Who tried Class D only to return to S/S or Tube

My Rowland 102 sounded the best after about 400 hours. Class D amps are very revealing and often don't work well with some "bright" components. I cannot say if class D works better when powered for a while since it is always on in my system. It doesn't even have on/off switch. Yes, trebles are different, more natural IMHO. Cymbals sound fuller and brassy and not "splashy" as it was with class AB amp. It requires some time getting used to. Midrange is just wonderful.

"Once the technology is there and they put that switching frequency way up higher, then and only then will they be able to filter the s**t out of it away from the audio band and leave a nice clean square wave, and only then will Class D have become of age"

Stereophile picture is about right:  1% of about 50V switched at 400kHz.  I’m not sure why it bothers you - you cannot possibly hear 400kHz.  Speaker will respond to average value.  There is distinctive possibility of tweeter modulating this wave with other frequencies, but this would be true only if tweeter’s membrane could move at 400kHz - not likely.  Also any effective radiation from speaker cable would require about 1/10 wave antenna that is 750m/10=75m.  I wouldn’t worry about that with few meter cables.  There is still possibility of cable to cable capacitive coupling but, because of shielding it would me minuscule amount further filtered by amp’s input filters.  There is some phase shift at 20kHz (about 20%), but it is usually the case of any amplifier bandwidth limited to 60kHz (my Rowland 102). Improvement in Mosfets speed will allows better initial linearity that is right now pretty good to start with.  Class D amplifiers suffer less delay (being one stage only) than multistage class AB amps reducing effects of TIM (resulting in softer sound).  Output impedance is also low by design, even without feedback.

Second generation of class D amps was better but I’m not even sure why.  Bel Canto’s Ref1000 got additional power supply caps and inductors.  That possibly reduced noise on the power cable (Power Factor Correction) but Icepower module used in both generations of this amp has line and load regulated power supply (uncommon in class A or AB amps).  My Rowland 102 is plugged into Power correcting Furman conditioner (big cap and inductor plus filters). Second generation of class D amps served one important task - it allowed some critics to remove foot from their mouth.

If top designers like Jeff Rowland, who is very sensitive to noise issues, don’t see any problems in class D or SMPS (that he uses in class AB amps), then I wouldn’t worry about it either and would judge it only on the merit of sound - it is the only thing that counts.  If it sounds good then it is good.

I’m  a little puzzled by all this "switching" talk.  My class D amp has 60kHz bandwidth while tweeters are almost completely quiet with my ear next to them (200W amp).   I’ve read the same in many reviews of different class D amps.  It was much worse with class AB amp in the same setup.   Highs in my amp are very clean while midrange is simply wonderful.   As for SMPS being noisy, nothing can be further from the truth.  New zero current/zero voltage switching SMPS are extremely quiet and that’s why Jeff Rowland uses them not only in class AB amps but also in preamps purely for their low noise (efficiency is unimportant in preamps).  Just to show an example:  Benchmark reduced noise floor in their latest DAC by 10dB using SMPS instead of Linear PS used in previous model of the same DAC.  It is because Linear PS is not linear at all.  It is simply unregulated noisy switcher that switches at 120Hz at max voltage producing not only hard to filter 120Hz but whole bunch of high frequency harmonics - since current is taken from the mains in huge narrow spikes.  Power supply is line and load unregulated and requires huge amount of capacitors to keep voltage steady.  These high inductance capacitors are in series with the output (circuit closes thru them).  Adding parallel non inductive caps might help but it will also create (with inductance of electrolytic caps) a parallel resonance circuit that will ring.   That’s why class D amps with SMPS are praised in  many reviews for keeping composure under heavy peaks (orchestra’s forte).  Lets forget about "switching" - speakers are unable to play 400kHz while we are unable to hear it at all.  It is imagined problem by "experts" that have absolutely no clue trying to find something wrong. Class D are linear amps where duty cycle, instead of voltage, is used as adjusted quantity.  

