Any experience with Class D amps and impressions?

[infullview]

I started a thread on class D amps a while back and it went on forever! Here's one thing I can add: I worked as a tech during the 80's for Data General while I was attending college. They used *many* switching power supplies for their equipment and they were all pulse width modulation (PWM) design which is the same tech used in class D. Even back then I could see the use for these as audio amps, but they do have some peculiar problems. 1) It's nearly impossible to completely get rid of harmonic interaction with the main switching frequency. 2) PWM power supplies are prone to be unstable at zero load (no signal). If you understand that PWM works by modulating the width of a square wave, it is easy to visualize what happens at very low levels when that square wave gets so thin it randomly starts to disappear. At that point, feedback controls go haywire, and the circuit becomes unstable. I'm sure that new class D amps try to get around some of these design issues by using some sort of DC bias and small constant load to keep the wave stable. Class D has it's uses, but for home audio I'd put my money on a good class AB system.

 

[Poultrygeist]

I started a thread on class D amps a while back and it went on forever! Here's one thing I can add: I worked as a tech during the 80's for Data General while I was attending college. They used *many* switching power supplies for their equipment and they were all pulse width modulation (PWM) design which is the same tech used in class D. Even back then I could see the use for these as audio amps, but they do have some peculiar problems. 1) It's nearly impossible to completely get rid of harmonic interaction with the main switching frequency. 2) PWM power supplies are prone to be unstable at zero load (no signal). If you understand that PWM works by modulating the width of a square wave, it is easy to visualize what happens at very low levels when that square wave gets so thin it randomly starts to disappear. At that point, feedback controls go haywire, and the circuit becomes unstable. I'm sure that new class D amps try to get around some of these design issues by using some sort of DC bias and small constant load to keep the wave stable. Class D has it's uses, but for home audio I'd put my money on a good class AB system.

Switching mode power supplies don't define a class d amp. Several of mine use linear power supplies. The technology has seen tremendous growth as of late.

 

[infullview]

Switching mode power supplies don't define a class d amp. Several of mine use linear power supplies. The technology has seen tremendous growth as of late.

You totally missed the point. I was working on PWM switching power supplies, but they are the basis for the class D audio amp design!

 

[gentlejax]

any of you have experience with the Wyred4Sound ST-500?

 

[gentlejax]

any issues with reliability ? long term...? with new class D ice modules specifically

 

[infullview]

any issues with reliability ? long term...? with new class D ice modules specifically

Very good question. I don't think high-end Class D amps have been around long enough to provide a good answer. My guess is that RF transients will probably raise hell with electrolytic capacitors used to filter out the carrier frequency over time, but these would be easily replaced and most of these class D amps have integral over current protection.

 

[fredgarvin]

any issues with reliability ? long term...? with new class D ice modules specifically

PSAudio, Peachtree and several other large Mfg. have been using icePower successfully for several years. The new IceEdge modules are about the best there is as well as the NCore.

 

[johnlcnm]

I started a thread on class D amps a while back and it went on forever! Here's one thing I can add: I worked as a tech during the 80's for Data General while I was attending college. They used *many* switching power supplies for their equipment and they were all pulse width modulation (PWM) design which is the same tech used in class D. Even back then I could see the use for these as audio amps, but they do have some peculiar problems. 1) It's nearly impossible to completely get rid of harmonic interaction with the main switching frequency. 2) PWM power supplies are prone to be unstable at zero load (no signal). If you understand that PWM works by modulating the width of a square wave, it is easy to visualize what happens at very low levels when that square wave gets so thin it randomly starts to disappear. At that point, feedback controls go haywire, and the circuit becomes unstable. I'm sure that new class D amps try to get around some of these design issues by using some sort of DC bias and small constant load to keep the wave stable. Class D has it's uses, but for home audio I'd put my money on a good class AB system.

Look at the Crown ITECH design. Differential 250 kHz square wave. In phase at 0 output.

 

[infullview]

Look at the Crown ITECH design. Differential 250 kHz square wave. In phase at 0 output.

