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  1. Old Comment
    abraxalito's Avatar

    My latest infatuation - transformers

    When I was married (going back now a decade and a half) my wife remarked I was one huge wooden spoon.... Seems we're cut from the same cloth. Yes let's wait and see
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    Posted 29th October 2014 at 02:08 AM by abraxalito abraxalito is offline
  2. Old Comment
    fas42's Avatar

    My latest infatuation - transformers

    When I was littl'un, we lived way out, in the bush, as we call it here. And I used to visit big ants' nests, where they were scurrying around, calmly carrying out their everyday activities - and I'd pick up a big stick, and poke it down the big centre hole of the nest, to see what happened ...

    Hmmm, don't know why I just mentioned that ...

    Ummm, ahh ... yes, bit of a shame, but will see how things develop, .
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    Posted 29th October 2014 at 01:53 AM by fas42 fas42 is offline
    Updated 29th October 2014 at 01:56 AM by fas42
  3. Old Comment
    abraxalito's Avatar

    My latest infatuation - transformers

    I was curious about Mooly's 'loaded opamp' test, tried to download the files but seems in China, dropbox is blocked so no cigar. Elvee has posted up an interesting way of doing a difference, one I'd not considered before - using LTSpice to subtract the .wav files.

    So far everyone is talking about the distortion but in my experience, loading the opamp sags the power supply dynamically and I wonder whether that's what Elvee's found with his cursor on the waveform? All very interesting stuff

    I see you're on the case with Audacity already - excellent progress!

    Later - looks like your analysis got the tumbleweed treatment too, in favour of Pano's
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    Posted 27th October 2014 at 11:41 PM by abraxalito abraxalito is offline
    Updated 28th October 2014 at 11:07 PM by abraxalito
  4. Old Comment
    wintermute's Avatar

    Repair of a vintage driver (Richard Allan)

    Nice job googlyone. Was it a success?

    Tony.
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    Posted 27th October 2014 at 11:14 AM by wintermute wintermute is offline
  5. Old Comment
    fas42's Avatar

    My latest infatuation - transformers

    Yes, I had similar sorts of concerns, the AP intrinsic distortion seemed to be interfering with the results - but I didn't wish to appear entirely ungrateful for him going to the effort of getting the extra info, in the first round anyway! My feeling is that a variation of the testing technique could bring up something genuinely meaningful, but unless I had all the apparatus myself to play with directly, I would be firing off too many wild shots and getting nowhere, suggesting other things to try ...
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    Posted 27th October 2014 at 09:26 AM by fas42 fas42 is offline
  6. Old Comment
    abraxalito's Avatar

    My latest infatuation - transformers

    I see Tom's put up the IMD results for you. He says the AP is responsible for most of the IMD but looking at his plots I must be missing something. On the first one (loopback) the 1kHz looks to be the highest distortion component at -115dB, the 18/21k tones are 1 or 2dB below that.
    Coming out of his amp there must be some anti-phase 1kHz to cancel the analyser's 1kHz distortion but the 18k and 21k tones are -84 and -82dB respectively. That puts the amp as having increased those components by at least 10dB. So the AP's 'raw' IMD is going to be below -112dB compared to Tom's own figure for his amp of -105dB. In actual fact perhaps the AP's ADC is responsible for some of the harmonics, depending on the range setting going into the AP. Really the test tones should be notched out for the FFT to have much meaning.

    My other concern is where the noise 'floor' sits. Normally the AP is using an FFT with at least 64k 'bins' giving an FFT gain over 30dB, typically around 36dB. So the noise 'floor' on the loopback test is then going to be at best -104dBr in the full audio bandwidth (-140 + 36). Not too resolving for testing a poweramp methinks.
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    Posted 27th October 2014 at 05:51 AM by abraxalito abraxalito is offline
    Updated 27th October 2014 at 06:02 AM by abraxalito
  7. Old Comment
    fas42's Avatar

    My latest infatuation - transformers

    Well, here you go then, LM337 datasheet pdf datenblatt - Linear Technology - Negative Adjustable Regulator ::: ALLDATASHEET :::. I have an old National Linear Databook - this has been a treasure over the years - and the graphs for the Linear version have been photcopied from there ...
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    Posted 26th October 2014 at 02:23 AM by fas42 fas42 is offline
  8. Old Comment
    abraxalito's Avatar

    My latest infatuation - transformers

    If you look at his PCB (since there's no released schematic) there look to be TO-92 3-terminal regulators adjacent to where the power comes in to the board. The 'THAT' chip is at the far end, furthest from those regulators. I'd hazard that the total load on the regs is going to be under 20mA (there are only 3 chips being powered) - regulator output impedance gets pretty shoddy when they're lightly loaded.

    Since Tom misconstrued my opening gambit - which was that there was going to be load-induced noise - and proceeded to defend the line rejection, this tells me there's probably a line rejection issue too, at HF. If the regs turn out to be LM317L/337L the line rejection at HF is notoriously poor.

    I just downloaded the DS for the LM337L - a pathetic travesty of a DS with no graphs whatsoever. So it seems to get graphic data on line rejection I need to turn to the LM137 DS. Line rejection looks from that to be 40dB at 20kHz falling to 10dB at 1MHz (assuming use of the ADJ pin cap).

