Multichannel Audio DSP Field Mixer Recorder

SEE PREVIOUS LOG FOR DESIGN NOTES

Its working but next revision I think I will change the priority to the mains ACDC power input if both battery and wall adapter are plugged in! basically swap the 2 power input wires around :)

it never ends !! :)

now I need a battery / acdc wall adapter power switch. I'm not charging the V-lock battery's on board as people generally use there own proper chargers.

I also include NTC protection [ edit ,  PTC resettable fuse , not NTC ! ) ]  in case there is ever a short somewhere !!

I'm going to try using the LTC4412 to switch a BSS308 ( P channel mosfet )

more cost and more complexity !

does anyone know of a cheaper simpler way that takes up little pcb space ?

I could just switch the mosfet with the mechanical DC barrel jack switch but seams a bit risky ?

My backplane is already pretty loaded with voltage level monitoring / ADC buffering so I'm making it 4 layer this time and with components will be on both sides. the vsense1 socket will send the buffered 0 - 3.3v voltages to the MC. the digi main board only has 6 ADC channels hooked up at the moment but eventually that will be upgraded to 8 , that way I can monitor battery Vin ( DC vin ) +-15 +-5 48v 3.3v and more

the diode in this example obviously doesn't fit this design !! I'm sure I will find a decent heavy duty

SMD version.

ahhhh , I'll make it all fit , there is always a way ! 

BB-Box gets a V-Lock pro battery 99 W/h , plane safe ! 

4 preamp cards and 4 balanced line drivers using OPA1637.

The Vcom and higher +/-9v rails for OPA1637 buffer are supplied through the stackable headers on the preamp cards. They are 1mm to long so not sure I will be able to squeeze all 8 preamps in there using those. 

The 6mm brass standoffs worked fine for the line drivers and I send the VQ down one line of brass standoffs and it works. 

I've started work on the case again and will print another in CF and will glue on some thin aluminium panels in recesses in the CF.

I am making plates for connectors that bolt to the chassis. so far only 2 plates , 1 for the mini XLR inputs / outputs and 1 panel for the AES IO mini XLR and 1/4" headphone sockets.

I made some slots in the digital and analogue main board mounts so I can slide in some EMI shielding between the 2 boards.

this is a very rough 3d thing of the assembly. you can click to make a cutaway.

https://grabcad.com/library/bbbox-1

I've started work on the case again and will print another in CF and add some aluminium panels.

I am making plates for connectors that bolt to the chassis. so far only 2 plates , 1 for the mini XLR inputs / outputs and 1 panel for the AES IO mini XLR and 1/4" headphone sockets.

I have no idea if this 3d grab cad page will embed but its a rough idea!

https://grabcad.com/library/bbbox-1

hopefully final video update. The filter is for half mclk frequency of 12.288 ( 6.144mhz ) 

some interesting notes on buffer filtering for ADC here..

https://www.ti.com/lit/an/sboa114/sboa114.pdf?ts=1625582064839

I was looking at frequency ranges just above audio but the filters job is to cutoff anything from 6mhz and higher ! 

update simulation using opa1637 as the preamp buffer to ADC.

better results in simulation at least.

Why I can't use the OPA1632 to attenuate the PG2500 with +/-5v power rails

OPA1602 attenuation and low 6mhz low pass filter

The main case is just one part with top and bottom access panels.

I'm making connector panels that can be changed so if somebody wants for instance 4 x full XLR

and a DSUB for the rest of the channels its just 4 bolts and an FPC connector to change it! 

I will make a panel for the AES sockets next. 

There is a recess for the oled confidence screen and Time Code clock battery. 

Also a recess for BT module and programmer connector.

I'll need to learn how to make a nice sliding panel for the battery change as I hate swing out doors that just snap off like a lot of sony pro cameras etc.

Later down the line when I'm happy with its functionality and I work out better waterproofing I will work on beveling the edges and work on the horrible sharp corners! The bottom edges have a routed channel for a waterproof strip the will mate with the bottom panel to form a water seal.

The preamps are working great with the new buffered VQ ( vcom ) going down a bolt that holds them in place.

I can't say the same for the headphone preamp at all !! 

I had not really thought through the bias injection to the buffer properly ( tried to inject it into each input leg of the negative rail which has the negative feedback loop. the positive was going to GND  ) wrong. 

ignore the note about 100uF decoupling ! 10uF c1,c2,c3,c4 should be enough to remove the DC bias :)

Now I need to incorporate this 50khz corner frequency filter in the negative feedback loop of my 4 channel OP4A134 or OPA1604 if this basic version works.

from a cirrus logic app note : 

A two-pole Butterworth with a corner
frequency of 50 kHz attenuates the signal at
20 kHz by approximately 0.1 dB and has nearly
ideal phase linearity within the audio band

As I type this I realise it may have been more sensible to breadboard the buffer with the filter and test it 1st before sending off for the pcb.. 

I can test the filter with a sweep  generated by the DSP from say 15khz -> 100khz and make sure the filter is doing the job.

I assume these ultra high frequency's can effect the lower ones harmonically if they are not removed? anyone know why i am actually making this filter ? it's just the recommended one in the datasheet !

I connect the differential DAC audio signals to the non inverting inputs of the 4 channel op amp IC and bias it with the buffered VQ (VCOM) form the DAC. I think if I need some attenuation of the DACS outputs I'll need to use voltage dividers on the inputs as the non-inverting inputs since any non-inverting op amp input has a minimum gain of 1 ? in other words in this configuration I cannot use the buffer for attenuation.

Please drop a message in the comments section if I am wrong there? thanks in advance :)

EDIT:  I just realised I can set the output level of the DAC using I2C registers , so would never need to set attenuation anyway.

I beefed up the upper lower case mounts where there was available space.

Also added some pcb mounts that don't require spacers. 

Good bad and the ugly, 

This will have to do while I work on more important updates with analouge boards.

Will try carbon firbre nylon composite later.