DIY Phonograph Preamplifier – How Hard Could It Be?

Like many of our hosts, Matthew is an aficionado of vintage technology. In this project, Matthew is completely rebuilding a Califone 1400 series portable phonograph from the early 1980s to improve its playback quality. The first obstacle he has to overcome is rebuilding the preamplifier circuit to bring the raw phono signals from the tonearm up to RIAA line level, but he’s having a little trouble with the op-amp chip. How hard could it be to build a simple preamp from scratch?

I had taken a bit of a hiatus from production (as I discussed in the last Surf Report), due to both a sense of being overwhelmed by my new day job and a general lack of enjoyment in the process. Building things became a job, and it stopped being enjoyable for a time. 2021 gave me an opportunity to reflect on my own goals, and I ended up scratching hundreds of projects from my list that I knew I would either never finish or had no interest in pursuing. I’m still culling that list, but the Califone stands firm. I’m still working on it, but I’m doing it slowly and on my own terms.

In an effort to get me back in the rotation on element14 Presents, the Producers and I agreed on this smaller-format video, showing a chunk of the project in the detail that I like to provide. It’s part of a new Friday series that highlights more conceptual projects, asking How Hard Could It Be? and following the trials that go into a simpler idea. In this case, I needed to build a phono-line preamplifier for the record player from scratch, and I made a fatal error along the way. The idea is to highlight how everyone makes simple mistakes and that it’s okay to ask for help.

The video was a nice transition back into work-for-hire and a way for me to warm myself back up for the next stage of the project. Now that I have a basic design for a power supply and preamp, I can get started on breadboarding a class-D main amplifier so these parts won’t have to spend another year on the shelf!

Single Stage, Single Channel Phono Preamp With Power Supply Schematic

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!

Single Stage Single Channel Phono Preamp With Power Supply

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.