Can you read your full last paragraph? Yes- in rereading it, it makes sense to me... However overly low impedance can accentuate ringing. I get the impression you would be familiar with snubber circuits? It is similar to a resistor in series with the capacitor. The resistance is needed to damp ringing. Usually those snubbers you see in amps are for the amp’s benefit, not that of the loudspeaker. But keep in mind almost any amplifier will provide damping to almost any loudspeaker. But the simple fact remains that there is no signal in audio where the ability to stop on a dime is important. As I just pointed out, once the woofer nears the zero crossing point it will want to keep going for a bit, but once it gets to the zero crossing point whatever signal that allowed it to get there will also be calling on it to continue in the same direction past the zero crossing point. There never is a point where the woofer stops unless the amp is shut off. So the two points of stopping are: *a DC pulse, or * the amp being turned off In either event no audiophile listens to either so its moot :) |
In theory the "signal" stops and starts as recorded so that is accurate.
The problem is the woofer does not stop right away. In a lower damping
factor amp the output resistance will help dissipate the energy in the
woofer which can give it a more controlled stop. This is of course the theory but in practice (IOW the real world) is not a thing. The woofer never 'stops'; its always in motion; there is no recording where the woofer 'stops'. The only way for it to 'stop' as often described abover is if a DC pulse is being reproduced, which is something that neither the amp or speaker will ever have to do. On this account I've often viewed this as a red herring. More to the point, the idea is that the woofer will continue to oscillate after the initial pulse. But in reality after the woofer moves the first way, the audio signal directs it in the opposite way; it never gets the signal removed and so can't ring. This is why amps with a low damping factor can do quite well in the bass. The real problem isn't ringing or distortion, but getting too much bass if the amp fails to reduce power into impedance peaks of the woofer's impedance curve. This isn't a control issue (which is also a problematic idea), its simply that an amp with insufficient low impedance will simply make too much power. |
My theory is that the very high damping factors unique to class D amps was truncating the decay times of the bass notes. IMO it is possible to overdamp loudspeakers and thus truncate the bass notes, ending up with a coloration called 'tight bass'. In this regard, amps have outstripped loudspeakers in terms of advancing the art. What I mean by this is for an amplifier to be a true voltage source, it has to have a very low output impedance, and while most speakers (certainly not all!) are meant to be driven by a voltage source amplifier, the simple fact is that they also should not be overdamped, and most high powered solid state amps do just that. No speaker made needs more than about 20:1 as a damping factor. But we see amps that have 500:1 and more- there is no way these amps can avoid coloring the bass as a result. IOW I agree with your observations. |
Have you made a decision yet on whether you’ll be offering free limited in-home trial periods for auditioning? We're not nearly that far along! With regards to GaN performance, they can be variable depending on who made them and their intended application. Many of them have a reverse diode conduction phase which can really throw the designer for a loop. While they are faster, MOSFETs have been steadily getting faster too so at practica switching speeds we're seeing now in another year or two MOSFETs will probably work just as well. One problem we're seeing is its very easy to build a class D with a very high damping factor, much higher than any speaker should really see. |
We're still working with prototypes. They have had 100 watts and double power into 4 ohms. Since last spring, they have had GaNFET output sections. We compare them directly against our tube OTLs as a reference. Mostly we've been sorting out layout issues; designing a class D from scratch using individual parts is challenging if you want high switching speeds! But we've been sorting things out, sometimes one at a time as problems are identified and solved.
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Would you say there is inherent limitation in the phase shift that is a
factor of the comparator speed, digital logic delays, and turn-off time
of the FET? What do you think a practical limit is on that? That may put
a practical limit of a few-10 degrees at 20Khz, but that would be
inaudible. The phase shift is indirectly related to the speed of parts like the comparitor (which typically has plenty of more speed than the output section). It is directly related to the filter at the output. As I understand it from you and Bruno Putzeys as a layman, there is
no such thing as too much feedback with class D amplification. The
generally accepted concept that feedback negatively effects the sound
quality of traditional linear amplifiers may be true, but this aversion
to feedback doesn't apply to class D amplifier design. Very high levels
of feedback, in an intelligently designed class D amp, are actually
utilized to optimize the sound quality throughout the entire audible
frequency spectrum.
Am I understanding this correctly?
That's it in a nutshell. The problem is phase shift in traditional designs prevents the application of the required amount of feedback in such amps; otherwise oscillation will occur. For this reason **every tube and solid state amplifier made has had insufficient feedback**. This has resulted in the applied feedback adding its own distortion- which is interpreted by the ear as brightness and harshness. This is why every amplifier with feedback up until now has sounded brighter and harsher than real life, and has fueled the tubes vs transistor debate all these decades. The industry has also been complicit in this problem, so when you see harmonic distortion measurements the fundamental frequency is usually fairly low (60Hz is common) so as to not get in trouble with the gain bandwidth product limitations of the amp under measurement! And this also explains why zero feedback amplifiers (like our OTLs) exist, as by designing a circuit that is linear enough to run without feedback the harshness and brightness of feedback is avoided. Transient Intermodulation Distortion is a symptom of poor feedback loop design coupled with poor gain bandwidth product and insufficient feedback. Can you define the time domain characteristics of feedback in a Class-D amp and linear amp? Yes. In a class D amp its all about propagation delay. In a conventional amplifier its all about phase shift as capacitive strays roll off the response (introducing phase shift). Effectively both have the same effect- at some high frequency the feedback is no longer negative so oscillation can occur. But unlike a conventional design, in a class D you can take advantage of that oscillation by using it as the switching frequency. |
I did not understand what kind of patent is this as class D amps are widely used for already more than a decade. 2 decades actually, but that does not mean that everything to be known about them is already known. The proof’s in the pudding when we see independent measured specs on.
what switching frequency used
output filter corner frequency
switching frequency residue on output
dead time performace
phase shift figures from 2khz to 20khz
rms output wattage just before clipping into 8ohm, 4ohm and 2ohm.
