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Power Up! - A Low Power Desktop Amp

Posted 28th April 2013 at 03:04 PM by BuildMeSomething
Updated 8th May 2013 at 06:38 PM by BuildMeSomething

The Amp boards arrived Saturday, unfortunately, I was just leaving for work so they'd have to wait. Everything looked OK on them.

Initial power up test was done with both LME49990 in place but only one LME49600 mounted on each channel [PSU used was a modified "The Wire" unit meant for testing the A/B Power Amp at +/-25Vdc, now set at +/-15Vdc. It was getting quite hot as it was dropping 10V per rail]. That went Ok, so the rest of the 49600 were put in place and powered up once more... Again, everything OK.

Measured offset:

Left channel: 27mV
Right channet: 8mV

Foam board speakers are progressing, and will be ordering a heatsink this week also.

Update [13-04-30]: Been a little bit worried about the higher offset value on the left channel, though it's not at a bad level overall. Once the amp has been on awhile, it appears to settle at 25.9 mV.

Made a quick SE input>volume pot AC coupled connector to try it out with some music. With one 3 3" FR driver in place, the play button was pressed and pleasant music did come forth... despite the shortcomings of using a small open driver just placed on the worktop, with a few bits placed around as an impromptu enclosure, the balance sounded ok... LF was very much lacking but there was a lovely amount of detail present.

Update [13-05-08]: Heatsink order has been placed with Schaeffer AG and the chassis is on its way also... Whole build should be completed by the end of next week
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Posted in Uncategorized
Views 1488 Comments 6
Total Comments 6


  1. Old Comment
    abraxalito's Avatar
    I'm curious - how does the PSRR of an array of paralleled LME49600s compare to the (datasheet) PSRR of a single one? Any ideas?
    Posted 29th April 2013 at 02:44 AM by abraxalito abraxalito is offline
  2. Old Comment
    The simple answer is I don't know, but would love to find out...
    Posted 29th April 2013 at 09:11 PM by BuildMeSomething BuildMeSomething is offline
  3. Old Comment
    My guess is that it would make no difference at all: if you consider each of the paralleled units as working in isolation with their own supplies, and then simply summing currents in the speaker load, then I can see no advantage or disadvantage to the method as far as PSRR is concerned ...

    But, the real benefit is that one is forced to engineer a more substantial power supply, to make sure that none of the chips are starved during a peak, and that's the real gain of the matter ...
    Posted 29th April 2013 at 11:06 PM by fas42 fas42 is offline
  4. Old Comment
    abraxalito's Avatar
    My initial thinking is that at HF, normally the PSRR is limited by parasitic device capacitances. So I figure that paralleling chips is only going to worsen PSRR because that's putting the stray capacitances in parallel. But then as a caveat, if the PSRR is capacitance dominated it will surely depend on what load resistance is used - the datasheet does not specify any along with the PSRR graphs.

    At the lower frequencies I don't know what causes the limited PSRR - is it related to the early resistance of the output emitter followers? If so then paralleling more followers will make the PSRR worse.

    Just my initial thoughts - would appreciate feedback!
    Posted 30th April 2013 at 03:17 AM by abraxalito abraxalito is offline
  5. Old Comment
    All food for thought... one would hope that each device would work on an individual bases and its PSRR wouldn't be effected by the outputs being in parallel with other devices. Abraxalito, I recently read a paper by Analog Devices about decupling techniques and it covers some of the areas you mention. However, the question still remains about any additional effect brought about by running devices parallel...

    I would hope that with proper decupling to a low impedance ground plane on each of the devices supply pins, plus with a suitable power supply, any issues would be kept to a minimum.
    Posted 30th April 2013 at 09:21 AM by BuildMeSomething BuildMeSomething is offline
  6. Old Comment
    abraxalito's Avatar
    Ground planes are great for RF work, I am not so sure though at audio they're the best solution. For audio we're not so concerned about inductance in the supplies as at RF - because the frequencies are so much lower. I tend to consider cross-coupling in grounding (common ground impedance coupling) a bigger issue to fix than supply inductance, hence I go for star earthing rather than planes. But each to their own!
    Posted 30th April 2013 at 02:03 PM by abraxalito abraxalito is offline
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