2

I designed a microphone preamplifier based on a single op-amp. It works perfectly using two 6-volt batteries. However, I need to power it directly from AC.

I found a 9 V AC rated wall-wart, but it is not center tapped so I need to find another way of making dual-voltage supply. How do I do that?

Nick T
  • 12,360
  • 2
  • 44
  • 71
Bartek Szablowski
  • 175
  • 3
  • 7

1 Answers1

4

If your wall wart outputs AC, then you need a bridge rectifier to convert it to DC. The bridge rectifier will drop the voltage somewhat (to 8.5 or so) so you will still need to regulate the voltage to 6 (I am assuming you are using the batteries in parallel). For that, you could use an LM317 linear regulator. Finally be sure to include some capacitors for filtering. There are lots of resources on the internet about converting AC to DC and selecting filter capacitor values.

One thing to check is the current requirement of your circuit. If your wall adapter is of too low a current rating, then it will cause problems. Measure the current use of your circuit on the batteries and make sure that the wall adapter can supply at least that much, with some overhead.

Edit:

Per your comments, you need a dual polarity power supply, (+6/-6V DC) so you actually are looking for something which can supply at least 12V on the DC side.

I would recommend finding a ~13.8V transformer with center tap and build your own supply using something like this:

Example Dual Polarity Power Supply

Please ignore the specific values in this schematic, I just "borrowed" it from the web. The idea here is that your center tap is your reference "ground." Both secondary legs feed a full-wave bridge rectifier. The positive half of the AC wave becomes the positive DC rail, while the negative half of the AC wave becomes the negative DC rail. Both regulators could for example, be LM317's adjusted for 6V.

Since you are building a microphone circuit you may want to experiment with capacitor values to reduce ripple as much as possible.

JYelton
  • 32,302
  • 33
  • 134
  • 249
  • Current draw of my circuit is 42mA, Wall wart is rated for 300mA so it's okay. I know how to regulate positive voltages. I need a NEGATIVE voltage for my op amp. – Bartek Szablowski Aug 20 '13 at 16:33
  • So you need both +6 and -6? You will probably want to get a transformer that is >13.8 volt center-tapped and build your own supply. You did say "dual," sorry I missed it. – JYelton Aug 20 '13 at 16:36
  • Yes, It's okay. Well. I'm sure there must be a way of separating positive and negative voltages. But what about ground.. – Bartek Szablowski Aug 20 '13 at 16:39
  • I'll update my answer with more info. – JYelton Aug 20 '13 at 16:43
  • Alright that clears some things up for me. So there is absolutely no way of getting both positive, negative and ground from a non center tap transformer ? – Bartek Szablowski Aug 20 '13 at 16:53
  • 1
    You don't need a center-tapped transformer. At such low current requirements, you can easily get +/-6V from a standard 9V AC supply using a voltage doubler circuit as shown in the following link: http://sound.westhost.com/project05.htm – us2012 Aug 20 '13 at 16:53
  • @us2012 thanks that's exactly what I needed ! – Bartek Szablowski Aug 20 '13 at 17:04