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Thread: Low-ripple (120Hz) 5V 3A AC adapter?

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    Default Low-ripple (120Hz) 5V 3A AC adapter?

    I've got an application involving a small MCU and an audio stage; both run from 5VDC.

    Much to my displeasure, I can hear a 120Hz hum. It's not stray signal pickup, as I can short the audio input to the audio stage and still hear hum. A small (2200 F) cap across the DC rails diminishes the hum quite a bit, but it's still there. Power is coming from an inexpensive (Chinese) SMPSU "wall wart".

    This came as a bit of a surprise because I'd expect a regulated SMPSU to have almost no line-frequency AC ripple. But it does, so obviously the primary side of the power supply still needs additional filtering.

    Would I be better off using a regulated linear supply?

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    [QUOTE=Chuck(G);606982]I've got an application involving a small MCU and an audio stage; both run from 5VDC.

    Much to my displeasure, I can hear a 120Hz hum. It's not stray signal pickup, as I can short the audio input to the audio stage and still hear hum. A small (2200 F) cap across the DC rails diminishes the hum quite a bit, but it's still there. Power is coming from an inexpensive (Chinese) SMPSU "wall wart".

    This came as a bit of a surprise because I'd expect a regulated SMPSU to have almost no line-frequency AC ripple. But it does, so obviously the primary side of the power supply still needs additional filtering.

    "Check the ripple on the DC side and you need to see less than 300mv."
    Surely not everyone was Kung-fu fighting

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    That doesn't really answer my question, does it?

    It's clear that if the AC filtering isn't up to snuff on the primary side, ripple will simply be carried over modulating the HF switching--and since the regulation on these cheap PSUs is little more than a PWM control, it'll all come through.

    I can see where cutting a few cents here and there is very attractive--and one of the costlier caps is the rectified AC filter cap--it needs to be high voltage. So cheap out on that and while your cellphone may recharge and your 5V digital widgets may recharge, it's not going to be so good for sensitive analog circuits.

    At least a linear supply with a pass regulator and sufficient headroom can be pretty clean.

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    Quote Originally Posted by Chuck(G) View Post
    That doesn't really answer my question, does it?

    It's clear that if the AC filtering isn't up to snuff on the primary side, ripple will simply be carried over modulating the HF switching--and since the regulation on these cheap PSUs is little more than a PWM control, it'll all come through.

    I can see where cutting a few cents here and there is very attractive--and one of the costlier caps is the rectified AC filter cap--it needs to be high voltage. So cheap out on that and while your cellphone may recharge and your 5V digital widgets may recharge, it's not going to be so good for sensitive analog circuits.

    At least a linear supply with a pass regulator and sufficient headroom can be pretty clean.
    Does it have any transistors with maybe TO-3 - TO5? I had cal lab experience back in the day and got paid on time study, so I knew most of the t/s short cuts. Hybrid and integrated took a little longer. You should be able to find the ripple with a scope I would think.
    Surely not everyone was Kung-fu fighting

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    Probably SOT23 or SMD IC for the inverter MOSFETs. Sure, I can clip a scope probe to the output and put a load on it and I'm sure it will show 120Hz ripple, but then what?

    Here's a reference design for a small SMPS:



    Note that you'll get line ripple superimposed on the DC output if the manufacturer cheaps out on C2 or C12.
    Last edited by Chuck(G); February 11th, 2020 at 07:26 PM.

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    Can you lift one end of the L1 and and pop the fuse out? I would check the bridge for a possible leaky diode first off.
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    A leaky diode would really amp up the hum. I'll get a scope on this thing and measure it, but I suspect it will be a lot lower than you'd expect. Thing is, I've tried three of these supplies (same model etc.) as well as a couple of other SMPSUs and all suffer the same issue to varying degrees. It doesn't take much power supply hum in a low-level audio stage to be audible.

    I've also got a couple of small 5V linear supplies to check.

    I was hoping that someone could speak from experience, but I guess the homework is up to me.

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    Quote Originally Posted by Chuck(G) View Post
    I was hoping that someone could speak from experience, but I guess the homework is up to me.
    I design audio stuff at work, using switchmode supplies. We always use linear post regulators...

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    Chuck - did you try this question over at the eevblog forums? I can't ever stump them with anything.

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    No, but I solved this one by inserting a pi-section filter to the analog stage 5V rail. The voltage wasn't too critical, so a bit of loss could be tolerated. Saves me from having to scrounge a PSU that I like.

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