What to make of these measurements: Tortuga LDR3.V25

[birkbott]

Considering trying the kit for this Tortuga product and came upon these measurements. Are they accurate? It's hard to believe that a product with generally favorable reviews could perform this poorly..?
https://neurochrome.com/pages/tortuga-audio-ldr3

There's no date on this article and no mention of the version (current is LDR3.V25 but it just says LDR3 in the post)

 

[solderdude]

There is talk about LDR volume controls HERE

There is no way around it... LDR's are , besides being light dependent also voltage dependent resistors so will add odd harmonic distortion... period.
It's a conversation piece and effect box which also controls the volume.
For high quality volume control use a high quality volpot or stepper.

Some people like distortion, others don't want it.
The @stereo coffee guy never sent his 'my version does sound good' LDR volume control to Amir to test.

 

[birkbott]

There is talk about LDR volume controls HERE

There is no way around it... LDR's are , besides being light dependent also voltage dependent resistors so will add odd harmonic distortion... period.
It's a conversation piece and effect box which also controls the volume.
For high quality volume control use a high quality volpot or stepper.

Some people like distortion, others don't want it.
The @stereo coffee guy never sent his 'my version does sound good' LDR volume control to Amir to test.

Are there any relatively inexpensive (like unter $1k) preamps that measure really well? I don't see a lot of measurements of just preamps out there.

 

[solderdude]

It is quite difficult to design/build a poor performing pre-amp but some do succeed.

The Schiit Saga in passive mode seems to do really well.

 

[LTig]

Are there any relatively inexpensive (like unter $1k) preamps that measure really well? I don't see a lot of measurements of just preamps out there.

If you don't mind the strange form factor I'd say the best performance/price ratio for preamps are very small analog mixing consoles. For less than € 100 you may find a console with two stereo line inputs including 2 band tone control; some even offer USB IO. The only parameter which may not be excellent is SNR, due to the mixing stage where the noise of all channels adds up.

I bought the Phonic AM-220 for rehearsal and it sounds quite good. It cost me incredible € 55 some 10 years ago. A Mackie 1402 VLZ4 offers 4 stereo inputs and 6 mono inputs (2 mono inputs can be used as 1 stereo input) for € 300.

 

[mansr]

It's hard to believe that a product with generally favorable reviews could perform this poorly..?

No. No, it is not. Sadly.

 

[scott wurcer]

Considering trying the kit for this Tortuga product and came upon these measurements. Are they accurate? It's hard to believe that a product with generally favorable reviews could perform this poorly..?
https://neurochrome.com/pages/tortuga-audio-ldr3
There's no date on this article and no mention of the version (current is LDR3.V25 but it just says LDR3 in the post)

Remember this post is by someone who has something to sell criticizing a competitor. I would like to see some DBT tests of the perceptual properties these devices with low order distortions that vanish at the origin (no discontinuities) rather than just the usual measurements. You won't find a speaker even close to the performance of SOTA electronics.

 

[pozz]

@tomchr wrote the article. He's put together some excellent gear as well. I'm sure he can delve in to the specifics of when and why he wrote it.

 

[Blumlein 88]

Having tried to use Light sensitive resistors for volume control I find these results not at all hard to believe. Whoever managed these results did pretty well.

 

[scott wurcer]

@tomchr wrote the article. He's put together some excellent gear as well. I'm sure he can delve in to the specifics of when and why he wrote it.

The LDR is a passive volume control no different than a pot or switched attenuator. It's easy to show that the pot or switched attenuator are far better with respect to THD. I don't see any competition here at all or any point in calling it competition. Tom does not sell a passive attenuator AFAIK.

 

[tomchr]

Remember this post is by someone who has something to sell criticizing a competitor.

I don't have a competing product, though. I may eventually offer enough building blocks that you can make a preamp using my products, but I doubt I'll do anything beyond an Alps "Blue Velvet" pot for the volume control. There are plenty of solutions out there for volume control, including some really bad ones.

I would like to see some DBT tests of the perceptual properties these devices with low order distortions that vanish at the origin (no discontinuities) rather than just the usual measurements.

I'd like to see that too. If you recruit the 250 participants and set up and run the experiment, I'll be happy to run the stats on the numbers for you.

One could also read past research on the topic. Belcher (1978) found THD down near -80 dB as being audible for some individuals. He suggested using a 3-tone IMD test as he found it to offer better internal validity. I forget where Olive & Toole landed regarding THD. I do remember that they concluded that better measurements correlate well with a positive listening experience.

You won't find a speaker even close to the performance of SOTA electronics.

No. But the >10% THD of the LDR3 is not SOTA by any stretch of imagination. It's more than 40 dB worse than the THD of my KEF R700 speakers at normal-to-loudish listening levels.

@tomchr wrote the article. He's put together some excellent gear as well. I'm sure he can delve in to the specifics of when and why he wrote it.

The LDR3 was provided by a client who wanted to know how it performed. I was curious as well, so I agreed. This was before ASR was formed - or certainly before I became aware of it - so I posted the measurements for everyone to see.

Tom

 

[tomchr]

The LDR is a passive volume control no different than a pot or switched attenuator. It's easy to show that the pot or switched attenuator are far better with respect to THD.

