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Review and Measurements of Massdrop THX AAA 789 Amp

odyo

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Nothing special to it, nope.


There is an SE passthrough if you use SE input. Could route that to active speakers.. but yes no active preamp. From the top of my head only Topping A90 has that functionality and same or better specs.
So i can use passthrough to my active speakers but can not control the volume via amp ? That's fine. Thanks.
 

Veri

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So i can use passthrough to my active speakers but can not control the volume via amp ? That's fine. Thanks.
As long as you use the RCA input then yes, you can use RCA passthrough :) XLR input to RCA passthrough will not work.
 

odyo

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As long as you use the RCA input then yes, you can use RCA passthrough :) XLR input to RCA passthrough will not work.
Well don't have enough budget for balanced dac for now. Going with the E30.
 

T3RIAD

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I have an interesting issue. I have three amps - Stax SRM-353x, Jotunheim R, and the THX 789 - for different types of headphones. I would like to have all three plugged in to my DAC at all times, so I don't have to switch the cables around when listening to different headphones.

So I bought four XLR splitter cables and wired everything up. Since the amp inputs are high-impedance, I figured this would work, and it mostly does. Except, when I'm listening to either the Stax or the Jotunheim and the THX is off, the sound coming from the Stax or Jotunheim is severely distorted. I have to unplug the splitter cable from the THX to make it listenable.

However, I discovered that if I turn the THX on, even if I'm not actually listening to it, the signal recovers and sounds normal again. With the THX on, the other amps sound the same whether or not the THX is connected. Only if the THX is connected and off does it drag down the signal.

Weirdly, only the THX seems to do this. I can listen to the THX while the Stax and Jotunheim are connected and off, and it sounds fine. Likewise, neither the Stax nor the Jotunheim seem to mind the other being connected. But if I listen to one of those two while the THX is connected and off, it sounds horrible.

Anyone know what's different about the THX circuit that makes it do this? I'm thinking maybe an ESD protection diode on the input.

I guess I'll have to get one of those XLR switch boxes, unless someone knows of a better way to fix this.
 

JohnYang1997

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I have an interesting issue. I have three amps - Stax SRM-353x, Jotunheim R, and the THX 789 - for different types of headphones. I would like to have all three plugged in to my DAC at all times, so I don't have to switch the cables around when listening to different headphones.

So I bought four XLR splitter cables and wired everything up. Since the amp inputs are high-impedance, I figured this would work, and it mostly does. Except, when I'm listening to either the Stax or the Jotunheim and the THX is off, the sound coming from the Stax or Jotunheim is severely distorted. I have to unplug the splitter cable from the THX to make it listenable.

However, I discovered that if I turn the THX on, even if I'm not actually listening to it, the signal recovers and sounds normal again. With the THX on, the other amps sound the same whether or not the THX is connected. Only if the THX is connected and off does it drag down the signal.

Weirdly, only the THX seems to do this. I can listen to the THX while the Stax and Jotunheim are connected and off, and it sounds fine. Likewise, neither the Stax nor the Jotunheim seem to mind the other being connected. But if I listen to one of those two while the THX is connected and off, it sounds horrible.

Anyone know what's different about the THX circuit that makes it do this? I'm thinking maybe an ESD protection diode on the input.

I guess I'll have to get one of those XLR switch boxes, unless someone knows of a better way to fix this.
I read it 3 times...
Summing it up.
Three amps connected to single source via XLR.
When 789 on, sounds good.
When 789 off, sounds distorted.

However I don't know how to make it work.
 
Last edited:

T3RIAD

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I read it 3 times...
Summing it up.
Three amps connected to single source via XLR.
When 789 on, sounds good.
When 789 on, sounds distorted.

However I don't know how to make it work.

The setup looks like this (all balanced connections):
Code:
----------
|  DAC   |
----------
    ||
    ||
    ||==============================
    ||            ||              ||
----------     ----------     ----------
|  Jot R |     |  Stax  |     |   THX  |
----------     ----------     ----------
                  ||
                  __
                 /  \
               ()    ()
              headphones

Note that I'm not actually listening to the THX 789. It's just plugged in to the same source.

If the THX 789 is powered on, the sound (from the Stax amp) is good. But if the THX 789 is off, it's badly distorted.

I don't have this problem with the other amps. E.g. in the above picture, the Jot R can be on or off, plugged in or not plugged in, and it doesn't matter. But if the THX 789 is plugged in and powered off, it distorts the sound.

I don't want to have to turn the THX 789 on every time I listen to one of the other amps, nor do I want to have to uplug it. It's not uncommon in pro audio to have one upstream device connected to multiple downstream devices; that's what these splitter cables are for.
 

odyo

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Hey i have a question :) Can i turn off this amp with the power button ?
 

odyo

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Well, yes? Would like to share what raise the question?
Thanks John. It's power button is a bit odd. It stays unpressed. I thought it might be only turn off with the auto off mode.
 

restorer-john

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Anyone know what's different about the THX circuit that makes it do this? I'm thinking maybe an ESD protection diode on the input.

This is a common issue for many, many amplifiers, preamplifiers, cassette and open reel decks. When front ends (inputs) are not powered up (unbiased junctions), various non-linear distortions can be produced. The problem has been around since Adam was a boy.

It is one of the reasons switchable source record outs on amplifiers and preamplifiers were introduced in the first place. Either turn on all the amplifiers or use a switchable source selector.
 

Veri

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Thanks John. It's power button is a bit odd. It stays unpressed. I thought it might be only turn off with the auto off mode.
Looks like your power button is not working correctly then, hm.
 

