Passive Volume Control for DACs/Sources

The Neurochrome preamp isn't really fully differential. The chain is Bal/SE -> ALPS Pot -> SE/BAL. Input and output are THAT chips (hence the name)

 

I don't think he's luring away any of the exotic attenuator fans with an Alps Blue Velvet. With that said, there are 10kUSD Nagra preamps which use the same pot. Me, I say that no pot sounds like no pot, so I attenuate digitally. Maybe I'll make an attenuator box with a 24 level TKD, so I can keep my digital high.

 

Digital attenuation also has problems itself since its basically interpolating and truncating data. While this might not show up in a thd measurement, it is reducing the quality.

 

Decimation is reducing the sampling rate. Interpolation is used to increase the sampling rate. Neither are concerned with volume. In principle though you are correct-ish, you will throw away bits past a certain point depending on the implementation.

 

You're right i meant truncation. I think there is interpolation though since you have to create new samples with a higher bit depth from the original say 16 bit samples, hence all the marketing with "64 bit lossless volume control". And then theres dithering and noise shaping, etc... In the end its ultimately based on whether dithering and interpolation is more transparent than analog attenuation when both measure well. And now we're getting into personal philosophies.

 

No offense, but the better answer is "Crap, I'm actually not really sure about these details, let me go and read up on it." instead of publically guessing more noise. I'm pretty sure @Hrodulf has a good grasp of it.

 

None taken. I honestly want his take on it too, since eq software and all that is digital attenuation...

 

I just listen and have no engineering background like many here but I do have a question. Is it possible that the distortion produced by your amp, headphones, speakers or records (if you listen to vinyl) make the less than great THD measurements of the Tortuga published by Neurochrome irrrelevant?

Neurochrome sells a Damn Good 300B SET amplifier with published THD up to 3% and says that:

 

Point taken @ultrabike thanks. I may wind up with a Slagle Autoformer but I have the money and time to test out the Tortuga and Truth for myself first. Both are refundable so no money is at risk and I am curious to hear for myself. The Truth may look worse than the Heathkit ham gear I built as a teenager but Arthur Salvatore likes it and I have found his advice to be solid over the years.

 

Here is what the Tortuga LDR3.V25 Passive Preamplifier looks like. Looks to be well made, fairly heavy, three SE inputs and two SE outputs:



Comes with an already paired Apple remote. 15 minutes after the last user command from the remote the display goes into screensaver mode and a Tortuga repeatedly slowly swims across the display until the remote is used again Too early for listening impressions need to leave it on a few days.

 

That's a rather truncated version of the truth. I measure THD vs output power to clipping, which I have decided to be 3% THD at 50 Hz. Many other vendors use 5% or even 10% THD as the clipping threshold.
The spec table includes figures for 1 kHz @ 10 W: 1.5 % and 1 kHz @ 1 W: 0.25 %. All of these measurements were done with 20 kHz measuring bandwidth.
You can see the graphs about halfway down this page: https://www.neurochrome.com/300b-set-amp/

Truncation should degrade both the THD and IMD.

A "passive preamp" (aka volume pot) is pretty hard to beat. With a low-noise buffer on the output you can make a pretty robust high-performance preamp.

Tom

 

I had already sent my Tisbury passive preamp back for a refund so all I have at hand right now is a Schiit Sys and the Tortuga. After three days of warm up at first listen the Tortuga is enjoyable to listen through, transparent and dynamic. I put the Schiit Sys back in for comparison and noticed a slight veil over the music with less precise imaging, and loss of sound stage depth. Found some tracks on Tidal and Quobuz recorded at lower levels so I can compare the Tortuga to running the Yggdrasil > Loki > Kenzie with the volume control turned up to at least 9 AM. Noticing some coloration with the Tortuga I will be able to comment on later after more listening.

 

Or a unity gain Grado RA1...