Tortuga Preamps

I purchased a while ago, the Luminous Audio Axiom, Series 2, with Walker Mods, 3 in, 2 out, single ended, with remote, and I love it. It is not LDR based. Enjoy ! MrD.

I have owned two LDR preamps. Extremely happy with mine.

Oz

The data seems to indicate significant distortion

Been there and done that with "active forced matching".

I use "quad matched" sets of the far more expensive NSL32RS2S "sorted" LED/LDR’s which also go lower in impedance than the non "S" version, then none are "over rev’d" as they all "rev" the same.

By using "active forced matching feedback type circuitry", which btw measures the the output resistance of the ldr’s, makes it an active preamp not true passive.

KISS: spend the "man hours to quad matched them" at different points, and then they all "rev" at the same, and also stay totally passive, and sound better for it.

https://www.digikey.com.au/products/en?keywords=NSL-32SR2S

https://www.digikey.com.au/product-detail/en/advanced-photonix/NSL-32SR2/NSL-32SR2-ND/5039808


Cheers George

George, 

Sounds interesting! Did you swap out these LEDs in a build or preexisting Tortuga. Please elaborate:-)

Does quad matching lower the distortion?

George manufactures the Lightspeed Attenuator. One of the the best bargains in audio. I have owned one for 10 years it seems. My old friend Pubul57 (RIP) started this thread years ago to discuss the Lightspeed:

https://forum.audiogon.com/discussions/lightspeed-attenuator-best-preamp-ever?highlight=lightspeed%2...

If only the Lightspeed had 3 inputs......I would sacrifice a " little " in sq for the convenience. George feels it sacrifices a lot in sq, and who am I to doubt him. Likely the best passive out there. Which, to me, means, it is the best, as I no longer hear the benefit ( s ) of an active preamp ( ime, imo ). However, my Luminous is quite " invisible ". YMMV.....Enjoy ! MrD.

If only the Lightspeed had 3 inputs......I would sacrifice a " little " in sq for the convenience. George feels it sacrifices a lot in sq

Like I say to all that ask for a multiple input Lightspeed, you can always get one of these to put in front of it, http://www.goldpt.com/sw4.html that way you still have the option to leave it out, when you want seriously listen without any source switching in the signal path.
As that’s what the Lightspeed Attenuator is all about, "no contacts at all in the signal path", volume control or input switches, from your source to speakers

Cheers George

George, I had thought that the Tortuga's calibration of the LDR's LED control current curve is invoked periodically by the user on an as-needed basis, not "actively" or on-the-fly.  If it's performed only periodically, how will this degrade SQ? 

Recluse, I enjoyed mine very much. It is very open and transparent. I was surprised to see that it measured so poorly. I have gone back to my active preamp due to selling my separate phono stage.

Love my Tortuga !

It’s an active circuit attached to the output, which is the audio signal, and not being match need calibrating at every increment of volume adjusted to stay calibrated.
If you have "say" 3 led/ldr’s (could be two or one, as they are not matched) of the 4 led/ldrs running at "x" impedance, the other/s may get over worked trying to keep up and visa versa which shortens their life span.
Also you need to use the NSL32SR2S to get low enough impedance to get the lowest volume.
Just look at the data sheets if you can understand them, of all that I have posted up many times here and at DiyAudio, and look at max 20mA brightness for the lowest impedance attained.

Cheers George

scm
Love my Tortuga !

That’s good stay with it, they are still way better than any other form of passive or active volume control.

Dartzeel use them the same way I do as their "Pleasure Control" in their $18k preamp
https://www.stereophile.com/solidpreamps/607dart/index.html
Half way down the page,https://6moons.com/audioreviews/dartzeel2/preamp_4.html explains the "Pleasure Control" (Lightspeed volume control), patents were never granted, I saw to that being the originator back in the 70’s 80’s

The only thing that can better them and the Lightspeed Attenuator, is direct connection between source and poweramp using the sources digital domain volume control, but these must be used at or above 75% to avoid any "bit stripping" IE: 16bit resolution down to 14bit and so on the lower you go with them.

Cheers George

Thanks for the elaboration,
Sounds intriguing.

Are these still being made and what is the link to purchase? 
Are these offered with real balanced connections.?

https://www.tortugaaudio.com/

Are these still being made and what is the link to purchase?
Are these offered with real balanced connections.?

Yes send a PM or there are many other ways just Google, or if you have the diy skills build one for yourself, as I've given all the instructions here.  
https://www.diyaudio.com/forums/analog-line-level/80194-lightspeed-attenuator-passive-preamp.html

And no I don't believe in xlr, unless your in a studio with with lots of noise problems, extra long runs of interconnects.
My customers use these if they need, XLR to RCA adapters, Neutirk being fair value for money, some are much less expensive 
 https://i.ebayimg.com/images/g/wrMAAOSwDk5UBLdo/s-l300.jpg

Cheers George 

I try desperately to stay out of the subjective vs. measured debate...it gets so old and tired.

I try to keep it simple. Do you like what you hear? Yes? Stay with it.

Don’t like what you hear? Try something else.

Is that so hard?

I know people read all these things about what’s good and what’s not and what are they to believe? Please, tell me what to believe!!!!

The neurochrome report screams one thing and one thing only in my view. It does a very good job at making the point that measurement (beyond a certain threshold point) is simply not relevant or informative.

One would think that neurochrome’s measurements would indicate that the Tortuga model that was measured would sound awful. It does not!

Moreover, we are several iterations beyond the model measured by neurochrome and it only gets better.

Trust your ears.

Cheers,
Morten

Regarding "active forced matching" mentioned above, the way it's described in this thread is NOT how Tortuga Audio implements this feature. During normal operation of the passive LDR preamp there's zero active matching going on. That measurement circuitry is disconnected and inactive when you're using the preamp. What we call LDR calibration or "auto calibration" only takes place when the preamp is essentially offline. Inputs and outputs are disconnected and the preamp operates in isolation during the calibration process that typically takes around 10 minutes for all 4 LDRs. This process is manually initiated by the user. I think it's fair to say that this calibration process is 90% all about the software behind it and only 10% about the hardware configuration. It took a boatload of software development work to sort this feature out and get this working smoothly. As a result, if/when an LDR goes bad, goes far enough out of spec limits, or simply shifts its performance curve, it's a simple matter of unplugging the bad LDR, popping in a new one, running a calibration cycle and Bob's your uncle ;)  I do agree that it would have been far easier for us to use pre-matched LDRs which we did do in the beginning. Easier for us but not so much the customer  if/when an LDR goes south. ;)