Making a phono preamp for a first electronics project

Nearly everyone’s first electronic project is something that blinks a LED. There are a million ways to go about this ‘Hello World’ project of electronic design; 555 timers, microcontrollers, or maybe even discrete components if you’re really cool. When [miceuz] was asked by a friend to help with his first electronic project he eschewed the usual blinking LED project and taught him how to build something he actually needed: a phono preamp for an old turntable.

Back in the day when vinyl was king, albums needed to be mastered to play on a record player. The mastering process cuts some of the bass and increases the treble. When the record is played, this process needs to be reversed. It’s a preamp that does this job by attenuating the high frequency sounds and boosting the thumping bass.

[miceuz] found a nice DIY RIAA preamp  project and found a nice little op amp  somewhere in his parts bin. After laying out the circuit, [miceuz] etched a few boards and taught his friend how to solder SMD components.

Of course the project didn’t work the first time around, but after poking around with a meter and checking out the old turntable, the preamp came to life with the clang of chords from an old record. If you’d like to build your own, you can get the files from [miceuz]‘s git.

Comments

  1. Tom Brusehaver says:

    The cool thing about this circuit, with a simple RC circuit on the inputs, and a couple LEDs on the output, it’ll make a cool blinking LED circuit too.

    (just kidding).

  2. Infidellic says:

    “Nearly everyone’s first electronic project is something that blinks a LED”

    I think mine was an AC rectifier which then powered a digital waveform generator. It was shockingly bad – discrete jumps in the waveform all over the place but I just wanted to point out that I doubt most people’s first foray is to get an LED to blink. After all if that’s all you can do I doubt people would come back for more

    • Sven says:

      The LED thing is true of micro controllers since the first thing you want to do is make sure your hardware chain is working, and the easiest way is to blink an LED.

      I don’t think i have met anyone who’s first non-programmable project was a blinking LED except if you count the people who build kits, but i wouldn’t count a kit as a project, more a soldering exercise…

      My first project with my dad btw. was an electric motor, one of those where you have to use the cardboard box for some of the parts. Yes i’m old…

    • Nick says:

      Before there were LEDs that were really affordable, the first circuit that most people built was a multivibrator of some sort. The intention was the same – make it do something that I can (see, hear, feel). I can remember wiring up two transistors in the classic configuration and feeding an earpiece from a telephone with it. I think it is the advent of the 8051, AVR and so on that has brought with it the blinky LED thing. Indeed the first time I used an 8031 at home I blew a ‘blink LED’ eprom to go with it – and I still have that eprom now, with its label.

    • Eirinn says:

      Analogue v.s Digital

      • Mikey says:

        It’s not analog versus digital it’s MCU versus no-MCU. My first project was an electromagnet. Get a bunch of D cell batteries, wire them in series to a bit of wire and wrap it around a nail, then watch it pick up paper clips and such.

  3. BrentB says:

    And it’s SMD! Kids, take note, this is how you do it. This could only be classier if they’d built it straight from an old national or analog app note.

    • pcf11 says:

      What does SMD do for it?

      • BrentB says:

        It shows that a beginner doesn’t need to be afraid of SMD parts.

        • pcf11 says:

          If fear is your only justification then you are saying that there is no good reason to use SMT with this project. I’ll stick with using parts that were designed for humans to use, not pick and place machines.

          • BrentB says:

            You can use whatever parts you want, of course! I personally find SOIC chips and say, 0805s (0603s… marginal), on a PCB easier to work with (note how little drilling they had to do with their board, and that they didn’t have to flip the board much to solder it up) than through-hole parts. That said, I can dead-bug through-hole parts faster than surface mount and at least as fast as I can work with a solderless breadboard, so there’s a place for them in prototyping when I can get ‘em… I just get frustrated with the pervasive myths that SMD parts are too hard for hobbyists to use, require special or expensive tools, etc.

          • pcf11 says:

            I get frustrated with people who fail to consider context when evaluating parts of processes. The industry certainly did when they decided SMT was good for them. I agree with their assessment, I don’t agree with yours.

            “pervasive myths that SMD parts are too hard for hobbyists to use, require special or expensive tools, etc.”

            Sometimes when a lot of folks say the same things there is something to what we’re all saying. Your denial does not change the truth either.

  4. M4CGYV3R says:

    “The mastering process cuts some of the bass and increases the treble. When the record is played, this process needs to be reversed. It’s a preamp that does this job by attenuating the high frequency sounds and boosting the thumping bass.”

    That’s not what a turntable preamp does, nor is that an accurate description of vinyl mastering. The preamp does not affect the EQ of the track whatsoever, or at least the sign of a GOOD preamp is that it has flat-profile amplification.

    Vinyl records are a series of grooves on a piece of plastic(vinyl) which cause a tracking arm(stylus/needle) to wiggle in the groove as it spins. This wiggling causes a small magnet on a lever(cantilever) to induce a current in coils in the pickup(cartridge) and send it down the tonearm of the turntable over wires.

