I had to try it. I hooked up my variable power supply to the voltage connector of one of my amps that is soon to be for sale. I was wondering what the current consumption would be for those of us that like 9 volt batteries.
To start off, 9 volt batteries do not function well below 7 volts. Without getting into the engineering aspect, the battery just can’t deliver enough current at this point. The LM386 will work at 5 volts, but once a 9 volt battery gets that low, its voltage sags dramatically. If you don’t believe me, do a Google search on the internal resistance of batteries.
Ok, so here’s another tidbit that you’ll find on the net. The Ruby circuit, which I use in all of my cigar box amps because it’s easy and I don’t have to design or redesign, works great at 9 volts. If you are able to take it to 12 volts, you can really get some great sounds from it. Just be sure it is a regulated 12 volts. Anything more than 15 volts will fry the IC. LM386 Datasheet
Here is what I found for the current consumption of the Ruby circuit.
At 9 volts and the amp at full volume, the quiescent current consumption is about 30mA. That is with the volume on your guitar turned completely off. If you turn the volume all the way up on the amp and your guitar and play heavy rock the peak current consumption can go as high as 200mA. If you are able to crank the input voltage to 12 volts, you can see peak current consumption as high as .250mA! That’s a quarter amp. that is a lot. Of course, the gain and volume control all of this. I merely show you the peak here since there are infinite settings between 0 and 11.
So what does this mean? Well, if you are playing heavy rock, you won’t get to play as long on one battery. Check out this Energizer datasheet for a 9v. This is probably fairly typical of 9 volts. If you play at an average of 175mA, you will probably only get about 2 to 3 hours of play. If you can play at moderate volume, which would average about 100mA, you will get about 4 hours of use. Now I see why having a good regulated voltage source is worth it. With good 9 volt batteries costing $3, it doesn’t take long to make a $20 voltage source well worth the investment. That is especially true if you play your amp a lot.
Note: This test was performed with a Fender Strat. A Piezo type pickup may have different characteristics, but the results should be fairly similar.