Passive Volume Control for DACs/Sources

Discussion in 'Digital: DACs, USB converters, decrapifiers' started by Madaboutaudio, Dec 20, 2015.

  1. GoodEnoughGear

    GoodEnoughGear Evil Dr. Shultz‎

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    For almost 24x the price we might expect a bit more though.
     
  2. allegro

    allegro Friend

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    The link is not easy to find but if you are not interested in DIY but a finished Slagle manual or remote autoformer passive preamp see MyEmia. Another interesting site compares the Truth with the Emia and other passives.
     
  3. bazelio

    bazelio Friend

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    I've built a DIY Emia. Find it earlier in this thread. This is what I've compared to the truth.

    I've found specs for the Truth today also, and it can't be an entirely passive signal path after all. I was probably mistaken. Tomorrow I'm going to open it up and have a look.
     
  4. allegro

    allegro Friend

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    Thanks @bazelio. If I don't like the Tortuga I will probably go for a Truth ($975) or a manual Emia Autoformer ($1250) but would email either [email protected] or [email protected] to check on availability and time to build before proceeding. Even though it has some quirks you seem to prefer the Truth to the copper Emia.

    Edit: the most recent review (November 2018) of The Truth preamp I could find is at this link. Also, the design appears to be in flux so the most up to date info can be found at the Horn Shoppe Forum.
     
    Last edited: Nov 25, 2018
  5. Thad E Ginathom

    Thad E Ginathom Friend

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    This is beyond my technical knowledge, but I have a preamp with two outputs, passive and buffered. The passive path is truly passive, and needs no power; the buffered path requires mains electricity. I was wanting to use it as my single volume control, but that just would not work well with a headphone amp on the passive out. I guessed that this was due to variable impedance, because it was fine on the buffered out. So it might matter in the real world.

    (My McCormack TLC1 is getting a makeover: trying to fix/replace the rough volume control and replacing the connectors which are all in a bad way. It won't be audiophool-price components, which most people would probably use for something that cost USD1,000 20 years ago, but I'm sure it will be good enough for my ears.)
     
  6. GoodEnoughGear

    GoodEnoughGear Evil Dr. Shultz‎

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    The rule of thumb is input impedance on the amp should be at minimum 10x the output impedance on the source. Many active monitors are a lowish 10kOhm input impedance and so a 5kOhm source (as such an attenuator is presenting to the amp) creates a problem.
     
  7. allegro

    allegro Friend

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    I had a nice conversation with Ed Schilling today and placed an order for a Truth preamp, expected delivery in about two weeks. He is customizing my Truth with a second set of outputs (only one is standard) and all OCC copper wiring. I will be able to make a side by side comparison of The Truth to the Tortuga LDR3.V25 and hopefully end any controversy on whether that negative article by Neurochrome is correct or just a hit job by a competitor. Both units can be returned for a full refund so no funds at risk. Ed told me he has never had a Truth returned, and he stays busy building them while his dad builds the speakers.
     
  8. k4rstar

    k4rstar Britney fan club president

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    Ed is very kind and a pleasure to deal with. The Truth would be best classified as a 'hybrid' line stage, as the actual portion for volume control ('photo cells') are not technically in the signal path. It is definitely not passive though as there are uber-simple solid-state input and output buffers (each with their own power supply!) which take care of impedance matching and ensuring the source presents enough energy to the load. This is great because it ensures compatibility in virtually any system where sufficient gain is present, whereas truly passive devices like the Slagle may have weird interactions with certain DACs and phono sources; irrespective of gain and impedance.
     
  9. bazelio

    bazelio Friend

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    Opened the TRUTH. The link above shows two PS boards, whereas mine has one.

    Overall, pretty simplistic. Input -> buffer -> photocell -> buffer -> output. The buffers are in metal can packages, maybe TO-5. I can't tell what type of devices are being used as buffers since Ed has glued heatsinks to them and (probably intentionally) hidden their part numbers. They each have 7 leads and pin 2 is snipped and left unconnected. Any guesses as to what these devices might be? Pin 2 is usually the inverting input for op amps, but then these are only 7-pin devices. The specs for the preamp are as follows:

    Input impedance is too high to measure at 6 trillion ohms.
    Output impedance is ~3 ohms.
    Slew rate is 200 volt/microsecond.
    Bandwidth is unrestricted from DC to ~60 Mhz (yes, Mhz).

    I'm now curious how AC measurements would look.

    Internals:

    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]
     
  10. bazelio

    bazelio Friend

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    In regards to (1), where did you get this information? The photocells being out of the signal path isn't intuitive at all. The actual volume pot is out of the circuit, as it controls the intensity of the LED. I just buzzed out the circuit. Per channel, it is as follows:

    Input -> mystery op amp pin 3 (non-inverting input) -> mystery op amp pin 6 (output) -> VT800* series photoconductive cell -> next mystery op amp pin 3 -> next mystery opamp pin 6 -> RCA output.

    The TRUTH is nothing more than a buffered variable resistor signal path implemented with a photo cell, with less than unity gain. I'm not even sure what value the photo cell adds to the circuit to be honest.

