1. Time for some upgrades in server hardware and software to enhance security and take AK to the next level. Please contribute what you can to sales@audiokarma.org at PayPal.com - Thanks from the AK Team
    Dismiss Notice

replacing batteries with a power adapter -

Discussion in 'DIY' started by rmunson, Feb 10, 2018.

  1. rmunson

    rmunson console destructor

    Hello group -
    I've put together a little microphone kit that uses three AA batteries - 4.5V
    I'd like to power the device off a small power adapter - an old phone power charger

    The power adapter I have is 5V - 0.7A --

    would this power adapter be too high for this small kit?

    [edit: I checked the voltages with my DMM -- on 3 AA battery power, I see 4.5 V
    I checked the small power charger - it reads 5.2 V

    Thanks!
    Reed
     
    Last edited: Feb 10, 2018

     

    Please register to disable this ad.

  2. elnaldo

    elnaldo Addicted Member

    Messages:
    7,346
    Location:
    Buenos Aires GMT-3
    I see it uses an Ne5532 opamp. 5V is OK to feed that ic.

    The mic seems a regular electret, I don't think 5V will damage it (I power a small electret with a 9V battery). Check the datasheet if you can, but 5V seems safe.

    I think you can try it and see if the PS filtering is enough (no hum) or if you need to improve the filtering.

    You can always drop 0.7V with a diode in series.
     
    Last edited: Feb 10, 2018
    Bill Ferris and redk9258 like this.
  3. rmunson

    rmunson console destructor

    Thanks!
    so I would learn these specs by looking up the specs for the IC - NE5532?
    would the 0.7V be a concern? What type of diode would I put in there - just a little LED?

    I just don't want to burn anything out --
    Thanks!
    Reed
     
  4. elnaldo

    elnaldo Addicted Member

    Messages:
    7,346
    Location:
    Buenos Aires GMT-3
    I think you'll be fine with 5V instead of 4.5. Actually I think the OPamp will work better with 5V than with 4.5V (the datasheet recommends 10V minimum)

    The electret microphone is in fact a capacitor, so it should be fine holding 4.5 or 5V, no big difference.

    The problem with cheap DC wall warts is the noise. The batteries are completely silent, but a power adapter sometimes is not. I'd try it to see how it works.

    If you are concerned about that little voltage difference, you can add a small diode (1N4148, or a bit larger, a 1N4001/4007) in series with the positive rail, between the adapter and your device (you can scrap a diode from some dead board you have around). It will drop approx 0.6 to 0.7 Volt from the DC supply. Many devices come with that diode to protect them from reverse polarity (in case somebody plugs an adapter with the wrong polarity), so it's a common setup, not something I've invented.
     
    rmunson likes this.
  5. rmunson

    rmunson console destructor

    very cool!
    Thank you!
    I'll give this a try --

    when specs talk about mA - is there a way to measure this with a standard DVM? - ie: the output of the wall adapter is 5V - .7A - how would you measure that .7A power number?

    Will post a demo of my DIY intercom when finished --
    Thanks!
    Reed
     
  6. stillspin'n

    stillspin'n Active Member

    Messages:
    338
    Location:
    southwest missouri
    If your DVM has a DC current function then just put it in series with one of the power leads, usually the positive, and see how much current it is drawing. I will guess it to be .1 amp or less, so your supply should be more than enough for it.
     

     

    Please register to disable this ad.

  7. rmunson

    rmunson console destructor

    ah!
    cool - I did not know that trick -
    so where would I put my DVM probes?
     
  8. elnaldo

    elnaldo Addicted Member

    Messages:
    7,346
    Location:
    Buenos Aires GMT-3
    Meter set to mA, probes between the adapter + and the device input. In series, the current goes to the meter and continues to the preamp. REMEMBER TO SET THE METER BACK TO OHM READING RIGHT AFTER MEASURING mA.
     
  9. rmunson

    rmunson console destructor

    good tip -
     
  10. rmunson

    rmunson console destructor

    ok - so I tried to record the amps used in the circuit using the above method - but I didn't get a measurement - just 0 - so I must be doing something wrong --

    I went ahead and tested the little mic preamp with the phone charger and I got a constant high buzz/ringing tone -- so the circuit must not be able to handle .7A --
    how would I reduce the amps to the neighborhood of 3 AA batteries?
     
  11. rmunson

    rmunson console destructor

    ok figured it out -
    my 3 AA batteries deliver 4.5V and the preamp circuit draws 9 mA

    now I'll try the phone charger again and see how many mA the circuit is drawing on the phone charger -

    ...
     

     

    Please register to disable this ad.

