About this project
Big News! Stretch Goal #2 has been improved! Volume Controls and Analog In and Stand Alone Operation, oh my!
Introducing the 503HTA - Hybrid Tube Amp!
Pi 2 Design is excited to introduce the first Tube Amp specifically designed for any of the 40-pin versions of the Raspberry Pi SBC Family. The 503HTA Hybrid Tube Amp HAT, designed and manufactured by Pi2Media (a division of PI 2 Design), is a High Quality I/O Shield designed to bring “Sweet Tube Sound” to the Raspberry PI ® family of Single Board Computers. This HAT allows the Raspberry Pi ® SBC to drive Headphones from 32 - 300 ohm with ease and fidelity.
Based on the popular Bravo V2 design with all recommended Mods, the On-Board PCM5102A DAC drives the single stage 12AU7 (or 6922) tube. The Class-A Output Stage is comprised of the IRL510 and LM317A. The dedicated heater supply insures zero crosstalk.
503HTA IN THE PRESS
"The 503HTA Hybrid Tube Amp by Pi 2 Design looks similar to other kit-based tube amps, but this one has a trick up its sleeve: it uses a Raspberry Pi — supplied by the user — for its brain.", Kristofer Wouk, Digital Trends
We just finished testing the prototypes of our 503HTA. a small Hat size add-on that connects to the PI I2S port. A PCM5102A DAC drives single 12AU7 tube gain stage with solid state Class-A output. It is designed to drive headphones from 32 to 300 ohm quite easily.
The design is derived from the popular Bravo V2 architecture with all the modifications recommended such as: IRL510 Output Mosfet; Regulated Heater Voltage to reduce cross-talk; Adjustable Plate Bias for Tube Rollers; Nichicon MUSE Output Capacitors; and Jumper-able output impedance of 1.5, 32 and 100 ohm and switchable gain selection to match your headphones exactly.
Second Revision Changes
In our second PCB spin we set out to improve upon our initial design specifications:
- Changed Output Mosfets (IRL510 & LM317) to TO-220 vertical through hole package. This will greatly improve the thermal solution.
- Added Output Relay protection to remove power on "thumps"
- Switched to bigger Anode Bias adjustable pots with mechanical stop
Rev. 2 PCB's arrived Monday 5/2 and we have built up the new revision:
- Raspberry Pi ® HAT Compliant Size w/40-Pin mating connector
- 24-Bit DAC - The well regarded -112db THD PCM5102 is used to convert the I2S Stream to drive the Single Triode Tube Stage at 24-Bit up to 192Khz Frame Rate
- Tube Rolling – The 503HTA supports both 12AU7( ECC82) or 6922/6DJ8 (ECC88) type tubes. Individual Anode Bias Adjustments plus selectable Series/Parallel Heater Drive insure the perfect setup for virtually any tube! See our friends at www.tubedepot.com for some alternate options!
- Headphone Out – Rugged 3.5mm Jack plus selectable output resistance between 1.5, 32 and 100 ohms.
- Gain Select Switch – 3 Gain settings allow the user to select the output voltage to the optimal level for each Headphone. Settings (based on 12AU7) provide approximately 2V, 4V and 6Vrms.
We are now over 250% funded thanks to all your great support. To keep the momentum going we are offering a stretch goal (perhaps more as we go along).
Stretch Goal # 1 - Audio Adapter Cables - $75K
Every 503HTA will ship with a pair of adapter cables. The first is a 1ft. 3.5mm male to 1/4" female. This allows you to plug your high end headphones into the 503HTA.
The second cable is a 1ft. 3.5mm Jack to a pair of RCA Female Left/Right Stereo. this is to allow you to drive an amplifier with the 503HAT, on the lowest gain setting, remember? ;-)
Stretch Goal # 2 - Upgrade Caps, Power Switch and Volume Control Option - $100K
If we meet this goal we will replace the Cathode Bypass caps with Nichicon Fine Gold (FG) series 220uf/10V caps. We will also replace the 24V Power Bypass cap with a Nichicon Aluminum Polymer low ESR cap. These changes will improve overall noise floor and PSRR (Power Supply Ripple Rejection).
In addition to this we will add a Power MOSFET to control the 24V input. A push-button switch will allow the unit to be turned on/off by the user. It will also allow the Raspberry Pi to turn off via software. This will be compatible with the standard Linux shutdown procedure to insure clean power down.
Finally we will add small .1" header that can be used to add an analog volume control/attenuator option at a later date. Maybe this is stretch goal #3? :-) Jumpers will be used to bypass this for the time being.
Risks and challenges
As we have already tested the initial prototypes, we are currently just tweaking the design so we can offer the best final product to our backers. We handle our own manufacturing so we control all aspects from beginning to end of the project.
We will work hard to meet all of our promised delivery dates and we welcome any questions or concerns you may have. They will all be addressed and answered in a timely fashion.Learn about accountability on Kickstarter
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