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  1. #11
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    Re: power amp tips

    Some amp I'm trying out



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    Pre amp: Ocatve Phono Module Power amps: Octave MRE130 Turntable: SAC Girati Grande. Tonearm: Dynavector 507mkII Cartridge: Dynavector Te Kaitora Rua Streamer: Auralic Aries LE DAC: Schiit Gungnir Multibit Power Distributor: Anzus Mains D8 Speakers: Raidho X-3 Speaker Cables: AudioQuest Rocket 88 RCA Interlink: Ansuz X, XLR Interlink: Purist Audio Design - Venustas Digital Interlinks Anzus Digitals A USB Phono interlink: Cammino Serie 1.0 - PH 1.0s Power cables: Anzus Mains Ceramic, two Pom, Aluminium, and two X series Audio Rack: Solid Steel H6

  2. #12
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    Re: power amp tips

    a.wayne ,


    “@TDSS,


    Hi Bob i have a question, could you explain or give your opinion as to why class-D amps have such poor squarewave responses and is this related to what is percieved as a wonky top end..”

    Thanks for asking. I’ll be happy to do what I can to answer your questions - for what it's worth. As far as “poor square waves” go, I first need to be certain with regards to exactly what you mean. If you are referring to the typical overshoot and ringing that Class-D amps exhibit, then I believe I can answer that. Although, if a given amp simply does not exhibit a sufficient Rise Time, then that just means that the amp’s audio bandwidth is limited to some upper frequency, possibly even down into the audio band below 20KHz.


    As an example, many manufacturers will test with a 10 KHz square wave as a means to check for amplifier stability at high frequencies. Excessive overshoot and wringing when driving a reactive load (moderately capacitive or inductive) can mean the amp is not sufficiently damped and may be prone to oscillation at some extreme high frequency beyond the audio band. This can be just as true for linear amps (Class A, A/B, etc.) as those of Class-D/Switching types, and in certain ways maybe even more so. In such cases, if the amp does “break out” into oscillation, it’s usually only a matter of milliseconds before it self-destructs.

    On the other hand, a bit of overshoot and ringing exhibited by a Class-D/Switching amp is considered to be somewhat “normal,” as typically it is all but impossible to completely eliminate the switching noise generated by the output transistors. If you analyze the frequency content of that energy you will find that it is usually some harmonic of the amp's fundamental switching frequency, which is almost universally well beyond the audio band and completely inaudible.

    BUT… that doesn’t mean that energy doesn’t have some effect on the final sonic presentation. In fact, “everything affects everything to some degree, it’s just a matter of degree.” The question I have is: “How does that energy interact with the speakers (drivers and crossover) and the speaker cables?” Specifically, does it cause any type of complex modulation and/or inter-modulation effects that would generate a-harmonic (not harmonically related to the music) “side-bands”? I’m a-think’n it just might.

    Then again, many of us love old high-quality analog recordings and those (other than direct-to-disc) where made using analog tape recorders. AND those virtually always employed application of a low-level “tape bias frequency” (usually somewhere near or around 200khz) that was laid down on the tape along with the signal being recorded. I won’t get into why, other than the fact that the bias was necessary to achieve a reasonable high frequency response.

    Are the two the same? No… but they are similar, enough so that based on the tape bias situation you’d think that the HF noise generated by Class-D amps should do no harm either. Whether it does or not, the jury is still out as far as I am concerned, but if you ask most any “academic-type” engineer he’ll likely tell you that the HF noise has no audible affect whatsoever. At least that’s what he’ll say when you ask him to figure out a way to get rid of it. Why? Humans don’t like their limitations being rubbed in their face and will often claim doing a certain thing is simply not possible – rather than admit their own ineptitude.

    Of course, it all comes down to distortion at some point. When it comes to that stuff, simple numbers are all but meaningless. What you absolutely NEED to know is what the total Distortion Profile (DP) looks like. In other words, what does the frequency response of the distortion products look like if they are plotted on a typical frequency vs. magnitude graph? That is a graph we are all familiar with, and we pay most attention to them when evaluating the response of a loudspeaker.

