cellphone charger supply in audio preamp

Discussion in 'General Electronics Chat' started by Precious Molina, Oct 12, 2015.

  1. Precious Molina

    Thread Starter New Member

    Sep 6, 2015
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    is cellphone charger 5volts noisy?
    noisier than wallmart 9volts?
    im going to use it in transistor preamp
     
  2. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,301
    880
    Some are very noisy; others, not so much. The worst ones I tested had almost 50 millivolts of high-frequency noise on their outputs, depending on load current. The best ones I had on hand (the "little white cube" chargers for iPhone) had only a couple of millivolts of noise. I don't know what Walmart 9V supplies are like regarding noise, but all the modern "wall wart" supplies I've tested are very noisy.

    I wouldn't use any of these to power an audio preamp or any other low-level amplifier without filtering out the high-frequency noise first.
     
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  3. Precious Molina

    Thread Starter New Member

    Sep 6, 2015
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    do you think 100ohm series and 100nf and 100uf in power supply is enough?
    is there a way to measure noise using DMM?
     
  4. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,301
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    It might be. I should think two such stages, with 100 ohms and 100 nF and 100 μF in each stage, would almost certainly be enough.

    I've never had any luck doing that. First, the noise has always been too low in amplitude to get meaningful readings from my DMM; and second, the noise waveforms are rich in high-frequency harmonics and the DMM doesn't measure such waveforms very accurately.

    Better to use an oscilloscope, in my opinion.
     
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  5. Sinus23

    Member

    Sep 7, 2013
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    If you don't have an oscilloscope and it is in a breadboard stage of development

    You could try to feed the preamp with a as clean variable sine wave you can get and connect the preamp to whatever the next stage is and listen to it with your best headphones available. Preferably closed headphones not the ear bud types.(This is a bit of ghetto style but when you don't have an oscilloscope it's better than nothing).
    First vary your frequency of the sine wave and listen if you can pin point a "note"(frequency) that doesn't change. then put the capacitors in the circuit as close to the cellphone charger as possible and listen if you can hear a difference. Try with different sizes until the noise has become inaudible.

    As unreliable as our ears are as test instruments. Small changes to a stable sine wave are very noticeable.
     
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  6. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The quality of the power supply has to be better and better as the signal you intend to amplify is weaker and weaker (higher and higher gain needed).

    Also, the power supply rejection ratio of your preamp and amplifier design. Example, using current mirrors on each side of the long-tailed pair will greatly reduce power supply noise.

    And the type of amplifier you use (class AB vs Class D).

    Using constant current Class D is less fussy about the power quality.
    If you start with a signal with much higher
     
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  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    I avoid the modern switch-mode power supplies and go for the older (heavy, real iron) transformer based ones. At least they put out "noise" at one well understood frequency (second harmonic of AC line freq) , and can be easily filtered. As people throw away their old electronic gadgets, the transformer based chargers/power supplies are very cheap at thrift shops...
     
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  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    How much current does your amp draw? 100Ω in series may drop too many volts.
     
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  9. Precious Molina

    Thread Starter New Member

    Sep 6, 2015
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    I have two option, iphone charger or desktop usb
    im gonna use it in dynamic mic

    can you give an example of that ihave lots of bc557 bc337

    is desktop switch mode? if the noise is still unacceptable, im gonna use 9v tranformer china

    i dont know, im going to build simple transistor with 30+gain preamp for dynamic mic
     
  10. Precious Molina

    Thread Starter New Member

    Sep 6, 2015
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    i saw in others schematic they used so many caps, is it worth it in audio preamp? i saw 1000u 100u 10u 2.2u 100n, is that gonna help?
     
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