Can 12V amplifier run on reduced voltage?
I have recently purchased the 12V Mini Stereo Amplifier from Jaycar (Cat. KC-5495, SILICON CHIP, May 2010). Will this amplifier run with a guitar as the input source and if so, will 9V be enough? (R. E., via email).
• While the amplifier will run from 9V, it won’t develop much power and a typical small 9V battery will not last very long powering it.
We recommend that you instead use a small 12V gel cell (sealed lead acid) battery. The best value is Jaycar SB2486 (7.2Ah, $24.95) and this will allow the amplifier to deliver reasonable power; probably all day, depending on how hard it is driven.
Its weight is 2.2kg though. If you need something lighter you can try the Jaycar SB2480 (1.3Ah, $19.95) which weighs about 570g but this may only last a couple of hours; possibly more if the output is not very loud.
These can be charged using virtually any lead-acid battery charger but some care must be taken to avoid overcharging them. Jaycar sell a number of suitable chargers (eg, MB3517 or MB3526).
As for plugging a guitar in directly, it depends to some extent on the type of pick-up but usually guitars have a relatively high output impedance and a low signal level. You can certainly try plugging a guitar directly into the amplifier and you should get some sound out but it may be too quiet for your liking. Changing the volume potentiometer to a 100kΩ logarithmic type will increase the input impedance of the amplifier and that may help.
If it is still too quiet then you will need to add a preamplifier between the guitar and the amplifier (it could be built into the same case). One possible candidate would be the High-Performance Microphone Preamplifier published in the September 2010 issue (Altronics kit Cat. K5514). This provides adjustable gain from 3-111 and is quite small so could be fit into the amplifier case.
Connect the guitar input to the microphone preamplifier input, then the microphone preamplifier output to the amplifier board inputs. It can run off the same power supply. The preamp gain knob does not need to be accessible outside the case as you can still use the volume knob.
We have also published various guitar mixers and other suitable preamplifier projects in the past. The latest one is the Versatile 4-Channel Guitar Mixer published in June 2007 which is available as a kit from Altronics (Cat K-5353) and Jaycar (Cat KC-5448).