amp that does not use the more common modules. So wondering if you believe it will also "season" and settle in past the 1000 hour mark. Honestly wouldn't surprise me if it does. Anything I've purchased fromDACs to speaker to amps have taken months to fully bed-in.
That's quite a coincidence...I just ordered a pair of Bel Canto's REF600M's to try them out. The ATI 544NC amp is equipped with dual NC-500 modules in bridge mode per channel, so plenty of power (900W in 4 ohms load) to drive any speaker out there.
I didn't want to end my quest for a 'perfect' Class D amp without auditioning the REF600's.
PS: I did find @georgehifi comment about REF600's little discouraging.
I'm so very happy with all of the participation on this thread, I'm also learning a lot, since I don't get to go to shows and dealers and listen as much as I would like to.
Would it be worthwhile to put together a web site JUST for class D amplifiers? It might give me the chance to listen to a lot more gear as a regular review feature, and break the mega-linear-amp stronghold.
Yes, George, Bel Canto Ref 600Ms are very good. I am right now listening to absolutely splendid sound of Mozart Symphony #40 with them driving Focal Alto Utopia Be speakers, sourced from Esoteric K-01 through an ARC LS 25 Mk II pre.
Thanks, Erik, great info. I had no idea it might mimic or be similar to other designs.
It’s my first adventure with class-D so I have no idea if it’s better or even as good as Hypex, B&O or Pascal. I do know I like everything about it so far. It’s the easiest amp to listen to since my octal pre/845 SET combo. Refined and smooth with no listening fatigue at all.
I hope you do check out NuPrime. Would love to get your thoughts on how they fare against some of the competition out there.
I looked at the NuPrime pages VERY briefly. NuPrime’s tech sounds a lot like the Yamaha EEEngine. Small class A amplifier sandwiched between Class D voltage rails.
Again, kind of goes back to Carver’s Magnetic Field amp and NAD’s switching rails technology.
Sure gets grate reviews. I would love to give them a serious listen to see what the benefits really are.
I mean, it’s quite different from Hypex/Pascal/B&O but I have no idea if it’s much better.
The challenge with this kind of hybrid approach is again, you rely on feedback to tell you everything about what you should do from instant to instant. With the voltage rails swinging at the same time the Class A amp is swinging it is like shooting an arrow from a moving boat.
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