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An a.c.-coupling power supply rethink |
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Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
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Posted: 03 Sep 2022 at 10:41pm |
The results of four years research into early solid-state amplification (1970s Design Indulgence) show that DC blocking capacitors do more than just block DC. They are in fact mini-power supplies that make up for the "missing third power rail". It isn't even covered in https://www.analog.com/en/analog-dialogue/articles/avoiding-op-amp-instability-problems.html by Analogue Devices engineer, Charles Kitchin. As Rod Elliott of ESP (Australia) writes, "While no-one gives this a second thought (or so it seems), it does require some explanation." And the "well-regarded supplier of 'high-end' equipment" he refers to is me, and it's OK that I'm not named as Rod has to make a living too. Mr Self also refers to halfwave class-B currents flowing in the grounds, describing single supply a.c.-coupled amplifiers (Audio Power Amplifier Design Handbook Fifth Edition). I have expanded Rod Elliott's mathematical investigation in the draft of my book about the "1970s" design. It may reach the light of day eventually. The mathematical outcome is proved by the reactive value of resistance that pops out of the equations, which match exactly that of the normal method of finding output capacitor capacitive reactance for a given frequency. It proves where the power for the negative excursion comes from, which I don't think Rod made abundantly clear, or it didn't appear that way to me. The capacitor recharges as it discharges, is the simple way to put it, but what that proves is the power supply is distributing enough of its energy to the DC blocking "mini-power supplies" to warrant investigation of its current flow. This will have a bearing on ground current flow which differs greatly to what is taught - that being in relation to three rail supplies, with no explanation of two rail supplies. As Rod writes "no-one gives this a second thought". It takes me all the way back to when I wrote to the editor of Wireless World regarding grounding issues. I cannot remember the exact reply, but I think the word derisive could have applied. Anyway, I think there is mileage in its investigation, as it might improve matters, even in DC-coupled circuits using DC blocking in the negative feedback. So, here is a spin-off investigation - another itch I need to scratch - which if it leads to better sound, will be well worth it.
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Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
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We've come to see certain techniques as being the right answer but I'm cautious, and maybe it's just another bandwagon? A purely analogue audio amplifier uses no switching. The arguments that "even glue logic" can generate immense emissions does not apply here. When you see a ground plane, do you think "flat earth"? I'm starting to realise that ground planes have no place in purely analogue audio, and I'm now regretting being taken in - also having been charged four figures for the privilege. It seems we were heading in the right direction around the time of the Decca Radio Research Lab in the '60s, but within a few short years, all was swept aside. A great contender, I thought, for the "rethink" would be my best work (this being based on the idea that progress is made over the years), but then I remembered that the Accession uses a ground plane! Comprehending science while being influenced is not an ideal mix. The politics of the influence works as a stranglehold on the physics. OK, the industry does need to get a grip and get back to doing things by the laws of nature, but after a lifetime of understanding, do we really need a nanny? It's that Jeux sans frontières effect I just mentioned in another thread, holding back real enlightenment. I'll just have to follow this path using another product.
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