Turn A Boombox Into An Amplifier

It’s been a few weeks since my desktop amplifier kicked the bucket and being a bit shy on money at the moment I decided to take a slightly more creative approach to replacing it. What I had on hand was an admittedly oversized Sanyo Boombox from the 90s. Stereos and boomboxes of this vintage are fairly plentiful (and cheap) these days as people are more and more abandoning them for sleeker, smaller and newer models. And this plentiful availability makes them fantastic for experimentation. Today I’ll be adding an input for using it as an amplifier for use with a guitar or other sound devices.


The front panel of these stereos can be removed by taking out a number of screws from the back. You will usually need a fairly long screwdriver as many of the screws are very deep set. One nice thing which Sanyo does (as well as a number of other manufacturers) is mark the screws for the front panel with small arrows to let you know which must be removed.
boombox amplifier

As I began to pull this guy apart I began getting excited. Internally I found a number of complex boards built entirely of through hole circuitry (perfect for modding and circuit bending). The above image is the amplifier board, The chip in center is the main amplifier chip (A 222003 chip). Also there were a number of very interesting mechanical features. Today my stated goal is simply to add an input jack but I will be returning to this board to see what else I can force from it.

In order to add the input jack there are three wires you will need to solder onto the board. The first is a ground wire which can be connected to any ground point in the stereo. I wired mine from pin 5 (ground) of the 222003 amplifier chip. Next up you will need to locate the left and right signal inputs. I initially planned on sending these into the amplifier chip as well but after some testing I found I got a much better signal sending them into the volume control board as shown above. To locate these points I built a small 555 buzzer circuit on my breadboard. I attached the ground from the stereo and touched the output to various points on the circuit.

NOTE : I strongly recommend powering the stereo using batteries when you are feeling around for these points. If you choose to do so with the stereo plugged into the wall I take no responsibility. Mains voltage can kill. Be aware of the location of the transformer and stay far away from it.

Boombox amplifier

Once I had located the speaker left and right inputs I attached lengths of wire to them. I connected these two wires to either side of a 100K (If you need to get more volume try a 50K or 10K pot) pot which will allow you to pan the signal between the two speakers. If you are not interested in having pan you could easily replace this pot with two static resistors. Then I connected the center pin to the tip of a 1/4 inch jack. In retrospect you may want to add resistors on either side of the pot to buffer the signal as it gets quite loud if you go to either end of the pot. I expect about 10K would work perfectly. If you are interested in adding a separate volume control or switch just add them in line in between the pot and jack. 
Boom Box Amplifier

From here you can drill and mount the pot and jack. Put the stereo back together and you are all set. You’ve just converted your tired old Boombox into a perfectly functional amplifier. I’ve had great results using this stereo with my guitar, circuit bent devices and my synth modules. The sound is actually quite clean. The stereo also has a 4 band equalizer and a bass booster which give you a fair bit of control of the sound.

Hope you guys enjoy this project, have a great weekend!

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Basic Bends – Little Tikes Pop Tunes Keyboard

The latest toy to come across my work bench was a Little Tykes Pop Tunes Keyboard. This toy stood out to me at the thrift store for a few reasons. First and most obvious is the awesome LED ring located between the power knob and speakers. Further it had some weight to it and appeared to be solidly constructed which gave me some hope for what may be found inside. Finally the song samples made it stand out from a lot of other toy keyboards I had played with. Rather than ear shattering renditions of “Old MacDonald” or “Mary Had a Little Lamb” the pop tunes keyboard featured recognizable (if a bit outdated) popular music  (“ABC” by the Jackson 5 for example).

Upon opening it up I found a fairly simple SMD circuit board with a black blob IC. There were quite a few transistors and resistors though a large quantity of them seemed to be associated with the LED functionality. Still there was plenty to play with so I set to it.

