Passive Volume Control for DACs/Sources

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

  1. gridmaster

    gridmaster Acquaintance

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    Funny you guys mention the Promithues Reference tvc. I actually ordered one a week ago.
    I'm supposed to get it 2-3 weeks, or something like that. I was super on the fence about ordering it because I read on forums the guy is super sketchy, and people waited for over a year for their orders....

    If I end up getting it anytime soon I'll post some impressions here. I actually didn't even buy it for the preamp capabilities, I wanted a good XLR balanced to RCA unbalanced converter haha. The volume control is def a plus tho.

    It is one of the cheaper TVC out there, why I bought it. Guy has an extensive background in transformer design so I assumed it might be good. Who knows.
     
    Last edited: Apr 4, 2019
  2. Thenewerguy009

    Thenewerguy009 Friend

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    A year? The design for it isn't super complicated.
     
  3. gridmaster

    gridmaster Acquaintance

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    It wasn't the build time, apparently according to the forum the guy just dipped on a sabbatical for a year leaving open orders unfilled. The guy responded to me promptly in emails tho, 3 or 4 of them, only reason I ordered it. But ya, most of the review links on the site are broken, no reviews or forum posts on his stuff from the last decade. I was surprised when he emailed me back tbh.
     
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  4. ogodei

    ogodei Friend

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    Good to hear. I sent him a custom request for a quote.

    Located in Kuala Lumpur per the old write up.
     
  5. mitochondrium

    mitochondrium Friend

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    Oh yeah I understand, wait a second let me have a look where I put my gear:


    [​IMG]

    [​IMG]

    works a treat without a remote, each to their own;)
     
  6. Thenewerguy009

    Thenewerguy009 Friend

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    Any updates, it should have been 3 weeks by now.
    Did you get any kind of order update?
     
  7. gridmaster

    gridmaster Acquaintance

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    Tracking says I get it this Wednesday. I'll update the thread with some impressions.
    I'm much more versed in DAC/Amp impressions tho, I only have 2 preamps (this one being my third). Goldpoint SA1X and Luminous Axiom II.
     
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  8. skem

    skem Friend

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    @Cspirou
    What’s FPE stand for?
     
  9. Cspirou

    Cspirou They call me Sparky

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    skem likes this.
  10. lcmusiclover

    lcmusiclover Almost "Made"

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    I look forward to your assessment of the Prometheus. Looks like you still own the Luminous? It'll be interesting to find out if there's any significant difference between the two. I mean passive preamps should be as close to unnoticeable as possible, right?

    Did you find much to differentiate the Goldpoint and Luminous? Those were the other two options I considered when I was shopping passive preamps, along with Hattor/Khozmo -- ended up with a Hattor Mini.

    As for only having owned two passives, that's 2 more than most folks have even heard, let alone owned. I've only owned the Hattor (along with my Emotiva XSP-1 active preamp).
     
  11. rlow

    rlow A happy woofer

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    @purr1n I noticed you’ve mentioned you’re using JBL Nano Patch passive recently.

    How do you find it - does it seem transparent? Are you using TRS to XLR adapters on the outputs? Considering maybe one of these over the Nobsound for going from Yggdrasil to my Vidar monoblocks.
     
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  12. gridmaster

    gridmaster Acquaintance

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    I got the Prometheus Reference TVC in yesterday, so I've had about 4-5 hours of intense A/B and listening on it.
    Take this review as is, it's only my opinions and ymmv.

    Let's get started with the functions.
    When any input is plugged into the TVC, it will output that thru all three outputs at once (1x XLR 2x RCA)
    It can do:
    #1 Input XLR -> Output XLR
    #2 Input XLR -> Output RCA
    #3 Input RCA -> Output RCA
    #4 Input RCA -> Output XLR

    I want to make a quick note on RCA -> XLR, I didn't even know this was possible without active parts (unbalanced to balanced conversion). I'm not sure what it's doing here, I think it's cutting off the equal and opposite wave form or something. Either way, while the first three functions all sound the same within function, #4 makes the sound brittle, harsh, and without dynamics. Not recommended.

    To start off why I even bought the unit. I own an EC AF that uses Cinemags CMLI-15/15B transformers to convert it's balanced XLR input to the native amps unbalanced RCA function. I never really liked the sonic signature the Cinemags imparted, slightly hashed treble, and general wonkiness in that range. The reason I wanted to use the balanced inputs is because I've read on multiple occasions that XLR is the native output of the Yggdrasil DAC, and sounds better than RCA out.

