Someone on DIY Audio brought up that the gain structure of the Input Buffer (and Purifi EVAL1) isn't optimal, so I'm considering a design change.
I would be curious for you folks' input on this:
By default my Input Buffer has 13.2 dB gain (4.57x). The 1ET400A input specs are (Purifi data sheet Rev. 1.0):
- Differential input voltage (pos to neg input): 12 V
- Common-mode voltage: ±5 V
So for differential input, the max input voltage in the default configuration is: 12/4.57 = 2.62 V (or just shy of 2 V RMS).
For the single-ended input, the max input voltage is limited by the common-mode voltage spec. So: 5/4.57 = 1.09 V peak (770 mV RMS).
I can easily change the 'custom' gain option to allow for 4 V differential, 1.5 V single-ended. That's the "fast to market" option. Unfortunately, this option would make the 'custom' gain option a bit harder to use, as you would need to remove an 0805 resistor and replace it with another 0805 resistor to change the gain.
Alternatively, I can redesign the Input Buffer to include a sort of "differential level translation". That would increase cost (maybe by $15), but would mean that the Class D amp always "sees" a differential input - even if the source connected to the Buffer is single-ended. That would be a nice market advantage.
I might be able to shave off a few cents from the board cost by eliminating the two 3-pin connectors for the optional external regulator and just leaving the 6-pin connector/jumper block in place. You can always connect a ribbon cable there if you want an external regulator.
I won't make this a design-by-committee project, but I would rather the Buffer provide good value and satisfy unmet market needs than be a "me-too" product.
Tom