The first step (as with all my toys) was to set up a kill switch and line out for the toy. For the kill switch I simply cut the battery positive wire and placed a switch on it. As far as the line out I cut the speaker positive wire and ran it through a toggle on – on switch. The other side of the on – on switch I ran through a 1K resistor and into the tip of a 1/4 inch jack. I then placed a 10K ohm resistor between the tip and ground and ran a wire from the ground back to the ground point on the speaker. I recommend experimenting with different resistor values to get the volume level you require as it can vary from device to device and also depending on where you are sending the signal.

Next up I set up a pitch bend on my Pop Tunes Keyboard. First off a big thank you to alienmeatsack who made a great post about this toy on Electro-Music.com and led me to this bend. This pitch bend is slightly different than the ones I’ve used in the past as it makes use of three points on the board rather than replacing a single resistor. For this bend I used R1 as the pitch base and soldered it to the center pin of a potentiometer. I then connected the outer pins of the potenetiometer to R07 and R011 which shift the pitch up and down respectively. The toy will crash if you shift to far to either side so you may want to buffer the pot with resistors on the outside pins. I found around 4K to 5K worked fairly well for this but there is no substitute for experimentation.

I was blown away by the low end bass when the pitch was shifted down on this device. The quality of the rumbling drones you can produce are just incredible for a toy like this.

Alienmeatsack also talked about getting good results from a voltage starve on this device so I may try that out. And I’ve found a few glitchy areas on the board which I would like to investigate. I should be back to update you on my progress soon.

NOTE : Shortly after writing this post I was experimenting on this board trying to force a loop when one of the resistors cooked itself… There was a little puff of smoke and many tears. I will come back to this toy and see if I can replace the resistor and get it running again but it may be on it’s way to toy heaven. This problem was not caused by the pitch bend which seems to be very stable and produced excellent results but if you are exploring the board be careful as some of the resistors appear to have a very low tolerance.

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Adding External Triggers – Kawasaki I-Soundz Drums

Trigger Inputs Drum Pad

A few weeks ago during one of my usual thrift store exploration I picked up a Kawasaki I-Soundz drum kit. Even before doing any bending I started having a great time with this toy. It has a large and varied vocabulary of samples and surprisingly high quality stereo audio driven by an internal TDA 2822 operational amplifier. I was also pleased to find that rather than the tactile buttons I’ve seen on many toy drum pads the Kawasaki kit is driven by piezo disks similar to higher end drum synthesizers. Unfortunately the device does not offer any polyphony but given the price point I did not expect it.

external triggers drum machine
The drum pads also contain a number of interesting on board rhythm samples. Unfortunately these samples only play for about 30 seconds before stopping (regardless of whether you are playing the kit). Since I was looking for something I could use to set up repeating rhythms while I played other devices this left me with a need. I wanted to set up external triggers for the drum sounds. By using these external triggers in conjunction with my recently completed 4017 gate sequencer I could turn the Kawasaki drum pads into an 8 step drum machine and unlock a world of new rhythms.

The Build

external triggers drum pads
The first step whenever you are setting up external triggers on any toy is to create a ground share point. This can be done by simply adding a banana jack or binding post and connecting it to any ground point on the circuit. If you are using grounded connections from your trigger source (such as 1/4 inch, 3.5 mm or RCA cable) then you can simply connect the ground point to the ground on your trigger input jacks rather than having a separate plug.


Because (in it’s normal operation) this toy is triggered by piezo disks most of our work is already done for us. When the pads are hit the piezo disk creates a trigger pulse. This pulse is sent to the base of an internal transistor (highlighted above), which switches the circuit on momentarily and causes it to play the sample associated with the drum pad hit. All we need to do to set up external triggers is send our external signal to the base of these transistors to switch them the same way the triggers from the piezo do.

External triggers solder points
One thing I am admittedly not incredibly comfortable with is soldering onto SMD (surface mount) circuits. That being said this is something that I feel I need to improve and develop my comfort with. Surface mount technology becomes more prolific and through hole circuitry becomes rarer and rarer each day. Further developing my comfort with SMD will open up near endless possibilities of new circuits I can work with. For this reason I have made it a goal not to shy away from these circuits. I will take the necessary care but am determined to become as familiar and comfortable with them as I am with more traditional components.

