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#12 - Destroy - Erase - Improve (or what MAD's future will be)

A project log for MAD - Modular Audio Devices

A set of Eurorack modules to control synths, make custom MIDI-compatible interfaces and USB audio, powered by Teensy 3.6

Michele PerlaMichele Perla 10/18/2017 at 17:290 Comments

After 6 months of thinking and overthinking, making layouts, changing schematics, making simulations, remaking layouts, making some more simulations, looking for components, making prototypes, breaking prototypes, making more prototypes I decided it's time to take everything good and bad in the project so far and start over, once again.

Here's a list of the good parts:

Now the bad parts:

While the whole project seemed like a novel approach to audio applications with the Teensy, I think I started with the wrong foot; I focused on audio quality but I probably ended up ruining it in the first place adding way too many parts on the signal chain. Also, focusing on modularity I ended up stripping away the CORE of most stuff, leaving it to be just a fancy breakout board for the AK4558, not very different from my Hi-Fi CODEC module board I did a while ago. 

With this in mind, I think that as it is right now the CORE is not really commerciable, because I think it doesn't add any added value to what's available for the Teensy (apart from the superior quality CODEC, that is). And the other modules kinda lose of sense without the CORE (apart for the PSU D which, with some modifications, could be a handy tool to have on a workbench).

So, that's why I started working on version 1.1 of the CORE. I will keep the CODEC, LDO, external power screw terminal, Teensy 3.2 connections, in/out filters, and configuration jumpers; I will definitely add  a single-supply headphone/line-out amp, change the I2S RX SMD jumper to a more reliable connector (like a thru-hole pin) connection, remove the 1.27 pitch breakout headers, add a RAM chip, add an micro SD slot, add onboard input/output jacks, add an output volume pot; I will probably add a single supply line receiver (if I find a decent chip), a full connection to the Teensy 3.2 (that is a connection that takes all the pins, even the backside pins), an input volume pot for each channel (either with yet again a Baxandall approach using a 4-opamp-in-one-package to retain board space, or using two log pots, either directly between the line receiver and the ADC pins, or using them with a buffer made with 2-opamps-in-one-package), an instrument/guitar buffer for at least one channel. 

A basic solution would be to have headphone/line-out on a 1/8 inch TRS jack, DAC volume control done digitally using the AK internal volume setting, and line-in on a 1/8 inch TRS jack with no attenuation/gain (that is, straight to the ADC pins);

A better solution would be the same as the previous but with analog gain/attenuation for the input;

A yet better solution would be to have headphones output on a 1/8 inch TRS jack, monitor output on 1/4 inch TRS jacks (in an impedance balanced configuration), independent volume control for the headphones and for the monitors (to be read as analog volume because using the digital DAC volume control would affect both outputs), inputs on two 1/4 inch TRS jacks (true balanced inputs) with independent gain/attenuation control

An even better solution would be same as before but with guitar/instrument buffers on the inputs, and a switch to select between line level and instrument level

The best solution would also have one input to be usable also as a microphone input through an XLR jack and with phantom power; the "bestest" would have both inputs with this option. Board real estate could be reduced by using combo XLR-TRS jacks.

Now I just have to pick one solution ... as if it was easy to choose :)

They're all good options because the first two will be cheaper and will be compact; the second two options will be more expensive, but will add a good amount of really useful features, especially for bigger or more pro-oriented projects; the last two will be surely the most expensive to build and more complex to design (i've been there already, lost a good couple of months), but users will have a full-fledged audio solution for ANY kind of project (the thing that comes first in my mind is: one Teensy + one CORE module like that = pro level, open source, USB Audio interface).

See you next update.

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