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2,000W into 1 ohm

Posted 12th February 2014 at 04:10 AM by fas42
Updated 12th February 2014 at 09:55 PM by fas42

I mentioned in a thread a while ago about doing an exercise of engineering an amp capable of delivering 2k watts into a 1 ohm load, with distortion aiming at the magical ppm figure. This is still happening, and making progress ... key problem as I saw it was managing crossover distortion intelligently - I'm looking at a conventional class AB output stage at the moment - and it didn't make sense to try and control it using a classic global negative feedback approach.

By nature I'm an excellent scavenger, I look to see what ideas are already out there - so I'm trying out some concepts in using local negative feedback. This is evolving, step by step - and showing promise: in a simulation of the output stage only, getting effectively 2kW in 1R, at 200kHz with reasonable stability - and the waveform at this stressful frequency looks pretty good, there is still some crossover glitching, but I'm reducing the visible level of it steadily.
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  1. Old Comment
    abraxalito's Avatar
    Strikes me that the challenge here in terms of audibility isn't the crossover distortion rather the power supply and PSRR. When you need a peak current of 63A even a 1mohm supply impedance gives rise to 63mV peak of noise.
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    Posted 12th February 2014 at 07:44 AM by abraxalito abraxalito is offline
  2. Old Comment
    fas42's Avatar
    Of course that will be a major issue, but I'm taking it a step at a time. In this first stage I'm using perfect power supplies, and developing the amplifying topology to work correctly within those ideal conditions. Once I'm happy with the core behaviour then I'll introduce more and more realistic PS elements, and do what is needed to keep the core circuitry sheltered, or robust, so that the performance is still maintained.

    It's the principle of divide and conquer ...
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    Posted 12th February 2014 at 10:03 PM by fas42 fas42 is offline
  3. Old Comment
    abraxalito's Avatar
    So what power supply ingress would be your target?
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    Posted 13th February 2014 at 09:32 AM by abraxalito abraxalito is offline
  4. Old Comment
    fas42's Avatar
    Purely as a rule of thumb, I would be unhappy if anything more than roughly the same level of distortion was injected through the non-perfect behaviour of the PS - via non-zero impedance and noisy mains, say well within the same order of magnitude ... as that contributed by the amplifying topology. I'm aiming at 120dB for the latter, so you could say that number is the target ...
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    Posted 13th February 2014 at 11:24 PM by fas42 fas42 is offline
  5. Old Comment
    abraxalito's Avatar
    OK then just putting some numbers to this - from one or two DSs that I've looked at, a typical EF output stage (single transistor pair) will have a PSRR between 60-70dB. Presumably to handle 2kW you'll be needing of the order of 20 pairs of devices - unless you have something a bit special up your sleeve in terms of advanced tech. With at best 50dB PSRR (degraded 20dB due to the paralleled devices) in your composite OPS then to achieve <1ppm it looks as though your supply impedance needs to be of the order of 0.3mohm. Does this seem reasonable?
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    Posted 14th February 2014 at 01:53 PM by abraxalito abraxalito is offline
    Updated 14th February 2014 at 01:57 PM by abraxalito
  6. Old Comment
    fas42's Avatar
    Those figures look reasonable in themselves, but remember I'm trying ideas in local feedback, which helps a great deal. In fact, I just tried seeing where the design is at, with regard to supply rejection on the output units - this at a still very rough stage, and will obviously depend on the accuracy of the models ... promising on first peek, well above 70dB at 200kHz, and 110dB in the audio band.
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    Posted 15th February 2014 at 04:46 AM by fas42 fas42 is offline
  7. Old Comment
    fas42's Avatar
    Steadily moving along ... the OPS is doing 2KW into 1R, at 200kHz, with a very clean waveform, no visible crossover artifacts, bias about 125mA per device. Harmonics are down at the 55 and 60dB level or so, and improve stongly once over a MHz, but still a little resonance in the drivers in the MHz range at maximum output to knock over ...
    permalink
    Posted 18th February 2014 at 08:45 PM by fas42 fas42 is offline
    Updated 18th February 2014 at 08:47 PM by fas42
  8. Old Comment
    fas42's Avatar
    Changing my approach a bit - worrying about behaviour at 200k is not helping distortion figures at 20k, so will focus on the latter for a bit. Next "issue" is that optimum performance, at the moment, comes with the penalty that the conventional means of maintaining thermal stability is compromised - 'tradeoff' time, or a new way?