Where possible I soldered my leads to resistors adjacent to the internal transistors as there was less risk of damaging these components.

To solder I held my soldering iron to tinned wires to heat them up prior to touching the board. Once the solder on the tinned wire was liquified I lowered the wire and soldering iron to the soldering point together. I raised the soldering iron from the board almost immediately after touching the two down and held the wire in place until the solder solidified. The key here is to spend as little time as possible with the soldering iron on the board. The components are significantly smaller and the solder connections are significantly weaker than traditional through hole circuitry. This means any excess heat on the board can damage components or loosen their solder connection knocking them out of place.

Since my solder points were weaker and the circuit was so crowded I also added a small amount of hot glue to each connection. This gives each connection added strength and also insulates the wire from the other components to ensure it doesn’t touch any other solder points.

For reference the solder points I used for the triggers were on R10, R13, R8, Q3, R2 and R4.

External triggers drum machine

Finally connect the wires from the trigger points to the trigger inputs of your choice. I was short on plugs so I have used bolts but you can easily use banana, 1/4 inch, 3.5 mm, RCA or any other type of input you have on hand.

Now that I have the external triggers set up on this drum pad I will be going back into the circuit and completing some more traditional circuit bending. I will be adding a pitch bend and hopefully will be able to find some other interesting bends and effects to give me an even wider range of sounds to use.

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Build A Simple Gate Sequencer

Gate Sequencer

For the past week or so I’ve been working on building a 4017 based matrix gate sequencer. I originally started thinking about this project after purchasing a set of Kawasaki electronic drum pads from a local thrift store. I wanted to create a tool I could use externally to trigger the drums in a continuous loop. As I began to design This build though i began to realize it’s full potential went well beyond that.

As this sequencer goes through each step it outputs a voltage (approximately 5V) at the top most pin on the matrix. By connecting this top pin to any of the 6 pins directly below it you can send this signal out through the associated output on the side of the gate sequencer. Because these signals are being sent at approximately 5V they are perfect for switching low voltage transistors such as the 2N3904 (essentially allowing it to turn on or off an electronic switch wherever you send it). By using these outputs to switch on or off transistors they could be used to trigger a sound from a toy, gate an oscillator, turn on or off a channel on a mixer, trigger an envelope or anything else you desire.

Click here for details on setting up triggers in a drum toy

Gate sequencer 4017

I’ve also included some basic controls common to more traditional sequencers like the Baby 8. These include a rate control to adjust the clock speed, a hold switch which pauses the sequence, a step selector switch which allows you to select how many steps the sequencer goes through before restarting and a clock out for syncing other sequencers or circuits to the gate sequencer’s clock rate.

Parts List:

  • 555 Timer IC
  • 4017 Decade Counter IC
  • 2 – 4.7 K ohm Resistor
  • 1 – 100 ohm Resistor
  • 200K ohm potentiometer
  • 1 – 10 uf Electrolytic Capacitor
  • 1 – 0.1 uf Ceramic Capacitor
  • LEDs (one for power and one for each step)
  • 1N914 Switching Diodes (One for each step)
  • Rotary Switch (number of positions equal to number of steps plus 1)
  • Toggle Switch – power
  • Toggle Switch – hold
  • Hook up wire (lots)
  • Ribbon Cable (Strands equal to number of steps)
  • Proto-Board
  • Clock out jack (I used 3.5 mm headphone jack)
  • Ground Connection Jack (I used banana)
  • 6 – Output jacks (I used bolts but banana jacks are ideal)
  • Matrix connections (I used pin headers but you can use whatever you have available, requires 1 out and 6 in per step)
  • 9V battery clip

Schematic:

This is the schematic I drew up while building the gate sequencer. For simplicity sake I did not draw out all of the steps but they will each mimic the first two shown on this schematic. Bear in mind though that the 4017 output pins do not go in order, make sure to check the pin out diagram to make sure you are setting up the steps in the correct order. For 8 steps you should be pulling from pins 3, 2, 4, 7, 10, 1, 5 and 6 in order.

