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(Intro here, you can skip this for the actual question)

As a hobby project I'm building an audio amplifier. It'll be the amp for my "home theater". I only have a small room so there's no room for 5.1 speakers. And honestly I don't care for 5.1. So I want just a small stereo system with a mono subwoofer, or as they call it, a "2.1" system.

I have decent speakers, (Yamaha NS-50F pair, rated at 80W), and a subwoofer I built myself. Testing has shown that an LM3886 (68W max @ 4 ohms) is more than enough to drive it. So I went ahead and built a simple, triple amplifier.

The results were disappointing. Not even remotely enough "bass". So I built a small "bass boost" circuit (more like a "treble cut" and raise the volume). It was a lot better. But I missed the EQ.

I would like to build an EQ. An analog one would be relatively simple to build with a bunch of opamps. I want this controlled by a remote control so I'd have to replace the pots with digipots. But it starts getting complex, as I'd need:

  • Input stage/buffering
  • Equalizer
  • Low-pass filter for sub
  • Output preamp (LM3886 needs to be driven relatively high)

This is rather complex, too many ICs involved. Layout becomes critical, opamp noise starts adding up... and I'm simply not that good at analog electronics.

So I thought of just using a DSP. I'm more or less familiar with these as I've played with FIR and IIR filters in the past. Building an audio EQ and output multiplexer (stereo to 2.1) with a DSP would make things a LOT easier, less noisier and overall (in my opinion), better. Well, except maybe for ADC-DSP-DAC delay.

Is there a solution that has:

  • High quality (high SNR, low distortion) ADC with at least 24bit/96khz
  • At least 32-bit DSP core (floating point prefered?)
  • High quality (SNR, dist) DAC.
  • FREE development tools, or at least, unrestricted trial versions.
  • Comes in an easy to manually solder package (not QFN or BGA. QFP is fine)

Bonus features:

  • Extra inputs such as S/PDIF, I2S, etc. And multiple analog input channels (to save on the input multiplexer).
  • It's available as a cheap "ebay board"

I've found several options but not one that fully meets all these requirements. TI is not very helpful (their audio DSP site seems broken). Analog seems to charge $5000 for their software, and the "trial version" only supports one part. Cirrus Logic needs me to "contact my local sales office" to download their development software.

Seems so far the only realistic option is to just use an audio CODEC and a 32-bit microcontroller such as STM32F4.

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    \$\begingroup\$ What kind of budget do you have? \$\endgroup\$
    – user6972
    Jul 6, 2014 at 17:28
  • \$\begingroup\$ Well, it's a one-off. So a $20 chip is no problem. But a $600 development board is too much. I'd rather keep the whole thing under $50 (EQ alone) \$\endgroup\$
    – hjf
    Jul 6, 2014 at 20:54

1 Answer 1

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Answering to myself, I found the perfect part for this, Analog Devices' ADAU1701. It does exactly what I need. It's cheap, really easy to program, and the software is free. All that is required is a 12.288MHz crystal and an EEPROM. It can even handle rotary encoders or potentiometers for volume control, or even pushbuttons. It can be controlled via I2C or SPI. It includes all the necessary DACs (four!) and ADCs (2). It's the perfect device for turning my stereo signal into 2.1, bass-boost it, LPF, control its volume, and much more.

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  • \$\begingroup\$ I would caution you with using a DSP for a hobby project. These DSPs have difficult architectures. In my experience, it is easier to learn and get help with by using a high quality audio codec, and then using a separate processor/controller. A codec paired with an M4 would do just fine like NXPs LPC4300 series. \$\endgroup\$
    – Funkyguy
    Jul 11, 2014 at 0:13
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    \$\begingroup\$ This particular family is programmed through a GUI, simply dragging and dropping components. You just "chain together" an input, process, output. That's it. I guess it could be programmed manually but that's not the point with these devices. See for example: i.imgur.com/fF2LyZC.png I don't think i need to explain what this does. \$\endgroup\$
    – hjf
    Jul 11, 2014 at 0:58
  • \$\begingroup\$ Analog Devices is doing it right! \$\endgroup\$
    – Funkyguy
    Jul 11, 2014 at 0:59
  • \$\begingroup\$ I know this is old, but do you happen to have a link to the family of devices that includes that one? I'm thinking of a project that requires a different number of ADC and DAC channels. \$\endgroup\$
    – AaronD
    Jun 8, 2021 at 20:26
  • \$\begingroup\$ Found it! The right side of this page has a list of devices that SigmaStudio supports. It's slightly annoying that the parametric searches don't include that option, but it's not too hard to figure out how they've organized things and cross-reference manually. \$\endgroup\$
    – AaronD
    Jun 30, 2021 at 3:13

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