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Arsenk
0
Feb 26, 2018
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Does anyone have a ballpark idea of how much it'll cost?
Feb 26, 2018
greatex
66
Feb 26, 2018
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it says 350
Feb 26, 2018
Motorrad
2422
Feb 26, 2018
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Note: The Massdrop x THX AAA™ 789 Linear Amplifier will launch March 1 at 6 a.m. PT for $349.99 with free US shipping and subsidized international shipping. Each amplifier is serialized and the first 490 purchasers are guaranteed a serial number under 500.
Feb 26, 2018
Arsenk
0
Feb 26, 2018
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Oh great. Thank you much!
Feb 26, 2018
Arsenk
0
Feb 26, 2018
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Thank you!
Feb 26, 2018
joshy_bennett
15
Feb 26, 2018
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I think I read somewhere that it would be close to $350. I would consider looking around though. There's one major engineering problem with the power supply for this unit already. There's no ground, and with audio equipment that could possibly be very bad.
Feb 26, 2018
Re lack of earth ground: that isn't a problem. The product's differential input architecture, even when using RCA inputs, takes care of that problem.
Feb 27, 2018
Anent
163
Feb 27, 2018
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Without seeing the inside or even a generalized circuit diagram, this is suspect (considering the low noise floor specs). How is bias current handled without ground...? Please elaborate as much as possible. Thanks.
Edit: found some background... (guessing your LinkedIn is blowing up!) https://www.stereophile.com/content/high-efficiency-non-switching-amp-thx
Feb 27, 2018
@Anent if you use the amp's RCA inputs, then yes tiny common-mode bias currents are induced by parasitic capacitance within the floating AC/DC (primarily 60 and 180 Hz) and pushed through the shields of your RCA cables. Multiplied by the shield impedance of these cables, you get a miniscule voltage (a "ground bounce") that is then amplified. But using an affordable Monster 3ft stereo RCA patch cable, it's less than 1 dB degradation in A-wt noise compared to balanced XLR inputs. If using RCA in, there's merit in selecting cables with a solid low impedance shield to minimize issues like this. Personally I like to use "double-shielded" BNC cables and then put BNC to RCA adapters on the tips. The "double-shield" refers to a foil shield AND a braid shield in parallel. The foil is for RF, but the braid has much lower impedance in the audio-band to minimize the ground bounce voltage. e.g. Pomona 2249-Y from Mouser.
Feb 27, 2018
Anent
163
Feb 28, 2018
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OK, the RG223 was a new one for me (not an EE). RG58 (typical 50 Ohm) are my standard in the lab.--yes, single-shielded. Not to go too far off-product (sorry!), but should I be concerned with matching a 50 Ohm coax with RCA connectors as I thought the RCA are typically used with 75 Ohm systems?--what's the 789's RCA impedance?
----- As an aside:
Andrew was suggesting a BNC ended coax cable (need 2) https://www.digikey.com/product-detail/en/pomona-electronics/2249-Y-12/501-1821-ND/736317 (various lengths possible but going short 12") along with a BNC to RCA adapter (need 4) https://www.digikey.com/product-detail/en/cinch-connectivity-solutions-aim-cambridge/27-8110/367-1012-ND/385337
Alternatively, for the DIY crowd... RG223 coax -- https://www.pasternack.com/flexible-0.216-rg223-50-ohm-coax-cable-pvc-jacket-rg223-u-p.aspx
Feb 28, 2018
@Anent 50 or 75 Ohm doesn't matter for home audio. Audio cables aren't transmission lines unless they're miles long. Wavelength of 20 kHz is ~ 10 km. Audio connections are instead optimized for max fidelity, not max power transfer. To achieve max fidelity: -low impedance upstream source (your DAC Zout) -low DC impedance cable (<< 1 Ohm) -high impedance downstream sink (789's Zin) All of the above maximizes fidelity by maximizing SNR and minimizing THD (due to not loading the source). See: https://en.wikipedia.org/wiki/Impedance_bridging
I'll try this analogy. Not sure if this is perfect, but here goes: Think of tying a 3" dia. 18' rope to a Costco ceiling 20' above (an impedance termination mismatch), then lie on the floor under the rope and move the rope end back and forth 2' faster and faster until you see the middle start to bow out back and forth - a standing wave (reflection with constructive interference). Call this freq the fundamental resonance freq of the rope and note how the rope itself is starting to actually pull your hand (the source) back-forth due to the standing wave - i.e. presenting a problematic impedance for your hand to move your hand as you desire (causing overshoot or undershoot, perhaps). Then shake it faster and faster, and observe as the standing waves magnitude reduces (reflections still present but not as constructive, easier to control with your hand), followed by a return of more dense standing waves at multiples of the fundamental freq and difficult to control again. Ok, so this establishes that mismatch of rope impedance to termination impedance is problematic for the source when near or above the rope's fundamental freq. Next, shake the rope end the same 2' distance back-forth but way slower: 1/1,000th the frequency. No more standing wave, and no fighting the rope standing waves to move your hand. So we can say that exciting the rope well below its fundamental freq means the impedance matching of rope to ceiling termination is no longer useful in predicting how it loads your hand.
Feb 28, 2018
Anent
163
Mar 1, 2018
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Thanks, Andrew. Super helpful! Indeed, RF issues with long cable runs in noisy places have been my (work) concern, hence the impedance matching slant, while impedance bridging wasn't on my (home audio) radar. So, theoretically perhaps a minuscule noise source, but in practice no way it's an issue.
I wonder how a portion of the audiophile community here might react to your take on 'high-performance' cabling?! ;p Nevermind. ;) Cheers.
Mar 1, 2018
Mar 1, 2018
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