MickMad
Well-known member
Hi there,
I've been thinking about this project for one month now and I already started laying out schematics and whatnots for this project, but I think that asking everyone around here for their opinion is the best way to know what is actually going to be useful and what not.
So, as you can see from the title, I want to design THE ultimate Teensy audio board; since the powerful Teensy 3.6 came out I think we need a better audio board, with hi fi level CODECs and other really cool features for audio projects.
What I have in mind is a sort of motherboard where you can plug the Teensy 3.5/3.6 in and then have a lot of plug-n-play modules that will allow to make any kind of audio project. You want to make a MIDI synth? Plug in your own knobs and switches; want to make a guitar effect processor? Plug in your own hi-z preamp on the inputs... etc. etc.
The features I think are a must are the following:
For the CODECS, I was looking at the AK4621EF (I worked with Ashai Kasei devices before and they make really good stuff), really hi-fi codec in a small package. The cool thing about this codec is that not only it has true differential inputs and outputs, but it also has really cool input and output bandpass filters that allow me to design the input and output connections without having to put a 6th-8th order active filter in front of every connector to cut away undesired frequencies. Oh and the SNR for both ADC and DAC is way above 100 dB.
For the supply I'd like to make a switching supply with dual outputs for +5 and -5 volts, that takes the input from a standard wall-wart supply. I'd like it to be a powerful supply, something that could source, say, up to 1 A easily, for more power-hungry applications (like, with a LOT of LEDs). I think the dual supply is mandatory for a hi-fi audio solution because if you want to make good preamps a dual supply makes opamp circuit design OH so much easier.
Nothing to say about the USB B plug: I saw that there are D+ and D- pads under the Teensy 3.6 and I'd really like to connect to it a huge and bulky USB B port because USB micro connectors (especially the male ones) fail miserably. I have USB B cables and connectors that after 15+ years of misuse are still working like a charm, while I have to buy new micro USB cables every 6 months.
For the jacks, I have two options: the first one involves using a balanced connector to be used as both a bal/unbal jack (you basically just short one contact to ground), or a more clean approach of having dedicated balanced and unbalanced jacks, with dedicated balanced and unbalanced buffers.
Headphones: I'd really like to have dual headphone outputs. Since maybe not everyone would need this, I could make the headphone out as an add-on module; if you only need one headphone out, use one module. You need two? Get two. You need none? Get none/ Easy as pie. The headphone out would be directly tied to one of the two stereo outs. QUESTION: the headphone out in this way would depend from the DAC out volume, do you think that would be an issue?
Inputs preamps: like for the headphones, I'd like to have a modular input preamp thingy; if you don't need any preamp and just want to use line level, then use the onboard connectors; if you need hi-z/mic preamp, you plug your preamp and use an analog switch to select between the preamp or the line input. The analog switch could be integrated on the board for ease of use and for ease of preamp modules development.
GPIO expander: I love multiplexers, and I've been overthinking about a way to use the SPI module on the Teensy 3.6 along with its hardware CS lines to read data out of a load of stuff while using a really low number of pins. It's a solution that involves using one SPI module, shift registers, multiplexers, 2 analog pins and a lot of connectors.
ILI display: nothing to say, it's just going to be a header where an user can plug its own ILI9341 TFT display. The best would be to have an on-board one in order to avoid any issues with non-standard sizes of display modules.
I remember reading about Paul's desire to use digital isolators between the Teensy and the ICs, and transformer coupled analog IOs for optimal performance, but this would ramp up the cost of the board a lot. I'm not saying it's not doable, I'm just saying if it is actually necessary to use them or not.
So, any comments? What would you like to see in a board like this?
Cheers.
Mick
I've been thinking about this project for one month now and I already started laying out schematics and whatnots for this project, but I think that asking everyone around here for their opinion is the best way to know what is actually going to be useful and what not.
So, as you can see from the title, I want to design THE ultimate Teensy audio board; since the powerful Teensy 3.6 came out I think we need a better audio board, with hi fi level CODECs and other really cool features for audio projects.
