PandorAmp - The first Open Source Stereo Vacuum Tube Amp
PandorAmp - The first Open Source Stereo Vacuum Tube Amp
We will develop and release the first HI-FI stereo vacuum tube amplifier utilizing an open source/hardware microcontroller design
We will develop and release the first HI-FI stereo vacuum tube amplifier utilizing an open source/hardware microcontroller design Read more
About this project
PandorAmp V1.0 : HiFi Tube Audio for the 21st Century
PandorAmp is an open-platform tube amp, designed to be integrated seamlessly into a modern audio system, giving the warm sound of a tube amplifier, and the control of a user-customizable microcontroller. First and foremost, I have designed a really awesome sounding, low distortion stereo tube amp. On top of this, I have overlaid a control scheme with a high-end microcontroller. The overall effect is a tube amp that allows you to monitor and control bias levels, power supply voltages, tube health, gain levels, feedback levels, and a bucket full of other things. The amp is really exciting, as this is the first time people can open up the box (thus the name) and mess with hardware and write code to control their amp. We can only offer a limited run of 75 assembled amplifiers for this first project, but we will release more kits if there is interest. Any funding beyond target will fund future projects at nielltronix.com
What we plan to do:
We are going to take the current lab demo unit and turn it into a real product you can enjoy, crack open, and hack. The project will also launch an online community at nielltronix.com/forum to provide support and share ideas and tweaks to the amp and future projects. There, members will see progress on the amp and the build. The amp, which is sitting on my lab bench cranking out tunes right now through studio monitor speakers, is in a fully functioning state. All the various bits and pieces work, but we need funding to integrate them into a whole. That entails final industrial design work, mechanical engineering work to design sheetmetal and woodworking prints, final circuit board (PCB) layout design, parts procurement, and software design work. Sounds like a lot, but that's why this project is on Kickstarter!
The amp will come with an LCD screen and a rotary encoder user interface, as well as a USB port and an onboard bootloader for uploading your own code. The code as-shipped, will be fully documented and tested, but released in the CC domain. www.nielltronix.com will maintain a forum for owners to share their own modifications to the code or hardware. This is a tube amp that embraces the DIY movement and brings tube audio into the MAKE-sphere in both a hardware and software domain. Once the amp is released, users will be able to log into the Nielltronix community and discuss their amp mods and experiences with others. The amp is fully developed on the bench, and with full schematics, and a USB port, you can modify it to your heart's content. It is a tube amp both for the hackerverse and the serious audio connoisseur!
The photo at the top shows what our industrial designer has come up with for the two chassis being offered. The chassis on the left is a hand-rubbed box-joint cherry wood frame made custom by Goose Creek Woodworks. The chassis on the right is made from custom-made anodized aluminum, made in New England. The amplifier itself is a 45W per channel stereo amplifier with user-selectable XLR or RCA inputs, switch-selectable 4, 8, or 16 ohm output, and a microcontroller. The microcontroller talks to the power supply, the amp, and the LCD and rotary encoder input device. Both designs have the same guts, but both designs cost a lot of money to produce. Any additional funds will to toward more Nielltronix amps or future kits and projects on nielltronix.com/forum.
Here lies the real driving force behind this project: I know there are people out there who are just as passionate about high-end audio and are tired of the same old stuff. Here's a new, exciting design with an open-source, open-hardware approach, but that costs a lot of money to get off the ground. Lots of parts are subject to minimum purchases, and custom metalwork and woodworking cost a lot of money too. Tubes must be secured in large quantity beforehand to ensure users will have replacements. All of these things add up to a daunting task. However, with just a little help at the beginning, we can make an amplifier that the entire DIY and high-end audio community can appreciate.
Manufacturing Plan and Details
Nielltronix's president's decade-plus-long history in the electronics business has forged relationships with vendors and engineering partners, enabling a very lean manufacturing process. The PandorAmp electrical schematics and layouts are all in OrCad, and mechanical parts, save for flat sheetmetal parts, are all created in SolidWorks. Once all the final details are ironed out, and the funding from kickstarter arrives, PCBs will be manufactured by Advanced Circuits in Colorado. SMT and thruhole parts kitting will be handled by Digikey's new hands-off BOM kitting service. In addition to the bare board manufacturing, Advanced Circuits' new turnkey manufacturing service will be handling the raw PCB stuffing. Additional soldering and manual wiring will be finished by Nielltronix staff, though if demand is large enough, connectors and cables will be handled through OSI Electronics in North Andover, MA. Code is being written currently to expand the amp's interface capabilities. All code is in C, compiled with the C18 compiler provided by Microchip.
