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Single Stage, Single Channel Phono Preamp With Power Supply Schematic

airbornesurfer
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.

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Anonymous
  • in reply to robogary

    The only dumb questions are the ones not asked! The tonearm connection is just for illustrative purposes, so don't worry about it on this diagram (Fritzing was a little sparse, so I found a generic part that I could just label). Assume the ground on the tonearm is also connected.

    I hadn't needed one yet, but I may end up adding one. This is still very much a work in progress.

  • in reply to michaelkellett

    Ay, no worries! It's taking some getting used to on my end as well. The plan is to completely rebuild the entire amplification circuitry from tonearm to speaker (as true stereo), but I'm working one step at a time right now. As for the pins being wrong, I'll chalk that up to a transcription error Sweat smile

    The "main amp" I referenced is an external powered amplifier driving the stereo system. I'm using it for testing before building my own.

  • I never really dug much into the guts of a turntable, so sorry if I ask dumb questions.

    Your signal from the tone arm is single ended ? Do you also have a wire to ground from the tone arm ?  

    I almost always seem to need a DC blocking capacitor in series with an external input to an amp. Did you have one, need one, and just didnt show it ? 

  • in reply to michaelkellett

    Sorry, just found your plan -(  I hate this new e14 website, can't find anything !).

    I've had a play with some of those cheap switching amps - dreadful - why not go the whole hog and build your new amp in the spirit of the old one ?

    One could easily design it to work from the existing transformer.

    MK

  • You do have the positive and negative supplies connected to the wrong supply pins on the NE5532, not good. The NE5532 is a low noise high performance op amp but not intended to drive speakers (but your schematic is labelled 'main amp' so I'm not sure quite what you mean.)

    Op amps need feedback to work correctly, without they have huge amounts of gain at DC and low frequencies.

    Why not fix the original amplifier ?

    MK