Nothing original and published yet. It was just a study to compare two topologies, of the same amp, one in Voltage feedback, one in so calling " Current feedback".
Bruno's UcD (e.g. Hypex designs) is very nice, and the more so for having evolved as a minimal-parts-cost approach when he was working for Philips. Incorporating the output filter as an integral part of the loop rather than a nuisance is a masterstroke. The latter company will be happy to let you use the patent if you use NXP semiconductors, last time I checked.
I don't know the inner details and probable personality conflicts that led to Putzeys' departure, but from an outsider's point of view it seems unconscionable that Philips/NXP didn't work on an integrated solution to embody UcD. But their conventional PWM parts are o.k. and a go-to for me when the requirements are not for something spectacular, and as well when someone insists on synchronization among multiple channels. UcD is inherently a self-oscillating variable-frequency approach, and although one can envision means to make it frequency-stabilized it's a lot of work and probably not worth it (Hypex people btw can get quite testy when you mention fear of heterodyne artifacts with multiple amps).
Where the overwhelming majority, if not all, of class D falls down a bit is with the brutal high-level adjacent-high-frequency-tone IM distortion test. If you work really hard to come up with material that has a lot of stuff going on up there you might find this to be a legitimate concern, particularly if the nonlinearities produce substantial difference-frequency energy that falls into a region of the spectrum where we have high aural acuity. But for most of the time, with real music well below clipping, this isn't going to knock you down.
I heard some music through the next-to-biggest Bryston and driving next-to-the-largest SoundLab electrostats a couple weekends ago, in a carefully designed room and with some good source components and material. Not long enough to form a strong opinion but certainly nothing to complain about, very good indeed, and no sense of grit and grain.
Why ? It is as obvious as the beat of two guitars same notes out of tune, and i would certainly not use any self-oscillating variable-frequency class D amp without a dedicated power supply (or well separated filtration) for each one.
We can found our hopes on this technology: Power devices will be faster and faster, commutation times will reduce very fast in next future (sample rates), and it is a very 'industrial' approach.
Synchronization among multiple channels will be very simple, as master-clocks and jitter correction are, at line level, once industry will provide the chips to do-it.
As power supplies with multiple phases computer like.
It is a virgin territory, with no virgins inside yet.
About IM, can-you provide more ? I do not see why we would have problems with IM, as long as you don't feed the amp with faster signals he can deal with. Just, on my point of view, there suffer from the same incoherency between ( relative ) limited bandwidth and very fast slew rate, as everything digital. Limiting the slewrate of the incoming signals can help for coherency ?
Thanks for your very interesting contribution. (no smiley here)
I'll collect my thoughts on IM a bit. Empirically, it's endemic to class D, but the details are complex.
I'm afraid !
Joke apart, how did Bruno managed to get rid of the delay and phase shift introduced by the filter in the closed loop ?
Demian, were these QUAD factory mods?
I'm certain the grey paint was after IAG (??) bought QUAD. They are the same mob that bought Wharfedale and turned it into a box stuffer.
I've had many arguments with Peter Walker over nerdy stuff but you gotta admire the old QUAD's technical integrity. They have their philosophy which has been finely argued and developed since the war and they stick to it. And all their stuff was so beautifully made it belongs in a Museum of Fine Art.
Of course there are those who would claim their stuff belongs in a museum cos the sound ...
Wavebourn, wanna buy a Golden Pinnae dust absorber? Hand carved from solid Unobtainium by Huntingdon virgins. Far better than your cheapo Chizhevsky lamps. As an added bonus, you can play beautiful music through them for the first 3 mths.
On films, just remembered I've done some measurements of a single layer of cling film on a Calrec Soundfield microphone. There is a measurable effect but its quite small. Other films were bad.
I'll cautiously suggest it as worth trying on a naked ELS-63 if you ever have one in that state. But don't strip off your existing dust covers unless you have something ready to replace them.
The cling film was part of an abortive attempt at a moisture barrier but that's what I'd use if I was stupid enough to take my Soundfield into the rain.
Demian, were these QUAD factory mods?
I'm certain the grey paint was after IAG (??) bought QUAD. They are the same mob that bought Wharfedale and turned it into a box stuffer.
I've had many arguments with Peter Walker over nerdy stuff but you gotta admire the old QUAD's technical integrity. They have their philosophy which has been finely argued and developed since the war and they stick to it. And all their stuff was so beautifully made it belongs in a Museum of Fine Art.
On films, just remembered I've done some measurements of a single layer of cling film on a Calrec Soundfield microphone. There is a measurable effect but its quite small. Other films were bad.
I'll cautiously suggest it as worth trying on a naked ELS-63 if you ever have one in that state. But don't strip off your existing dust covers unless you have something ready to replace them.
The cling film was part of an abortive attempt at a moisture barrier but that's what I'd use if I was stupid enough to take my Soundfield into the rain.
The dust covers are part of the Crosby Quad mod. it was the thinnest film DuPont could make at the time that was wide enough to work as a dust cover. We did a number of changes (grills, delay line caps wiring etc.) that all made a difference and some later surfaced in newer versions of the Quad.
I have used Quads without the dust covers and I know the limits.
On the old ESL the factory sprayed paint on the dust covers I believe for cosmetic reasons.
This is similar to my own experience.
I had a Bryston power amp on my system and I agree with your description of its' sound. It was replaced by TeddyAmp MB100 monoclocks, which were replaced by Pass Labs XA30.5. Should I win the lottery, I'll change the power amp to a more powerful one of the same series and make.
The low pass filter is inside the loop so it falls out of the final response, until you get to the systems overall band limits. Bruno claimed to have a 5 or 7 pole compensation scheme. He also claims to have controlled the interaction between the modules as long as you don't do something stupid. He is a strong advocate of control theory and its detailed application.
Of course, anything before the feedback line will have distortion canceled, and there, the filter. A good thing.
But you have to compare the original signal to the amplified one somewhere. An there, the delays and phase shifts induced by the filter are an issue. Because you will compare signals with phase differences at the upper range of the audio bandwidth..
And this filter cutoff (because the switching frequency) is not far enough, on my point of view. So i wonder how...
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