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Say you're going to spend your two week vacation in a small cabin in middle of nowhere, where there's nothing but time. Well, what a great chance to practice your guitar playin'. Only problem is your amp. You can't drag your 4x12 cabinets with you, and hate the feeling of playing unamplified. Or you left your Marshall Stack to your bassist's garage, and wake up in the middle of the night with a great riff humming in your head. Or maybe you want to warm your fingers backstage when your stack is already on stage. Well, build yourself a battery powered practice amp that you can take with you anywhere.
You don't need to have a preamp in a practice amp. On the other hand, a simple preamp would make the amp much more versatile, but also it will increase cost and complexity a little bit. A simple preamp would just be your favorite distortion effect pcb mounted in the same housing with the amp. Or you could make a two channel opamp preamp so you could have a separate clean and distortion channels. Len Galasso has a nice design called PracticeMan. It has a two channel preamp with a mixer control. You might also want to add a walkman input for playing along to your favorite CD's. Adding a simple preamped mixer (like the one at AMZ) to the circuit after the preamp will do the trick.
There are many different type tone controls. You can have a simple one band tone control circuit like explained in the Cook Your Own Distortion-article. A Big Muff Tone Control will also make a nice and simple EQ. If you want a no compromise EQ, use Len's Three Band Active Baxendall EQ on his Practice Man.
A practice amp can drive a pair of headphones or a few watt speaker. Any way you want it. If you want to practice without disturbing others, choosing headphone operation might be an ideal choise. Battery life is long and the amplifier will be cheap and can be fitted into a small housing like a cigar box. If you want to play along with others, you might want to have a little speaker mounted in your amp. Remember that the bigger speakers you drive the shorter the battery life will be.
There are several options for a practice man power amp. For headphones, I'd use this simple dual opamp design. There are few IC's that could be used with just a few external parts to do the trick. National Semiconductor's LM386 is a very simple chip that will give you ½ watts of power with battery operation. It's also very simple to alter its gain. Check out my LM386 power amp design. You can use LM386 with headphones and a small speaker. Check out LM386 datasheet for more info. ST Microelectronics' TBA820 is also an intresting piece of silicon. It can put out 2 watts with +12V supply. Study TBA820 datasheet for more info. For a little bit higher power I'd go for National Semiconductor's LM383 chip. It can put out 8 watts with +12V supply. Drives nicely even a little bigger speaker. Kills batteries, but ok with wall wart supply. Here's my LM383 power amp design. Check out also Craig Anderton's LM383-based practice amp. For more info check out the LM383 datasheet. You don't have to have anything but the poweramp to make a nice and simple practice amp.
|Headphone Amp||Simple headphone amp.||Gif||N/A||Ckt|
|LM386 Power Amp||A half watt power amp to use with headphones or small speaker.||Gif||N/A||Ckt|
|LM383 Power Amp||8 watt power amp for driving a bigger speaker.||Gif||N/A||Ckt|
|"Electra" preamp||My Electra variation combined with the Big Muff tone control. Simple and Cheap.||Gif||N/A||Ckt|
|Craig Anderton's LM383 practice amp design.||8 Watt practice amp by Craig Anderton.||Gif||N/A||Ckt|
|PracticeMan||Len Galasso's half watt practice amp. See downloads.||N/A||N/A|