    There is no amplification in most(older) turntables(the power cord supply is exclusively to turn the motor on the platter), so the voltage that is actually output from the turntable’s audio cable is MINISCULE voltages and currents created by that wiggling magnet alone.

    A preamplifier for a turntable takes the microvoltages and converts it to a higher level that a line-level input (headphone level) can accept such as a stereo without a phono input(which is just a built-in preamp)or a DJ mixer without phono inputs. Without this preamplification, the audio coming through even the nicest stereo systems would be so quiet it would be virtually inaudible.

    • John says:

      Actually a good preamp does effect the frequency response. Records are cut with the RIAA equalization (http://en.wikipedia.org/wiki/RIAA_equalization) which is then reversed during playback.

    • PJ Allen says:

      “Vinyl records are a series of grooves…”
      One continuous groove – It’s a spiral.
      (Likely unique, one side of “Monty Python’s Matching Tie and Handkerchief” was cut/pressed with two grooves.)

    • Garbz says:

      If you’re going to try and sound smug it would actually help if you knew what the hell you were talking about. Records are mastered with one of several different EQs pre-applied usually looking something like -20dB at 20Hz and +20dB at 20kHz.

      Without this it would be almost impossible to fit any decent size song on a record the bass would cause grooves to be massively wide and with serious potential to launch the needle into a different track, and the treble would be completely drowned out by recording noise.

      You’re idea of a “Good” pre-amp would sound ear-screechingly horrendous. There’s a reason all vinyl pre-amps have listed in their specifications an “RIAA equalisation accuracy”

    • mstone says:

      If you read a little farther in the article you’ll see that the author says there are various schools of thought about reversing the RIAA equalization, and that he *chose* to put it in the preamp.

      You’re perfectly welcome to make a preamp with a flat frequency response if you want, but that just means you’ll have to pass it through an RIAA compensation filter somewhere further down the signal chain. Since the article is written explicitly for people who want to pull sound off vinyl but didn’t get their uncle’s turntable and HiFi along with his collection of leisure suits and shag carpet swatches, it makes sense to roll everything into one circuit.

  5. Spacedog says:

    WOW!!!! you mean to say that once upon a time the RIAA actually did something useful for musicians/consumers!!!!! I’m stunned.

    • M4CGYV3R says:

      Yes, they once had standards and quality behind them as they were actually a Recording Industry instead of a legal aid office for corrupt label execs.

      They penned a lot of the standards for phonographs and even some later technologies like cassettes and LaserDiscs. Now it’s just ‘dump that file on that master disc’ so there’s no art or difficulty to it.

    • I know. First and last time here, folks.

  6. Cold_Turkey says:

    Just for the ‘record’, vinyl is still king! ;-)

  7. plasmator says:

    For a really excellent discourse on RIAA preamps and equalization, check out “Valve Amplifiers” by Morgan Jones. The fourth edition came out not too long ago and it adds a cool hybrid (tubes and solid-state) preamp design to the material present in the already-excellent third edition. Jones is a retired BBC engineer who knows what he’s talking about when it comes to this stuff, and as a bonus has that dry British sense of humor which he uses to great effect when busting ‘audiophile’ myths.

  8. Galane says:

    Instead of a first program that simply prints HELLO WORLD on an output device, how about one that synthesizes the music and lyrics to The Partridge Family theme song? ;)

    The lyrics start with Hello world… (Except for the pilot episode which had different lyrics but the same music.)

  9. Steve0 says:

    That is a terrifying soldering job.

  10. M0nsieur2 says:

    This is a great project to learn about op-amp circuits. Good luck finding a preamp at an audio store. Watch the employees at any electronic shop scratch their heads when you say you’re looking for a phono preamp.

    Side note: I tried to open files in Eagle 6.3 and get an error.

  11. David says:

    Hey M0nsieur2, Radio Shack still sells a phono preamp. There are also lots of them on eBay.

    Congrats to the builder, but this might be a little frustrating as a first project for some people. The reason is that if it doesn’t work, it would be hard to figure out why without test equipment.

    Also, those ceramic capacitors really are too nasty for audio.

    The output should be AC coupled, unless you know for sure it’s driving an AC coupled input.

    • M0nsieur2 says:

      I actually bought a TCC brand preamp from Amazon several years ago since it had favorable reviews. It comes with a decent power supply and an AC ground connection to get rid of the 60HZ hum (something you don’t need to worry about if you decide to run off of battery power).

      I wanted to build my own preamp back then, and this makes me want to again.

      I also agree on the difficulty factor. Your circuit could have problems, your phono cartridge could have problems, your stylus (needle) could have problems, or the wiring between the cartridge and ouputs could have problems. I had to troubleshoot all of those before I got my vintage turntable working. If you tackle this project I would recommend starting with a turn table known to be working properly.

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