# +/- 9V DC Power Supply

I am working on a simple audio mixer project. The op-amps I am using (LM741) take +9V and -9V inputs. I am currently getting this by wiring two 9V batteries together, connecting + on one to - of the other, and grounding the pair. This seems to work, but I would now like to switch to a power supply. I can find a bunch of 9V power supplies, but they all only have the one polarity. Is there an easy way to get +9V from a center-negative power supply? I figure I probably need an inverter circuit or something, but I don't really understand what I'm doing.

• By the way, are you sure you need +/- 9V? I thought the 741 went down to +/- 5, which you can almost do by splitting the 9 V supply. – pingswept May 22 '10 at 22:17
• There are a lot of single supply opamps available. 741 may not be the best candidate for audio to start with. – XTL Jun 27 '10 at 17:27
• @XTL - agreed about the 741. With single supply opamps, however, you need capacitive coupling, and audio aficionados don't like that in the signal path. – stevenvh Jun 3 '11 at 8:42
• @pingswept: ±9V supplies gives you +6 dB signal-to-noise compared to splitting a 9 V. If you went to the ±22 V limit of the 741, you'd get 14 dB more signal. Higher is generally better. – endolith Jun 4 '11 at 20:11

Assuming at least one of the power supplies is isolated (which will be true if they are good power supplies, but maybe not if they are cheap), you can make a stack of two supplies, so you have outputs at 0 V, 9 V and 18 V. Then, connect your op-amp circuit like this:

• Power supply 18 V -> 9 V op-amp pin
• Power supply 9 V -> 0 V op-amp pin
• Power supply 0 V -> -9 V op-amp pin

Another way to think about this is to understand that ground (0 V) is a relative choice; you can change your choice, as long as you're consistent within one circuit.

One other note-- I should explain what I mean by "isolated." A cheap power supply will have its ground pin connected to the neutral wire of its power cord. Most of the time, that's fine. But if you want to stack supplies, you need an isolated supply, i.e. one where no output pin is connected internally to the power cord.

If you do try to stack non-isolated supplies, you'll be connecting the positive output to the neutral wire, which is the same as shorting out the supply, so its internal fuse will blow. (If it's a really cheap supply, this will destroy it.)

## One other solution

If you can't lay your hands on an isolated supply, you could use a switched capacitor voltage converter. This is a chip that charges up a capacitor, and then quickly shifts the pins so what was previously the positive lead is connected to ground, and the old ground becomes a negative output. It does this charge/switch routine at 10-100 kHz, so to humans, it looks like a negative supply.

1. LM741 is a terrible op-amp for audio. The NE5532 is something like 15 dB quieter, and still cheap. Notes on Audio Op-Amps
2. You should be using rails as high as possible for best dynamic range. The mixers I work on are always ±15 V, because that's the highest voltage most op-amps are recommended for.
3. Power supplies with split center-tapped outputs exist, though I can't find any right now. That's essentially the same thing as stacking two isolated supplies. Also, you could always build your own with a center-tapped transformer and learn how to build linear supplies. It's not too complex: High Quality Audio Mixer - Stage 3 - Power supplies
• Right, there are lots of pin campatible amps that are much better. If you are really cheap and have the 741 you can use it. – russ_hensel May 24 '10 at 14:43
• The linear supply is probably the best choice for a high-quality audio system. Switching will work, but there are more ways to go wrong than with a simple center-tapped linear supply. – markrages Jun 4 '11 at 17:17

One thing you can do when you aren't going to source or sink a lot of current into your 0V, and only have a single supply, is to use a 'virtual ground'.

For example, suppose you only have an 24V supply - two leads, supply and return. You can get -12V/0V/+12V by dedicating an op-amp to stabilize the half-way point and then use that as a 0V reference. In other words, the 24V supply becomes your +12V line, the 24V supply return becomes your -12V line, and the dedicated op-amp produces the 0V line. All you do is put a 50% voltage divider across the 24V to give you the half-way point, and then buffer it with a voltage follower configuration.

Basically your op-amps are powered from the +V/-V rails anyway, the problem is not having a ground. The half-way point voltage follower effectively 'regulates' the midpoint for you. The limitation is that you can't source or sink more current into the virtual ground than the op-amp delivering it can handle, or you lose the reference.

