While completing my first set of mods on my Yamaha PSS-30 I noticed that the internal amplifier driving the mini keyboard was a 386D amplifier chip. This chip has an identical pin out and seemingly identical function to the popular LM386 which gave me some ideas for possible bends I could try. If I was able to apply some common modifications or adjustments which work with the LM386 in amplifier applications like the LM386 guitar amp I may be able to further expand the versatility of the instrument.
The first modification deals with the gain of the amplifier. If you’ve worked with LM386s in the past you may already know that the gain of the amplifier is set using pins 1 and 8 of the chip. Essentially by placing a resistor (usually 1 ohm – 10K ohm) and a capacitor (typically 10 uf) between these two pins you can set the gain. The higher the resistance of the resistor, the lower the gain. Upon inspection of the circuit I could see this is exactly how this 386D chip was set up. Pin 1 is connected through a 10 uf capacitor, which connects to a 1.1K ohm resistor (immediately to the left of the chip) And then to pin 8 of the chip. In order to replace this system with a variable gain control I removed the 1.1K resistor and replaced it with a 5K ohm potentiometer. By reducing the resistance you can get a slightly crunchier and more distorted sound, and by raising the resistance you can get a cleaner more polished sound. Note the gain level will influence the volume of the output so you will need to compensate for this either at the volume control added in Part 1 or at your mixer/amplifier.
While I was under the circuit board attaching the leads for the gain pot I also connected a few more wires for use with my second mod. I connected the first wire (yellow) to pin 1 (gain control pin) and two blue wires to pin 5 (output). These will be used for the second modification I had in mind. This is a slightly less used LM386 circuit modification but still one which is fairly well documented. By sending a signal from the output pin (5) through a small capacitor and resistor to the gain control pin (1) you can create a bass boost effect. To accomplish this I attached the first blue wire to a switch on my panel. I then ran it through a 0.1 uf capacitor and a 10K ohm resistor. I attached the yellow wire to the other side of the resistor completing the circuit. Now by flipping the switch you connect the bass boost circuit.
Though the bass boost is audible I am not overwhelmingly impressed with it. It is far from the thumping low end I was hoping for. This may be a limitation of the device itself but I feel like further experimentation is needed. I will be going back in to experiment with some other cap/resistor values and other circuit options to see if I can get a better effect. I will report back here if I find better results.
The second blue wire I connected through a resistor to an LED and tucked it behind the circuit board. Because I used a transparent panel this creates a cool back lit effect and since it is powered from the amplifier output it pulses along with whatever is being played on the keyboard.
That about raps it up for the PSS-30 for the time being. I really love the small form factor of this device and would love for it to make it’s way into my regular instrument lineup. In spite of this circuit being a bit limited as far as bend points and options, I still had a lot of fun and got to try out some interesting new things on the circuit. I’m going to keep this device in the back of my mind as I work on other projects and hopefully I can return to it down the road with some new ideas to further mangle it’s square wave outputs. That’s all for tonight but thanks for reading and happy soldering!