![]() The third is to limit slew rate at the input, before feedback summing point. Second is to avoid deep long global feedback and use more of the local feedbacks. The first obvious one is to use quality design and components instead of very deep feedback. Before it was discovered in 70s amps had no distortions (very deep feedback) but unpleasant bright sound. Late summing of the fast changing input signal has the same effect as amplifier having higher gain for a moment, causing overshoot. but 40dB of negative feedback means that amp with open feedback has gain of 3000 instead of 30. Negative feedback improves everything THD, IMD, output impedance, bandwidth etc. TIM is an effect of phase shift (time delay) thru the amp causing negative feedback to feed back late, causing overshoot with fast changing input signals (unpleasant higher order odd harmonics). It appears that people mix two different types of distortion IMD and TIM (Transient Intermodulation Distortion). Trust John ( to find a great example of "old school" tech like that Sansui amp that used feedforward (and feedback) correction from the heyday of Japanese designs. All the audio class G (switching output supply rails) or class H (tracking rails) amplifiers I have seen use a class AB core, including Benchmark, Emotiva, etc. I have only read a few papers on applying class G/H topology to class D amplifier cores, and am unaware of any audio amps (the ones I saw were for low-power RF amps, not sure any made it to market). ![]() The class H RF amps were for applications that had relatively easy distortion specs, more like -40 to -60 dBc and not the -100 dB and under audio amps provide, and relative to the carrier frequency (>10 GHz) the envelope was fairly low in frequency (<100 MHz). Switching between two rails is hard enough in my limited experience. He cited the same concerns I had and have hit when trying to design them myself (not for audio, again my day job has been at higher frequencies), primarily the difficulty in controlling the tracking rails while making them fast enough and low enough in impedance to provide good performance. I'd much rather buy products from companies that speak to the past, celebrate it, and demonstrate what they've done better, instead of claiming it was 100% their own brilliant idea.Ĭlick to expand.In the post I found in a quick search here, and am too busy/lazy to find again, he clearly said it switched between two rails (class G in the USA, class H in Europe, IIRC) and was not a tracking supply. Standing on the shoulders of giants who rarely get credit is something that really upsets me. Nor have I seen the Benchmark schematic (I'll ask for that when one lands on my bench for repair if ever), but there's not much new in this space- it's all been done in the past by much smarter people than me. I haven't seen or read the THX AAA patent and I wonder what prior art mentions there are in it. Benchmark clearly saw what the FFWD could do and figured they could save a ton of quiescent dissipation and kill Xover distortion at the same time, whereas Sansui attempted to take an existing, already excellent topology of theirs, to the next level. The Sansui's feedforward is contributing much less than the Benchmark's feedforward correction stage, but the principles are likely similar. The Benchmark uses both feedforward and feedback as does the Sansui. I wrote about TIM distortions because of the claim that since class D measures quite well (THD, IMD) different sound has to be caused by something else, like TIM. Yes, it got lost, thanks to unnecessary comments like yours. I'm also familiar with construction of AHB2 since I read everything I could find before I bought it. I'm quite familiar with John Siau reading his comments and articles for many years. Did you also object to others quoting Bruno Putzey? I don't quote experts and only referred to Douglas Self, since he tested distortions at different bias levels. I did not volunteer with low bias distortion comment, but only responded to another post claiming low bias created mess in Benchmark. I also suspect that you are not one of the "experts" who design am[plifiers for living, but even if you are - if you disagree - please say so, but stop making patronizing comments. I have suspicion that people like you occupy this forum as "resident experts" and cannot tolerate anybody else making technical comments. In contrast to you I'm not presenting myself as an "better expert" or any expert. ![]() I do not "argue" with anybody, but rather receive unpleasant patronizing comments from so called "experts".
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