    LM337 OP impedance looks to be around 30mohm at 20kHz, but note the graph is plotted at 500mA output current. A first approximation would suggest that at 20mA the Zout rises by a factor of 500/20, so around 750mohm would be my guess. Subjectively this fits with experience as when I upgraded my integrated amp's input opamp with shunts it cleaned up the sound substantially, that was previously fed with the standard 317/337 pair.

    Seems there's been revision on TI's part of these reg DS - no output impedance graph any longer for LM317, but there is a new one - ripple rejection vs OP current and that shows a substantial fall off in ripple rejection at light loading for the LM317 - looks to be around -58dB at 20mA. A quick sanity check on my Zout estimate for the LM337L is made by comparing with the LM317L which does have the requisite graph, taken at 40mA and shows just under 1ohm for 20kHz. This is the point where the OP cap takes over the impedance curve.
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    Posted 26th October 2014 at 12:16 AM by abraxalito abraxalito is offline
    Updated 26th October 2014 at 12:57 AM by abraxalito
  9. Old Comment
    fas42's Avatar

    My latest infatuation - transformers

    Indeed, yes. In fact, if you check earlier posts I've been pushing him to do extra, but unfortunately still standard, tests in the hope that perhaps something of significance might appear.

    It is remarkable that people can't seem connect that ultimately it is all one circuit - they always break it down into submodules of functionality and behaviour, and treat each as isolated from anything real ...
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    Posted 26th October 2014 at 12:00 AM by fas42 fas42 is offline
  10. Old Comment
    abraxalito's Avatar

    My latest infatuation - transformers

    Good point yes, I also wondered about this. Trafos have leakage inductance so perhaps its isolating problematic capacitance from the amp's output? Another aspect is that speaker cables act as an antenna hung on the chipamp output - I haven't seen any series inductor in the speaker itself. So there may well be marginal stability issues which I'm fixing up by running through the trafo.

    Incidentally are you following Tom's thread about his 'Modulus-86' chipamp design? If so have you noticed the tumbleweeds there in response to my probing about opamp OPS-induced power supply noise? Interesting how Tom's put up so many deflections - like citing cherry picked regulator PSRR numbers to 'defend' his design, plucking regulator output impedance seemingly out of a hat.

    This all goes to tell me power supplies are a major lacuna when designed by 'objectivists'
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    Posted 25th October 2014 at 11:44 PM by abraxalito abraxalito is offline
  11. Old Comment
    fas42's Avatar

    My latest infatuation - transformers

    I would also be curious as to whether the transformer in some fashion is acting as a filter, which allows the circuit to function more optimally - as an off the top of the head example, whether RF muck on the speaker side of things is blocked more from feeding into the output stages, say?
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    Posted 25th October 2014 at 10:07 PM by fas42 fas42 is offline
  12. Old Comment

    NwAvGuy odac 24/96 DAC review

    Quote:
    Originally Posted by dimkasta View Comment
    I recently modified one for a friend isolating the +5V from the USB cable (cutting the pcb track) and using a salas BiB for the power supply.
    Quite an improvement.
    Could you describe in what way did the mod improve the sound?
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    Posted 25th October 2014 at 09:59 PM by hallom hallom is offline
  13. Old Comment
    abraxalito's Avatar

    My latest infatuation - transformers

    I am really curious now how far I can push that trafo ratio. Chipamps tend to max out below 100V - LM4766 (74V) is a bit below the LM3886 (94V) but its a dual so more convenient to use. I reckon I can run that in bridged with a 3:1 step down ratio. So then each chipamp side sees 18R (for a 4R drive unit).

    Beyond that caps go on getting better - in terms of energy stored per unit volume. I found photoflash capacitors have about the best Joules/cc ratio available. Not as good as EDLCs but those are too low voltage to be useful except in large paralleled arrays in high power amps. LME49810 goes up to 200V operation but beyond that it will have to be discrete....
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    Posted 25th October 2014 at 12:20 PM by abraxalito abraxalito is offline
  14. Old Comment
    fas42's Avatar

    My latest infatuation - transformers

    Good work there, ... . One gets blasť at times, thinking one's got the SQ in pretty good shape - I'm as guilty as anyone of doing this - and then something is done that pops the level so much further forward, yet again!

    The key is knowing, really knowing, that no matter what barrier seems to be in front of one, in terms of "fixing" the sound - that it can always be bypassed, in some fashion ...
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    Posted 25th October 2014 at 09:31 AM by fas42 fas42 is offline
  15. Old Comment
    abraxalito's Avatar

    Taobao has a TDA1387 DAC (apparently)

    Well spotted! So then it'll need a minor bit of modding to function as a DAC. OK
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    Posted 23rd October 2014 at 06:00 AM by abraxalito abraxalito is offline
  16. Old Comment
    miklos's Avatar

    Taobao has a TDA1387 DAC (apparently)

    The label over the rca sockets says in.
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    Posted 23rd October 2014 at 04:27 AM by miklos miklos is offline
  17. Old Comment
    rjm's Avatar

    Voltage Regulators for Line Level Audio. Part VII : The k-multiplier

    Good point, it seems R1=220 ohms is a better value.
    permalink
    Posted 3rd October 2014 at 12:17 PM by rjm rjm is offline
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