As I explained in the How Hard Could It Be? video, the first objective in getting sound out of a record player is amplifying the phono-level signal from the tonearm (about 5mV) up to line-level (1V). This pre-amplifier stage uses a low-noise operational amplifier to boost the signal to the appropriate level. For Project Califone, I'm building the preamp stage using a Texas Instruments NE5532 OpAmp chip. Of course, I was having a little bit of trouble getting the device to work because I neglected to realize that I needed to apply both a positive and a negative voltage to the chip in order for it to function.
After realizing my mistake, I sourced a 10:1 AC-AC transformer that I could use for prototyping purposes. From the wall, I can get down to a manageable 12VAC and with a simple rectifying circuit, split that into +/-12VDC. I will have to adjust the power supply circuit to account for the 30VAC output from the transformer already installed in the phonograph, but that is a problem for another day!
At this point, I have a minimum-viable amplifier circuit for a single audio channel. Note in this schematic that there is no resistance on the input signal, so there is effectively no gain control at this point. The signal is horrendously over-driven--and when piped through the main amplifier becomes so over-modulated that even Luigi Russolo would shiver--but it works! From here, it's a matter of adding some resistors to control the gain before feeding the output to a single-knob tone control, the second pre-amp stage, then the main amplifier.
This post was also featured on AirborneSurfer.com