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Massdrop x THX AAA™ 789 Linear Amplifier - Impressions Thread

Discussion in 'Headphone Amps (full-size)' started by XERO1, Mar 1, 2018.
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  1. PopZeus
    I thought it was the opposite, where the unbalanced RCA input is turned into a balanced before getting amped, and the balanced XLR input is just maintained throughout the signal path. Either way, I’m okay with it.
  2. MikeW
    The 789 always outputs signal from its balanced headphone out regardless of input. But it converts balanced input to single ended before amplification stage.

    So yes, that does mean balanced in > single ended conversion> amplification > balanced output.

    As opposed to its single ended input which skips the conversion stage.

    Single ended > amplification > balanced output.

    The SE input avoids an entire unnecessary summing circuit.

    This is why I caution people who are asking what balances Dac to pair with 789. Makes no since
    Last edited: Apr 10, 2019
    PopZeus and eeagle like this.
  3. Ken G

    Can't really go wrong with either one. They both measure incredibly well especially for their price. Below are links to reviews and measurements of the 789 and the Atom. Decision would likely come down to budget and power requirements of your Headphones.

    THX 789

    JDS Lab Atom
    eeagle likes this.
  4. alpovs
    How is that? Ground is ground - it's Earth. Do they ground left and right to different planets? :)
  5. phthora
    From their Application Notes: "We do offer a 4-pin XLR connection on the HPA4 headphone amplifier, because this 4-pin connection offers lower contact resistance than a traditional 1/4" TRS connection. For performance reasons, this connector is not driven with a voltage-balanced signal. Instead, it provides isolated left and right ground returns that are individually connected to the ground reference points in the left and right amplifiers."

    Maybe my wording wasn't entirely clear, but that's seems to qualify as "separately."
  6. alpovs
    This is totally wrong. It makes no sense whatsoever. I never heard this before. A properly designed balanced amp outputs 4 times the power out of balanced output compared to single ended.
  7. alpovs
    Individual parts connected to ground individually, or separately. Of course. How else? It's just a matter of speech. There is no significance in it. It's like saying "my friend and I drank coffee" or "my friend and I individually drank coffee". We couldn't drink coffee from the same cup simultaneously. "Individually" is implied.
  8. phthora
    Uh... what? I'm not following you.

    Separate ground returns are different than a single ground return offered by a TRS. That's supposed to be one of the benefits of balanced headphone output. Which brings me back to the overarching question: What makes the Benchmark not unbalanced?
  9. MikeW
    There is benefit in balanced headphone output stage. It is gear dependant. As many pieces of gear that offer balanced output, do so with lower noise, and distortion and higher levels of power on their balanced output sections. In these cases the advantage of "Balanced" is very very real. This is not to argue that an equally good single ended circuit could not be created, but only that for that particular piece of gear, the balanced section is superior. Like Schiit Jotunheim, Monoprice Liquid Platinum.. THX789, they all offer sinigifcantly more power on their balanced output, all three also offer single ended input with balanced output, but only two are properly "differential" balanced from end to end. (Jotunheim and Liquid Platinum). The 789 converts it's balanced input to single ended as a cost cutting measure. It's performance is excellent so it likely does not matter to the ear. However, as previously stated multiple times in this thread, certainly don't go out of your way to run balanced input into the 789, you gain nothing, and actually lose on technicals, likely not actual performance you can hear. As opposed to Jotunheim and Liquid platinum, which could potentially actually have something to gain from running proper balanced connections into them. The only thing running balanced into 789 accomplishes is pushing your signal though another set of op-amps, and a whole circuit that gets avoided when using single ended.
    Last edited: Apr 10, 2019
  10. domho7
    Hi all. Besides massdrop where can I purchase this amp. Tks.
  11. AndrewM888
    Depending on the rest of your rig, 789 MAY benefit from supplying balanced signal to it, regardless of whether using its bal or unbal output stage.

    On 789 there's no "another set of opamps" if using bal vs unbal. It's the same input stage opamps either way, reconfigured with relays.

    The advantages of using bal input and XLR cabling for 789 input (or any amp) is really because of a possible deficiency in RCA unbal cabling.

    Depending on how noisy your environment or connected gear is, RCA cabling can pickup noise currents from this that flow in RCA cable's unbal grounds. Since unbal ground wire is also used as signal-, and since the RCA ground wires have non-zero resistance, a noise voltage appears on this signal- that directly pollutes the audio signal (aka ground bounce). Depending on the noise source it can sound like hum (50/60 Hz harmonic), or machine noise, etc.

    In contrast XLR-3 cabling eliminates this ground bounce by separating ground from the +/- audio signal wires. Noise currents flowing in the cable ground wire don't induce any voltage across the cable +/- wires.

    The noisier your gear/environment, the more you benefit from XLR cabling.

    Even if you only have unbal DAC with 789, we recommend using RCA-to-XLR cables to achieve most of the aforementioned benefit. Search for "Monoprice RCA to XLR" on amazon and you'll see a 1.5 ft option.

    This entire topic highlights exactly why pro audio uses XLR-3 balanced cabling everywhere. If they didn't, the noise pickup induced ground bounce on 25ft runs of RCA cabling would be obscene.
    Last edited: Apr 11, 2019
  12. stimuz
    Don't forget https://www.audiosciencereview.com/...monoprice-desktop-thx-headphone-amp-dac.5420/

    even though its a fantastic dac/amp this non-review killed its traction lol.
  13. domho7
  14. stimuz
    Pretty sure its the same THX module x2. Monoprice just managed to rub the reviewer the wrong way to dodge the review completely. Either that or Massdrop rubbed them the right way.
  15. Ken G
    Others may disagree but I put a lot more faith in the build quality of Massdrop made products than those made by Monoprice.
    omniweltall likes this.
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