JohnYang1997

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Looks like your power button is not working correctly then, hm.
It's a digital button without latch. It's pretty hard to use tbh. Especially when it needs a few seconds to charge up before it works.
 

ZeDestructor

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Thanks John. It's power button is a bit odd. It stays unpressed. I thought it might be only turn off with the auto off mode.

That's normal - as JohnYang1997 said, it's a non latching switch. In full bypass mode, press the button and it triggers the circuit inside to poweron. Press again and it turns off. I'd check auto/timeout mode if I was at home, but I'm not.
 

odyo

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Comparing E30+THX789 vs SBX AE5 soundcard. THX 789 4x more powerful but i'm not feeling it. Using both at high gain
AE5 at %50
THX 789 at 12 clock assuming it's %50. AE5 actually sounds louder ?
 

JohnYang1997

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Comparing E30+THX789 vs SBX AE5 soundcard. THX 789 4x more powerful but i'm not feeling it. Using both at high gain
AE5 at %50
THX 789 at 12 clock assuming it's %50. AE5 actually sounds louder ?
Gain is very different from power. You can have a 10mW amplifier set with 40db or 60db gain. Yet it's less powerful.
 

odyo

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Gain is very different from power. You can have a 10mW amplifier set with 40db or 60db gain. Yet it's less powerful.
Oh that's interesting. So AE5 outputs more db's with a less power 33ohm: 330mW. THX 33ohm: 1.3W.
I'm going back and forth i noticed some differences but it's not volume matched. AE5 sounds more visceral, thumpy and closer. E30+789 combo sounds more open, distant, less fatiguing, deeper soundstage. I'm getting the feeling of better resolution in the mids and highs but bass is lacking a bit.
 

Marakas

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I have an interesting issue. I have three amps - Stax SRM-353x, Jotunheim R, and the THX 789 - for different types of headphones. I would like to have all three plugged in to my DAC at all times, so I don't have to switch the cables around when listening to different headphones.

So I bought four XLR splitter cables and wired everything up. Since the amp inputs are high-impedance, I figured this would work, and it mostly does. Except, when I'm listening to either the Stax or the Jotunheim and the THX is off, the sound coming from the Stax or Jotunheim is severely distorted. I have to unplug the splitter cable from the THX to make it listenable.

However, I discovered that if I turn the THX on, even if I'm not actually listening to it, the signal recovers and sounds normal again. With the THX on, the other amps sound the same whether or not the THX is connected. Only if the THX is connected and off does it drag down the signal.

Weirdly, only the THX seems to do this. I can listen to the THX while the Stax and Jotunheim are connected and off, and it sounds fine. Likewise, neither the Stax nor the Jotunheim seem to mind the other being connected. But if I listen to one of those two while the THX is connected and off, it sounds horrible.

Anyone know what's different about the THX circuit that makes it do this? I'm thinking maybe an ESD protection diode on the input.

I guess I'll have to get one of those XLR switch boxes, unless someone knows of a better way to fix this.


I have similar issue, with my thx789 and smsl m200.
If both outputs in the dac are connected (xlr thx, espressivo-e rca), sound in rca is heavily distorted if thx is off. When I turn it on, everything's fine.
I already thought it was a m200 fault. Good to know...

T3RIAD what's your dac model?
 
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odyo

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Not super precise but i volume matched the two with a sound meter app.
AE5 low gain %34: 85db pink noise from Ananda
789 mid gain 12 clock: 85db pink noise from Ananda. Switching to low gain it's 75db as it should so i guess it's not totally off.

Still evaluating the sound. Not sure which one is better. There are obvious differences. Slam better on the AE5, smoothness better on the E30+THX789. Now i have first hand experience i can confidently say they sound different but not much. AE5 treble is a bit glary which can hide some detail, 789 treble is calmer and more treble detail is visible. Biggest difference is in the bass punch. 789 is softer here. In the mids i noticed 789 have a bit 3D like sound. With 3D i don't mean it's enveloping surround 3D. I mean sound comes from a flat plane like normal but i can feel the 3d physicality of the instruments.

So basing my opinions from AE5 i can say that THX 789 sound smooth, less fatiguing and deep(soundstage). ''Smooth'' and ''deep'' highlights of the show. It's like a calibrated display. It doesn't have brightness, contrast, gamma, saturation issue which helps showing out more detail.

No difference between E30 filters at first glance. No difference between low, med, high gain settings either.

I've tried using E30 dac with AE5 amp but faced with very nasty ground issues. I will try AE5 dac with THX 789 amp.

Edit: At higher volume THX 789 destroys the AE5. AE5 soundstage completely squashed at high volumes. 789 flawless here. However i don't understand why AE5 have more physical rumble with the bass.

Currently using AE5 dac connected to 789 amp. This is harsher than E30+789 but still closer to E30+789 than AE5 only.

AE5 have no problem at higher volumes with the high impedance Beyer headphones. So i think AE5 have good voltage swing but bad current capability.
 
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T3RIAD

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I have similar issue, with my thx789 and smsl m200.
If both outputs in the dac are connected (xlr thx, espressivo-e rca), sound in rca is heavily distorted if thx is off. When I turn it on, everything's fine.
I already thought it was a m200 fault. Good to know...

T3RIAD what's your dac model?

My DAC is the SMSL SU-8.

According to another poster some amps just do this. They short the inputs when they're turned off. I'm trying to find the Nobsound MC-103 XLR switch box for sale somewhere to switch between the amps.
 
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