Found an interesting thread on Audio Asylum regarding Tortuga Audio. What got my attention were the comments on whether our LDRx passive preamps are indeed true passives or actually actives....or perhaps some type of hybrid.
http://www.audioasylum.com/cgi/t.mpl?f=amp&m=193154The answer may depend on one's definition of either term but I think the arguments for either distill down into something like this...
It's a passive preamp because:
1) There's no gain (no amplification of the audio signal)
2) All the audio signal sees is a variable resistor (LDRs)
3) Although it has a power supply, the power is used solely to control the variable resistor (LDRs) which is optically isolated from the audio signal.
4) There's no buffer stage (high impedance in, low impedance out)
It's an active preamp because:
1) It uses a power supply, the power controls the LDR light source, the light source modulates the variable resistor, the variable resistance influences the audio signal, thus the audio signal does indeed "see" the power supply, therefore it's active.
2) If an improved dc voltage regulator can improve the preamp's sound quality, how is that not an active device?
It's a hybrid because:
1) It has characteristics of both a passive and active device without being classically either.
To be clear, I'm not taking a firm position on either end of the argument because in my view these are all valid observations and characterizations. That said, if we consider a continuum of 1-10 with 1 being pure passive and 10 being unequivocally active, I would submit the LDRx preamps are somewhere in the 2-4 range; a hybrid but more passive than active.