The LDR is different than a pot or switched attenuator. As you can deduce from my measurements, the capacitance of the LDR is rather nonlinear, and its voltage coefficient is significantly higher. That makes the LDR a much, much worse choice than a pot or attenuator for volume control.

You can measure the voltage coefficient of the LDR pretty easily. They're cheap on Mouser/Digikey/et al. Measuring the distortion of resistors is a little harder in that it requires a distortion analyzer. I've measured the THD of thick film SMD resistors versus thin film SMD resistors. The thin film perform better. Some are within measurement tolerances of leaded metal film resistors.

I don't see any competition here at all or any point in calling it competition. Tom does not sell a passive attenuator AFAIK.

Correct. I don't offer a product that competes with the LDR3 and I have no plans to.

Tom

 

[scott wurcer]

The LDR is different than a pot or switched attenuator. As you can deduce from my measurements, the capacitance of the LDR is rather nonlinear, and its voltage coefficient is significantly higher. That makes the LDR a much, much worse choice than a pot or attenuator for volume control.

You're missing the point this is all well known (though I doubt the capacitance figures in at all). These criticisms of the Pass Labs First Watt posse or the SET amp crew are just farts in the wind. Why do you care? Why can't you take your "I have -130dB SINAD stuff" and leave others alone.

 

[tomchr]

As you can tell from the date in the plots, I measured the LDR3 nearly three years ago. The measurements have been available, peacefully and without incident, since December 2016. I provided the measurements because I believed it would be in the public interest to have them available. I believe information is power, and more information is better. Would the LDR3 have measured any better had Amir tested it? Would you have been as upset had he published the results?

I firmly believe in science and in the self-correcting nature of the scientific process. The science is pretty darn clear: On average, in scientifically controlled blind testing, audio equipment that measures well tends to be rated as better sounding. One of the reasons I enjoy this forum more than other forums is that science tends to be valued here.

And in an attempt to get back on topic: If the capacitance of the LDR does not change, how do you explain that an attenuator made with two LDRs in series has different bandwidth depending on the attenuation setting?

Tom

 

[Blumlein 88]

snip.....
One could also read past research on the topic. Belcher (1978) found THD down near -80 dB as being audible for some individuals. He suggested using a 3-tone IMD test as he found it to offer better internal validity. I forget where Olive & Toole landed regarding THD. I do remember that they concluded that better measurements correlate well with a positive listening experience.
snip.....


Tom

Which Belcher paper did he discuss this and the 3 tone IMD. I've seen a few Belcher papers on distortion from those years, but not one with that info in it.

 

[tomchr]

I could have the year wrong. It may have been his earlier work. 1974? I'm pretty sure it's in Belcher (1978). I could also be mis-remembering or flat-out wrong. That's happened before. I'm human. A quick glance at the 1978 paper looks promising, though.

I'll take another look at the paper tomorrow. I need to go play hockey now. Teams tend to appreciate it when the goalie shows up on time.

Tom

 

[amirm]

You're missing the point this is all well known (though I doubt the capacitance figures in at all). These criticisms of the Pass Labs First Watt posse or the SET amp crew are just farts in the wind. Why do you care? Why can't you take your "I have -130dB SINAD stuff" and leave others alone.

??? Leave people alone? People want clear, comparative information. People are not technical to know the true performance of things. I know when I read the LDR measurements from Tom, I found them useful. It is not like he charges us for it and we are unhappy with what we got.

 

[Blumlein 88]

I could have the year wrong. It may have been his earlier work. 1974? I'm pretty sure it's in Belcher (1978). I could also be mis-remembering or flat-out wrong. That's happened before. I'm human. A quick glance at the 1978 paper looks promising, though.

I'll take another look at the paper tomorrow. I need to go play hockey now. Teams tend to appreciate it when the goalie shows up on time.

Tom

Okay, I'd read that one earlier this evening. I didn't find them suggesting 3 tone IMD. Which is why I thought I had the wrong one.

Just came in from watching the Unicorn meteor shower which didn't live up to expectations. Darn.

 

[solderdude]

I am quite sure that if I would design a pre-amp using an AD797 and regular volume control the performance would be much better than when the same circuit and using LDR volume control.

The problem is that the LDR volume control performance becomes worse when the volume is turned down.
With SET and ACA amps the performance becomes worse when the volume is turned up.

See it this way.. when one listens to music just to enjoy it your volpot is usually not turned all the way up but rather all the way down.
For a relaxed listen we do not listen at loud levels so the SET and ACA will actually perform decently to good and audibly transparent.

With the LDR volume control turned down to 'relaxing' listening levels the distortion is highest. The opposite of said amps that only perform best at low levels.

Ultimate effect box may be the balanced ACA + LDR Tortuga so you can have odd harmonic distortion added at all listening levels that reaches audible levels. That is if one is into this kind of things and enjoys such over 'less modified' music.
Otherwise a regular pre-amp (with or without the AD797) and a < 0.01% distortion amplifier is all one needs for music listening.

 

[Daverz]

You're missing the point this is all well known

I didn't know it. I've always seen the LDR attenuators being touted as being superior to pots.

(though I doubt the capacitance figures in at all). These criticisms of the Pass Labs First Watt posse or the SET amp crew are just farts in the wind. Why do you care? Why can't you take your "I have -130dB SINAD stuff" and leave others alone.

Such a very strange post.