FUQ (eff -yu - cue) - FAQ's with an EDGE

Tagged As: Small Guitar Amp

Question:
I've recently built a small guitar practice amp circuit found here: http://www.generalguitargadgets.com/richardo/practiceamp/lm386.gif I've seen generalguitargadgets.com mentioned in ths ng before, and there are a few players here, i see. I'm betting that someone has come across and possibly built this circuit themselves, or that it's simple enough (and probably generic enough) that most of you can look at it and understand it :-) (i'm still working on understanding *how* things work as a whole, myself). Anyways, it soldered it up and it works. I've put it together exactly how it is in the diagram, except that for the output i've got it driving a 40W 6x9 4-ohm speaker instead of headphones. It's surprisingly loud for being 1/2W- not earth shaking, but you have to raise your voice to talk over it. Not expecting stellar sound quality, yet the distorted guitar tones are pretty good for what it is- probably better than most really small, solid state practice amps. However, speaking of harmonic distortion, there is a LOT of it. It's great for rocking out (Toni Iommi In A Box!) but i can't clean it up at all. There is no volume or gain control for the circuit, so it relies on the guitar's volume knob for net output. Turning down from the guitar will reduce the distortion just a little bit, but it's still very present all the way down to 0. There is a noticeable change in overall volume by toggling the switch (between pins 8 and 1) but the relative level of distortion doesn't change. The guitar i'm using is a stratocaster with stock, passive singlecoils, and i'm not using any sort of effect pedal or preamp with it. Just guitar-->cord-->amp. Distortion is great, and desireable for some guitar applications, but seeing how this OpAmp was possibly created with Walkmen, Discmen and other small audio circuits (Cheap T.V.s?) in mind (or not), is this level of distortion sound excessive? Or does it sound as if i may have overheated something whilst soldering? (I'm getting better, but i'm not perfect yet). Or have i overlooked something re: speaker type and compatibility? (I won't lie, it was less than $10 and it's targeted at the piss-poor car audio market).

Answer:
The part is not a high performer but the -4 version can output 1 watt of power although that was into a 32 ohm load with a 16 volt supply. Using a lower impedance speaker isn't going to help. Looking further, they show a .2% THD at a paltry 1/8 watt output. Take note that power amps usually have a voltage gain of less than 1, they are designed to deliver large currents to the speaker for power. There are other designs that might serve you better though most of what I find is toob based. The Total Harmonic Distortion is 10% ... quite poor.