I wanted to mention as well as it is not clear on this schematic. If you are using fewer than all 10 steps from the 4017 counter you will need to wire the output of the next pin higher than the ones you have used to the final position of your rotary switch so that the counter resets after going through the steps you have used rather than the full 10. For example my gate sequencer uses 8 steps (outputs 0 to 7 on the 4017) so I wired output 8 (pin 9) to the final position of my rotary switch.
AND gate for gate sequencer
If you are using this device to trigger circuit bent toys you may also run into an issue where you are not able to trigger the same noise for two consecutive steps. This is because if you send the signal to the same output for multiple steps the output will remain high rather than sending a pulse for each step. I was able to find the fix above from Peter Edwards of Casper Electronics who used it in a similar project he built a few years ago. In order to correct this you can place an AND Gate on each output and send the clock pulse into the second input on each AND gate as shown above. This will cause the output to pulse in time with the clock when the signal from the matrix stays high for multiple steps.

The Build:


The first step of the build was to populate the circuit. Following the schematic I had created while testing and designing my gate sequencer I placed and soldered all of the on board components. I also used a number of short leads to put the steps in order on the board so that I could work with them easier going forward. One thing I want to mention is the row of diodes shown in the above picture were actually removed and placed on a secondary board (more details to follow) to simplify the finished product.

At this point I also mapped out the surface of my project box and populated the off board components (switches, knobs and LEDs). Due to the number of components on the box I used a piece of graph paper cut to the size of the surface to plan the device locations then used a pin to mark each one through the paper. From here I drilled the holes for the larger components and secured them in place.

pin headers

Due to the sheer number of connectors required to build the matrix I was not able to use banana jacks (which would have been ideal). What I did have on hand though were a number of male to female jumper cables and a pile of pin headers. I cut 8 of the female heads for the top posts and used individual pin headers for the connections. To mount the individual pin headers i ran fairly high gauge solid core wire through the holes and soldered them to the short ends of each pin header. Next I pulled the wire back down the hole until the plastic guards on the pin headers sat securely against the top of the box. To secure them I poured a substantial amount of hot glue onto them from the underside of the box.

gate sequencer ribbon cable

In order to limit the rats nest I foresaw forming between the top of the box and the main board I used a small scrap piece of proto-board as a junction. From here I ran all the connections needed for each step. The ribbon cable shown here is attached back to the main board (orange is step 1 through to black for step 8). From the board there is an orange cable for each step to go to the rotary switch (attached to the reset pin), a green wire to connect to the LED for each step and a red wire to go to the top pin of the matrix for each step. Note the red wire is after a diode on each step while the orange and green are before it.


Here is a picture after each wire has been soldered to its place on the back of the lid. I also used hot glue to attach the small proto-board to the lid of my gate sequencer. Make sure you test all of the connections thoroughly prior to gluing it down. Check for any bleed between steps and that all the solder connections are strong. Once you glue it down it will be very difficult to modify.


I connected all of the pins on the matrix (excluding the top row) in rows and connected each row to the corresponding output on the side of the box. One more coating of glue and I was ready to make the final connections. First I connected the ribbon cable to each of the 8 steps on the main board. Then I worked my way around the components which needed to be connected to the main board. Once everything was connected I wired up the battery, power switch and power indicator LED. I secured the battery with some Velcro and after some brief troubleshooting (There was a faulty switch I had to change) it was ready to go.

I am currently in the process of setting up trigger bends on my drum pad. Once they are completed and running smoothly I will have another article up describing how you can use your new gate sequencer to trigger noises from circuit bent toys  and down the road you can expect to see me using gate and trigger voltages to control a variety of other devices. Within the next week or so I should also have a demo video of this device uploaded for you to check out.

Thanks for visiting and happy soldering!

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