What I have in mind is a sort of motherboard where you can plug the Teensy 3.5/3.6 in and then have a lot of plug-n-play modules that will allow to make any kind of audio project. You want to make a MIDI synth? Plug in your own knobs and switches; want to make a guitar effect processor? Plug in your own hi-z preamp on the inputs... etc. etc.
The features I think are a must are the following:
- 2 stereo ins, 2 stereo out
- External power supply with dual rails (really useful for designing good preamps and output amps)
- USB B plug (I personally hate USB micro connectors)
- Balanced and unbalanced connections (there's no hifi system without balanced connectors)
- 2 headphone amplifiers (one for each stereo output)(may be designed as a module)
- Hi-z/line input selector (instrument or mic preamp that brings the signal to line level)(may be designed as a module)
- GPIO expander (to be able to connect LOTS of analog or digital IOs to the Teensy without using too many pins)(more about this later)
- ILI9341 headers (so to be able to use a nice display for any GUI needed)
For the CODECS, I was looking at the AK4621EF (I worked with Ashai Kasei devices before and they make really good stuff), really hi-fi codec in a small package. The cool thing about this codec is that not only it has true differential inputs and outputs, but it also has really cool input and output bandpass filters that allow me to design the input and output connections without having to put a 6th-8th order active filter in front of every connector to cut away undesired frequencies. Oh and the SNR for both ADC and DAC is way above 100 dB.
For the supply I'd like to make a switching supply with dual outputs for +5 and -5 volts, that takes the input from a standard wall-wart supply. I'd like it to be a powerful supply, something that could source, say, up to 1 A easily, for more power-hungry applications (like, with a LOT of LEDs). I think the dual supply is mandatory for a hi-fi audio solution because if you want to make good preamps a dual supply makes opamp circuit design OH so much easier.
Nothing to say about the USB B plug: I saw that there are D+ and D- pads under the Teensy 3.6 and I'd really like to connect to it a huge and bulky USB B port because USB micro connectors (especially the male ones) fail miserably. I have USB B cables and connectors that after 15+ years of misuse are still working like a charm, while I have to buy new micro USB cables every 6 months.
For the jacks, I have two options: the first one involves using a balanced connector to be used as both a bal/unbal jack (you basically just short one contact to ground), or a more clean approach of having dedicated balanced and unbalanced jacks, with dedicated balanced and unbalanced buffers.
Headphones: I'd really like to have dual headphone outputs. Since maybe not everyone would need this, I could make the headphone out as an add-on module; if you only need one headphone out, use one module. You need two? Get two. You need none? Get none/ Easy as pie. The headphone out would be directly tied to one of the two stereo outs. QUESTION: the headphone out in this way would depend from the DAC out volume, do you think that would be an issue?
Inputs preamps: like for the headphones, I'd like to have a modular input preamp thingy; if you don't need any preamp and just want to use line level, then use the onboard connectors; if you need hi-z/mic preamp, you plug your preamp and use an analog switch to select between the preamp or the line input. The analog switch could be integrated on the board for ease of use and for ease of preamp modules development.
GPIO expander: I love multiplexers, and I've been overthinking about a way to use the SPI module on the Teensy 3.6 along with its hardware CS lines to read data out of a load of stuff while using a really low number of pins. It's a solution that involves using one SPI module, shift registers, multiplexers, 2 analog pins and a lot of connectors.
ILI display: nothing to say, it's just going to be a header where an user can plug its own ILI9341 TFT display. The best would be to have an on-board one in order to avoid any issues with non-standard sizes of display modules.
I remember reading about Paul's desire to use digital isolators between the Teensy and the ICs, and transformer coupled analog IOs for optimal performance, but this would ramp up the cost of the board a lot. I'm not saying it's not doable, I'm just saying if it is actually necessary to use them or not.
So, any comments? What would you like to see in a board like this?
Cheers.
Mick