one part case & DAC headphone buffer / filter design

A project log for Multichannel Audio DSP Field Mixer Recorder

bluetooth app controlled professional portable DSP mixer.balanced audio IO,phantom power,flexible routing,ISO recording. timcode. AVB audio.

ben bilesben biles 05/14/2021 at 03:460 Comments

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.