Building the A5500 Microphone Kit from Aurycycle
A major portion of Audio Electronics III at the Art Institute of California, San Francisco is building and testing a DIY microphone kit with a tube cartridge amplifier. The instructions that come with the kit are a little sparse, so we are writing our own, below. You should read these completely before you start building.
You should also read Mac’s A5500 Build Report and browse other posts on this forum regarding this kit as a way to familiarize yourself with the components, construction steps, and potential difficulties.
- Identify all components
- The blue background of some resistors make the colors look different. For instance, yellow might look somewhat greenish, and purple might look black. You should confirm with a multimeter. Remember that your body has a resistance, and if you touch the multimeter probes while measuring a very high resistance you are in fact putting your body in parallel with the resistor, which the multimeter will faithfully measure. If you want to measure just the resistor without your body, make sure you don’t touch the resistor leads or multimeter probes.
- Identify the main circuit board
- Locate the main circuit board. It might be attached by 4 tiny screws to the microphone frame. If so, remove the 4 screws suing the correct Philips screwdriver. Using the wrong screwdriver might damage the screws. Put the screws somewhere safe so they don’t get lost.
Main Circuit Board Assembly
- Install the components that lie flat on the main circuit board
- Insert the components R3, R4, C9, C10, R1, R2 onto the main circuit board and fold the leads over so the components don’t fall out.
- Note that these capacitors are non-polarized, so orientation does not matter. Some other capacitors are polarized and must be oriented correctly.
- Solder the leads
- Trim the excess leads and SAVE THE TRIMMINGS! We will use them later.
- Install Resistors R10 and R6, and capacitor C4
- Note that the capacitor must go into the hole in the middle of the ceramic insulator, and that one each the resistor leads must go in with it.
- Twist the resistor and capacitor leads that are in the same hole together and solder.
- DO NOT TRIM THE LEADS FLUSH!
- Later you will have to solder to the leads in the ceramic insulator, so leave about 1/8″ protruding when trimming
- The other side of the resistor can be trimmed flush
- Install the polarized capacitors C1, C2, C6, C8, and C11, paying careful attention to the polarity
- Identify the negative indicator on the capacitor
- Identify the positive indicator on the schematic and assembly drawing
- Install the remaining components
- Which ones are these?
Tube Circuit Board Assembly
- Prepare the tube socket
- You must trim off the pointy bits so that when you solder the socket onto the small printed circuit board (PCB) the terminals do not extend beyond the edge of the board. You might have to trim beyond the pointy part a little and extend into the flat part.
- bend the remainder of the pin inwards so that it fits through the hole in the circuit board
- Insert the socket through the PCB so that the pins are on the same side as the solder pads
- flatten out the solder pads
- rotate the socket around so that pin 2 goes nowhere and pins 5, 6, 7, 8, and 9 go to ground
- solder the tube socket pins to the circuit board. Make sure you heat up the pins well enough that the solder flows completely and creates an excellent solder joint. Be very careful not to let solder fill the holes on the flat side of the circuit board.
- test fit tube and straighten pins as necessary to fit socket. The pins on the tube might have bent a little. Use the socket to determine what needs to be straightened. Be careful bending the pins so as not to break the tube. You can also loosen the socket opening a little with a screwdriver but be very careful here as well.
- insert lead trimmings into the 5 holes on the flat side of the tube circuit board. Solder lightly with equal lengths protruding from the front and the back.
- bend leads parallel to circuit board and insert into main circuit board
- Solder the leads into the main circuit board, and then re-solder the leads in the tube circuit board. Note that two positions on the main circuit board have no through holes, and so the lead is just positioned flat against the solder pad and soldered to the pad.
- Trim all leads carefully
- Install the tube at this time. It’s a lot easier without the microphone body getting in the way.
Installing Circuit Boards Into Microphone Body
- Carefully solder the wires from the socket and the transformer to the bottom of the circuit board assembly. Pay careful attention to the color order. Note that there are two white wires – these are the same and can be attached in either order.
- Screw the circuit board assembly into the microphone body so that the tube and components are on the inside
- Attach the wires to the microphone cartridge. Pay attention to the connection points: the red wire goes to the leads protruding from the ceramic insulator, but the blue wire goes to the empty hole adjascent to the ceramic insulator.
- Attach a piece of wire (not included) between tube socket pin 2 and the other ceramic insulator (the one to which the red wire from the microphone cartridge is not attached).
Carefully screw the microphone body and end cap in place
Pictures are in unison/pictures/microphoneKitAU2333