[doctorcilantro]

What's the input sensitivity of the LDR? Is there any flexibility here or is it expecting 2v?

[kernelbob]

I don't think sensitivity is applicable here since the LDRs are passive-- no gain, only attenuation.  The volume you get at the output of the power amp is dependent on the gain of the power amp and the output voltage of the source device such as the DAC or phono stage.

Before I purchased the Tortuga LDR1-V2B, I was concerned how it would mate with my power amps since they have a lowish 25 kohm input impedance.  My speakers are rated at around 89 dB @ 1 watt, 1 meter.  I have no problem getting all the volume I could want and typically have the Tortuga set to the mid 30's, occasionally in the low 40's in my system.  I've tried the Tortuga in various other systems and never needed to turn up the volume setting above 50 to get levels that were overloading the rooms.  Given that the max level is 70, that gives lots more output if needed.

[doctorcilantro]

thanks, this is why I asked. I had an inkling of this, but it's all over my head. I'm looking at the Decware ZBIT and the Empirical Audio Final Drive units as I really want to use the ORFEO based balance doutput on my Vega DAC.

Waiting to hear more from Morten.

cheers!
dc

[doctorcilantro]

Morten will probably chime in, but he did mention to me that the LDR can become overloaded at some point.

I found out my Vega DAC's SE output is the same as XLR, 4v, so I am going to try XLR>RCA adapters first.

[kernelbob]

I had the same question on overloading.  I'm running the PS-Audio PerfectWave DAC MkII (still shopping for a replacement).  That DAC's output is quoted as 10 volts, unusually high for a DAC.  I've had no problem using the Tortuga LDR1B (or the LDRxB which I also auditioned) with that input level.  I'm running my system balanced throughout.

I've also tried the Tortuga in other systems with DACs rated in the 2V output range.  The only difference was that those systems required a bit less attenuation.

I've found the selectable input impedance to be very useful to get the most out of a system.  For mine I using input impedance settings of 7, 8, 10, 15, and 20 kohms.  Given that the impedance that the source sees using the balanced input is doubled, that's actually 14, 16, 20, 30, and 40 kohms.

Using the PS-Audio DAC as source, the 20 kohms setting is a good middle ground.  At the lower impedance settings, the bass becomes a bit more full and articulated with the overall sound becoming more lush.  Below 14 kohms more harmonics are present though I suspect this may be a bit higher harmonic distortion in the DAC when loaded into low impedance.  At the moment I'm using 14 kohms input impedance (shown as 7 on the display).  At the 30 kohm and higher settings, the bass is leaner.  At 40k the soundstage starts to have a bit less depth.

I expect that an optimal impedance setting will vary depending on the system in which the Tortuga is installed.  It's a great feature that this can be interactively customized and controlled by the remote.

[tortugaranger]

My understanding of the term "sensitivity" in audio is that it's synonymous with voltage gain or just gain for short. For example an amp with ~22.6 V/V sensitivity (20*log(22.6) = 27 dB) connected to a preamp with a 2 V RMS output would produce 22.6 x 2 = 45.2 V RMS output (max) assuming the power supply had sufficient headroom and doesn't clip. Connected to an 8 ohm speaker load, your get 45.2/8 = 5.65 amps which equates to a power output of 45.2*5.65 = 255 Watts. Which would be quite a monster noise maker with most speakers.

Back to LDRs. Since LDRs are variable resistors they really don't have sensitivity per se. At least not as defined above. Or perhaps it's more accurate to say they have a sensitivity of 1 or even slightly less than 1 since there's always some loss through a resistance device but this has a lot to do how they're configured and used within an attenuator.

However, LDRs do have limitations in terms of V RMS input levels. All LDRs exhibit increased distortion as you increase the voltage drop across their photoresistor. So for example if you have a hot source punching out 5 V RMS line compared to a more nominal 2 V RMS, then there will be greater V drop across the LDRs to achieve the same dB level of output from an LDR attenuator. In absolute terms, the distortion will be higher as a result of using a source with a higher line stage voltage (all other things being equal).

I don't know at what level the distortion level would become an issue with LDRs but I do know that in the 2-4 V RMS range it's not a problem subjectively - meaning whatever distortion is there  hardly matters if what matters to you is how it sounds. The manufacturer's info I have on the LDRs we use provide distortion info up to 5 V but results are highly dependent on how the LDRs are configured (attenuator topology).

One thing I find both amusing and irksome at the same time is you can easily find LDR naysayers among the online audio cognoscenti declaring them entirely unfit for audio due to their published distortion specs. No doubt they believe this to be true. And no doubt anyone who actually owns and listens to an LDR preamp will shrug at this, smile and carry on enjoying their LDR preamp. Another example of the "multiple parallel universe" (MPU !!!) syndrome that's all too common in audio land.

Final point. To the extent high input V RMS were ever to become a limiting issue with LDR attenuators, the "fix" as it were would simply be to add additional LDRs such that the V-drop across each LDR is reduced (cut in half) to an acceptable level. This is more critical for LDRs in series and less so for LDRs in shunt. Adds complexity but entirely doable.

Best,
Morten