Class D amps - use them where they are at their best

I think the important issue here is to make the most of the Class D amps, as one should do with any others.

I triamp a modified Tympani with Bohlender Garebener midrange drivers.

The Midrange carries the bulk of the music at 250 hz to 5 khz, and I found it did very very well with EVS modified NuForce 8 amps, better than modded Triode Dynaco MkIII, a Classe Dr-9 with substantial vibration damping and waaay better than Strickland designed brideged differential amps from Fosgate. However, the Nuforce requires a heavy application of viscoelastic damping to produce sharp transients. In part this is because of my loud listening volume.

What I consider "natural sound" is where I differ substantially from most audiophiles. I had been an amateur classical musician and come from a musical family where a Beethoven quartet was something you played in the kitchen with your buddies from the youth orchestra. Where Row 4 to
7 center is a normal perspective on an orchestra, and 6 ft away from Rampal in an impromptu concert is not a special sound situation, where sitting in on most of a piano competition is not taxing even if you are hearing Le Tombeau every half hour for six hours straight, and are only 20-30 ft away. Where you, or one of your parents is playing the piano alone or accompanying a soloist preparing for performance and you listen from a few feet away.

Though I was used to listening from within an orchestra, as clarinetists sit in the middle back normally, I had more than enough passive listening from the front of the orchestra to "fix" my reference there.

Unfortunately for me, the tactile physical presence of a musical experience that I was used to and remains my reference, is not the target of the recording engineers, the high end manufacturer/designer, nor most other audiophiles. I had been in a few dozen recorded sessions of chamber and symphonic music and wondered how the front balcony perspective of the microphone placement could ever capture my experience. Indeed, the foreshortened deep arc perspective of the orchestra, chamber ensemble or soloist just a few feet in front and at eye level would land a high end designer or recording engineer in the line of fire of most reviewers.

Nevertheless, with some modification and careful speaker and listening seat setup and a few kw of amplifier power hooked up directly to mostly resistive speaker drivers without passive LC components, it is nearly doable at sufficient volume due to the non-linearity of instrument's frequency response and attack characteristics with distance, you can get the close-up perspective by playing a recording made for listening at Loud = 90 db at an SPL of 100db and adjusting the trebble shelf on the crossover to correct gross disproportions caused by the recording or the jacked up volume.

That said, the NuForce amps were well beyond what the other amps could do at a normal output of 5-10 watts into 94 db efficiency and was the only amp I had ever heard, including a Bryston 4B NRB, that could do the midrange tutti and loud piano passages without particularly noticeable distortion when outputing 50-100 watts or more. And that is where their strength lies. For a moderate cost, you can get a clean 10-100 or even 150 watt midrange that is normally only available in $5000 per channel and up with pure class A solid state designs or megalithic tube amps that weigh as much as a Stonehenge stone block.

What they don't do is good trebble, where the high current Classe DR-9 outdoes the NuForce amps even without the benefit of the great advances made in solid-state's trebble since it was introduced over a decade ago.

If you compare the bit-rate of the PWM to the PCM of early digital at 500 khz to 700 khz and modern digital 24/192 at over 4600 khz, and compare sound quality to a "good" analogue master tape from which those are derrived, then you get an idea of where (in frequency) you want to cut off the use of a class D amp - at about 4-5 khz for class D amps operating in the 500-800 khz range.

As with old digital, one could coax extra performance out of the low bit rate technology, but one does not have an extra decade of frequency range above the treble signal (only one decade available) to make room for the detrimental effects of imperfect components (transistors, inductors, capacitors, processing chips). When 2-3 Mhz technology is available, it might be possible to get a reasonable cost class D amp to operate in the trebble without undue compromise.

I have no idea what the bass sound of the switching amps is like from personal experience, nor how it integrates with the midrange since my bass power requirements dwarf the NuForce amps entirely at about 1 KW per channel into 4 ohms delivered from a pro-audio Crown 5002VZ amp (2.5 KW into 4 ohms rating). Reviews comments tend to indicate a disproportionaly powerful bass tight, but occasionally over-ripe.

I can say the the NuForce amps are sensitive to reactive components in the speakers where I placed large inductors or capacitors that should not have made a frequency response difference in the operating band of the Nuforce amps (only 250-5000 hz is fed into them - above the inductor's pass band effects, below the capacitor's pass band effect) to "see what happens". Both components had very nasty sounding effects similar to the white glaze some complain about on the caps addition, and indecent levels of compression and higher frequency roll off with the inductor. This may have something to do with the great preference some planar speaker listeners have for the class D amps where despite LC active components in the speakers, the speaker provides a near resistive load.

My own experience and experimentation indicates that the class D amps should not be used with strongly reactive speakers, and are best when hooked up directly to drivers in multiamp setups with line level crossovers. so as to avoid introducing distortion from LC active components.