[tortugaranger]

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=193154

The 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.

[Davey]

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.

You sound like a politician. 

I think some folks get wrapped up in a topic like this because they have a predisposition against an "active" approach.  (Usually for reasons they don't understand.)  Most probably don't realize that even in an "active" preamp the attenuator control is most likely a passive device.

I don't think "passive" or "active" is the primary issue here.  The question is how the device interfaces with upstream and downstream components and whether it's appropriate (or not) for a specific application.

I think "hybrid" is the best label because that covers all bases.    I sound like a politician. 

Dave.

[konut]

  I enjoy these types of discussions, but do not often participate. They are the result of a fertile imagination and having too much time on ones hands.
    On the one hand, since there is no amplification circuitry(gain) or buffering, then it must be a passive device. That was the traditional classification that was used.  On the other hand, since there is a part of the device which requires either AC or DC power for the device to work then it must be an active device. As with any activity where old devices are put to new uses, and new technologies are applied to old problems, some degree of overlap occurs. Hybrid is the perfect word. Kudos Davey!
      This is all fine and dandy when one is interested enough to actually investigate the specifics of what is going on, but causes problems when trying to communicate with others who lack the understanding of the specifics of the device in question.
      Morten, would it be accurate to say that:
The active circuitry of Tortuga LDRs is only used to control a micro-possessor to determine the equivalent resistor values of a light dependent diode.
       If that is not completely accurate, how would you better restate it?

[tortugaranger]

You sound like a politician. 

Oh gawwwdd! I start my 12 step program on Monday. 

[glynnw]

I guess asking people to decide about a piece of gear based upon how it sounds is just too much to ask.  One of my best friend's favorite quotes is about a conversation he overheard a few years back between two teenage girls who were discussing rock groups.  One said  "I hate White Snake but I love Poison."  Seems to sum up a lot of things.

[tortugaranger]

      Morten, would it be accurate to say that:
The active circuitry of Tortuga LDRs is only used to control a micro-possessor to determine the equivalent resistor values of a light dependent diode.
       If that is not completely accurate, how would you better restate it?

Simplified, it looks more or less like this diagram.

The audio signal passes "passively" through the variable resistance of the LDR insofar as there's no deliberate attempt to do anything to the audio signal except attenuate/modulate it's signal strength going to the amp. Passive in that the power supply does not directly interact or "touch" the audio signal as is the case with a buffer or active preamp with/or without gain.

However, the resistance of the LDR is indeed actively regulated by the software driven control circuitry which adjusts the light intensity of the LDR's internal LED (light source).

Where the "better tasting, less filling" debate comes into the picture is the fact that the active circuit is communicating with the variable resistor optically. What is that? Active or passive?

In my view it stops being purely passive given that artifacts in the active circuit's power supply can be optically communicated to the variable resistor and thus imposed on the audio signal. I'd initially thought that  the power supply couldn't possibly impact the audio quality because any artifacts in the control voltage would get filtered out and not be "seen" by the audio signal. It appears I was wrong about that.

Optically isolated or not, that old rule in audio that it's all about the power supply applies here as well.

How about actively controlled optically transmitted variable resistance passive preamp?  ACOTVRP(squared).     Nah. Passive Preamp. 

[tortugaranger]

I guess asking people to decide about a piece of gear based upon how it sounds is just too much to ask. 

So I've discovered. I've had people tell me that unless I could show test data or specifications that showed how the LDRx preamps were better than other preamps that there was no way they'd buy one. Wouldn't even audition one. 

Aside from how you design a system around LDRs, I think the "magic" behind LDRs comes down to the physics of the photoresistive materials used in their construction and thus their inherent ability pass audio signals with fewer downsides compared to other resistive materials. And while it would be interesting to find our why that is (I've not found that explanation btw), I frankly don't care how they work or why they're better, just that they do and that they are. All you have to do is listen! 

[Davey]

You're side-tracking your own thread. 

These LDR-based attenuators may have some "magic" sound associated with them, but that's a different topic than whether the implementation is active or passive, is it not?

