The output stage of the Saucer Platform
Design Goals
- Resonant, broad-spectrum output with piercing highs and booming lows
- External amplification capability
- Low noise
- At least as loud as a typical acoustic guitar (80db at peak frequency?)
Key Platform Questions
Split Output - when plugging some headphones directly into the STM L432KC's DAC, the board fails as the headphones draw too much current. Is a headphone pre-amp the solution to this situation? Can it be the same amp that drives the on-board speakers with a resistor inline to control volume?
Amplifier
How much power should the amplifier pull from the board? From the power supply?
Is a breakout board best, or SMC and some board layout?
Speaker connectors? What is smallest and most available?
Filtering
What analog filtering is needed? How is it best designed? Laid out on a board? What if we just skip it?
Aux Headphone/Line Out Jack
Will a headphone jack with pre-amp and volume control make sense?
Can the MCU provide enough current for this?
Or will this design make more sense, powering the headphones/line out as well?
Can we put a resistor inline to limit headphone volume?
How can a make-or-break jack be wired to seamlessly switch between speaker and jack outputs with appropriate leveling?
External DACs
Not investigated in this project, as the MCU's built in DAC seems sufficient.
Possible Wiring
The speaker could be wired through the make-or-break circuit on the jack in parallel with the headphone jack's audio pins. Only one of the 2 output circuits will be closed at any given time without having to use the amp's mute or shutdown pin.
Amplifiers
TPA2005
$7.95
https://www.sparkfun.com/products/11044
Good-sounding, loud enough for a toy, easily available at a low price. The workhorse of the fleet, I have a bout 5 or 6 in inventory.
PAM8302
$3.95
https://www.adafruit.com/product/2130
A very cheap alternative top the Sparkfun part, currently out of stock on Adafruit. Available from Amazon at about $7.
VMA408
$4.95
https://www.velleman.eu/products/view/?id=435574
Would be a nice alternative to the above 2 boards, but very hard to find. You Do It had one in stock once and I grabbed it.
MAX9744
20W Stereo
$19.95
https://www.adafruit.com/product/1752
Untried. That barrel jack looks like serious business!
LM386 Kit
$3.95
http://www.oddwires.com/lm386-amplifier-kit
Untried. Looks a bit crunchy - might be worth laying out an output board before getting this.
LM4971
https://www.ebay.com/i/283032764309?chn=ps
TDA1308
LM1875T
Gravity: 386 Amp module
$12
6V power supply
static power consumption of only 24mW
Sparkfun Noisy Cricket
$10.95
1.5W Mono (4 ohm load)
2.4-5.5VDC
GBA Audio Amp
$24.99
24dB gain at max volume
1W speaker
GBamp
source available
Instrument Classes
Toy instruments come in a variety of sizes - they can be roughly classified by their length in feet.
2-3 feet - guitar hero controller
1-2 feet - larger-class guitar shaped toys
0-1 feet - smaller toddler-sized guitars
Each class should have a commensurate-sized output stage for its purpose. Smaller instruments could have a 3.5mm instrument jack(which can easily be adapted to 1/4").
Headphone/Line Out Jacks
Digikey search for switched 3.5mm jacks
Question: How can we wire them to each amp to defeat when plugged in?
A typical switched jack layout looks like the following:
Assuming:
- 1 is ground
- 2 and 3 are L/R(and bridged for mono operation)
- 4 and 5 are the make/break connections(normally closed, and open when a plug is inserted)
The signal path out from the MCU (@ +- 1V) should go through a leveling capacitor and into 2. When headphone or amplifier is connected, the signal will go directly to it and the amp will be disconnected. When the plug is removed, the connection from 2 to 4 is made, and the amplifier receives the signal, passing it to the onboard speaker.
This assumes that when the amplifier is connected to power and receiving no signal, it consumes no (or very little) power/current.
What size capacitor should be used?
Do we need to pre-amplify the instrument signal?
What is the power and current draw from the MCU?
On-Board Speakers
What makes for more loudness in a pre-made plastic-molded body? Bigger magnets and cone? Better body resonance?
Can speakers like these be dropped in? https://www.amazon.com/dp/B01CHYIU26/ref=sspa_dk_detail_2
Will they shake the whole thing apart with their power?
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