Once mechanical details are finalized and SolidWorks models completed, sheetmetal and woodwork final drawings will be executed. Woodworking drawings will be sent in a build-to-print state to Goose Creek Woodworks. Bent sheetmetal drawings will be shopped to a few different sheetmetal shops in New England with whom Nielltronix has had experience on projects in the past. Machined parts will be handled primarily by Clematis Machine in Waltham, MA. Flat sheetmetal parts, such as the top, front, and rear panels will be fabricated by Front Panel Express. Artwork for the flat sheetmetal can be seen below.
Custom ferrite transformers will be supplied by Renco, while custom iron transformers will come from Edcor. Mechanical hardware will of course mostly come from McMaster-Carr, but other hard-to-find hardware will come from a number of smaller vendors.
Final assembly, programming, and test will occur at Nielltronix headquarters. The Nielltronix assembly team (Fred, Dale, Jerry, Mike, the other Mike) has access to over 1500 square feet of manufacturing space, with all the necessary tools (programmable reflow oven, several soldering rework stations, oscilloscopes, etc.) to build this hardware.
- 2-channel stereo amplifier, 45W/channel
- Custom output transformer with 4, 8, 16 ohm taps
- Push-Pull Class AB2 with novel phase splitter
- Integrated silicon audio front-end with microcontroller interface
- Menu-driven intuitive click-and-scroll interface
- < 0.5% THD 25W, <1% THD 45W, -3dB 10Hz, 45kHz
- Signal input: RCA or XLR, clipping at 3V
- Power input: Universal input, 100-240VAC, 200W
- Power supply: <100mV 120Hz ripple at 425V, <75mV 120Hz ripple at 300V, <50mV 120Hz ripple at -75V
- DC Grids, soft start buck grid circuit for long tube life
What about this t-shirt?
Our awesome designer spent a lot of time on this t-shirt design. We are using locally-sourced t-shirt printing in Boston, MA from http://www.hemlockink.com/.
Above is the Nielltronix tshirt, available in both army green and heather gray. It captures the awesomeness of the PandorAmp project and the cool nielltronix.com online community in a single cool t-shirt. Special thanks to @derekcascio for help with design.
So where is the project now?
Good question. I'm listening to the amp right now, but that doesn't answer the question. There are a number of things that must be addressed before you can listen to this amp in your livingroom. Here's a list of the various amp systems and subcomponents that must be finished before product release.
- Power Supply: We haven't made any changes to the power supply in about 4 months, during which we've been testing the amp in all kinds of situations, from techno to Bach, and shorted output to low-line input. We just have to do a final iteration of the power supply PCB.
- Controller Board: The interface circuit audio parts sit on a vector-board in the prototype chassis. The digital controls sit on breadboard, as seen above, with wires going to the audio interface board. A final PCB must be designed and created for this.
- Chassis: Two chassis possibilities are shown above. The wood chassis is relatively well defined, and final manufacturing drawings need to be finished and sent to the woodworkers. The inset custom anodized aluminum chassis panels for the wood amp are designed and ready to send to the custom sheetmetal shop. The custom sheetmetal amplifier chassis is in concept phase, and will need to be fleshed out, detailed, and manufactured.
- Parts Acquisition: Many parts must be acquired - from nuts and bolts to vacuum tubes and microcontrollers. Custom output transformers have been designed, and must be ordered. All of the electronic components required to make these amplifiers must be ordered, sorted, and made ready for assembly.
- Power Amplifier Section: The hardest part, to be certain. Luckily, this part is pretty well taken care of. The tubes all work great and most importantly, they sound great!
Q: This amp seems cool, but I've never heard of the 1625 tube. Can you make it work with other tubes?
