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quadrophonic 3 channel mixer with front to rear pan


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mkay, that was a mouthful.

 

I'd like to upgrade my quad amplifier by adding a 3 channel mixer controlled by an MSP430. remote control, display, internal health check all these will be added one at a time.

 

volume levels for the channels - PC (front & rear), spdif in (front), line in (front) and for the front-to-rear pan will all be set via a few PGA2311 chips via SPI.

my first thoughts have been laid out in the following schematic [1]

 

I thought of placing the project here for peer review. if you see anything stupid in the audio circuit your comments are more than welcome.

 

[1] https://raw.github.com/rodan/ampy/master/hardware/mixer.png

 

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Sounds like a fun project.  Daisy chaining the PGA2311 would save you a ton of pins. I'm not good with audio so I may be sticking my foot in my mouth but reading the data sheet for the OPA2134 it says that the distortion goes up if the input impedance is above 2k so that might be a factor for you. Someone please correct me if I am wrong.

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Sounds like a fun project.  Daisy chaining the PGA2311 would save you a ton of pins. I'm not good with audio so I may be sticking my foot in my mouth but reading the data sheet for the OPA2134 it says that the distortion goes up if the input impedance is above 2k so that might be a factor for you. Someone please correct me if I am wrong.

 

thanks for your reply.

 

my hope that the op-amp buffers were a panacea for all impedance problems just went up in flames.

I am reading a book on op-amps, so I am hoping that this input impedance problem (and the cure) will start to make sense to me soon :rolleyes:

 

regarding the daisy-chaining of PGAs, I've seen it's possible however I really don't like the idea. pins are not in short supply since I'll end up using at least 3 uCs in this DIY amplifier project:

 - one F5510 on this mixer module

 - one F5510 in the power module - this will probably have the user interface and will control all the other uCs

 - one atmel mega that comes in the e-ink display I got from seeed [1]

 

[1] http://www.seeedstudio.com/depot/BADGErv4-p-1587.html?cPath=34

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No problem.

 

The impedance problem shouldn't be too difficult. 

 

I also noticed that with your summing amplifiers it sums the voltages out of the PGAs which has a maximum output is 7.5V p-p.  your opamps have a -5v to 5v(10p-p) rails so you risk railing the opamps pretty easily when summing the signals. I don't know if you have a specific output you want. So I would recommend either increasing the rails to probably -15v +15v for the opamps or averaging the summing amplifiers so you can still use the -5v +5v rails?

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  • 3 months later...

all the amplifier module pcbs have arrived, so I went ahead and built the 2 power circuits:

 

post-34829-0-28242000-1401644158_thumb.jpg

 

the one in the back is for the power amp and provides unregulated -24V, +24V DC

the one in the front is used for everything else - -5V, +5V x 2, -15V, +15V, +18V

 

next were 2 power amps for my quad setup. if 6 channels would be needed another one of these can be stacked on top.

 

post-34829-0-73679600-1401644178_thumb.jpg

 

started populating the enclosure. that spdif DAC will be part of the final setup.

 

post-34829-0-12888000-1401644198_thumb.jpg

post-34829-0-68604800-1401644208_thumb.jpg

 

sound tests went great

 

post-34829-0-44987500-1401644218_thumb.jpg

 

 

 

more pics here: https://plus.google.com/photos/106706955427383613204/albums/6020010464235749729?banner=pwa

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Update #3  - msp430 makes the sounds

 

 

It took a while to get the mixer board ready.

mostly because I was soldering and testing core functionalities one at a time - tweaking the input filters, debugging current consumption to get the fuses right, writing code for the PGAs.

the final result is amazingly good. surprisingly good given the complexity of this module. also no bodgewires needed, much wow.

 

here it is in all it's glory:

 

post-34829-0-19448700-1402131202_thumb.jpg

post-34829-0-58182300-1402131213_thumb.jpg

 

and installed into the enclosure:

 

post-34829-0-31217800-1402131222_thumb.jpg

 

did I mention it works unbelievably well? :biggrin:

an uart interface is used to send volume levels for all 6 PGAs. this will be the job of a second uC that will also write to the display.

 

now I have a question or two for you guys:

 

what cad-like software should I use to design the front and back covers? the design files will probably reach an aluminium laser cutter.

(extra points for something that works in linux)

 

what are the file types/extensions 'all' laser cutters support?

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  • 2 weeks later...

Update #4 - firmware done

 

this was a roller-coaster of good and bad feelings, but finally it become a nice amplifier package.

 

lessons learned:

 

 * I had run into problems with the eink display - it was going grey and fuzzy 1-2 minutes after every refresh.

at first I blamed the toroidal transformers - so I moved all the power sources into another enclosure, but believe it or not the 'ink' particles were dislodged by light, not magnetism. if I shine my cheap ikea led desk light to the screen it will go grey in 2 minutes max.

I don't understand why, but it's perfectly reproducible. I still have not pealed off the protective film, that might be a factor. weird.

 

 * port mapping on my msp430f5510 reserved a nice surprise:

    PMAPPWD = 0x02D52;
    // set up spi port mapping
    P4MAP1 = PM_UCB1SIMO;
    P4MAP3 = PM_UCB1CLK;
    // set up i2c port mapping
    P4MAP4 = PM_UCB1SCL;
    P4MAP5 = PM_UCB1SDA;
    ...

is not possible (aka using the same USCI module in 2 different modes at the same time)

 

however

    P4MAP1 = PM_UCB1SIMO;
    P4MAP3 = PM_UCB1CLK;
    // set up i2c port mapping
    P4MAP4 = PM_UCB0SCL;
    P4MAP5 = PM_UCB0SDA;

is perfectly fine. even if UCB0 pins are normally allocated to PORT3 and they are not even present on the outside of the 48 pin chip I'm using.

whoever decided to make this possible is my new hero.

 

this is how the amp looks like now:

 

post-34829-0-71655800-1403098019_thumb.jpg

 

inside there is a CC430F5137 devboard that gets commands via an RC5 ir remote and talks to the display via UART and to the mixer board via i2c.

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  • 2 weeks later...

Update #5 - panels!

 

@@bluehash thanks for the pointer to front panel express. they also have a partner in Germany so I did not have to pay extra taxes and expensive shipping for the same service.

 

last peek inside

 

post-34829-0-02502800-1404323448_thumb.jpg

 

and the finished product

 

post-34829-0-92772700-1404323456_thumb.jpg

 

post-34829-0-31776100-1404323493_thumb.jpg

 

everything is open source and available at https://github.com/rodan/ampy

more pictures available here

 

I will probably end up documenting the mixer board and eventually sell populated/tested copies it if anyone is interested.

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