# Review and Measurements of Massdrop THX AAA 789 Amp

#### AresHarvest

##### Active Member
Forum Donor
Very good info, curious, how does one calculate the voltage based on ref level and volume knob level?

What’s the benefit of using auto ref with the typical consumer equipment vs running fixed?
The advantage to auto ref level is so that you don't need to change gain modes to get the full range of output levels with the pot. It's not useful in this case.

As for calculating the voltage:
• Volume is shown in dBr is somewhat of an open scale, where the reference value is implied. In this case, we can assume the reference value is the user-set Ref Level. So with ref level of 13 dBu, and a volume of -4.5 dBr, we have an output of 8.5 dBu (13 - 4.5 = 8.5).
• dBu uses a reference value of 0.775 Vrms
• Calculation for converting voltage to decibels is 20 * log(V2/V1) where our desired voltage is V2 and our ref voltage can replace V1
• Plug our values in with a desired voltage of 2.1 V... 20 * log(2.1/0.775) we get a gain of 8.658 dB, and I rounded down to the nearest 0.5 dB to account for the increments of the RME's volume control

#### Ty89m

##### Member
Thanks a ton for running through that! Will toy around with that formula, supremely helpful.

Sounds as though running fixed ref for a specific voltage would be the simple way forward in my case.

Thanks again, truly. Great forum.

#### AresHarvest

##### Active Member
Forum Donor
Sounds as though running fixed ref for a specific voltage would be the simple way forward in my case.
That's my thinking, too. If using the headphone amplifier built into the ADI-2 DAC, having auto ref enabled would be good when you have a mix of high- and low-sensitivity headphones. That way you can get the level you need for any one of your headphones with a single control (volume) rather than two controls (ref level and volume). If using an external amplifier as you're planning to do, this convenience is moot because your level setting would be fixed so that the volume control is done with the amplifier instead.

Edit: another consideration to avoid clipping is accidental volume changes. If you set your ref level to +7 dBu and your volume to 0 dBr, then you're not quite getting the maximum possible voltage swing, but conversely you are not risking any clipping if you happen to accidentally brush the volume control with your hand - since the volume is already maxed out relative to the 7 dBu reference level. I don't know if the RME has a lock feature that prevents accidental changes.

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#### Ty89m

##### Member
Is there any real danger to the equipment if clipped beyond potential audible distortion etc? Would using your recommendation of 13 dBu, and a volume of -4.5 dBr be preferable to the manual stated 7dBu and 0 dBr?

Adding another complication into the mix, seems running XLR vs RCA out to amp increases the db by +6, I assume that changes what I'd set as far as ref level? Also mentions in the manual that running 7dBu and 0 dBr on volume limits ones ability to use EQ or Bass/Treble, at least boosting as opposed to cutting. Boosting pushes the DAC into overdrive, so seems you'd have to back off the volume regardless.

I guess I'm just shooting for the best compromise and simplicity while getting the most power output/voltage swing.

Again, I really appreciate all the input.

#### Ty89m

##### Member
Alright, confirm if I'm understanding this correctly - when using RCA out, +7dBu at 0db on volume knob equates to 1.73v etc. When using XLR with that equates to +6 over RCA, I'd actually set ref level to +1 instead of +7 for the same output voltage to account for +6db increase with XLR? If I were to use the +7dBu setting with XLR, I'd actually be outputting +13dBu - or 3.46v, yes?

#### MC_RME

##### Technical Expert
Technical Expert
Manufacturer
As for calculating the voltage:
• Volume is shown in dBr is somewhat of an open scale, where the reference value is implied. In this case, we can assume the reference value is the user-set Ref Level. So with ref level of 13 dBu, and a volume of -4.5 dBr, we have an output of 8.5 dBu (13 - 4.5 = 8.5).
Nearly nailed. The reference is the max level available at the respective output, +19 dBu at XLR, +13dBu at RCA. If the user sets a ref level there is no dBr anymore, the unit will show dB as volume/gain value.

#### AresHarvest

##### Active Member
Forum Donor
Nearly nailed. The reference is the max level available at the respective output, +19 dBu at XLR, +13dBu at RCA. If the user sets a ref level there is no dBr anymore, the unit will show dB as volume/gain value.
So volume is shown at dB relative to max output regardless of the Ref Level? That changes the numbers required to get in the neighborhood of 2.1 Vrms. -10.5 dB or -4.5 dB would do it, then, depending on whether the XLR or RCA outputs are in use.

Ref Level is only relevant when using the auto ref? I did read the manual, but I guess I should have read it more closely.