    Here's the trade-offs (for me, again ymmv). Yggdrasil -> XLR -> EC AF had better dynamics, air, and soundstage.
    Yggdrasil -> RCA -> EC AF had better texture, plankton, and sounded more organic. For my personal tastes, I liked RCA better.
    Thus, I thought to myself, can I get the best of both worlds? The dynamics and soundstage of Yggdrasil XLR with the texture and organics of the RCA out? So I bought this TVC. (Note that Yggdrasil XLR supersedes RCA in every way if going to a truly balanced amp without the need for conversion)

    Let me quickly compare the TVC, Luminous Axiom, and Goldpoint in terms of warm->neutral->bright.
    Goldpoint was never my jam, I always felt it imparted a bright signature wherever I used it. Although it was relentlessly transparent. The Axiom has a (very small) touch of warmth to it, with overall exceptional transparency (but a hair shy of the Goldpoint). The TVC is, warm, but not by signature, it's because it's a little bit rolled in the treble. Transparency doesn't match either the Goldpoint nor the Axiom. I wouldn't call it "dark" tho.

    This means that while the the chain Yggdrasil -> XLR -> TVC -> RCA -> EC AF fixed the hashed treble for me, it was overall not as crispy or defined as Yggdrasil -> RCA -> EC AF direct. The XLR still does give slightly better dynamics and soundstage when converted using the TVC, but it created my problem in the opposite direction. Instead of a bit hashy treble, I now have a bit dulled treble.

    As for as direct XLR -> XLR or RCA -> RCA with this TVC, it has the same signature as reported above with XLR -> RCA.
    There doesn't seem to be any audible differences within those three functions. Still has the slightly rolled sound to it with a minute decrease in transparency.

    I also want to make clear, these are all < 1% changes to the sound. I get far greater tonal changes rolling tubes. As with anything tho, it's all things being equal, and using top end gear you're always striving to expand performance within that < 1% range (of course if you're crazy enough to spend $1k on a volume control when you can get a good pot for $30).

    My take aways:
    #1 It's very hard to convert balanced to unbalanced without somehow changing the tonal signature of the source device.
    #2 The TVC has great performance in functions #1,#2,#3 but falls slightly short of much cheaper preamps (Axiom).
    #3 If you want a passive preamp without the need for conversion, simple designs seem to make the most sense. Like Goldpoint's stacked thin film resistors in a stepped configuration, or Luminous's pot to shunt single resistor design.

    I can only envisage if better transformers in a different TVC would be more ruthlessly transparent or neutral, but transformers by design will always impart their own sonic signature, in my experience to a greater degree then simple resistor based preamps.


    [​IMG]
    [​IMG]
    [​IMG]
     
    Last edited: Apr 24, 2019
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  13. Thenewerguy009

    Thenewerguy009 Friend

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    Do you think burning in the TVC for a few weeks of continuous use, might open up the treble?
     
  14. purr1n

    purr1n Finding his inner redneck

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    Things to consider:
    1. How do we know the Cinemags are responsible for the "slightly hashed treble, and general wonkiness in that range?" It could be possible that the Yggdrasil balanced outputs sound that way relative to its SE outputs. We really cannot isolate this to the Cinemags unless we have an amp with native balanced inputs (and then compare to Cinemags wired balanced to balanced).
    2. TVC to yet another line-level transformer (Cinemag) to SE is likely to present some impedance mismatches resulting in soft sound, dull sound, rolled highs.
    3. Cheap-ass solutions under $100 like JBL Nano Patch (will support balanced), Schiit Sys, JDS OL Switcher, etc.
    4. Depending upon source and load impedances, a single resistor and shunt pot design may be screwy because the input impedance will change depending upon the position! This design actually doesn't make much sense. Audiophiles think of all sorts of weird stuff like, "oh looky, the pot isn't in the signal path". Everything is in the signal path.
    Magic eight ball says no. It's a passive device with no moving parts, no equilibrium state to reach, etc. It's also possible that the TVC simply just rolls off. Many specialty boutique audiophile transformers do this. You know, to take the edge off the highs.
     