    In regards to (2), what are you talking about? Please be specific.

    *As best I can tell
     
    Last edited: Nov 25, 2018
  11. purr1n

    purr1n Desire for betterer is endless.

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    Cool. I'm so happy that somebody reinvented the definition of passive. I didn't know active devices (chips in tin cans) running without gain are now known as passive devices. This is as hilarious as people saying certain R2R discrete ladder DACs process DSD natively.

    FWIW, the stuff in the tin cans does sound better than the plastic packages.
     
    Last edited: Nov 25, 2018
  12. Cspirou

    Cspirou They call me Sparky

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    Tom is very opinionated but I have no reason to doubt his measurements.
     
  13. purr1n

    purr1n Desire for betterer is endless.

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    Agreed. @tomchr did a great job on the measurements, looking at realistic use cases (consumer and pro-DAC outputs, typical volume knob positions) and the distortion from different angles (frequency sweep, spectrum analyzer, and level sweep). The picture is pretty complete. I'd trust his measurements on his AP-525 one thousand times more than Amir with his AP-555. He didn't say anything bad about the product and simply left it as people "draw their own conclusions from the data."
     
    Last edited: Nov 25, 2018
  14. Hrodulf

    Hrodulf Prohibited from acting as an MOT until year 2050

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    AFAIR, they use Light Dependant Resistors rather than photovoltaic cells. With that said, it would’ve been insanely exotic if they would opt for a signal driven light source shining on a photovoltaic surface and then attenuate optically with a variable natural density filter.

    I don’t see much of a reason for @tomchr to try assassinating Tortuga. They both cater to different lines of thought. I wouldn’ t shy out of calling his measurements “damning”. I have a fascination with engineering that approaches the theoretical limits of performance, but at the same time I can appreciate an interesting idea to solve a problem. Tom hasn’t been the first to point out the inherent linearity problems with LDR’s, to my knowledge it was actually Pass who posted measurements on DIYA. As for Tom’s measurements, they show a level of performance I deem unacceptable for any device participating in a high fidelity playback chain. The overall distortion levels would be acceptable for a transducer, but the harmonic structure with 3rd order dominating is a “no from me, dawg”.

    P.S. That Truth preamp looks like someone’s first DIY project with modules scraped from eBay. Bare mains wiring is asking for trouble and the ungrounded case is the f'ing sugar coated “pretty please”. Flying untwisted signal wires, Lumpy Space Princess school of soldering and the really class act of gunking the opamps... You don’t have to be Dr. Gilmore or an Icelandic baker to hate this kind of practice.
     
  15. Armaegis

    Armaegis Friend

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    I know if you dig hard enough, you'll find a couple posts with Schiit bashing on the opto/LDR/blahblah as well, though no specific names were ever given.
     
  16. allegro

    allegro Friend

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    Has anyone ever seen another manufacturer besides Neorochrome present reviews with a negative tone on their home website for other competitors' products? Under "Resources > Measuring Your Gear" Neurochrome not only critically reviews the Tortuga, but also the MiniDSP 4×10 HD, O2 Headphone Amp, and Sjöström QRV08 current feedback headphone amp. In the Sjöström QRV08 review he says:

    That is damning with faint praise. Makes you wonder what comes next, negative reviews of Schiit gear by Neurochrome?

    I am not a member of the subset of the audio enthusiast community that believes gear has to measure well to sound good and dismisses any gear with less than stellar measurements. That being said, if I had read that negative review of the Tortuga before I ordered one it would have given me pause ... I probably would have passed. Since i have one on order I am now really intrigued to see if my ears agree with the measurements.
     
    Last edited: Nov 26, 2018
  17. purr1n

    purr1n Desire for betterer is endless.

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    Fricking Amir has only made the measurement obsession worse.

    What constitutes a good THD profile? I'd say somewhere between 0.1 to 0.01 is good enough. Other considerations are the characteristics (even to odd order) and high order.

    There is stuff that measures like shit. Then there is stuff that measures good enough where humans cannot hear it under normal circumstances. Then there is stuff that only AP-555s or AverLABs can detect.

    Not too tight and not too loose. Too loose is well... Too tight, and it becomes religious fundamentalism.
     
  18. bazelio

    bazelio Friend

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    @tomchr doesn't even have a competing product, does he? It really didn't read like a malicious review to me.

    The Truth, yep it looks like a 5th grade science project on the inside and outside. But it doesn't sound like one. When I get a chance, I'm going to breadboard AD826 -> resistor ladder -> AD826 both in unity gain. Feel free to suggest a more appropriate device. I'll bet it sounds very close to the Truth. Why shouldn't it?
     
  19. Cspirou

    Cspirou They call me Sparky

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  20. bazelio

    bazelio Friend

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    Oh shit. That bastard!

    I guess if you want to nitpick, one is balanced and differential only while the other is unbalanced only. But.... yeah. EDIT: And reading further, looks like you can build the Neurochrome either way... So much for that thought.
     

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