  12. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    27,876
    Location:
    uk.. the middle bit
    circuit will take what in needs current wise ..buzzing and stuff likely bad supply for project . battery chargers are not real power supplies in my book .
     
    dB happy, rmunson and I LIKE MUSIC like this.
  13. N8Nagel

    N8Nagel AK Subscriber Subscriber

    Messages:
    6,742
    Location:
    Sterling, VA
    Rather than go with a power supply why not invest in some NiMH cells and a charger?

    Only downside is, NiMH has lower internal resistance than alkaline despite the lower voltage so in SOME circuits they may cause the smoke to come out. Only device for which that has happened to me so far is an Amprobe brand non contact voltage detector - and I use the damn things in EVERYTHING because I'm sick of alkaline batteries leaking and destroying my shit.
     
  14. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    27,876
    Location:
    uk.. the middle bit
    a charger in conjunction with a rechargeable battery pack may well be a good plan . best of both worlds that way .
     
  15. rmunson

    rmunson console destructor

    tested with the phone charger and I measured 10 mA
    the sound quality is noisy - not as good as with the batteries -- so I guess I need to get that mA value down -- I'll get a diode as recommended by elnado and see if that helps --

    my hope is to use this as a one way intercom - so I would prefer to have it on a power adapter and not deal with replacing batteries...
     
  16. I LIKE MUSIC

    I LIKE MUSIC Super Member

    It is not the current (mA) that is causing you a problem.

    It is likely that as others have mentioned, the charger is generating the noise.

    With a device like you have you need to power it with well filtered, non-switching power supply that will supply an operating voltage that is clean, that is does not contain a lot of noise.

    Again, it is not the current (mA) that is causing you problem.

    You circuit needs a very clean, noise free power source, this is why it is set up to use batteries and not a power adapter, although it may work okay with a well filtered, clean output, power supply.

    The data sheet for the IC used in your device listed is operational current from 8 mA to 16 mA with no audio signal and as much as 60 mA with the output of the IC shorted.

    Again, that extra 1 mA of current is not your problem.

    And the cable of your power supply can act as antenna an pick up unwanted electrical noise.
     
    rmunson and mbz like this.

     

    Please register to disable this ad.

  17. elnaldo

    elnaldo Addicted Member

    Messages:
    7,346
    Location:
    Buenos Aires GMT-3
    You can't "lower" the 700mA. That's the current the PS can deliver, but your device just draws 10mA. Actually it could be the PS is more noisy because the low current draw. Try drawing around 100 mA from it with a resistor around 50 ohms from + to -, in parallel with your amp. It's half a Watt, I'd use a 2 watt resistor. Or go with 22 ohms to draw around 220 mA, I'd use a 3 watt or larger resistor. But if the PS is noisy you need to improve the filtering, adding inductors and capacitors, or get a better PS.

    If you read my first answers, the noise was my first concern. Batteries are silent.

    I think a regular transformer with a full wave rectifier can be silent with a proper filtering, easier to do than dealing with a noisy switching PS.
     
    Last edited: Feb 11, 2018
  18. I LIKE MUSIC

    I LIKE MUSIC Super Member

    Any noise in the power supply will be coupled to the input of the IC because it supplies bias to the microphones.

    The picture below shows at a minimum the amount of power supply decoupling needed for the microphone bias supply. R1 and C1 provide some additional power supply decoupling (noise filtering), more may be needed. Any noise that is not decoupled (filtered) from the power supply (red line) goes to the IC and is amplified. This is why good power supply decoupling and a clean, noise free power supply are important.

    Again, this is why your kit is set up to use batteries.

    upload_2018-2-11_20-55-21.png

    You need a good low noise power supply. A linear power supply would likely work the best, but you may still need some additional power supply decoupling (filtering).
     
  19. dB happy

    dB happy Active Member

    Messages:
    228
    Yeah, noise is from the power supply. I'd recommend a lm7805 voltage regulator, with a nice big filter cap on the bridge rectifier. Then another small filter cap after the 7805.
     
  20. I LIKE MUSIC

    I LIKE MUSIC Super Member

    If you are not up to building you own power supply from scratch, there are a number of them on the, you guessed it, internet.

    upload_2018-2-11_23-25-50.png

    This one happens to be adjustable. It was the first one that I came to that included the transformer. I have no first hand knowledge of this device, but there is not much that they could do wrong although I would likely fuse both the primary and secondary of the transformer if it is not already fused.

    I took a quick look at the page and I did not see that it is fused, but it does come with an AC power cord.

    Anyway take it for what it is worth, it is just an example.
     

Share This Page