    Last edited: Apr 24, 2019
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  15. purr1n

    purr1n Finding his inner redneck

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    Honestly, this is an area that could really benefit from measurements:
    1. Load and source impedance at different volume positions of the volume control.
    2. Frequency response into different loads at different source impedances.
    3. Distortion into different loads at different source output voltage levels from compatible gear.
    I sense too much audiophile wankery with these AVC / TVCs (or even steppers). I am not against them because I've heard some fantastic ones, but I dislike the lack of objective information because it then becomes of matter of experimentation of what works and what doesn't with these not-so-cheap solutions.

    I am not a measurement extremist, but measurements and objective data would allow us to short-list solutions that would work in our particular applications without spending $500 to $1000 to $5000 for the privilege of experimentation.
     
  16. purr1n

    purr1n Finding his inner redneck

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    TRS to XLR cables from Guitar Center. I'm really liking Guitar Center cables. Super flexible, tough, and high quality. Used by musicians and sound engineers. I'm getting so sick of audiophile cables that weigh 5 tons and wreck RCA sockets or are so stiff that they cause gear that weighs less than 20lbs to rotate or tilt up.

    JBL Nano Patch load impedance is 5k ohms (I assume 10k using balanced inputs). A little on the low side, but this shouldn't be an issue for most sources. Pot seems good. No channel imbalance - considering that the pot is always under 10 o'clock, usualy just under 9 o'clock, and most counterclockwise position is at 7 o'clock.

    Perfectly willing to forgo 0.132% of sound quality for sensible cables if that won't cause me to die a lonely man (rejected by wife and kids, and with no real friends) with just a temple of audio (with speaker cable risers and all that) in my living room. Seriously, there are dudes like this.
     
    Last edited: Apr 24, 2019
  17. Cspirou

    Cspirou They call me Sparky

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    Needs to be measured independently but SAC in Thailand posts some of their own measurements

    http://www.sacthailand.com/Transformer_Attenuator.html

    Frequency response and phase but no distortion. I've read a few members here like SAC too.
     
  18. purr1n

    purr1n Finding his inner redneck

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    I've been considering their TVC, but have several concerns. Devil is in the details.
    1. "From 10Hz to 35,000Hz the frequency response is only -1dB deviation from 1KHz reference point. Source impedance is 12K ohm, load impedance is 10K ohm." WHO THE HELL RUNS 12K OHM SOURCE INTO 10K OHM LOAD? Please give me FR on the scope for a more realistic use scenario. Bandwidth WILL change depending on these factors.
    2. FR has a slight rise before rolloff. Behavior correlated with a bump on the rising edge of the square waves. This could be problematic. I've measured and seen enough of these from transformers to give me pause. Might not be an issue, but I prefer to see a slightly underamped or square wave than one with slight overshoot.
    3. The 10kHz square wave at "high voltage level (11.3V peak to peak)" already looks visibly distorted. This is 4Vrms. It seems a lot of recent DACs SBAF has gotten in run a bit hotter than this.
    4. The better looking measurement at the bottom uses somewhat more realistic source and load impedances and hotter input, but it's configured in 1:2. I'd rather use an active if I needed gain. Again, I'd like to see measurements provided for realistic use cases.
    5. As you said. No distortion.
    If people like their stuff or find synergies, that's great. but I at least want to put some science into it. And again, I'm not a 0.000001% THD+N or 20-20khz +/- 0.0001db guy either.
     
    Last edited: Apr 24, 2019
  19. purr1n

    purr1n Finding his inner redneck

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    Example here:

    http://softone.a.la9.jp/english/Model5/m5-1.htm

    I actually had this in for testing. It was a complete piece of shit (for my application) feeding a Crest CA2 (set to x20 gain) with 10k input from a 2V source impedance. Speakers were 92db efficient.

    At least Softone was honest. The changing source impedance is LOL. Probably some weird artsy Japanese thing to color the sound depending upon volume level. The darn thing sounded duller and duller as I turned up the volume.

    The proper amp would have been something with JFETs or tubes at the input with 100k or 1MM load impedance.
     
    Last edited: Apr 24, 2019
  20. frenchbat

    frenchbat BritishBat's arch enemy - Friend

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    No need for magic eight ball actually. Prometheus does have the (well deserved it seems) reputation to be on the lower end of transparency, and to favor "euphony". In a nutshell, this is the TVC you want if you don't care about details, resolution, and/or properly mastered music. EQ would likely make more sense than this.
     

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