Headphone Amp Features A Tiny CRT

[ErikaFluff] needed an amp for her Grado open cans. Rather than build yet another boring black box, she built what may be the most awesome headphone amp ever by adding a tiny CRT which displays the current audio waveform. She packaged all this up in a customized Hammond box which makes it look like it just rolled off the line from some audiophile studio.

The amplifier in this case is based upon the CMoy, a common headphone amp design. [ErikaFluff] added a MOSFET on the output to drive his relatively low impedance (32 ohm) Grado headphones with reasonable volume. The CRT is from an old video camera viewfinder. Before LCDs were advanced and cheap enough to include in video cameras, CRTs were the only show in town. These tiny black and white screens use high voltage to scan an electron beam across a phosphor screen just like their bigger brethren.

In action! - ImgurSince she was going with an oscilloscope style vector scan rather than the raster scan the screen electronics were originally designed for, [ErikaFluff] had to create her own horizontal and vertical deflection circuits. Horizontal scan is created by a 555 timer generating a sawtooth wave at 75 Hz. Vertical deflection is via an LM386 driving a hand wound impedance matching transformer. The high voltage flyback transformer and its associated driver circuit were kept from the original CRT, though repackaged to make them as small as possible.

You might think that having a few thousand volts next to a sensitive audio amplifier would cause some noise issues. We also worried a bit about shorts causing unexpected shock treatments through the wearer’s ears. [ErikaFluff] says there is no need to for concern — the signal is fed to the CRT circuit through optocouplers. The audio circuit is also electrically split from the CRT and runs on a virtual ground. Judicious amounts of shielding tape keeps the two circuits isolated.

This may not be the most practical project, but we think it’s pretty darn cool. The response over on Reddit’s electronics subreddit seems to be positive as well. We hope [ErikaFluff] is sitting down when this post gets published!

40 thoughts on “Headphone Amp Features A Tiny CRT

  1. This is nothing short of epic!
    The only thing I would add is a green gel overlay with a reticle pattern to complete the retro-oscilloscope aesthetic.
    I can’t help but wonder how much the CRT adds to the power-consumption, though… At least he made a matching battery-pack to go with it.

    1. Probably not all that bad considering what it is… the old camcorders were fairly decent on batteries, but the main load was the tape drive and the scanning for it. The view finder was peanuts. No match for today’s Li+ batteries.

      Pretty darn cool.

    1. CMoy is crap indeed but O2 isn’t a flawless design either. I guess I’m just not a big fan of using 2 weak 9V batteries (350mAh or so — often overstated), along with a trickle charger (with dual polarities, no less). I’m sure it works great for some people and it probably sounds a TON better too, but for something portable I prefer something with LiPo and efficient class D (portable, easy to recharge from a USB port anywhere, efficient).

      1. I’m mostly interested in using the O2 on the desktop, with a built-in ODAC, so the batteries aren’t something I’m really worried about. However, I have wondered how difficult it would be to replace the two 9V NiMH with some small Li-ion or even LiFePO4 batteries.

    1. MY sentiments exactly. When I was younger I used to set up a CRO when drinking with friends. It was really trippy with synthesised music in x-y mode like Craft Work – Jean Michel Jarre and the older Pink Floyd.

      So that is what we did instead of drugs lol.

      Reading this tempts me to make an electro-static deflection version (for x-y mode) for my son complete with headphone amps like this one.

      But winding the high voltage transformers kinda turns me off the idea. I don’t remember all the formula’s for winding transformers now.

    1. That’s pretty much it, though you might try fleabay… Maybe some sellers from Russia or one of the bloc countries with some bulk stock to unload. Ymmv, and you’ll probably have to translate the data sheets, but you might get lucky.

    2. There are some 1 inch CRT tubes on ebay. They look more like a valve and are intended for CRO’s so they’re electrostatic deflection. You would need to build the HT transformer (LOPT or Flyback) and higher voltage electrostatic deflection. It would be much easier with the electro-magnetic deflection of a viewfinder and you will probably get the HV transformer and drive with it.

      The advantage of electrostatic deflection is that you can have a x-y mode.

        1. Try being one of us, it really gets confusing. I’ve been here since 2005 or so (my memory fails me), so other Rob is clearly a whippersnapper trying to profit from my good name. Or something like that.

          Fortunately, it’s a universal law that all Robs must get along at all times.

    3. I’ve used a couple. They’ve connected to the rest of the camera with a CVBS and power.
      Maybe I just got lucky.
      But such a small package already done for you, mad not to re use something old.

  2. I have a few of these old CRTs in my junk-bin that I had removed from old camcorders. I’d love to find a use for them, this isn’t something I would be interested in, but I would like to do something with them. Hmm…

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