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switchers and amps = bad?

Discussion in 'Electronic Design' started by Michael (Micksa) Slade, Jan 26, 2005.

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  1. While at the local parts shop doing my routine skulk around I asked about
    making an RIAA preamp and powering it with a switched (single-rail)
    supply, and one of the guys there went "nonono, bad idea, switchers are
    too noisy for audio applications".

    Is this true? I'm no veteran but as far as I know any supply noise should
    be able to be filtered out with sufficiently large/numerous capacitors,
    should it not?

    Mick.
     
  2. There are plenty audio amplifiers using switched power supplies, but
    a RIAA preamp does not need much power and using a switcher seems a
    lot of hassle. I'd go for a simple linear regulator.
     
  3. Dave VanHorn

    Dave VanHorn Guest

    True to some degree.. It's possible to make a REALLY noisy switcher, but
    it's also possible to make a really quiet one. I did an application a while
    back, reading the signal from a MICR head (microvolts) with an amplifier
    powered by a switcher, and sitting next to the controller for a 24W stepper
    motor with chopped current regulation.. No problem at all. It just took
    careful attention to design.
     
  4. Mark Jones

    Mark Jones Guest


    Thanks Dave. Read: switchers can make all kinds of nasty
    electro-magnetic interference, from the high frequencies used in
    switching to the magnetic fields used in the inductors. I agree that a
    RIAA preamp should use very little power, there is no need to use a
    switcher.

    Still, if you must use a switcher, you can get really clean power out
    of it (at small currents) by using four RC lowpass filters. i.e.,


    [in]---\/\/\--+--\/\/\--+--\/\/\--+--\/\/\--+---[out]
    R1 | R2 | R3 | R4 |
    + | + | + | + |
    --- C1 --- C2 --- C3 --- C4
    --- --- --- ---
    | | | |
    | | | |
    --- --- --- ---
    - - - -

    You'd want to select the R/C values to reject the lowest frequencies
    present in the source. You could use say 1uF/3.75K and it will block
    out almost all frequencies, but the voltage risetime will suffer as
    each stage slowly charges up. And (3.75k x 4 = 15k) of DC impedance
    would leave a +15v supply able to provide only 1mA. But this might be
    acceptable for an RIAA preamp.


    -- "The problem with participating in a rat-race is that everyone is a
    rat." MCJ 200312
     
  5. Guest

    I would tend to agree with the guy at the parts shop. Discrete changes
    in voltage or current generally spell trouble with line and radiated
    noise that can couple onto your signal unless great care is taken. Why
    not make it easy on your self an build or pick up a linear supply.
     
  6. Dave VanHorn

    Dave VanHorn Guest

    Funny thing is, at low currents, the Rs are more effecitve than inductors,
    especially if you consider the cost.
    Or scale the Rs down by 10, and the caps up, and add a 0.1 on the input
    side.


    On most of my designs, where I have analog and microcontrollers together, I
    use a single stage RC on the analog supply, right at the chip. As long as
    you watch the current demands, it works nicely.
     
  7. Pooh Bear

    Pooh Bear Guest

    Don't forget that switchers generate plenty of HF magnetic field too.

    It concerns me more that in today's world you might be making an RIAA pre-amp
    ( any pre-amp in fact ) using a single supply rail.

    The switcher I've just designed for a semi-pro audio mixer generates a mere
    couple of hundred microvolts audio band noise on the supply rails btw.
    Slightly quieter than the previous linear regulator based design but I did
    apply extra filtering compared to a stock swicher. The magnetic field still
    needs a small amount of taming - but you'd get even worse problems with a line
    frequency transformer when the PSU is mere centrimetres from the pcb.

    A line frequency supply is a lot easier to design though.


    Graham
     
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