Disclaimer:- These are my solutions, they may not suit everyone bit maybe they will get some ideas.
1. Simply Buy One
The first would be to simply buy a battery driven mixer such as Behringers 1003B for around £90.
It takes two PP3 batteries and the manual says they will last four hours. An extra battery is needed for phantom power.
Mind you, the manual also says it takes 8 watts. From two 9V batteries that is 0.44 Amps.
PP3 batteries are only rated for around 0.6Ampere hours – that’s around an hour and a half.. So take plenty of spare batteries.
2. Use what you have
I have a Wharfdale 802USB mixer (around £90) It is similar to the Behringer above, withe the addition of a USB input and output (which is why I bought it, but that is another story)
It is not designed to run from batteries.
But I thought I’d try it anyway.
The power supply inside the mixer probably (I have not actually looked) looks like this:-
With the wharfdale the power supply IS the transformer, and the connector feeds straight into the Bridge. So I thought if I connected two PP3 batteries in place of the transfomer, would it work?
It did! The output was understandably lower.
With the wharfdale transformer the output can reach 22V peak (That will amost certainly saturate the input stage of any following equipment). Interestingly, the indicator leds are based on ratios of the supply voltage. So if I use a lower voltage, the leds still let me know if it is close to saturation.
With PP3 batteries it can peak at around 3.5V which is still easily enough to drive a following amp. It takes around 90mA from the batteries indicating a life of around six and a half hours. Other batteries could be used instead , anything up to 24V would be OK.
It does work, but I don’t use it.
3. My solution
I also have a little Behringer MX400 mixer (around £12)
It worked fine as standard, but I needed to add a few things.
- A battery supply
- A master volume control that I can use independent of the mix
- An indication of clipping.
The standard power supply is an external 12V supply. I found it runs quite happily from 9V. In fact it will run with a supply as low as 4V.
I bought a PP3 battery box, which conveniently come with a switch and fixed it with screws to the side of the box. Then I simply took the wires into the box through a corner, and soldered them to the circuit board.
The MX400 circuit as standard has input stages with a maximum gain of 100/4.7 or 21.3, followed by a mixer stage with a fixed gain of 47/10 or 4.7. This gives an overall gain of 100.
I wanted a bit more and I wanted it to be variable. I managed to increase the gain and provide a volume control by replacing R2, the 47K feedback resistor on the mixer stage with a 500K pot.
Max gain is then 100/4.7 x 500/10 or 1064.
OR it would be if the GBW product of the mixer op amp did not reduce the gain from 50 to 47. at the high frequency end So max overall gain is now 1000.
Apart from gigging or busking I wanted to be able to use it to provide the audio when shooting videos on my iPhone or Macbook
Apple do not publish safe limits, but probably, based on general guidelines for consumer audio inputs, dc voltages of >5V or ac transients of >10V could damage the iPhone
I also wanted a clipping indication. I decided to use two red LEDs connected in anti parallel, across the output.
The Macbook, and the iphone, unhelpfully, put a 2.5V voltage on its microphone input connectpr which is meant to power electret headphones. Ca is fitted to avoid this signal turning on Da. They both also need to see a dc path to ground on the inputs before they will recognise an external mic source. Hence Ra.
From experiment, these leds start to glow at 1.6V (when they are taking only 0.02mA). They are glowing quite brightly at 1.74V which is when the iPhone will clip. At the maximum current that the amp can provide the leds limit the voltage to 2.02V peak which is well below the iPhone safe limit. In practice non symmetrical signals may only light one led. As the supply voltage drops, the maximum signal will drop too, but clipping indication should still work.
The Modified MX400 (or MMX400)
Current taken is 17.6mA (measured) so a single battery should last around 34 hours.
The MMX400 in use
The mixer is quite rugged and the box is made out of steel. This makes it very easy to mount to a steel bracket fitted to the mic stand using magnets. I use couple of neodymium magnets removed from an old hard drive. The bracket is actually designed for a microphone.
I think that an acoustic guitar sounds much better through a mic than through a pickup, and although all of my guitars have pickups, I’ll use the mic when I can.
The bracket can be easily situated to put the mic in the ideal position for the guitar while having the mixer just in front of the guitar.
Conclusion
If you need tone controls or equalization the last solution is not for you. Personally I don’t think mic inputs for performing need any, and for recording it can all be done in the processing in something like Audacity, Garageband and even some video editors (Like VideoPad.
It’s small, light neat, economical, and fits in a guitar bag.
Works for me.