Tuesday, 24 December 2013

Micro guitar amp mixer

It's been nearly three weeks since we went out to Berlin. The Arduino workshops were a success and from a group of about eight people, we reckon at least three (maybe four) are already well on their way to becoming electronics and Arduino aficionados!

Since we got back, there has been quite a bit going on, but - as often happens around this time of year - work, real life, and Christmas have tended to get in the way a little bit. But something else has also come up, which demands some attention:

While out in Berlin, Andrea (remember him, the screen printing guy who lives out there?) had lined up a gig at a night club on the Saturday after arriving. A few of us took to the stage at about 1am and played a 45 minute set, after just one-and-a-half days practice (and for some of us, a break of about 12 years since last appearing on a stage!)

We had a great time, and some of us were inspired to actually learn to play our instruments properly (it's like the Sex Pistols and 1977 all over again). Practicing an instrument can be tortuous for friends and family - and neighbours - especially since the only amplification equipment we currently have is a 300W PA and 15" speakers that we bought off eBay almost immediately after getting back!


Of course, there's no way anyone can use this for learning how to play a few guitar licks (though it will sound amazing if we ever get a gig in Brighton!). So, personally, I've been watching videos on YouTube and twanging out a few licks on an unplugged electric guitar.

One really interesting video (and one that convinced me to shell out some proper cash for a learning course) is this one by Griff Hamlin. He shows how to take a single, simple lick, with just a few notes, and apply it to a variety of different songs.


After watching this video, I'm even more convinced that just practicing licks is as pointless as repeating scales and patterns up and down the guitar neck. It's all well and good being able to play a lick or a scale - but to sound good, they need to be played in context. They need to be played with a song, not just on their own!

Also, electric guitars are made to be amp'ed up. They just sound awful "acoustically". And many of the lead licks I've been learning only really sound half-way decent if played through a powered speaker. So I got to thinking about making something which would not only amplify the guitar signal, but also mix in a backing track, to practice along with.

Being able to hear a backing track, as well as the electric guitar signal (not just the twanging of the steel strings) would be a real bonus. Either through a speaker, or headphones, just being able to hear a sound coming back would be really cool. So here's what I came up with



The input signal(s) are mixed using a simple resistor-based passive audio mixing circuit. This allows us to give each incoming signal it's own input volume. Then we use a DPDT switch to route the incoming signal to one of two different amplifier circuits.

It might be possible to merge these two amplifier circuits into one hybrid circuit, but for simplicity, we're keeping them as two different circuits: this means that the entire project can easily be replicated and altered to suit different needs. We used a DPDT switch so that during switchover (as the user changes the output from speaker to headphones and back again) the amps are powered down, to reduce any loud popping or crackling sounds.

I used two different LM386 op amps because the gain on these can be adjusted from the default of 20 to a whopping 200 (x10 output volume) depending on the resistor/capacitor combination between pins 1 and 8 (no components between pins 1-8 sets it to the lowest amplification value 20).
If we route this output to a set of headphones, sometimes even this signal can be too high and create nasty clipping and/or distortion. This can be taken out by adjusting the volume on the guitar, but if the guitar does not have a decent volume control, the gain from the LM386 might still be too high for a nice clear sound on the headphones.

So I created a completely separate circuit just for headphone use (the DPDT switch routes the incoming signal to one of two different amplification circuits) and used a fixed RC combination to force the LM386 to use a lower-than-default gain of about 11. Just for good measure, I also added a "choke" on the incoming signal for the headphones amp, since different headphones may have different sensitivities and use different speakers in the earpieces. The low-pass filter on pin 7 of the headphone circuit helps reduce any low frequency resonance (feedback) introduced by using such a low input signal.

It's probably too close to Xmas now to actually etch a PCB and try the design out, until Santa's been, polished off the mince pies and drunk my best sherry - but if there's a few hours over Christmas between everyone else watching the obligatory James Bond mid-afternoon movie, and Doctor Who saving the world at about 7pm, I'll try to get up in the loft and put one of these together to see how it performs.

In the meantime, the schematic is above for anyone interested in trying it out on a breadboard.