It started with the chassis of an old ASEA traveler’s radio from 1953, and with the Altec Lansing subwoofer saved from the scrap heap when the management closed down the Uppsala branch of the company where I worked. My idea was to do reverse engineering on the subwoofer and convert it into a guitar amp, and to use the old radio as the housing.The main components of the subwoofer are the standard IC KIA324 (LM324) 4 x Op amp, a compressor circuit (NE572) and the KIA 6210 stereo power amp cirquit, and of course the twin coil Altec Lansing 6“ Speaker. When I had downloaded the power amp data sheet I discovered that each channel contained two bridge coupled power amps, which is a common car stereo design to get high power from low power supply voltage (2 x 22 W at 14V), and with a max voltage of 20V. Promising…
Note: ASEA was the name of the big Swedish electric/electronic company now called ABB - Asea Brown Bowery. Of course they do not manufacture radios now.
Note 2: This is not a step by step instruction, but I hope that it can be used as a starting point for your own design - which always is more fun than copying an existing one.
Step 1: Implementation - Reverse Engineering and Preliminary Design
After some preliminary tries the first serious reverse engineering step was to take photos of the front and backside of the printed circuit board. With the help of the free software Inverted Image I inverted the backside photo so that it would be easier to trace the circuit diagram. And with the help of the enlarged photos it was fairly easy to get a complete diagram. I put the photo inside a plastic folder, and drew the PCB lines on the plastic (see above). From the front side photo I could identify the components, which I drew as ordinary electronic symbols on the plastic folder.From this drawing I was then able to draw an ordinary schematic of the subwoofer amp.
The schematics showed that there was one preamp stage with a low pass filter (consisting of two OP amps), then a third order low pass filter, a compressor stage made up of the fourth op amp and the KIA power amp. The signal was fed to both channels of the power amp, and each output was connected to its own loudspeaker coil. I suppose that when the company name is Altec Lansing it is easy to manufacture a speaker with two coils so that you can use a stereo power amp. I have not included the complete original schematics here for the simple reason that I mistakenly threw it in the waste basket…
After some removal and bypassing of the low pass filters I made a try to keep the compressor, but I did not get any significant effect, so I removed the compressor circuit. At this stage the amplifier worked, the input stage was fixed, and one op amp was remade into a better Voltage/2 reference for the op-amps. The power supply provided one single voltage (14 V). The second op amp sourced the volume pot, and then the last op amp upped the gain a little.
Step 2: Implementation - Modifying the Circuit Board
The circuit board photo is shot at some time at the start of the project when I realized it was too difficult to perform reverse engineering by just looking at the circuit board and draw the wiring diagram. Some tests had been made, e.g. bypassing the low pass filter part and increasing the input impedance to a more fitting 500k. On top of the photo you can see the power amp with the original low budget Aluminum L-profile heat sink. This photo was used together with the inverted photo of the soldering side for the reverse engineering.
Step 3: History - the Tuning Capacitor
So beautiful - the tuning capacitor of the ASEA radio
Step 4: Implementation - the Tone Control
The amp needed a tone control. I had decided to try and use the tuning mechanics of the radio and the dial with all the old time middle wave radio station names. The first attempt was to build a standard Fender tone stack, and replace one of the capacitors with the tuning capacitor of the old radio. This did not work at all, too much hum and no good performance. Another drawback was that the tone stack design requires three controls (bass, mid and treble) on the front, which would alter the minimalistic look too much. I searched the Internet, and of course I found the perfect design with only one potentiometer - the tone control of the classic dist stomp box Big Muff Pi. The first source I found was Big Muff Design (http://gaussmarkov.net/wordpress/tools/software/ltspice/ltspice-ac-analysis-with-the-bmp-tone-stack/). Thanks a lot.
The differences between the Marshall and the Fender tone stacks inspired me to add a switch to be able to change the characteristics of the Big Muff tone control. With the help of the Tone Stack Calc 1.3 application, which provided the Big Muff frequency response curves for different component values it was easy to add the changeable “Marshall/Fender” switch that increases the low mid present in the Marshall sound.
Step 5: Implementation - the Marshall Tone Control
Frequency response with the Marshall characteristic
- White – max bass
- Blue – 15 % up
- Yellow – mid position (50%)
- Cyan – max treble
Step 6: Implementation - the Fender Tone Control
Frequency response with the Fender characteristic
- White – max bass
- Blue – 15 % up
- Yellow – mid position (50%)
- Cyan – max treble
See "Step 9" for the complete design of the tone control.
