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Raspberry Pi gets a hybrid tube audio amp HAT

May 4, 2016 — by Eric Brown — 5,692 views

Pi 2 Design’s 503HTA Hybrid Tube Amp is a HAT add-on for 40-pin Raspberry Pi’s that taps a 24-bit, 192Khz DAC for that old-time tube amplifier sound.

The Raspberry Pi has inspired a variety of retro technology hacks, from resurrecting ancient televisions to breathing new life into vintage gaming platforms. So it’s not surprising to see the SBC matched with the guts of an old-school tube amplifier system, as it is in Pi 2 Design’s 503HTA Hybrid Tube Amp.

This isn’t even the first RPi tube radio hack, although that one leaves the tube aspects up to the old radio. Yet, while most other tube amp projects have been about reanimating old tube radios, the 503HTA aims to bring what many believe to be a richer tube amp sound to any audio project.

503HTA Hybrid Tube Amp first gen prototype from different vantage points
(click image to enlarge)

The 503HTA Hybrid Tube Amp is past the halfway mark on its Kickstarter journey, and there may be a few more early bird packages available at $99, or $198 for a two-pack. After that, the price rises to $109 before settling at the retail price of $149.

Shipments are due in July or August, depending on the package. Prices include a 24V @ 2A power supply with IEC input, plus standoffs and screws and an acrylic case. Customers bring their own Pi and tube — the 503HTA supports both 12AU7 (ECC82) or 6922/6DJ8 (ECC88) type tubes.

3D models of second-gen design
(click image to enlarge)

The 503HTA Hybrid Tube Amp is a HAT (Hardware Attached on Top) extension to any Raspberry Pi with a 40-pin expansion connector, including the Raspberry Pi 2, Raspberry Pi 3, Raspberry Pi Model B+, and presumably even the 40-pin Raspberry Pi Zero. HAT boards differ from standard Pi shields in that they enable the Pi SBC to automatically configure its GPIO signals and drivers for use with external devices.

The 503HTA incorporates a 24-bit, 112db THD PCM5102A DAC that taps the Pi’s I2S interface to drive a single 12AU7 tube gain stage at a 192Khz frame rate. The setup enables audio from 32 to 300 ohms on headphones connected to the 3.5mm jack. A gain select switch offers three gain settings of approximately 2V, 4V, and 6V rms.

503HTA block diagram
(click image to enlarge)

The 503HTA design is based on the Bravo V2 tube rolling architecture, and includes recommended mods such as:
  • IRL510 output MOSFET
  • Regulated heater voltage to reduce cross-talk
  • Adjustable plate bias for tube rollers
  • Nichicon MUSE output capacitors
  • Jumperable output impedance of 1.5, 32, and 100 ohms
  • Switchable gain selection

The 3D model above shows the second-generation prototype. The upgrade is said to offer better thermal and output relay protection, as well as bigger anode bias adjustable pots with mechanical stops.

Pi 2 Design is currently providing patched versions of the Volumio and OSMC open source media players, with plans to add Rune Audio. The company will supply patches to the Linux kernel, so the players should eventually include built-in 503HTA support.

Previous Pi 2 Design Kickstarter projects for the Raspberry Pi have included the CSB502SSD multifunction SSD shield and the 502IoT Shield.

Further information

The 503HTA Hybrid Tube Amp is available on Kickstarter through June 6 at prices starting at $99. More information may be found at the 503HTA Kickstarter page and Pi 2 Design’s website.

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3 responses to “Raspberry Pi gets a hybrid tube audio amp HAT”

  1. former ERIMite says:

    The 12AU7 uses a very common tube pinout which means there are several different tube types with the same base. In particular, there is the 12AX7 which is less noisy and more linear than the 12AU7. I wonder why they are not suggesting that.

    • QZM says:

      Looking at the design I would have to say the reason they dont choose a 12AX7 is that they do not appear to have much of a clue at all, other than a ‘w00t, putting in a tube makes it audiophile l33t dude!’ approach.

      If increased distortion is your thing, then go ahead, I guess…

      They appear to be simply using the tube as a ‘distortion stage’, so any possible advantage is lost anyway, and they drive a nice big solid nail through any possible quality by having output resistors, hence giving the system no damping.

      Hell, few of the components even appear to be audio grade. I have approximately zero confidence in them being able to correctly implement what is a rather tricking D/A also..

      Sorry, back to audio design 101 with these guys, but hell, idiots will pony up for the look, and convince themselves it sounds wonderful, as usual.

      • omgchips says:

        Exactly. The only reason for this product to exist is to separate some people from their money.
        The tube won’t add anything to the sound, except cost. Also, at 24V supply it is surely a “starved plate” design which means it won’t even sound tubeish, unless one is aiming at fuzz distortion.
        I would put all my horses that in a blind test with this board against a traditional hat with an old light bulb mounted just for the look, nobody would tell the difference if operated clean.

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