    OK, so you can easily imagine that such a graph may well have a number of peaks and dips all along its length, and with respect to amplifier distortion measurements, they usually do. So let’s use the speaker comparison a little more to get a better picture.

    So for “hypotheticals,” we’ll say a given loudspeaker has a response graph that exhibits a continuous rise from say... 3 KHz on up to 20 KHz. In that case, we’d expect that the speaker would sound very bright, with too much energy in the highest octaves. Well, if a given amp’s DP curve does the same thing, then you’d likely perceive the sound that it reproduces to be “too bright” and possibly irritating as well.

    Nevertheless, if you take a look at that same amp’s STANDARD Frequency Response graph (which is actually the same thing as the graph of a typical loudspeaker Frequency Response measurement), you will find that it might very well measure with little more variation than +/- ½ dB from 20 to 20KHz. Therefore, just looking at that alone you would never know there might be a problem, let alone what the amp will actually sound like.

    Now to complicate things even more, the DP will vary dynamically. At low output power-levels there will be a certain shape to the curve, and as the power levels increase the shape of the curve will change. So that means the various distortion products are constantly changing along with the output power level, as the ongoing dynamics continually change in the music.

    Then like one more bullet for good measure, let’s add another necessary level of complexity just to make sure the thing is really dead. Besides all of the above, we have to track how the DP changes with respect to frequency and the how the varying impedance of the loudspeaker affects current draw from the amplifier. Along with that, we have back-emf being generated by the speaker and energy storage/release driving signals back to the amplifier as a result of its complex reactance, hysteresis effects and magnetic materials saturation.

    YIKES! Even Bill Gates would have his hands full trying to develop an all-encompassing test that would quantify all of that! In fact, it would take some serious Artificial Intelligence computing power to even try, and even then, it still wouldn’t really tell you the whole story.

    So I guess the next best thing is to use the Natural Intelligence located between your ears and just LISTEN… because that’s about all we have to go by. Then we all know how listening is a subjective process, so hence we find the many different audio products out there to choose from, not to mention the headache of trying to glean some reasonable amount of truth in an industry dominated by hype.

    At this point, we find ourselves back where we started. Why do Class-D amps have a “wonky” top end? I dunno. Do they? If so, it’s most certainly has something to do with their DP, and I’m guessing it’s because in some amplifiers there are more complex HF distortion artifacts in the region above 3KHz. Is that situation unavoidable due to the nature of the technology? No, because personally I have heard otherwise, and I know others have too. You should hear a pair of our TDSS upgraded Ref-9V3s or possibly the NuPrime Evo-1, and maybe you’ll form a different opinion. I know that I can live with them playing night and day without developing any sense of listening fatigue, and I’ve been into high-end for over forty years, so go figure.

    I’ll finish by making my point with a little story. Back when I was the Support Manager for NuForce I had a gentleman call me for some reason or another, and we got to talking. In the course of our conversation he told me that he just loved the sound of his NuForce Ref-9s and how he could tuck both of them under one arm as he went out the door. In particular, he mentioned how he got a real kick out of taking them to his various audiophile friends’ houses to do comparisons with their amps. He said, “I plop them down next to my friends’ huge, 100+ pound, mega-buck mono-block tube amps, hook them up, play some good music… and watch their jaws drop and their frick’n eyeballs pop right out of their heads in disbelief.”

    So there ya go. What’s the truth of the matter? Whatever you happen to hear, I suppose. Perception is reality” – right? Why do you hear what you do? I’m betting it has something to do with the DP of the amp in question (most likely at high frequencies), or… your ears may be a little broken, or… your personal bias is skewing your perception so it’s simply your overactive imagination. Take your pick.

