Power Supply MK2
I wanted to build a power supply that would run my germanium radios converted into guitar amps reliably and flexibly. Needed something that would allow me to compare amplifiers from different radios without worrying about power supply, but it would also allow me to test amplifiers under different conditions – change voltage and/or introduce voltage sag.
So I designed a power supply based on a simple variable voltage regulator (VVR). Having variable voltage will give some control over volume and compression at the same time as the voltage drops, but it would also allow me to crank up the voltage above the battery level and increase headroom. It uses the standard LM317T variable regulator and has a range from 1.5V to 11.5V. Standard value for R2 (program resistor) is 240 ohms, but I didn’t have a small enough pot to give me the voltage range I wanted, so I bumped up the value of R2 to 1.5K. 240 ohm resistor in series with the pot controls the minimum voltage when pot is turned down to zero.
With low power amplifiers you can do some funky stuff that would otherwise be hard or impractical to do on high power amps. Variable “sag” control is one of those things I always wanted to have but couldn’t be bothered to do. If we introduce a series resistance with the power supply, we will sag the voltage every time the amp pulls more current, so we’re effectively trying to quiet down the loudest signal peaks. And that’s compression! This is only possible with class B and class AB amplifiers because they pull more current with higher signal levels. Most germanium radios are class B, so this should work nicely.
You’ll notice a DPDT center-off switch in the schematic. In the first position it just shorts input to output, so there’s no resistance and no sag. In center position there’s no connection with the output, so power is off completely. In third position, a 100 ohm trimmer is injected in series with the output. I put a trimmer so I can adjust the level of sag. Two things to keep in mind here are trimmer value and power rating. All radios I played with draw somewhere between 10mA and 50mA of current. Taking that into Ohm’s law formula, we see that 100 ohm resistor would sag between 1V and 5V under those loads. Worst case power dissipation is 0.25W (5V * 50mA), so I put a 0.5W rated trimmer to be on the safe side. If you plan to implement sag control in a higher powered amplifier be aware of this and adjust resistance and power rating accordingly. Sometimes it’s more practical to put a fixed power resistor or several resistors that are switchable with a rotary switch.
I used two 100uF capacitors for filtering, keeping in mind that none of the amplifiers that will be powered with this circuit draw more than 50mA of current. If used to power circuits with higher current consumption (LM317T can provide up to 1.5A with a proper heat sink), increasing filtering capacitance may be a good idea. Rule of thumb is to have at least as many uF’s of capacitance as mA’s of current draw. I doubled it for extra filtering.
Usage in Positive Ground Circuits
Note that this is a standard negative ground regulator and almost all germanium amplifiers (and some fuzzes) are positive ground circuits. Standard negative ground supply can be used to power one (or multiple) positive ground circuits, but it cannot be used to power positive and negative ground circuits at the same time because it would short out and something would burn. To power a positive ground circuit, just flip the + and – leads keeping in mind that no other ground reference should connect to our positive ground.
Power supply is squeezed onto a 1×1″ proto board from MeasureExplorer, my favorite board for simple and compact circuits. Voltage control pot is mounted on the board, so I don’t need to worry about mounting the board to the chassis. Originally I planned to use the power supply with a wall wart DC adapter, but in the meantime I changed my mind and decided to use the hefty 12VAC 1.5A wall wart I had lying around. The only change I needed to make is the added bridge rectifier at the input of the circuit and it was relatively easy to implement it right on the input jack using four UF4007 diodes. If used with a regular DC adapter (min 14V DC input if maximum voltage of ~12V is needed), the bridge rectifier should be omitted.
I drilled a small hole in the enclosure right above the sag trimmer, so it can be accessed without having to open the enclosure. To be able to measure the voltage, I exposed two makeshift test points at the top of the enclosure.