Ait, Choke at the input as well as choke and big capacitor in my Furman Power Factor correcting conditioner serve as filter. Charging Linear PS capacitors is still done in narrow spikes of high amplitude - no escape from that.  Current is switched on  and off when voltage is the highest (worst possible moment). Choke filters out spikes from the power cord but problems inside still exist.  Switching noise propagates thru the circuit affecting sound.  The only linear part in "linear supply" is that it is unregulated.  Output voltage still depends on the load while line regulation is zero.  I wrote about "linear" supplies, to show that switching can be done smarter in SMPS that has very tight line and load regulation - usually not possible in power amps linear supplies because of power dissipation in voltage regulators.  There are great sounding amplifiers with linear supplies, even without any input choke.  I’m only stating that it can be done better with quiet SMPS.  Switching supplies designed by Jeff Rowland are extremely quiet.  FAQ on his website explains why he abandoned traditional "linear" supplies.

http://jeffrowlandgroup.com/kb/questions.php?questionid=145

Ait, Perhaps I'm missing something.  Imagine rectifier charging capacitors to 50V with 5mV ripple present. 

http://i.stack.imgur.com/BLGnf.gif

Capacitors are charged ONLY during voltage increase on capacitor.  This period for 5mV ripple (1% of 50V) is:

Arccos(0.999) x 16.6666ms / 360deg=0.12ms  

Charging current has to be huge (hundreds of amperes) during such short time to meet average current draw.  In addition to this huge current spikes rectifiers are not fast enough to turn off at the peak of the voltage conducting current for a moment in opposite direction to snap back to zero.  That creates extremely short spikes that large inductive caps won't filter and that will couple to any LC circuit.  Using "softer" diodes (faster switching - slower snap back) helps to reduce this noise (makes pulse longer).  There is a lot of switching noise from Linear PS polluting from 120Hz to tens of kHz

In addition to noise created by linear power supply huge spikes of current have also huge rms/average ratio causing big losses in transformer windings while high frequency component causes losses in the core (eddy currents, hysteresis).  You might filter these spikes with chokes but capacitor itself can be only charged during very short time (fraction of ms).

Ait, I understand that choke helps storing and releasing energy, but it won't affect charging of capacitors that has to happen in very short time.

Ait, I think it is possible to place very large coil in front of capacitors in order to obtain constant charging current in form of sinewave.  The problem I can see with this scheme is size of the coil (huge inductance with low resistance) and the fact that voltage on the capacitor will depend on the load.  Voltage with load should be roughly 2/3 of peak voltage, but without any load it could climb up to 100% (peak value). I guess it should be possible in class AB amps to provide minimum load with additional resistor (if necessary).   Interesting concept, but I suspect seldom used because of the cost and size.  On the other hand high end audio is never practical.  Thanks for the info - I will find some articles to read on this subject.

tfats, I've never heard Channel Islands but I know they were based on Hypex modules and had great reviews.  Bel Canto used Icepower modules (that my are in my Rowland 102 as well). Hypex had dual supply (+/-) but only two output Mosfets switching (thru Zobel filter network) one end of the speaker (the other end at GND).  Icepowers used single voltage but four Mosfet output bridge switching speaker direction between + and GND.  Somebody mentioned sound difference between two families, Icepower being closer to tube amp while Hypex closer to very good SS amp.  I'm not sure if output configuration has something to do with it or it is just different signal processing (modulator).  Hypex configuration is more practical (output can be further bridged), but I'm happy with Icepower.  It sounded different with different speaker cables - a little thin and colder with older AQ cable and fuller/warmer with Acoustic Zen Satori.  Lower midrange got stronger, cello sound fuller, male voices have depth (chestiness).  

kuribo, it appears that we need some common language here.  Saying "technology is mature" is a polite way of saying "It hit dead end" - therefore don't expect major improvements and place your order today. There are many technologies still improving in exponential rate HDTV, Phones, Computers, DSP processing etc. that most likely will keep improving, but it doesn't mean they are inferior to such mature technologies like B&W analog TVs.  The fact that effect is achieved by switching instead of steady value doesn't mean it is worse.  Plasma HDTV operates on class D principle and to many people it looks more natural (colors, motion etc) than LCD/LED HDTV.  Modulator used in class D amps resembles Delta-Sigma A/D converters that already went thru many years of improvements (PWM is a byproduct of Delta-Sigma processing) and in my opinion will keep improving for a long time.  It is not even a question what class amps are the best, but rather how good is the amp I can afford.  Many class D amps offer a lot for the money.

There is only about 1% of 500kHz switching noise on the speaker cable, and that is absolutely irrelevant. Placing an ear next to tweeter proves only that people cannot hear 500kHz (almost complete silence). Modulating 500kHz by the tweeter would require its membrane to move at this frequency - not possible. There is no sizzle of any kind in my class D amp. If anything, class D amps are reviewed as "dark sounding". Yes, higher bandwidth would be always nice, but 65kHz -3dB sounds fine to me (some class D amps go twice higher).