    Take care,
    -Bob

    PS. The DP issue regarding amplifiers as outlined above holds true for loudspeakers (which happen to be particularly horrible) in most ways too, as well as every other component in your system. Therefore, if you weren’t already pretty much screwed trying to figure all this crap out before now…
    HA! Welcome to the asylum!!!
    Bob @ TDSS

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  4. #13
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    Re: power amp tips

    Hi Bob, you are right about the TDSS modified Model 9v3 se amp. You modified my friend's amp and it sounds great. No problems with the top end and I never noticed a problem before they were modded. The only issue with it is that when they are not playing music they tend to whistle occasionally. It drives my friend nuts. He is probably going to sell them and go with the PS Audio M700 mono blocks. I am trying to convince him to try the Nuprime evo's.
    My Gear- Mains System-Pass X250 amp, BAT VK-51se preamp, Luxman DA-06 DAC, Magnepan 1.6's, Thorens TD-145 TT, Dual Martin Logan Subs, Vintage Luxman T-110 Tuner, Other systems- Parasound A21 amp,Van Alstine Ultra Plus Hybrid tube DAC and Preamp, Magnepan MMG's, Monitor Audio S1's, PSB B6's, Def Tech Pro Monitor 1000's, Velodyne sub, Adcom GFR-700 AVR, Music Hall 25.2 modified CDP, Cables by Cardas Parsec, AQ Columbia DBS 72v, Wire World.

  5. #14
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    Re: power amp tips

    Mechnutt,

    Your friend has a pair of my upgraded Ref-9s and they are "whistling"? That ain't right, and in fact it's a sign the amp module is failing. They should be DEAD SILENT at all times (except for maybe a split second at Turn-ON). Sell them and get Pass Labs? Not a bad alternative, but NOOOOO!

    Doesn't he understand that they are covered under our TDSS Warranty? Have him send those puppies back to me and we'll give them a good once-over. All he might have to pay for is the part (under $200) IF (and even then maybe not) we have to replace one of the modules. Those are my "babies" and if they aren't right we'll do what we have to in order to make them RIGHT. Please let him know. Thanks so much for letting me know and...

    Take care,
    -Bob
    Bob @ TDSS

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  7. #15
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    Re: power amp tips

    Quote Originally Posted by TDSS View Post
    Mechnutt,

    Your friend has a pair of my upgraded Ref-9s and they are "whistling"? That ain't right, and in fact it's a sign the amp module is failing. They should be DEAD SILENT at all times (except for maybe a split second at Turn-ON). Sell them and get Pass Labs? Not a bad alternative, but NOOOOO!

    Doesn't he understand that they are covered under our TDSS Warranty? Have him send those puppies back to me and we'll give them a good once-over. All he might have to pay for is the part (under $200) IF (and even then maybe not) we have to replace one of the modules. Those are my "babies" and if they aren't right we'll do what we have to in order to make them RIGHT. Please let him know. Thanks so much for letting me know and...

    Take care,
    -Bob
    I will let him know. He lives in Albuquerque, so you might remember taking to him.

    Larry
    My Gear- Mains System-Pass X250 amp, BAT VK-51se preamp, Luxman DA-06 DAC, Magnepan 1.6's, Thorens TD-145 TT, Dual Martin Logan Subs, Vintage Luxman T-110 Tuner, Other systems- Parasound A21 amp,Van Alstine Ultra Plus Hybrid tube DAC and Preamp, Magnepan MMG's, Monitor Audio S1's, PSB B6's, Def Tech Pro Monitor 1000's, Velodyne sub, Adcom GFR-700 AVR, Music Hall 25.2 modified CDP, Cables by Cardas Parsec, AQ Columbia DBS 72v, Wire World.

  8. #16
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    Re: power amp tips

    Quote Originally Posted by TDSS View Post
    Hi Bob i have a question, could you explain or give your opinion as to why class-D amps have such poor squarewave responses and is this related to what is percieved as a wonky top end..”

