Class D at low volume

03-04-15: Kijanki
Bombaywalla, Linear power supplies have regulation in preamps, DACs etc. but MOST of power amps are unregulated because of the amount of heat dissipated in power supply (and loss of efficiency).

agree. There might a small change happening here - Sanders Sound Systems Magtech amps have a regulated power supply (& I believe Roger Sanders has applied for a patent for that?).

Look at fig. 2 showing how PWM signal can be obtained by just using linear ramp. Icepower modulator uses sinewave, has two feedbacks etc. - but only for improvements.
http://en.wikipedia.org/wiki/Pulse-width_modulation

thanks for this reference, Kijanki I'm quite familiar with class-D PWM using a ramp waveform.
My question was using delta-sigma instead. There might be some info on that same page you sent the link for. I'll read up.

Also had a question re. how class-D PWM can be designed without feedback like you wrote in your initial post. Got any references that I can read?

My amplifier with bandwidth of 60kHz shifts phase of the 20kHz signal by 20 degree.

agree! The "funny" thing here is that I wrote something very similar in another thread but had Almarg state a contrary view stating that it wouldn't be such a big deal given that the hi freq energy is much less at these frequencies. Perhaps that is true - it's true that the there is generally less energy at the high frequencies. So, even tho' your amp produces a 20 deg phase shift at 20KHz, it might matter very little(?).
What I wrote in that thread specifically was

....This also means that since the ARC Ref 150 bandwidth is just 3X (rather than 8X or 10X) the music bandwidth (of 20KHz) one can expect to hear the amp impart its own phase shift onto the higher frequencies of the music. This can manifest itself in a few ways - the highs could sound rolled-off or they could sound warmer or there could be less sparkle/shimmer compared to an amp of higher bandwidth.

.
Almarg's comments to my post:
http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1423113432&openflup&50&4#50

Went to class D 12 years ago when replaced my Bryston 3B-ST with Sunfire 300(class T which is same as D) and never looked back.

Bombaywalla, fig. 2 shows exactly how it can be done. For
any input voltage corresponding duty cycle is created.
Feedback would only improve performance. Modulator itself
might be not 100% linear but most likely linearity error is
created by the Mosfet switches. Their positive and negative
slew rates are different and vary a bit with the load
(creating timing errors). Negative feedback improves timing
but also lowers output impedance increased by the presence
of common mode choke. Early D class amps (tripath) had this
choke outside of the feedback and had loading problems with
many speakers. Icepower uses two different feedbacks. One
is from the output of modulator (improving timing), the
other from the speaker output. At least that's what I
remember from Karsten Nielsen doctorate.

Icepower uses single supply. Speaker (thru Zobel network)
is placed in the bridge made of 4 N-channel Mosfets. Bridge
is connected to GND and +Vs. At any given moment speaker is
always connected between +Vs and GND only direction changes.
Mosfets are turned on in pairs diagonally. At 50% duty
cycle average, filtered voltage on the speaker is zero. Half
of supply voltage is always present on the speaker's
terminals and the output cannot be bridged. Hypex used only
two Mosfets switching speaker between GND and +Vs or -Vs.
This allows output to be bridged.

As for the phase shift - 20 deg is a substantial phase shift
but I have no idea how much it (summing of harmonics) is
audible. Obviously Jeff Rowland wants no phase shift,
extending bandwidth to 350kHz.

I've read few times people saying "I know that class D
requires a lot of negative feedback" Now I hear that it
cannot even operate without feedback. How do they know
that??? If anything, class D requires less feedback since
voltage-duty cycle conversion seems to be more linear than
nonlinear bipolar transistors in class AB output stage.

SACD is an example of class D without any feedback.
Incoming 2.8MHz train of pulses is averaged and becomes a
sound.

Kijanki your right on the money,the Tim distortions can be very can be very fatiguing without noticing it

Serge,

You asked:

"Thanks Noble100. Speaking of feedback, what do you think of Benchmark AHB2."

Sorry, I've never listened to the Benchmark AHB2 or read any reviews on it yet. I've read very positive reviews on Benchmark DACs, however, and knew they recently came out with a couple of new class D amp products. If I were to venture a guess, I'd bet that the obvious care and attention to detail they devote to their high quality DACs is likely to also be devoted to their amps.

Mapman,

I've read your comments about class D, on this thread and several others, and typically find your impressions on this evolving technology closely mirror my own. I think that 2 independent users of class D amps having such similar impressions of their high quality performance, among other benefits, speaks volumes about the excellent high-end bargains these amps actually are. It seems like we're both now drinking the class D Kool-Aid.... but only because it's delicious.

Bombaywalla and Kijanki,

Thank you both for explaining things in more detail.

Bombaywalla, you stated:

"The way class-D architecture was invented/designed/formulated, global negative feedback is part of its entity. So, just because you read global negative feedback you shouldn't relate it in the same way as you would to GNFB in a class-A, AB power amp. The class-D architecture needs GNFB while class-A, AB architectures have topologies that can do with little or no GNFB."

I realize that many traditional amp designers have considered the use of GNFB in their amps to be something to be avoided since the early tube designs, believing having zero feedback improved sound quality. Solid-state amp designers seemingly just adopted this low/or no GNFB principle in their next generation class A and A/B amps.

I also realize class D amp designers have taken a radically different approach, embracing GNFB and making it an integral circuit within the amp to lower noise and distortion levels as well as to ensure the input signal faithfully matches the outputted signal.

In my visits to various hi-end audio shops over the years, I've listened to many systems powered by tube, class A and A/B amps (with zero and low GNFB design) that sounded excellent. However, most of these amps exceeded my budget by a large amount.

From my layman's perspective, good class D amps that utilize high levels of GNFB represent a great alternative that is more cost effective and energy efficient means of achieving excellent sound.

I'm not technically savy enough to know what the main reason is for my class D amps' excellent performance. I just know they are lightweight, small in size, sip electricity, run cool and sound great.

But I think Paypet's comment probably drives the point home the best:

"Listening to wyred for the last month , difference between wyred and wytech ruby mono blocks , wyred $2995 , ruby $25000 , can't pick out the differences they are that close ,doing A B tests for 3 weeks nothing stands out in either one , will keep trying."

That is a very honest and powerful testimonial.

I apologize for getting a bit off topic from Serge's original post question and am now stepping off my class D soapbox.

Tim