Review and Measurements of Massdrop THX AAA 789 Amp

[olds1959special]

Have you considered routing the RCA outputs from your DAC to the Monolith as well? And then use the front switch to select between the (a) the higher voltage XLR input when listening to headphones using the 4-pin XLR, and (b) the lower voltage RCA input when you are routing "line-level" output to your amplifier / subwoofers?

That would provide a much greater range on your volume knob when using the THX 789 as pre-amplifier; or even when using it as headphone amp for easier to drive headphones.

Thanks, this works well.

 

[olds1959special]

Have you considered routing the RCA outputs from your DAC to the Monolith as well? And then use the front switch to select between (a) the higher voltage XLR input when listening to headphones using the 4-pin XLR, and (b) the lower voltage RCA input when you are routing "line-level" output to your amplifier / subwoofers?

That would provide a much greater range on your volume knob when using the THX 789 as pre-amplifier; or even when using it as headphone amp for easier to drive headphones.

When doing this (using RCA inputs) I can use the II gain setting as well. I'm not sure if it makes any difference to the sound quality? I guess II is 0dB and I is -10dB. I'm not sure but this might sound better. I'm not going to try III though because that would be too much.

edit: I went back to using the gain I setting.

 

[rcstevensonaz]

When doing this (using RCA inputs) I can use the II gain setting as well. I'm not sure if it makes any difference to the sound quality? I guess II is 0dB and I is -10dB. I'm not sure but this might sound better. I'm not going to try III though because that would be too much.

For an unexpected reason, you will most likely gain stereo sound quality using the RCA inputs.

The reason is that often there is a degree of channel imbalance on analog volume POTS when they are close to zero. When you feed the high voltage input, you were having to turn the volume far towards zero — "It’s a bit hot so I can’t turn it up past 9 or so". By lowering the voltage input so that 12:00 becomes the nominal position (whether that is level I, II, or III gain setting), it allows you to manage the volume POT without having to dip down into the lower range below 9:00".

Further, by gaining a greater range of motion, you are more able to fine tune the volume to the exact level you want it at. That would have a perceptual audio benefit.

But that should be the only audible and perceptual audio quality impact assuming the DAC's balanced and unbalanced amplification stages are both sufficient high quality with undiscernible noise floors. And if the two are not equal, odds are good that the unbalanced audio would be the one with higher sound quality. [Edit: I just looked at the Amir's review of the SMSL DO100, and the XLR output was measured just an (inaudible) smidge above the RCA; so no inaudible penalty either way on this device]

(side note: I have the Monolith THX 887 which is virtually identical to this)

 

[olds1959special]

I stopped using this as a preamp. I'm using it as a headphone amp though, and noticed that I prefer the high gain setting with my DAC set at 80 to avoid distortion.

 

[rcstevensonaz]

I stopped using this as a preamp. I'm using it as a headphone amp though, and noticed that I prefer the high gain setting with my DAC set at 80 to avoid distortion.

Are you using the XLR or RCA interconnect?

 

[olds1959special]

Are you using the XLR or RCA interconnect?

I'm using the XLR but I'm not really sure I'm right about this.

 

[olds1959special]

I’m back to using this as a preamp. My DAC direct to power amps didn’t work so well (ground loop buzz in right channel.) Here’s a pic showing the cables going to my amps and subwoofer. Using the balanced inputs for the DAC seems to work best for me. While I can use the medium gain, the low gain works the best, and there’s enough room to turn the volume. I have my amps and subwoofer on a power strip, so when I want to use headphones, I can just turn them off and plug in my balanced HD650’s into the XLR output, and switch to medium gain. Is there any reason at all this is not desirable as a preamp as well as a headphone amp?

 

[olds1959special]

Using a multi meter I measured the XLR output of my DAC so I could figure out what volume to use to get full output and prevent clipping on high gain (THX AAA 789). I was surprised to find the correct volume setting is 83. This is so good to know because I like listening on the high gain setting to get the full voltage swing. For some reason, while using medium gain the sound feels sterile.

 

[olds1959special]

Once again I'm finding I prefer gain III while attenuating my DAC to under 2.1 volts.

 

[olds1959special]

For an unexpected reason, you will most likely gain stereo sound quality using the RCA inputs.

The reason is that often there is a degree of channel imbalance on analog volume POTS when they are close to zero. When you feed the high voltage input, you were having to turn the volume far towards zero — "It’s a bit hot so I can’t turn it up past 9 or so". By lowering the voltage input so that 12:00 becomes the nominal position (whether that is level I, II, or III gain setting), it allows you to manage the volume POT without having to dip down into the lower range below 9:00".

Further, by gaining a greater range of motion, you are more able to fine tune the volume to the exact level you want it at. That would have a perceptual audio benefit.

But that should be the only audible and perceptual audio quality impact assuming the DAC's balanced and unbalanced amplification stages are both sufficient high quality with undiscernible noise floors. And if the two are not equal, odds are good that the unbalanced audio would be the one with higher sound quality. [Edit: I just looked at the Amir's review of the SMSL DO100, and the XLR output was measured just an (inaudible) smidge above the RCA; so no inaudible penalty either way on this device]

(side note: I have the Monolith THX 887 which is virtually identical to this)

I'm using this amp unbalanced from my Apogee Duet 2 now. Going balanced didn't seem better.

 

[olds1959special]

I replaced the stock PSU with a 24V 5A one but it made no audible difference to the sound.

 

[Malaj]

I missplaced the power adapter? Anybody know some affordable decent to use with it. any 24v/and above 1.8A output? Is it fine to use lower rated power adapters?

 

[FrantzM]

I missplaced the power adapter? Anybody know some affordable decent to use with it. any 24v/and above 1.8A output? Is it fine to use lower rated power adapters?

Amazon can (sometimes) be your friend :
Click here

 

[Malaj]

I found one that output 24v and 1.75A in the attic that had the right size connector. It powers on and dont instantly kill some cheap headphones I put on it. May actually be the actual one I cant recall. Trying my pro 900 at gain 1 and I need to run the volume to the max. But gain 1 is for iems I suppose? I just cant recall how I used to use it.

 

[Malaj]

I got some crazy distortion maxing the signal to +19 from my rme adi-2. Use gain setting 1 on the thx 789aaa. Reducing to +13 or below it seems to work better on my d9200. But surprised there is such a massive difference at similar volume.

Seems like the rme output more bass at higher outputs? Used rihannas umbella bad song but the distortion was just crazy at +19.

Still I really dislike it. The midrange is so bad. Voices never sounds right. Its like every recording is poor. But it do bring out more bass on my d9200 then other amps I have if its just for my hunt for mids making me increase the spl.

 

[Antagron]

I got some crazy distortion maxing the signal to +19 from my rme adi-2. Use gain setting 1 on the thx 789aaa. Reducing to +13 or below it seems to work better on my d9200. But surprised there is such a massive difference at similar volume.

Seems like the rme output more bass at higher outputs? Used rihannas umbella bad song but the distortion was just crazy at +19.

Still I really dislike it. The midrange is so bad. Voices never sounds right. Its like every recording is poor. But it do bring out more bass on my d9200 then other amps I have if its just for my hunt for mids making me increase the spl.

Do you have EQ going on the RME? You don’t have any headroom left if you max out the volume.

 

[Malaj]

No eq at all but kind of curious trying some eq to simulate 7200

 

[Malaj]

I found the stock one in the cellar unexpectedly