Review and Measurements of Massdrop THX AAA 789 Amp

[olds1959special]

I started using the middle gain setting for my HD650's. It's powerful enough and shouldn't be distorting this way.

 

[olds1959special]

I'm not sure but maybe I like listening more on gain 3 but with attenuating my DAC to 75%. There's more distortion but the sound feels warmer and bigger.

 

[unpluggged]

There's more distortion but the sound feels warmer and bigger.

Better use EQ for that.

 

[olds1959special]

Today I listened to well-recorded and mastered album on gain II with my HD650's and realized the sound is great! I just think I am not used to hearing such low distortion in an amp. Gain III is soft clipping with the DAC on full volume, but it's not needed. My volume knob is pretty close to 12 o'clock.

 

[solderdude]

Today I listened to well-recorded and mastered album on gain II with my HD650's and realized the sound is great! I just think I am not used to hearing such low distortion in an amp. Gain III is soft clipping with the DAC on full volume, but it's not needed. My volume knob is pretty close to 12 o'clock.

Volume knob at 12 o clock means it is set around -20dB of the full range.

There is no difference in distortion between all 3 gain settings, just a level difference (at the same volpot setting)

 

[olds1959special]

Volume knob at 12 o clock means it is set around -20dB of the full range.

There is no difference in distortion between all 3 gain settings, just a level difference (at the same volpot setting)

I want to think this is true, but the manual states that you can't use the max gain setting when using a balanced dac at full volume?

 

[olds1959special]

I am listening again and don't hear a problem with the III gain setting with the HD650's with a balanced cable, using the balanced inputs. If it's clipping, I can't hear it!

EDIT: I tried a louder album and I needed to use the gain II for the best sound, I think.

 

[rcstevensonaz]

I want to think this is true, but the manual states that you can't use the max gain setting when using a balanced dac at full volume?

Was that a statement for all three levels, or just for level 3?

 

[solderdude]

a balanced DAC when gving out 4V and with +16dB gain (6.6x) will be clipping at full volume.
When a DAC (balanced) is 5V it will also be clipping.
That's probably what they mean.

I don't think this amp has a gain stage before the volume control (like the O2)
Looks like it does... silly design.

A balanced DAC with the volume control at 12 o clock will never clip this amp, also not in high gain mode.

 

[olds1959special]

Was that a statement for all three levels, or just for level 3?

level 3

 

[solderdude]

Ah... turns out XLR and RCA use the same input.
In high gain the max. input voltage is merely 2.1V both for RCA and XLR so any balanced DAC will clip the XLR input on gain III.
On gain 1 and 2 the inputs can handle 7V.
It looks like the gain is done before the volume control (ala O2) and the RCA and XLR inputs are the same.
Weird decision, probably noise floor related.
Probably because of the numbers game.
Looks like Amir did not test the XLR input in high-gain mode.

A balanced DAC (4V), max. volume can clip the output of the 789 on 2 setting so there is no need nor benefit.
mode 3 is only needed when you want clipping levels using RCA (max 2.1V)

So... XLR balanced max. setting 2
RCA max setting 3

 

[unpluggged]

Weird decision, probably noise floor related.

Gain selector positions I and II are actually an attenuator (−10 dB gain) and a buffer (0 dB gain). Only position III amplifies the input (by 10 dB, which is roughly the ratio between 2.1 and 7 volts, so this can somewhat explain what is going on here).

So... XLR balanced max. setting 2
RCA max setting 3

Depends on your source's output level. Many studio gear can overload the THX 789 XLR input at any gain setting.

 

[olds1959special]

I am selling my other headphone amps now, this amp is great!

 

[olds1959special]

Also it seems to work as a pre-amp too. I use the balanced inputs with my DAC at full volume then set it to low gain and run a cable out of the single ended output to my power amps.

 

[rcstevensonaz]

Also it seems to work as a pre-amp too. I use the balanced inputs with my DAC at full volume then set it to low gain and run a cable out of the single ended output to my power amps.

Can you please describe how you have setup this configuration in a bit more detail? I've never been able to it to work that way, and I'm very concerned about blowing out my speakers.

 

[olds1959special]

Can you please describe how you have setup this configuration in a bit more detail? I've never been able to it to work that way, and I'm very concerned about blowing out my speakers.

SMSL D0100 Pro balanced out to THX amp at full volume. Then I run a mini to rca cable from the SE headphone out jack to the two amps. I set the gain to low (1) and then control the volume with the knob. It’s a bit hot so I can’t turn it up past 9 or so but it works even at low volumes. If I did turn it up too much it would definitely be bad though. Is that what you mean?

Also I can leave the mini to rca cable plugged in while using the xlr out for headphones with no detriment to sound quality.

 

[rcstevensonaz]

SMSL D0100 Pro balanced out to THX amp at full volume. Then I run a mini to rca cable from the SE headphone out jack to the two amps. I set the gain to low (1) and then control the volume with the knob. It’s a bit hot so I can’t turn it up past 9 or so but it works even at low volumes. If I did turn it up too much it would definitely be bad though. Is that what you mean?

Also I can leave the mini to rca cable plugged in while using the xlr out for headphones with no detriment to sound quality.

Thanks for clarifying. Your language was ambiguous at first and I though you were referring to using the single ended RCA bypass to the amp.

I've summarized your wiring configuration with a bit more detail, where you are routing to the amplifier from the headphone output jack on front panel:

• DAC (100% volume) XLR Output --> THX 789 3-pin XLR input (back side)
• THX 789 3.5mm (or 1/4" TRS) output (front side) --> Amplifier RCA input

 

[olds1959special]

as a preamp (described above) I'm using the volume at 12 o'clock and it's loud but not crazy loud, so the range of the knob is good.

 

[olds1959special]

Thanks for clarifying. Your language was ambiguous at first and I though you were referring to using the single ended RCA bypass to the amp.

I've summarized your wiring configuration with a bit more detail, where you are routing to the amplifier from the headphone output jack on front panel:

• DAC (100% volume) XLR Output --> THX 789 3-pin XLR input (back side)
• THX 789 3.5mm (or 1/4" TRS) output (front side) --> Amplifier RCA input

Also I have the second SE output (front side 1/4") going to my subwoofer so I can have that working too.

 

[rcstevensonaz]

Also I have the second SE output (front side 1/4") going to my subwoofer so I can have that working too.

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.