Tuesday 16 June 2015

Fern LM386 Amp (for Tane)

Just put the finishing touches on this beautiful amp. It was previously a Vox VT-15 which had died. The Valvetronix series of amps had a modeling section, with amp selection as well as various effects. It then ran the signal into a 1 watt valve amp (12AX7) to get that phat toob toan and then into a basic solid-state power-amp. I couldn't figure out what was wrong with it (only a tiny amount of signal came through the reverb, apart from that there was nothing. Oh and someone had slipped the wires off the speaker lugs) so rather than going through lengthy research and testing (too much time and cost, plus proprietary digital circuits are a nightmare) I completely gutted the whole thing and did away with all of those knobs. There were really too many.
This is the finished amp. It looks great, IMO. You may notice that I've flipped the VOX logo upside down, which I mostly just did because it was there. I cut the original chassis in half (it wasn't needed anymore) and covered the remaining section in a thin layer of card and then some trippy fern-patterned patchwork fabric my mum had lying around. Cutting the holes for everything with a scalpel took a while (and I was nervous about slipping and damaging the fabric) but visually it turned out very well.


Starting from the left (sorry about the rotation) we have the original input jack, the Gain knob (A10k with a 222 cap across lugs 1&3), the Sag/Voltage Drop knob (1k pot with one lug omitted), the green LED and the original power switch. I used as many original parts as I could salvage. The input jack has also been reinforced. The Gain knob is my favourite, and has a lovely sweep from clean and clear through to downright disgusting, much like any of my creations. Often it gets left on the latter setting :)

I wasn't sure whether the LM386 could handle a 4 Ohm speaker or not, but here it performed swimmingly. Also when I measured it, it said 3.2 Ohms, (that might just be the fact that I was measuring resistance though) which was interesting because the datasheet says something along the lines of "please don't take it below 4." I really like the way the grittiness works with an LM386: With a high impedance load (so a smaller load) it amplifies very cleanly and generally just makes a good representation of the input sound. Get it down to 8 Ohms, though, and it starts to bug out a bit. Lower frequencies start to take precedence over the higher ones. Higher frequencies get modulated by lower ones. It doesn't quite get up to full monophony (just one note, like many synths and some fuzzes i.e. fuzz faces/tone benders) but that's a good thing because it still maintains the integrity of whatever chord you're playing, And with a 4 Ohm load, everything is made even more toasty.

On the back we have the battery (standard 9V, use a knife to pry it out) and the Power In jack, which can take anything from about 5V up to 11V or so. It is centre negative, and I've been running it off a pedal power supply as I've been building and testing the amp. The power jack overrides the battery, like any good pedal. It's a lot of fun actually, because I can stick a daisy chain in the back of the amp and run a couple of pedals just sitting on top of it. It worked nicely with a Hotcake -> Hotone trem -> Strymon reverb setup. Because of the nature of steel and the fact that the whole chassis was made out of it, I didn't have any tools that could put new holes in the metal. So all of the external components use holes from the original design of the amp. My favourite one is where the jug lead socket used to be: It has been replaced with a 9V battery caddy that was almost exactly the same size. It's still the power source, but it's just a little different now. Also in this photo you can see the effect of the fabric quite well. If it gets dirty or wet, I reckon a damp cloth would sort it out.

The Voltage Drop knob was originally going to be a volume knob, but I ditched the idea because most guitars have a volume knob on them anyway and I'm sick of turning things down. The amp runs, by default, at a nice comfortable listening level that is musically immersive. It may not hold up well with drums around, but then that just gives you an excuse to turn up your fuzz pedal (guaranteed awesome results, the amp gets a nice cloud of fuzz around it when you hit it with a really hot signal). So I put in a knob that lowers the input voltage, starving the chip of the power it needs. This results firstly in a drop of volume (in case you reaaally needed it). Then it starts to break up and develop gating and monophony (epic synthbass possibilities) and finally it turns into an amazing mess of subharmonics and misrepresentations, all the way to the left.

This nice green LED works whether you're running on the battery or the wall supply. On the battery there's less current available, so it starts to flicker if you put a hot enough signal into the amp. It's fun watching the amp respond visually to the kinetic/electrical energy you're pumping into it. Also if the battery's flat enough, the LED will start to dim down a bit. This picture is of the LED running at the full 9V from my pedal power supply.

All in all, I'm really happy with how this amp came out, and I'll be sad to see it go. That said, there were a few problems with it. The clean setting, combined with the wrong guitar, sometimes still has a bit of noise coming through, which can ruin the musical immersion. This can be fixed by better isolation and shielding and with better, closer circuit layout. Also the speaker having such a heavy, low-impedance load probably isn't helping with getting the cleanest sounds possible. I've got a couple more speakers lying around waiting for cabinets so I'm keen to revise the design.

Finally, here is a demo of me playing some riffs and such through the amp. I talk a bit throughout this but due to the mic positioning it might not be so easy to hear. One of my favourite things that this amp does is the oscillation. It only really works at 9V, but without any input, if you turn the gain knob, the chip starts to oscillate. There is a demonstration of this in the recording. The oscillation also makes different sounds when you have different length guitar leads in the input jack. Any input signal will override the oscillation. One thing to note is that the oscillation is a lot quieter than when the amp is driven by a signal, which I have compensated for in the recording.

No comments:

Post a Comment