    Thanks for asking. I’ll be happy to do what I can to answer your questions - for what it's worth. As far as “poor square waves” go, I first need to be certain with regards to exactly what you mean. If you are referring to the typical overshoot and ringing that Class-D amps exhibit, then I believe I can answer that. Although, if a given amp simply does not exhibit a sufficient Rise Time, then that just means that the amp’s audio bandwidth is limited to some upper frequency, possibly even down into the audio band below 20KHz.
    I'd like to add to Bob's exceptionally detailed and helpful response regarding an amp's square wave response by defining what frequency components actually exist in a square wave. As explained in more detail in this reference, "square waves are mathematically equivalent to the sum of a sine wave at that same frequency, plus an infinite series of odd-multiple frequency sine waves at diminishing amplitude." Therefore limiting the bandwidth of an amplifier can significantly impact its ability to accurately reproduce a square wave since the higher frequency components will be decreased in level, as illustrated by some of the graphs in the linked reference. Since most Class D amps use a low pass filter on their output to reduce high frequency switching noise, that will impact their ability to accurately reproduce a square wave particularly at higher frequencies. Whether bandwidth limiting could be the cause of what a listener may perceive as a "wonky top end" in some Class D amps is not definitive, there would however be an impact on a such an amp's measured square wave performance due to the filtering of its higher frequency components. Amp manufacturers that espouse exceptionally wide bandwidth electronics design often claim that a more limited bandwidth negatively affects the phase response even though the frequency response may be fine.

  9. #17
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    Re: power amp tips

    I'll most likely end up buying the mono's MRE130. They sound really good

    But I'm figuring out how to finance it.
    Pre amp: Ocatve Phono Module Power amps: Octave MRE130 Turntable: SAC Girati Grande. Tonearm: Dynavector 507mkII Cartridge: Dynavector Te Kaitora Rua Streamer: Auralic Aries LE DAC: Schiit Gungnir Multibit Power Distributor: Anzus Mains D8 Speakers: Raidho X-3 Speaker Cables: AudioQuest Rocket 88 RCA Interlink: Ansuz X, XLR Interlink: Purist Audio Design - Venustas Digital Interlinks Anzus Digitals A USB Phono interlink: Cammino Serie 1.0 - PH 1.0s Power cables: Anzus Mains Ceramic, two Pom, Aluminium, and two X series Audio Rack: Solid Steel H6

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  11. #18
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    Re: power amp tips

    audio.bill ,

    Thanks so much for chiming in and adding that additional information!

    I feared going that far might be getting a little too deep in the weeds for some folks, so I hesitated. Nevertheless, it would be a good thing if the devout enthusiasts out there could take a good look at that info and manage to wrap their heads around it, because much of what we deal with on both the engineering and listening sides of this stuff surrounds the square wave response issue.

    I’d like to comment on your previous statement though:

    "Since most Class D amps use a low pass filter on their output to minimize high frequency switching noise, that will impact their ability to accurately reproduce a square wave particularly at higher frequencies."


    In my experience the low-pass filter mentioned above has not been a significant issue in the Class-D amps that I have been involved with. Frankly, I forget exactly what it happens to be, but in the case of the NuForce amps I do know that they have a POWER bandwidth (as opposed to only a small-signal bandwidth) that extends out to 50 KHz (sine wave). In fact, back some years ago we had a customer that purchased a few of them for scientific use in an ultra-sound technology application, and it was specifically for that reason. Now maybe they didn’t do a thorough search, but FWIW the customer told me that our amps were the only ones they could find that would fit their requirements in that regard.

    That said, due to Nyquist the low-pass corner frequency had to be around 100 KHz or a bit higher. Well, there are a whole lot of linear amps out there that can’t even come near achieving such a high frequency power bandwidth, and yet in the context of this discussion the perception is that “linear is better”? Maybe so, but not for that reason. At least not when it comes to the NuForce (and Crown) gear that I am familiar with.

    Furthermore, the NuForce Ref-9s had an advertised Damping Factor of 20,000… (from DC) clear out to 20 KHz!!! I’d (just about :-) bet my fat frick’n behind there’s not a linear amp out there at any price that can even come close to that! So, you can imagine what the loop & power bandwidths had to be to pull that off.