#### MC_RME

##### Technical Expert
Technical Expert
Manufacturer
It would be more easy to understand having a unit

The user sets a ref level, the volume is shown as dB and relates to that specific ref level.

In Auto ref level mode that dB value would change as soon as a different ref level is chosen by the unit (automatically), which doesn't make sense and is totally confusing. So that mode uses dBr and refers to the highest ever possible output level, no matter what ref level is currently active. I can assure you it's as simple as logical.

#### Ty89m

##### Member
So to clarify, running XLR out, and shooting for the typical 2v (give or take) what would one set ref level to (manual, not auto) as well as actual volume dB to achieve that. Or was I on the right track with my previous statement?

#### AresHarvest

##### Active Member
Forum Donor
It would be more easy to understand having a unit
You guys already got my money when I bought a Babyface. And again when I bought a UCX and UFX II

The user sets a ref level, the volume is shown as dB and relates to that specific ref level.
That's how I described the operation... Before you corrected me. "So with ref level of 13 dBu, and a volume of -4.5 dBr, we have an output of 8.5 dBu (13 - 4.5 = 8.5)."

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#### MC_RME

##### Technical Expert
Technical Expert
Manufacturer
That's is not what I 'corrected'. You wrote:
> Volume is shown in dBr is somewhat of an open scale, where the reference value is implied. In this case, we can assume the reference value is the user-set Ref Level.

With dBr shown there is no 'user set ref level'. I did not say anything about the numbers, which are indeed correct. There are several examples in the manual, page 62:

@Ty89m: all what you wrote is correct.

#### AresHarvest

##### Active Member
Forum Donor
I read the manual before I posted. So the thing I got wrong was about the ref level, since it is auto set instead of user-set, then? Why not just say that?

#### JohnYang1997

##### Major Contributor
I read the manual before I posted. So the thing I got wrong was about the ref level, since it is auto set instead of user-set, then? Why not just say that?
If I understand him correctly. There are two modes.
1, 0dBr = 0dBFS = 19dBu/13dBu
2, 0dB = user set level
Where imo the second one should also be dBr. And would be better if the first change to dBFS.

#### AresHarvest

##### Active Member
Forum Donor
That's what I thought too, which is the behavior I initially described - user can set the output reference level to either 13 or 19 dBu, and the volume is shown in decibels relative to that level.

#### JohnYang1997

##### Major Contributor
That's what I thought too, which is the behavior I initially described.
I think what he meant was in user set mode, it doesn't show dBr but only dB.

#### MC_RME

##### Technical Expert
Technical Expert
Manufacturer
Correct. But showing dBFS would be wrong. This is a volume setting, means gain control, from +6 dB down to -97 dB, not a digitally measured level.

#### JohnYang1997

##### Major Contributor
Correct. But showing dBFS would be wrong. This is a volume setting, means gain control, from +6 dB down to -97 dB, not a digitally measured level.
Ok so it caps at a set dB number. I get the logic.

#### Ty89m

##### Member
Wow, a lot of smart folks - quite the forum. Wish I'd stumbled across it sooner

Anyway, not to keep beating this horse - but just for curiosity sake...

Any benefit to running +13dBu and -4.5dB volume (2.06v rms) vs +7dBu and 0dB volume - I realize the latter provides a lower voltage, but does starting higher and dialing back via volume a better option? Perhaps more headroom for EQ etc?

Really enjoying the RME, paradise for something who likes to fiddle with things ha!

#### trl

##### Addicted to Fun and Learning
King of Mods

View attachment 16962

As with my previous tests of Benchmark DAC3 and ADI-2 Pro, there is significant degradation in low frequencies due to reverse EMF current from the headphone driver getting strong.

Note that the above is at low output level (1 volt) and with double the bandwidth of the dashboard. As such, THD+N is much lower even with the dummy load (33 ohm).

On the plus side, the ruler flat response is impressive with dummy load. There is no frequency dependent components.
Very interesting indeed, I wonder if you could please include such a test with all future headamps, because it's very relevant. 1V RMS should do, I guess. I would call it "headphones driveability" and one planar + one dynamic headphone should be good enough. I guess such a test will help some of us to better understand if an amp "sounds" better or worse than another amp, with real headphones attached instead of a dummy resistor.

Funny thing, apparently my Objective2 with MUSES8920 used as buffers (mostly because of the lower output DC) seems to be measuring a bit better between 20-4000 Hz than Atom (e.g.: THD @40Hz is 0.05% instead of 0.15%). However, I don't have SR60e, so I used instead the SR60i (I've heard there are the same drivers inside):