All of which are known Achilles Heel’s of class-d
This type of generalization is problematic! A self-oscillating class D amp can have a fairly low switching frequency (400KHz for example) and consequential low filter frequency (80KHz) and yet little or no phase shift at audio frequencies. This is because a self-oscillating class D amp can run so much feedback that it can correct phase shift and even distortion caused by the application of loop negative feedback. In order to do this the feedback has to be in excess of 35dB which is nearly impossible with conventional amplification due to poor gain bandwidth product. But gain is easy to create in class D circuits. |
As for ecological concerns, at low power levels, where most amps operate
during most listening, even class D amps are inefficient. This statement is incorrect. The efficiency of a class D amp is such that at higher power levels its power draw is similar to conventional amps. But at lower power levels a class D amp draws considerably less power! |
This is wholly, completely, and utterly false,
speculative, and prejudiced not only have some of us "listened' we have
also actually "measured" using reliable, repeatable, objective
measurement techniques that are recognized by engineers, scientists, and
industry as fitting, proper, and appropriate but of course if you are
happy with Class D you should absolutely enjoy it but you're reasoning,
arguments, and suspicions are unfounded and false. Wow. Because of the unqualified way in which this is stated, it renders the statement false. I have a little Topping class D amp that makes 30 watts per channel. It has a lower signal to noise ratio than a Realistic SA-175 amp (despite the fact that the Realistic only makes 10 watts) we have in the shop that is rebuilt (being nearly 50 years old). In case anyone is wondering why I have an old Realistic amplifier hanging around, I put myself through college working at the regional Radio Shack repair center in the Twin Cities, and I enjoy troubleshooting and rebuilding work as a hobby; this amp is definitely a bit of nostalgia and its cute. I rebuilt the matching tuner too. In our work the main thing that we've seen that contributes to noise in a class D has to do with the encoding scheme- in our case, Pulse Width Modulation (PWM). PWM relies on the use of a triangle wave generator and a comparitor that compares the incoming audio signal to the triangle wave and thus has an output that is either on or off. If the triangle wave generator is not perfectly steady in its frequency, or if there is a bit of DC offset at the input of the comparitor, the result can be a bit of white noise hiss in the loudspeaker. With fairly simple techniques this noise can be reduced to noise floors that are less than conventional amplifiers. |
Like some one said here they have one that goes to 1mhz in bandwidth!!
this one will certainly over time cook or quickly the tweeters voice
coils.
I even leave them on most of the time as recommended.
I don't think this is a good practice, as you don't know or
hear how much switching noise is being let through to the tweeters, if a
bit, it can slowly temper and blue the voice coils, as what happened to
my friends Wilson tweeters, they still worked, he was just complaining
that his highs had deteriorated, and yes those well known expensive
Class-D's were left on 24/7 for over 1 year on his speakers.
This is all nonsense. A typical residual waveform might be 1/2 volt; into 8 ohms that's 0.03 watts. A lower powered tweeter might be rated at 2 watts and would never see any temperature rise with such a small signal. This anecdote is misleading, false, is apparently calculated to cause alarm; its not based on an understanding of how class D amps operate. |
Read again from Texas Instruments, far more authoritative than you
Ralph, and this is still with the output filter in place, but with it
set too high, without it and you have a tweeter meltdown.
From Texas Instruments:
On output filters of Class-D amps that are set too high corner frequency.
"A concern with the switching waveform being dissipated in the speaker is that it may cause damage to the speaker"
This
is what happened to my mates Wilson 8’s above, as Wilson does not use a
Zoble Filters on it’s tweeter and I on any of their speakers tweeters.
Oh, sure- that is indeed a concern for the *designer*. Not the finished product! Your friend's speaker was not damaged by the residual of a class D amp; that idea is ridiculous. If indeed the anecdote is real and this isn't a made up story then the tweeter was damaged by an amplifier being driven into overload. That is the most common way tweeters are damaged. |
From Texas Instruments on output filters of Class-D amps that are set too high corner frequency.
"A concern with the switching waveform being dissipated in the speaker is that it may cause damage to the speaker"
This
is what happened to my mates Wilson 8’s above, as Wilson does not use a
Zoble Filters on this tweeter and I think all their speakers.
This statement is false. There are no class D amps that run without filters- such an amp is theoretically possible, relying on the inductance of the load to sort things out, but so far even with GaNFETs the switching frequencies are far too low. More likely the reason for the tweeter failure (if even real) was that the amp used with the speaker was overloaded. That's a classic source of tweeter failure! |
Go near one with a portable am radio tuned off station around 600khz (or
whatever your switching frequency is) and see what you get out of the
radio.
If you look at their output on a oscilloscope they are
more than just noisy as heck! And that (undetectable to human) noise
goes through to the tweeter if it’s not Zobel’ed as many hi-end ones
aren’t This is all nonsense. |
