freeDSP-aurora - DSP with 8 I/Os, USB Audio, S/P-DIF, ADAT, Bluetooth and Wifi contro

[JayShoe]

Here is the recommended input stage of the AKM AK5558 32bit-ADC.



And the input stage of the Aurora.



====Preamp?
So it's implimenting the recommended input. My question is whether this input can (and/or should) be modified to allow for preamp gain for my dynamic microphones. I'm also curious if a potentiometer can be rigged to the aurora's existing input stage to give an adjustable control. Please note that for my purposes, I can be OK with a "set it and forget it" gain. I would adjust the overall levels in the DSP parameters, not on the preamp. For some users, a digitally controlled preamp would be nice. Yes, very nice. :-)

====Single Ended
I'm also noticing that the datasheet calls for a single to balanced conversion in the case of "BNC Connectors". I'm sure this is a common question, but should we actively convert the single ended inputs? Or should we just connect + and ground -?

====Active or Passive
I'm trying to understand what's required of these add on boards and what should be done to get them made. Are we designing a simple passive "jack on a PCB" with Aurora-IO headers to make hookups easy? Or do these input boards require some more active conversions?

[JayShoe]

Any chance you could move the "Getting Started" guide to a google doc? It may help with contributions. Also, I'm trying to grab the BOM so I can price out the parts on digikey, but I can't copy it out.

[Windforce85]

It is bad idea to use vol. pot on R7, use pot instead of R5. Generally it is bad idea to use pot anywhere around feedback loop of opamp.
EDIT: nevermind, you've changed image in meantime..

[dspverden]

In case you want to use microphones, I would recommend to add another opamp stage like you have shown.
The maximum gain of the current input stage is a bit limited. The maximum negative swing is limited by the virtual ground of +2.5V required by the ADC. Therefore, I have also replaced the NE5532A by another rail-to-rail-opamp: OPA1652.
Having said this, an additional stage for the mics will preserve the option to connect line signals.

[dspverden]

Originally Posted by JayShoe

====Active or Passive
I'm trying to understand what's required of these add on boards and what should be done to get them made. Are we designing a simple passive "jack on a PCB" with Aurora-IO headers to make hookups easy? Or do these input boards require some more active conversions?

Yes, that is actually what needs to be done. Make a passive PCB with e.g. some XLRs and a pin header with aurora pin layout. You should also place some RFI stuff close to the XLRs. RFI has to be as close as possible to the input.

Regarding balanced vs. unbalanced: While you can do it passive an additional inverter stage on the input board would improve the signal quality.
And here you can see why I did not make a decision for the input and output connectors on the aurora board: There are so many variants and I did not want to limit freedom.

[dspverden]

Originally Posted by Windforce85

Great project! Would you like to add inverted all-pass for crossover phase shift correction? How does analog output stage, clock distribution and DAC Vref power supply looks like?

The inverted allpass is already there. It is splitted in an inverter stage and an allpass stage for phase manipulation.
Power supplies are all LDOs with DSP, USB, analog power for converters and opamp power having each a regulator. The +2.5 virtual ground is provided by TLE2426ID.

[JayShoe]

case you want to use microphones, I would recommend to add another opamp stage like you have shown.

I wonder if a THAT 1580/5173 or THAT 1583/5173 chipset with an i2C to SPI controller would be the correct choice for my mic preamp application with the FreeDSP Aurora? I wonder how much they would cost (read: $$$).

Alternatively I could choose an OPA1652 mic preamp circuit, not sure if it could be digitally controlled, not sure it matters for my application or not.

Hmm... This is why I was originally designing with the PCM1865, it has all the front-end input stuff incorporated into it. I might continue to develop that part with the aurora FreeDSP Expansion Connector. I'm ultimately looking for an inexpensive way to add the mic preamp to this design. Another idea could be to design a simple single ended to balanced converter board for the Aurora. There are many cheap balanced input to single ended microphone preamps on ebay.

[JayShoe]

INA103 is another option.
mic pre amp INA103 - EasyEDA


ADI - SSM2019, TI - INA103, INA163, INA217, THAT – 1510,1512, 1570 and possibly others


Mic Preamp Kit - Electronics DIY Projects PCB


Tiny Professional Microphone Preamplifier - Electronics-Lab

PGA2500 or PGA2505

[JayShoe]

Looks like THAT 6261, 6263, 6266 may actually be the best option and cheapest. 2 channel boards makes sense. Still need an i2c to SPI conversion.

[JayShoe]

I had an idea while I was sleeping that might solve the need for both of our purposes in one board. What if we were to create a preamp module that would have a series of "input inverters". These input inverters would also have a simple gain control in either fixed resistor or variable resistor format. If designed correctly, we would solve two birds with one stone.

Single ended inputs would be sent through both inverters, one after the next. This would thus create a balanced signal. Balanced signals would go through the same two inverters but in parallel, so that the balanced signal is just reversed. Both the single ended version and the balanced version would have gain control on the inverters (in the form of a fixed, or variable resistor), so you can add gain coming from the single ended or balanced input signal. The idea would be that at the right gain, this circuit would also be a mic preamp.

The result is an input stage board that would offer single to balanced conversions, input filtering (for standard balanced connections), and gain options (for microphones and instruments); all in one neat little package.

I started a project to draw out my idea.... This is a start. Where would the gain control variable or fixed resistor sit on the circuit? Is this a valid way to approach this situation? This would add gain to the mic input correct? Would it be of any decent quality?