see https://sarvel.xyz/Projects/AnalogAudioAmplifier for more details.
This project is a realization of audio amplifier with separate gain (10V/V to 100V/V) control for 3 frequency ranges (midfrequencies: 0.1kHz, 1kHz, 10kHz). The project uses OPA1678 opamps as they provide reasonable tradeoff of cost and quality of amplification. It was realized as part of curriculum for Analog Electronic Circuits course at our university.
The project consists of:
- LTSpice simulation in
spice/ - Kicad schematic, simulation, and PCB in
kicad/
Both should work out of the box without any further configuration.
Two simulations are provided because
- it was required that we provide LTSpice simulation
- kicad simulation made development much less error prone
Note that stuff in kicad includes real components that we picked to optimize the costs.
The processing consists of 3 stages:
- initial filtering and separation
- band filtering
- amplification, summing, and separation
- voltage followers to minimize load on the signal
- band pass RC filters to pass only audio frequency, with a bit room to spare to minimize attenuation within audio range
This stage does not employ any amplification
All filters are implemented as 4th order Sallen-Key topology https://www.linkwitzlab.com/filters.htm. It was necessary to use 4th order filters as they provide 40dB/dec rolloff, which is necessary if each frequency is to be controlled fully separately with gain up to 100V/V. Additionally we had to use filters such that different band filters have similar phase-frequency response between them, this lets us have almost flat gain-frequency response. Low pass and High pass filters are essentially taken 1:1 from that website, while band pass filter is simply composition of high and low pass filters.
We combine summing and amplification of signals in this stage with standard opamp summator.
And finally, just before the output, another follower is implemented to reduce the strain on the amplifying stage.
The only somewhat interesting misc component is power supply, we provide two ways:
- direct connection of bipolar power supply
- USB power supply
The latter gets transformed into +-5V power supply, providing dual port power supply. Note that our footprint is wrong, fortunately this was fixable during board assembly :D