In the "audiophile" world, I don't think passive or active or hybrid operation is correlatable to the subjective sound of any device.  Unless an audiophiles preconceived notion agrees with what they hear.  Then, it IS correlatable. 

Cheers,

Dave.

[kernelbob]

I agree that the evaluation of the preamp/controller/attenuator (pick your term) should be based solely on its performance, not on whether it is active/passive/etc.

As far as whether the Tortuga devices are active, passive, or some hybrid, I look at what is happening to the signal path.  My understanding is that the only active circuitry in the Tortuga is setting the light level that, in turn, sets the LDR resistance to a determined fixed level.  There is no manipulation beyond the setting of that value on the audio signal.  To my mind, the attenuation of the signal path is exclusively passive.  [please correct me if I'm missing something here]

If that changing of the signal path resistor value is considered "active", then wouldn't the attenuating mechanisms in the variety of passive attenuators accepted to be passive via switching from one resistor (or network) value to another via a potentiometer and/or selector switches also have to be designated as "active"?

I'm more interested in how/whether the elimination of switches and potentiometers from the signal path impacts sound quality.  Another variable in the mix is the sound quality of the LDR resistors themselves compared to top shelf fixed value resistors.  As I said, the proof will be in the listening.  I'm looking forward to it.

[werd]

The biggest advantage of a passive is the absence of a power supply. All power supplies add noise, in or out of the direct signal path.  I look at this LDR passive as an active for that very reason.  I am sure it sounds great but it defeats the greatest advantage in system strategy for using a passive. It needs to plug in.

[tortugaranger]

The biggest advantage of a passive is the absence of a power supply.

I'd argue that the biggest advantage of a passive is that a well executed passive can sound better than most actives and perhaps as good as even the best.....and for a heck of a lot less money. 

If the mere presence of a power supply is your litmus test for passive vs. active...then......it's an active...and we can all go home. My view is it's not the presence or absence of a power supply but what that power supply is doing if present. If it's amplifying or buffering the audio signal, it's clearly active. If it's there simply to control a variable resistive element, I think it's fair to characterize it as an actively controlled passive. For example, a motorized Alps pot controlled via a remote is still fundamentally a passive attenuator, the presence of a power supply notwithstanding.

All power supplies add noise, in or out of the direct signal path.

Indeed.

I am sure it sounds great but it defeats the greatest advantage in system strategy for using a passive. It needs to plug in.

If it does indeed sound great then I'd argue the system strategy for using a passive (or any preamp of any type whatsoever) is to accomplish that end. Plugged in or no. 

[werd]

^^^^^^^^^^
What my TVC did was give me an excellent view into my front end components. They tend not to play boss in the sound of your gear and ultimately your soundstage. The main reason is the lack of any powered circuits in dc. Or amplified gain stages.

if you want to hear what your dac sounds like get a TVC passive ( this isn't criticism of yours in any way btw). Its the main benefit for a passive imo. You get to hear your dac or source in its purity.

Now a hybrid such as yours may offer great results as do many line stages but it doesn't pass the "litmus test"
for a passive. Not that it matters if it sounds good.

That's how I look at it.

[tortugaranger]

if you want to hear what your dac sounds like get a TVC passive ( this isn't criticism of yours in any way btw).

What has frankly surprised me is several of our customers who own (or did own) various TVCs, have reported unequivocal better performance with our LDRx Passives relative to the their TVCs. Surprised them too!

[glynnw]

I sold my TVC after hearing the Tortuga.  Never regretted it.

[Randy]

I have a TVC, too, but plan to get rid of it once my balanced Tortuga gets here. Yes, I auditioned the Tortuga, directly compared it with the TVC, etc. It was no contest, even if I had to plug it in.  My amp is battery powered, so I prefer to be off the grid, but I am sure this won't be an issue with the Tortuga.

[Shakeydeal]

What has frankly surprised me is several of our customers who own (or did own) various TVCs, have reported unequivocal better performance with our LDRx Passives relative to the their TVCs. Surprised them too!

Count me as one. I have owned TVCs using S&B transformers and the Sonic Euphoria PLC, using autoformers. Both designs are very good and easily beat most active preamps. The LDR, however, is king of the hill for me.

Shakey