    ... some reading and thinkin' to be done ...
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    Posted 22nd February 2014 at 02:44 AM by fas42 fas42 is offline
    Updated 22nd February 2014 at 02:46 AM by fas42
  9. Old Comment
    fas42's Avatar
    Back to business ... as is usual when investigating achieving certain goals, the really interesting stuff emerges when one is considering how to deal with, best optimise some behaviour that may be a problem. And one of the considerations is the world of temperatures, thermal behaviour. At the moment I'm looking at a bipolar output stage, and these things love losing the plot if you're not careful, thermal runaway.

    There are standard ways of dealing with this - but how about thinking outside the square? Why not give the devices excellent thermal inertia, so that no matter how electrically active they are their internal temperature varies as little as possible - this can only lead to improved overall performance.

    Partially inspired by the Symphonic Kraft 400 priorities, I'm looking at some materials engineering at the moment ...
    permalink
    Posted 25th February 2014 at 11:20 PM by fas42 fas42 is offline
  10. Old Comment
    fas42's Avatar
    ...
    permalink
    Posted 3rd March 2014 at 06:56 AM by fas42 fas42 is offline
  11. Old Comment
    Okay, this blog has inspired me to try and build a stand alone linear power supply that can deliver 1000W into 1 ohm for 4 channels. I have a slightly used 5kVA house transformer (frame type +/-24V) that I will rubber mount in an aluminum box with the diodes and 4x35mF caps. Which rectification scheme and diode selection is likely to get me the best supply?
    permalink
    Posted 28th August 2014 at 06:54 AM by yldouright yldouright is offline
  12. Old Comment
    fas42's Avatar
    Good on you! As is obvious from the lack of further input from me here I managed to get sidetracked from pursuing this - this is an endemic problem for me these days - but I encourage you to go for it!

    As regards the best way to produce the rails, in part I would want to know the type of amplifier circuitry it was driving - if a typical circuit I would separate the rails feeding the output stage from all the prior circuitry all the way back to having separate rectifiers for each part. If I wanted to go all the way, I would go an extra step and use a separate, optimal and obviously quite small transformer just to generate the rails for the pre-output circuitry.

    Also, I would like to determine the parasitics of the transformer fairly well, there are methods outlined for determining these; these parameters would be very useful to know.
    permalink
    Posted 28th August 2014 at 11:15 AM by fas42 fas42 is offline
  13. Old Comment
    Noted. I had the krill and Aleph-X in mind for this supply but since it will remain in a separate box, I'd like to use it as my universal reference supply for 30-35VDC rails. Having it in a separate box allows me to use a high capacitance mains wire which should remove HF parasitics from the delivered power line and get me a respectable S/N ratio. I don't want to use additional transformers in it. As far as the rectification scheme goes, dual half or full wave and what kind of diodes for either? How do you feel about regulation after the diodes? What are the best tests to describe my transformer and in what order should I perform them. My equipment may not be up to the task, no distortion analyzer but I do have a 35MHz/2mV scope and 6.5 digit meters.
    permalink
    Posted 28th August 2014 at 11:53 AM by yldouright yldouright is offline
  14. Old Comment
    fas42's Avatar
    A separate box is good, and bad. Good, because it keeps some of the nasties away from the amplifying circuitry; but bad because it means that further filtering, regulation is required inside the amplifier boxes to prevent significant voltage fluctuations, caused by current transients through the impedance of the umbilical cord - that cord should be the lowest resistance, lowest inductance, highest capacitance type you can devise, for that reason ...

    The 'goto' thread, IMO, is here: http://www.diyaudio.com/forums/power...voir-size.html - a huge amount of very useful input, and wrangling about what's important, and why. In there is good discussion of ways to "measure" transformers.

    Note that there are a lot of people in diyAudio with very extensive, hands on experience with what devices are best to use in various areas - I don't have anywhere near that sort of experience; my approach is to start with a target performance, and use what's needed, that's easily available, to give me that - I don't do casual experimenting, if it works it works. In other words, I don't like to give advice on the 'right' part to use; I work from the angle of a troubleshooter, rather than a designer.

    Regulation is good, but the power you're dealing with is great - a smart design will be needed be to it make happen efficiently.

    Hope this helps a bit !!
    permalink
    Posted 29th August 2014 at 11:10 PM by fas42 fas42 is offline
 
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