    As an aside, back when the first Ref-9v3s came out, NuForce's engineering guy (long since gone) obviously didn’t know his stuff like he should, because the things were blowing up and/or frying the tweeters in customer’s speakers. Actually, the guy was just a “fill in” for “Tran” who was the original designer of the basic circuitry. Tran had left NuForce a couple years or so earlier, so the fill-in guy was just making tweaks to his designs and somehow managed to come up with the V3. In any case, he left the job without working the bugs out, so Casey Ng asked me if I could fix the problem.

    Anyway, I took a look at the thing and yep… the first thing I did was run a square wave at (to be safe) 1 KHz through the thing. Sure enough, that puppy was just-a ringing away, with at least 1/4th of the top of the square wave looking like one big UHF “burst.” At that point, I removed the worthless Zobel Network they had soldered to the output terminals, grabbed some small caps, coils and power resistors, and went to work stabilizing the thing. A couple of hours later all that was left on the oscilloscope was a cycle and a half of small overshoot > undershoot> and little damped bump – and that was it. Textbook perfect, and that was that. Never another problem (in that regard) since. After that, the Ref-9v3.0 became the v3.01

    The point being, while I was scoping the thing out, just for the hell of it I decided to check the Slew Rate. Well, knowing that F = SR/ 2*pi*V, it wasn’t hard to figure out what the upper limit of the bandwidth was (which I never actually got around to doing). It’s been so long now (fall of 2009) that I don’t remember what the exact numbers were, but FWIW I do remember thinking “WOW” after I measured the SR, because it’s not all that hard to do the conversion in your head.

    You see, in years past at Crown I had taken so many SR measurements and done the conversions that I got to the point that I didn’t bother to work the formula out anymore. Just check the SR and if it’s pushing 8V/uS or better when you’re outputting 100-watts into 8-ohms… and you’re good (~ 50 KHz bandwidth). Well, from what little I do remember, that early production 9v3 in back in 2009 was doing at least 15V/uS or thereabouts, so again… we’re talking somewhere around 100 KHz or so.

    Now, I can’t tell you about any other products though, because I have never worked on anything other than Crown & NuForce amps. Maybe the L-P filter is an issue with some? I dunno. I just know that it doesn’t necessarily HAVE to be.

    Thanks again and…

    Take care,
    -Bob
    Bob @ TDSS

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  13. #19
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    Re: power amp tips

    Bob @ TDSS - Thanks for your followup and detailing the low pass filter implementation in the Class D amps you've been involved with. I can certainly appreciate that the chosen frequency of any low pass filter and therefore the amp's overall bandwidth will determine the impact on its measured square wave performance. Taking your 50kHz bandwidth for example, with a 10kHz square wave signal only the third and fifth harmonics would be within the 'theoretical' pass-band. With higher order odd harmonics missing or at least reduced in level the resultant square wave would show some distortion, however whether that level of distortion would be audible or of any significant impact for most listeners is debatable.

    While I'm an EE my career focus primarily specialized in telecommunication software development for AT&T Bell Labs, so I have nowhere near the level of real world experience in hardware design as you do. I sincerely appreciate you so openly sharing your design expertise in the field with us here on AudioShark!

  14. #20
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    Re: power amp tips

    Today I bought the Octave phono module, Octave MRE130’s, Anzus mains P, Purist Audio Venustas 6m XLR interlink.

    It sounds great IMHO
    Pre amp: Ocatve Phono Module Power amps: Octave MRE130 Turntable: SAC Girati Grande. Tonearm: Dynavector 507mkII Cartridge: Dynavector Te Kaitora Rua Streamer: Auralic Aries LE DAC: Schiit Gungnir Multibit Power Distributor: Anzus Mains D8 Speakers: Raidho X-3 Speaker Cables: AudioQuest Rocket 88 RCA Interlink: Ansuz X, XLR Interlink: Purist Audio Design - Venustas Digital Interlinks Anzus Digitals A USB Phono interlink: Cammino Serie 1.0 - PH 1.0s Power cables: Anzus Mains Ceramic, two Pom, Aluminium, and two X series Audio Rack: Solid Steel H6

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