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A project log for Multichannel Audio DSP Field Mixer Recorder

USBmidi, bluetooth app controll 8 chan portable DSP mixer.balanced audio IO,phantom power,flexible routing,ISO recording. networked Timecode

ben bilesben biles 11/03/2019 at 16:150 Comments

excited about my new smaller thinner preamp cards using 1.27mm headers.
so far they seam to work as before. I kept some space by the optical relay as I expect there to be more front end circuitry. I hope to find a smaller 1 form c opto relay to use. found a toshiba that looks promising that's 1/4 of the size. The 60v 1Amp clamping protection diodes have started to loo very large so working on finding some smaller diodes to.  the gold caps to the right are DC blocking caps before the ADC. The PGA2500's output some DC which needs filtering out.


The dual rail cards are quite long at 80mm but output +/- 1 amp at incredible low noise using lm43602's and TI ultra low noise regulators.
I have 1 card for +/-5v and one for +/-15v ( or I might use +/- 9v )  depending
on how much power I want on the master outputs of the line drivers.
I should change the conductors for +/- 15v setup from 6.8uH to 15uH.
The negative rails needs a lower in series resistance inductor 150megOhms. the + rail works with 75megOhms.
both cards are set to  run at 1mhz switch frequency but are not running in sync yet.
I'm interested in knowing how you can use an ADC on say the ARM micro to monitor the DCV of each power channel and then even using a digipot fine tune the accuracy of each rail. fine tuning the Voltage is perhaps overkill but monitoring the rails could be good way to self test the power is healthy before potentially frying amp chips in case a power card went down. maybe i'd need to use a higher voltage capable adc to feed the results to the micro ? and how about the negative voltages? would I need an ADC capable of neg voltage ? or use a voltage buffer for each channel and invert the neg rail?

Although I was fairly happy with the shrinking down of the 1st cards ( still more to do ) the 3.3v 2amp power card was a complete fail!!!

1st I messed up the voltage divider traces. I tried adding the correct resistor to ground to set the TI tpsm84209 output voltage to 3.3v

Without the regulator IC in circuit the output fluctuates all over the place.

I'm thinking so far using the through hole resistor made the voltage divider loop to large. ( the datasheet warns against making that loop large ! )

Although the caps are within voltage spec I'm not sure about the in series resistance. This is were to be honest I'm entering a new area. I don't have a LCR meter and I don't even know if there is LCR mode on my keysight 34461A bench meter. ( better check the manual but dont think so ) 

maybe if I generate 100khz square wave with the mixer recorder I could get the in series resistance of the capacitors I used that way ( the old mixer recorder prototype is still working ) . never used an LCR , no idea yet how it works. looks like I'll need one eventually.

Even the picture I took of this little 1 x 1 cm board failed. it came out upside down and blurred. it cost 1$ , so I think the best thing is to make a better effort with the pcb design and start from there!

These 4 cards will slot into a backplane to make

+/-15v(+/-1A) line drivers and headphone

+/-5v(+/-1A) preamps and adc + dacs

48v(135mA) phantom power

3.3v(2A) digital power

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