Step 7: Implementation - Drilling a Hole
Now I had something that worked fine, and it was also possible to replace the tuning capacitor in the radio with the 50k tone control potentiometer of the Big Muff tone control. The only difficult thing was to drill a 2,5 mm hole in the axle of the pot and then make the thread with the M3 thread tool. The screw in the middle of the big tuning wheel (as seen in the photo) is screwed into the threaded hole in the axis of the pot. When I had mounted the potentiometer on a L profile and fixed it to the radio chassis I could use the old tuning dial to get the "high precision" tone control (where every city name provides a different tonal response).
Step 8: Implementation - the Built in Stomp Boxes
When I told my son in law (who is in the Gypsy Punk music business as a singer in the Swedish band Crash Nomada) about my project he told me that he owned an original Big Muff stomp box, and we spent an evening looking at schematics on the Internet. I thought it would be fun to add the Big Muff distortion to the amp. It is a very simple design consisting of two transistor stages with diodes used as voltage limiters (square wave factories). When I described the project to a friend of mine, he revealed the he was the owner of the first version of the Ibanez Tubescreamer. Electrosmash.com provided schematics and the addition of two diodes and a gain potentiometer to the input stage realized the Tubescreamer. Since I did not want to use the transformer of the subwoofer I looked into other sources, and I tried different laptop power supplies. I found that the Dell 90 W laptop supply has a stabilized voltage of 19,2 V and can provide all the current I need. , The voltage is below the max voltage limit of the power amp but it raises the output power to 2 x 35 W. I tried other Dell supplies but they induced hum and noise. The 90W supply is really silent. It also has the advantage off short circuit protection, and has high efficiency. So the ring transformer of the Altec Lancing was left out.
Step 9: The Complete Design
The input stage also makes up the Tubescreamer circuit. Since there is no bypass function, I excluded the Tubescreamer input buffer stage. The Tubescreamer switch turns on the diode circuit and switches off C4 to get the high pass filter cutoff at 720 Hz. (R4 & C3 connected to IC1.). In normal position the high pass frequency is 72 Hz. Gain is adjustable roughly from 10 to 100 times which is sufficient for normal guitar pick ups. The roll off frequency provided by C6 and R5-R6 is lowered as the gain is increased to get a softer clipping.
The Big Muff distortion stages (the two transistor stages) are implemented with the original component values. I added a PCB (printed circuit board) switch to be able to select only the first Big Muff stage. As you can see, the Tubescreamer and Big Muff can be turned on simultaneously, but without noticeable effect. Big muff has already turned everything into a square wave.
The Buffer stage and the last preamp stage is nothing special. Gain is adjustable, but that is only because the original board had a trim pot. The output of the buffer stage can be tapped off as echo send and return. I could not resist the temptation of separating the stereo output stages to enable two signal sources and two loudspeakers. The additional input jack is placed on the backside of the amplifier. The input level can be adjusted with a pot and is fed directly in one of the power stages.
The power amp stage is not modified. On the speaker outputs I have tele connectors so that I can connect external speakers and disconnect the internal speaker (or use one external and one coil of the internal speaker).
The amplifier can also be battery or AC powered via the existing diode bridge input.
Some notes:The component numbering is made by the DIYLC-3.28 Circuit drawing program (https://code.google.com/p/diy-layout-creator/w/list) and does not correspond to the PCB numbering.
Components that does not do any harm has been kept on the board, e.g. L1.
All diodes are 1N4148 or 1N914.
Step 10: The Hardware
The main parts are seen in the photo: The Computer heat sink, the original printed circuit board, the back side of the new Veroboard with the stomp box circuits, the Altec Lansing loudspeaker. Note the extra baffle in front of the loudspeaker. The original heat sink (Aluminum L-profile) was replaced with a heat sink I had salvaged when I scrapped an old Dell computer.
The row of switches consist of:
- Big Muff Stomp Box switch
- Tube Screamer Stomp box switch
- Fender /Marshall tone switch (The Input stage gain pot has been moved to the front since the photo was taken)
- Dual input switch
- Stand by switch
The Dell Power Supply is located behind the black cable at the bottom.
Step 11: The Speaker
The original 6" Altec Lansing speaker with its two coils. A really good speaker that works surprisingly well as a guitar speaker.
Step 12: The Result
This is the ASEA Guitar Amplifier together with my old Telecaster. The Black Shadow loudspeaker works well if you want more output. I made some videos describing the amplifier and also a demonstration of the sound of the amp. Maybe I will provide these later...
Hope that you have enjoyed