Nevin,
1. The mic pre on the SGTL 5000 is MONO. It will have significantly worse noise and distortion performance than the THAT chip and it is VERY BAD practice to have a preamp and then reduce it to mic level again and re-amplify. You basically multiply the noise and distortion of the two preamp stages (ours, and the one in the SGTL) together.
2. The THAT chip has an absolute max supply of +/- 20v so running it from the 48V supply won't work. While creating a virtual ground works OK in some circumstances, it is an additional source of design issues for a mic preamp. Routing from pin 1 on the XLR to the virtual earth becomes critical, as does making it very low impedance to ground and dampening supply noise. I can't see anywhere in the data that says +/- 5V will produce poor results (but also haven't tried +/-5v with this chip) - but happy to be directed to where it does! I will substitute the -5V converter for a +/- 12V one. No big deal, and likely a much better result.
I'm also being careful not to run out of 5V USB supply - the switching supplies, have a no-load current and <100% efficiency (generally a design factor of 70 - 80% at light loads, is appropriate). We can rely on a total of 500mA, and 1000mA from some ports. Let's say 100mA for the Teensy, leaves us 400mA for our preamp, the Audio Board and any other draws on the supply. I'd be keen to keep our static draw < 100mA, and dynamic < 150. BTW, each phantom leg sources around 5 mA. So two preamps = x4 = 20mA.
What's your thoughts on likely USB loadings?
On other design matters:
1. There is little need for an additional volume control on the THAT chip, and we will simply confuse users by having two gain controls for each channel. Mic outputs are 5 - 15mV RMS, with a gain of 50 that puts us in 250-750 mV RMS or ~1.5v p-p. The THAT chip will handle that OK with +/5V supplies, even more so on +/-12V. The volume control on the LM833 amplifier can reduce those, if needed, to appropriate levels. It's the mic preamp that is critical for noise performance - we're at instrument/line level after it. LM833s have a long history of working well in demanding line-level audio applications (5534's are the other chip that is often used, but they have a much lower p-p swing and about the same noise performance at line levels.)
2. Why would we go for a GP opamp (Joemeek) in a mic preamp????? You selected the expensive THAT chip as it's the best available, specially optimised, microphone preamp chip around and rejected the cheap TS472 module as being below spec. Happy to go down the low cost/low spec path if you want to save $$$. the LM33078/OPA2604... simply do not have the noise performance required. 3.5 - 4.5 nV/root(Hz) is a good noise target.
3. I'm keen to mount the switching regulator modules under the PCB - both to reduce the space they take up and to have a solid ground plane between them and the audio signals. Does this pose any mounting difficulties for you. The stand-offs needed would be about 1/2".
4. BTW, I'm rethinking the capacitance multiplier idea - I re-read an old Analog Devices paper on the idea and they reduced phantom supply hash from 80mV p-p to around 4mV. I'll see how the module performs when it arrives, and make allowance for cap multiplication if the noise is more than the existing filtering and preamp's CMRR can handle with ease.
5. We still haven't landed on a setting for the critical price-performance tradeoffs. Do you have a maximum, all-up, price ceiling for parts?
1. The mic pre on the SGTL 5000 is MONO. It will have significantly worse noise and distortion performance than the THAT chip and it is VERY BAD practice to have a preamp and then reduce it to mic level again and re-amplify. You basically multiply the noise and distortion of the two preamp stages (ours, and the one in the SGTL) together.
2. The THAT chip has an absolute max supply of +/- 20v so running it from the 48V supply won't work. While creating a virtual ground works OK in some circumstances, it is an additional source of design issues for a mic preamp. Routing from pin 1 on the XLR to the virtual earth becomes critical, as does making it very low impedance to ground and dampening supply noise. I can't see anywhere in the data that says +/- 5V will produce poor results (but also haven't tried +/-5v with this chip) - but happy to be directed to where it does! I will substitute the -5V converter for a +/- 12V one. No big deal, and likely a much better result.
I'm also being careful not to run out of 5V USB supply - the switching supplies, have a no-load current and <100% efficiency (generally a design factor of 70 - 80% at light loads, is appropriate). We can rely on a total of 500mA, and 1000mA from some ports. Let's say 100mA for the Teensy, leaves us 400mA for our preamp, the Audio Board and any other draws on the supply. I'd be keen to keep our static draw < 100mA, and dynamic < 150. BTW, each phantom leg sources around 5 mA. So two preamps = x4 = 20mA.
What's your thoughts on likely USB loadings?
On other design matters:
1. There is little need for an additional volume control on the THAT chip, and we will simply confuse users by having two gain controls for each channel. Mic outputs are 5 - 15mV RMS, with a gain of 50 that puts us in 250-750 mV RMS or ~1.5v p-p. The THAT chip will handle that OK with +/5V supplies, even more so on +/-12V. The volume control on the LM833 amplifier can reduce those, if needed, to appropriate levels. It's the mic preamp that is critical for noise performance - we're at instrument/line level after it. LM833s have a long history of working well in demanding line-level audio applications (5534's are the other chip that is often used, but they have a much lower p-p swing and about the same noise performance at line levels.)
2. Why would we go for a GP opamp (Joemeek) in a mic preamp????? You selected the expensive THAT chip as it's the best available, specially optimised, microphone preamp chip around and rejected the cheap TS472 module as being below spec. Happy to go down the low cost/low spec path if you want to save $$$. the LM33078/OPA2604... simply do not have the noise performance required. 3.5 - 4.5 nV/root(Hz) is a good noise target.
3. I'm keen to mount the switching regulator modules under the PCB - both to reduce the space they take up and to have a solid ground plane between them and the audio signals. Does this pose any mounting difficulties for you. The stand-offs needed would be about 1/2".
4. BTW, I'm rethinking the capacitance multiplier idea - I re-read an old Analog Devices paper on the idea and they reduced phantom supply hash from 80mV p-p to around 4mV. I'll see how the module performs when it arrives, and make allowance for cap multiplication if the noise is more than the existing filtering and preamp's CMRR can handle with ease.
5. We still haven't landed on a setting for the critical price-performance tradeoffs. Do you have a maximum, all-up, price ceiling for parts?