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
The HTA503 Hybrid Tube Amp comes with the following items:
1) The 503HTA Hat
2) 2-piece acrylic open side case
3) All standoffs and screws to mount the 503HTA onto a Pi B+, 2 or 3
4) 24V @ 2A Universal Desktop style power supply with IEC input
You must supply:
1) Raspberry Pi B+, B+ Model 2, B+ Model 3 or Raspberry Pi A+
2) IEC Power Cord for your country
3) Phillips head screw driver
4) Favorite Headphones
When you set the 503HTA gain switch to low (or x1), the 503HTA drives approximately 2Vrms. This is compatible with the input on most amplifiers. Just be careful as there is nothing to prevent you from setting the gain switch higher and thus potentially damage your other equipment!
Personally we recommend you get our 503DAC1 - PCM5102A DAC HAT for those situations.
The tube multiplies the input voltage from DAC by 1, 2 or 4 times. The MOSFET passes the same voltage out (also called unity gain), but provides the ability to drive up to 125ma of output current. This is equivalent to 650mw of power into a 32 ohm headphone.
First you go over to our friends at Tube Depot (www.tubedepot.com) and get your tube of choice. We use the 6DJ8 and 12AU7 because we can get them new from JJ Electronics through Tube Depot. But there are many NOS (New Old Stock) tubes spanning many years that have become audiophile favorites.
Once you have your tube, while the power is off, do the following:
1) Carefully remove the current tube and place your own.
2) Set the gain switch to x1 and set the two bias pots to the middle of their range.
3) Plug in a set of headphones that are not very important to you!
4) Put in the power plug and wait for the Pi to boot up.
5) Using a voltmeter, adjust each bias pot until you get 12.5V on the bias test point
Depending upon the tube you may want to set the bias point higher. For example, for the 6922/6DJ8 setting the bias to 16V results in lower distortion. But because your are now only 8V from the top rail (24V) you can only swing about 7V P-P or 2.4Vrms. Great for low ohm headphones (50 ohm or lower), but possible not enough for higher ohm headphones depending upon their efficiency.
A great spreadsheet done by Rob Robinette can help you decide what voltage and current you need for your headphones, depending on how loud you like it! You can get his spreadsheet here:
We are supporting Volumio and OSMC now. We plan to add Rune Audio at some point. If there are others you want to see, please ask.
Note that initially we will have to supply you patched versions of Volumio and OSMC. We will submit our patches to the Linux Foundation, and once accepted they will be automatically included when the Player Software owners update their release. This could however, take several months before it is in the default release for any given player. We will also make the patches available for the folks out there who like to tinker on their own!
There will be a backer survey taken at the end where you make that choice.
The 503HTA uses the following GPIO's:
GPIO5 - PCM5102A MUTE pin
GPIO6 - PCM5102A Filter Pin
GPIO12 - Power Supply Good Signal
GPIO13 - PCM5102A De-emphasis Pin
In addition, we use the following GPIO in I2S mode:
GPIO18 - I2S Bit Clock
GPIO19 - I2S Left/Right Clock
GPIO21 - I2S Data Out
No. The 503HTA receives it's power from the supplied 24V @ 2A supply. On-board regulators create the 6.3V Tube Heater Voltage as well as the 5V @ 3A rail for the Pi. A very low-noise 3.3V LDO is used to power the PCM5102A DAC.
There is a "back-power" protection circuit that prevents any damage if you have the Pi power plugged in as well the 24V Power. No harm will come from this, and the unit will operate normally, but the Pi power supply is redundant.
No, the 503HTA only works with the Pi. The input to the tube section is from the Digital to Analog Converter (DAC) which gets it's digital audio data from the Pi. There are no other inputs.
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