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PGA2500 digitally controlled pre amps

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 09/14/2015 at 17:340 Comments

Ordered 5 x PGA2500 digitally controlled TI preamp IC's today and some Low noise +/-5v bipolar power LTC3260 dual power supplys.

The PGA2500 Mic PreAmp PGA's look good. There used in a fair bit of pro audio gear RME and others so they should be ok for thiis mixer.. they are balanced inputs and the IC attenuates form 10db to 65db of gain wirh Total Harmonic Distortion plus Noise(THD+N) with Gain = 30dB: 0.0004%.. should be ok !

I also ordered 1 x 8 channel ADC CS5386 should work well with the CS4385 DAC I have wired up.

I started work on the schematics in CircuitMaker ( Altium designer hobby version for makers etc ) Its really strange to work with since its in the cloud.. so its quite clunky, at least it is here in Japan , perhaps i'm a long way from the server or something. ) I'm just on the basic internet package which is 65mbps down / 45 mbps up real world speed.. so can't be me I think :)

anyway, soon as I get the parts I will solder the whole lot up and do a demo video of the Pre amp with MIDIusb surface control opperating the gains and channel volume controls.

If I get really carried away I'll throw in the master / heaphone control volumes too :)

the week area I feel is the 8 : 2 summing part. no routing just balance and going single ended for master volume control and headphone cotrol just seams booring..

ADC and DACs all differential... think I need to change that master control volume !! I thought of using two digitally ganged together, one for each phase, but I think that could introduce phase probelms since there not locked together with a clock since there analouge at this point ..

I'm hoping the copied 8 channels in the 2 DACS will be locked together since they will share the same I2S master clock from the 1 ADC.

Next :

I need to learn what 48volt phantom power actually is and how you make it :) I am assuming its a very low power 48v bias current, but how does that flow over a differential audio connection ? better I don't assume anything ...

ok, off to read more..

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