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Perhaps you do not understand how these amps work.

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In Reply to: Re: I found it . It says "NO FEEDBACK-No coupling capacitors . . . posted by morricab on October 13, 2006 at 01:48:03:

A full-bridge PWM amp can be built with a single Vdd supply. The output of the low-pass filter is the audio-frequency signal with the switching frequency artifacts removed. However, it is at one-half Vdd above ground.
With zero audio input signal, the voltage difference between the (+) and (-) speaker terminals is zero. However, both terminals are at 1/2 Vdd above the input audio reference (audio ground). The amplifier case is generally connected to AC safety earth, and the audio ground is at a negligible DC voltage with respect to safety earth.
If feedback is to be used, this DC offset must be blocked. Just as in a multi-stage tube amp, the DC can be blocked by capacitors, use of a transformer, or balanced out by resistive coupling to a separate DC supply below audio ground. Each method has its own drawbacks. Capacitor coupling has the least of the drawbacks overall, but the capacitor sonic character is imposed on the feedback signal and necessarily colors the output of the amp. It would cause phase shift according to the natural output impedance of the switching stage and filter, but if this is small, the phase shift would be negligible.
As I understand the Glacier site statement ("No Feedback - No coupling capacitors used"), the Raptor amps do not use feedback, and avoid the use of coupling capacitors that would color the sound. I believe they do use capacitors in the low-pass filter, and I will ask Mark to clarify this when time permits.

Moreno article on Elliot Sound Products site. (Open in New Window)

Follow Ups:

Re: Perhaps you do not understand how these amps work. - morricab 07:45:59 10/15/06 (4)
Hi Al,
Well I understand, in principle at least, how Class D amps function and to my knowledge they all needed feedback, thus my surprise. I am not sure how they are filtering the ultrasonic crap but usually you need an inductor and/or capacitors to make a low pass filter. So there may not be a coupling cap in series but there is for sure an inductor (not necessarily better) at least.
If you find out more info on their design then fill us in.
- Update. - Al Sekela 13:45:46 10/16/06 (3)
  I spoke with Mark about your questions. His intent with the Glacier web site comment was that the amps do not use global feedback in the audio domain, and that all the stages are direct-coupled. The only capacitors used are in the power supply filters and the output low-pass filters.
  These amps (including switching amps in general) use some form of digital control of the dark time, which is the time between one polarity of output transistor turning off and the other polarity turning on. Ideally, this time would be zero. However, due to the differences in available power NMOS and PMOS channel characteristics, including changes with ambient temperature variation, passive setting of the dark time results in too much distortion if it is too large, or the risk of output transistor destruction if it is too small and goes negative.
  Thus, one could call this control a form of feedback. However, it is not applicable to the analog domain, and does not relate to the output impedance versus frequency characteristic.
  The Raptor amp output impedance is specified at 50 milliohms (I may have been incorrect in another post), and actual measurements stay below 25 milliohms over the audio band.
  - Al please ask Mark to - GL 15:57:22 10/16/06 (1)
    either explain or correct this web page that states that the Raptor contains no transistors and no capacitors. It does not inpire credibility in his product.
    http://www.glacieraudio.com/new Glacier Audio Site/discussion2.htm
    I appreciate this discussion between you and morricab. My take is that Mark is using a pretty standard Class-D (PWM) module is his product. The feedback is around the comparator and output section like everyone else does it. You can argue, reasonably that this is not the same as ordinary feedback in the strict analog domain sense.
    What Mark appears to do, which is different, is that he replaces the analog front end (analog amplifying stage) that everyone else uses with a DAC/DSP/ADC front end. Pretty clever idea, because you can claim no feedback and you can also apply any frequency or phase correction in the digital domain to compensate for problems elsewhere. But I'm just guessing.
    Cheers,
    Graeme
    - Correction: ADC/DSP/DAC.. - GL 16:00:29 10/16/06 (0)
      Sorry about that.
  - Re: Update. - morricab 14:14:17 10/16/06 (0)
    Ok this makes more sense. THere are caps on the output as part of the HF rolloff. Not quite the same as a cap in series to block DC since the caps should be shunt to ground and the inductor(s) is in the signal path.
    I don't know if some other Class D amps are using "traditional" global negative feedback or not but the latest stereophile review of a tri-path amp shows a rise in distortion with increasing frequency, which is characteristic of global feedback. I would be interested to see some raptor measurements to see if it is possible to tell what the dead time control is doing.

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