I like the way they solved the control problem with the square waves being in phase at zero output - at this level both are still creating enough of a wave to keep them stable. These look great for professional use at places like TD Garden in MA. THD is respectable at .35%, but still not as good as most A or AB designs which are usually less than 0.1% (.05% at rated power is typical for a decent solid state amp). Big name makers will have a field day with this tech because it's so cheap to build relative to analog. Their profit margins will be through the roof. Compact form factors, no heat sinks to speak of, and if they were really clever, they could use HF transformers on the output of the switcher to provide line isolation and power the switcher directly off line voltage We actually made a power supply like that at Data General. It was internally named the "Cobra" and had a Lexan shield on it to protect people if it blew up. Feedback on this design was accomplished via optical transducers.

 

[johnlcnm]

I suspect the biggest problem with any PWM is the signal to noise and/or a lack of dynamic range. I think the distortion is rated at less then 0.1% for the HD series. Don't know if that is at full rated output.

 

[infullview]

I suspect the biggest problem with any PWM is the signal to noise and/or a lack of dynamic range. I think the distortion is rated at less then 0.1% for the HD series. Don't know if that is at full rated output.

This may be a dumb question, but what would limit the dynamic range? If you believe specs, S/N seems very respectable. The .1% rating is almost always given at 1kHz as distortion generally climbs on switchers at higher frequencies.

 

[johnlcnm]

The rise time of the amplifier itself. You are asking the PWM to reproduce, lets say a 90 dB dynamic range. Pulse widths become very narrow at the lower levels. Hence, fast rise times. This makes the assumption that the output rail voltages remain constant.

 

[infullview]

The rise time of the amplifier itself. You are asking the PWM to reproduce, lets say a 90 dB dynamic range. Pulse widths become very narrow at the lower levels. Hence, fast rise times. This makes the assumption that the output rail voltages remain constant.

These amps are switching at a very fast rate compared to the analog signals they produce. They also employ feedback to insure that output is linear and equivalent to input. A class D amp should be as capable any class A or AB circuit within the linear voltage range of the amp. In your previous post about the Crown ITECH design, you explained that max stability was obtained at zero output by using differential PWM quiescently biased with in-phase stable square waves at zero output. Shouldn't that solve the problem?

 

[david1111]

@kurtgo
I do really enjoy researching audio gear, and analyzing and comparing the various products and designs that fall within my research range at the time.
But, when it comes to trying to ascertain whether a comparatively inexpensive, new design of gear is as good as the foregoing design; you could easily just buy a used one, and sell it on, for little or no loss, if you don't like the sound.
Or keep it, and ditch yer old rig, while you research and find your ideal Class D amp.
Just a (simple) thot.

Dave

 

[kurtgo]

Dave - kinda with you on that thought. I have since bought 3 class D amps from $89 to $2,900. It has awakened me to what these things can do and in a small package. Since then I have bought 3 of the $89 amps to drive patio speakers, center channel and another "cheaper" system via their Bluetooth connectivity. This experience has been enlightening. My tube amps however are still a step ahead sound wise. Yea - no fear of selling what I don't need.

 

[david1111]

That's excellent Kurt. Sounds like you have the best of both situations.
My main setup is A/B, but I have an older Technics receiver that is Class H, which I believe is very much like Class D. It sounds very good, and throws a nice sound stage. I can't properly compare it in my system cuz it doesn't have pre outs and amp ins, and I'm pretty sure my tube pre would make the amp sound even better.

Dave

 

[Pio1980]

I think that "class 'H' " was a commutating output power rail arrangement of some sort, not pwm switch mode class 'D',

 

[david1111]

That sounds like a good thing. Great.
It has the same lighter weight as a class D, although it's still quite heavy where the power supply is.

Dave

 

[Pio1980]

https://en.m.wikipedia.org/wiki/Power_amplifier_classes

 

[david1111]

Thanks Pio, I'll read the part on class D amps. Hopefully they cover G and H amps as well.

Dave