If we fed a cleaner 12v like a paul hynes or teddy psu. Will the effect of low noise be negated by these 5v regulators, good as they may, is not the lowest noise regs currently available to acheive the lowest noise possible?
While I generally subscribe to the maxim that better/quieter power supplies means better audio performance, I'm of the view that this may not apply unequivocally to LDR preamps.
Lets unpack that.
The Tortuga LDR preamp design utilizes 2 overlapping circuits. The primary control circuit is an open loop circuit that regulates voltage to the LDRs in order to set their resistance level. That control circuit does not electrically interact with the audio signal. What we're actually controlling is the brightness level of an LED which in turn is optically coupled with a photoresistor (the definition of an LDR or optocoupler). The control signal is steady state DC except when volume level is changed when there's a smoothed out step change. The reaction time of photoresistors is not instantaneous therefore LDRs act as low pass filters. Upstream the control signal is itself filtered with a knee of just a few Hz. So lots of boring steady state DC. I submit that when this circuit is powered from a source with roughly 10-20 mv of ripple most of which is well over 100 kHz that further improvements in this power source is unlikely to improve audio performance. Can I prove that? No. But that is the current state of the design with the existing 5 V switching DC-DC regulator.
The second circuit is the auto-cal circuit which overlaps with the control circuit but is otherwise isolated from the control circuit during normal operation. During auto-cal, these circuits operate in a closed loop fashion. This closed loop involves DACs, op amps and ADCs. It's a closed loop control, measurement and calibration system that acutally measures the resistance of the LDRs. It is far more sensitive to noise than the regular open loop volume control circuit. As I recently discovered, feeding the LDR3x.V2.1 board with crappy wall-wart power did noticeably impair the functioning of the auto-cal process. As a result, we've changed to a superior regulated linear wall-wart power supply for our LDR3.V2 and V2K preamp products. Our LDRx/LDRxB products use an internal hybrid switch mode 12 VDC supply which is extremely clean and comparable to to the regulated linear wall-warts. Roughly 20 mv ripple/noise on a 12 V signal. Ultimately the auto-cal circuit is the limiting factor in how precise we can match left/right channel attenuation which impacts sound stage and stereo imaging. Further improvements in power supply for this function may be worth exploring.
Best,
Morten