A: That's the whole reason behind running this project as an open source/open hardware project! You can modify this amplifier to use a variety of beam-power tetrodes or even pentodes. The power supply has enough juice and the transformer is designed around a common plate impedance, allowing for a variety of tubes. Don't like the 6SN7? Plenty of tubes are pin-for-pin compatible!
Q: How does it sound?
A: Insanely awesome.
Q: This sounds like a complex project and a pretty ambitious build. Have you ever done anything like this before?
A: Products Nielltronix has designed are key components in semiconductor manufacturing processes, and are installed around the world. The president, Fred Niell, has released several different nixie tube clocks before, and is comfortable with this type of project. We're confident we can get all of this hardware and software out the door in the time given.
Q: I love my amp's sound, but I found a better bias point for the tetrodes when listening to techno. I'd like to share my data with others - what's the best way to do that?
A: The Nielltronix website will go live soon with a forum specifically for the open source modification of the amps, along with future projects. PandorAmp users can share schematics, data, code, and other ideas relating to the amps, and help guide us toward new projects.
Detailed Design Brief:
The input stage is based on a professional grade differential audio line stage receiver, the INA2134. The gain is set with an ultra-low THD stereo audio attenuator chip, the PGA2311. Additional gain processing and feedback processing is handled with National's performance audio dual opamp the LM833. The input stage is on a PCB in mixed SMD and thruhole technology.
The power amplifier is a push-pull class AB2 amplifier using either Russian G-807, American 807 or 1625 tubes for the power stage (user selectable), and 6SN7s for the phase splitter and gain stages. The output stage uses a high-grade M6 steel based transformer with a low-leakage-inductance bobbin, giving excellent high-frequency response. The AB2 operation point requires a fair amount of grid current on the output tubes. As such, cathode followers are used to drive the grids in high-power excursions. This is a departure from many commercial tube amps that simply use a capacitor to couple from phase splitter to output tube grid - doing so seriously affects transient response introduces IM distortion at high levels. Additionally, since the cathode followers don't care about the change in grid-circuit impedance, there is a seamless transition from A to AB2. The phase-splitter uses a novel active-tail cathode circuit, giving extremely flat gain and phase, as well as extraordinary bandwidth. Components are mounted on a PCB with all tube connections are made with MOLEX minifit and microfit connectors.
Wrapping around the tubes and the input stage are a high voltage power supply to power the tubes and a microcontroller and associated user interface hardware. Both the microcontroller and the high voltage power supply present significant departures from traditional vacuum tube amplifier design. The microcontroller adds incredible flexibility to the design. The high voltage supply is not the typical plate transformer, rectifier tube, and huge capacitor bank, rather, it is a sophisticated switch mode power supply.
The high voltage power supply is a high efficiency current-mode flyback circuit, utilizing a UC3845 and a custom-wound transformer. The supply achieves roughly 82-85% efficiency depending on bias settings. The main plate voltage of the amplifier is 425VDC, the preamp tubes use 300VDC, and the fixed bias sits at -75VDC. Each voltage is supplied by the high voltage feedback amplifier. The supply has a hardware-programmable transient response and loop characteristic, meaning that the supply can sag and be just as slow and underdamped as a typical tube and cap supply, or it can be incredibly stiff and respond to transients in less than 10microseconds. The amplifier as shipped will be set for ultra-fast transient response, giving incredibly tight bass response and good speaker damping factor. Additionally, the fully regulated 300V that feeds the screens of the power tetrodes gives excellent dynamics and stability.
The microcontroller can act in "hands-off" mode, where the tubes really shine and the micro just reports things, or in a more "active" mode where the micro is in charge of controlling feedback or bias levels. The power supply is controlled by the microcontroller, as is the soft-start of the filaments. Bias points are reported as well as total hours for each tube. The microcontroller has both USB functionality and a bootload capacity, making it possible for the user to change the firmware inside the chip. The micro also has control of the filaments of the tubes - the tube filaments are protected from inrush current by the microcontroller driving a buck converter that provides a controlled current to the filaments during warmup phase.
The microcontroller implements a simple, intuitive click-and-scroll type interface through a rotary encoder plus pushbutton control on the front panel. Additionally, a USB jack is situated on the control board to allow users to bootload code from their computer.
Special thanks to (in no particular order):
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