• TI makes a 'rail splitter' just for this application: search.digikey.com/scripts/DkSearch/… if you choose to use a generic op amp, be careful of stability issues if your using decent sized bypass capacitors on your +/- supply lines, adding a resistor to the output of the op amp will increase stability but increase resistance to ground which can have negative effects on audio circuits (increased cross talk in a multichannel system for instance) – Mark May 22 '10 at 22:31

I usually use an inverting charge pump to supply my negative voltage rails, do pay attention to your noise requirements, some times i use an LC filter on the output to reducing the switching ripple. In digikey category lingo what you want is an "inverting switched capacitor DC-DC regulator". This will turn your +9V input into -9V (more like -8.8V or something close).

If your working with audio, choose an IC with a switching frequency well above the audio range, 400khz is a common value but they range from 10khz to 2mhz. If your circuit also has some digital circuitry on it and is compact, consider a charge pump with switching synchronization input. Keep the switching capacitors as close to the IC as possible to minimize noise.

If you search digikey / mouser there will be tons of choices, just enter your desired parameters/package they come in everything from a SOT-23-5 to a DIP-8.

• is it possible at all to use a switcher to supply +15/-15 for opamps like the NE5532 without introducing noise? The NE5532 has a high PSRR, so I guess if there is noise on the rails it would be cancelled out? – b20000 Oct 11 '17 at 6:14

You can use a high current buffer to create a virtual ground:

The BUF634 used here can supply 150 mA (250 mA in the TO-220 version), which will be more than you want to draw from your 9V battery.

(image shamelessly stolen here)

• If your buffer can't deliver enough current (like if you're using an opamp) you can add a push-pull pair of transistors. – Federico Russo Jun 3 '11 at 9:17
• Forgive me for asking a question in such an ancient post, but why would you recommend using an extremely expensive and obscure part for a simple audio mixer project? Using a centertapped transformer would still be less expensive and easier to replace, yet there are still better solutions. If that buf634 was cheap and commonly found in every corner shop, then sure, no problem. – Christoph B Sep 25 '12 at 14:37
• @Jonny - I guess at the time I didn't realize how expensive it was. Anyway, I think it's still a valid answer that indicates this type of ICs exists. Types with less than the 180 MHz bandwidth will probably be less expensive. Thanks for your feedback. – stevenvh Sep 25 '12 at 15:00

While it's nice to have +/- supplies for ground-referenced signals, it's not necessary in many cases including yours. It can make the design conceptually simpler, but a more complex power supply will likely be more trouble than designing the amp to work with a single ended supply, particularly in your case.

The thing to note is that audio is AC only. You can ignore, in fact should ignore, the DC component of any input. The output should similarly not have any average DC component. With this observation, you can realize that the input and output can both be capacitor coupled to some other voltage the amp works on internally.

You didn't say where the 9V value came from. It's not clear if this was due to careful consideration of the voltage overhead you need, or merely a convenience due to available power supply or battery voltages. Without additional information we have to assume you need the output voltage range. Since wall warts are readily available in a wide range for voltages, I'd design the amp to run from a single ended DC supply in the 18-28 volt range.

As others have pointed out, a LM741 is pretty poor for audio. There are some opamps specifically for such applications, but for hobby purposes you might look at the very available and cheap TL07x (TL072 for dual, TL074 for quad). These have much lower noise and higher slew rate. They need a few volts headroom on each end, but with a 24V or so supply there is plenty left. The 741 needed some headroom too.

Design the circuit with all the inputs capacitive coupled with the internal signals biased to half the supply. Make sure to filter this half supply signal so that the inevitable power supply noise doesn't get into the output. It doesn't really need to be a full virtual ground since it doesn't need to sink or source much current. It's more of a "bias" than a ground.

If you're interested in persuing this direction, let me know and we can get into more details.

simulate this circuit – Schematic created using CircuitLab

Figure 1. Regular 9 V PSU wall-wart. Figure 2. Modified for +9/0/-9 V.

You can easily modify a standard 9 V wall wart supply as shown in Figure 2. Ripple voltage will be worse and max current on each supply will be half of original specification so I'd recommend some large caps or voltage regulators to eliminate hum.

If you want to keep a standard jack on the PSU then convert it to an AC PSU by removing the rectifier and capacitors and put the diodes and caps into your project case.

• +1 This works in australia 12VAC plugpacks are commonplace and ideal. – Autistic Jan 13 '16 at 20:43

you can use a virtual ground circuit to get a +/- vcc for your opamp with this circuit