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RME User Forum → ADI-2 & 2/4 Pro series, ADI-2 DAC series → Feature request: Lock Reference level
I'm building an electrostatic headphone amplifier and plan to mate it to the RME ADI-2 DAC. I will be feeding the balanced output of the DAC into the headphone amp. It would be nice to keep from overloading the amplifier with the DAC if the user accidentally chooses too high a reference level. Thus my idea to offer a reference level lock: e.g. if +13 dBu balanced is the highest I should go.
What do you think?
The following assumes you want to use ADI-2‘s Digital Volume Control:
In this case “Auto Ref Level” function is quasi mandatory for best signal quality.
“Auto Ref Level” works best when it can use it‘s full working range, all existing reference level steps.
Therefore calibrating your estat amp‘s clipping point to ADI-2 DAC‘s full scale output would be the way to go.
ADI-2 DAC‘s full scale analog line output level (manual page 20):
XLR balanced:
(nominal Ref Level +13 dBu) + (6 dB balanced gain) + (3 dB Volume‘s headroom) = 22 dBu
RCA unbalanced:
(nominal Ref Level +13 dBu) + (3 dB Volume‘s headroom) = 16 dBu
If you don‘t want to use ADI-2 DAC’s Digital Volume Control, the “Lock Volume” - function does the job.
The following assumes you want to use ADI-2‘s Digital Volume Control:
In this case “Auto Ref Level” function is quasi mandatory for best signal quality.
“Auto Ref Level” works best when it can use it‘s full working range, all existing reference level steps.Therefore calibrating your estat amp‘s clipping point to ADI-2 DAC‘s full scale output would be the way to go.
ADI-2 DAC‘s full scale analog line output level (manual page 20):
XLR balanced:
(nominal Ref Level +13 dBu) + (6 dB balanced gain) + (3 dB Volume‘s headroom) = 22 dBuRCA unbalanced:
(nominal Ref Level +13 dBu) + (3 dB Volume‘s headroom) = 16 dBuIf you don‘t want to use ADI-2 DAC’s Digital Volume Control, the “Lock Volume” - function does the job.
Thanks for your response. I do not want to use auto reference level. First of all, as mentioned in the RME manual, any improvement in signal to noise ratio by automatically optimizing the analog output reference level will not be perceived. Second, I work with a calibrated level system called the K-System (which I invented) where a given dBr monitor attenuation will produce a predictable SPL with a given audio source. My main mastering loudspeaker system is calibrated in this way and makes the process of evaluating audio compression much easier.
For the external headphone amplifier, I would like to have at least a predictable SPL with a given source with a given dBr attenuation. Therefore I need a single, fixed analog output reference level from the ADI-2. I've already calculated that the external power amplifier will clip if I use the +19 dBu reference (balanced) from the ADI-2. The optimum reference in my situation to prevent clipping will be the +13 dBu reference (with 3 dB additional attenuation that I will build into the headphone amp). Obviously I would like to prevent someone (not me of course :-) accidentally setting the reference level to +19 dBu. That's why I would love to be able to "lock" the ADI-2 to the +13 setting in options.
…I work with a calibrated level system called the K-System (which I invented) where a given dBr monitor attenuation will produce a predictable SPL with a given audio source. My main mastering loudspeaker system is calibrated in this way and makes the process of evaluating audio compression much easier.
I have your book and know K-System.
For the external headphone amplifier, I would like to have at least a predictable SPL with a given source with a given dBr attenuation.
…The optimum reference in my situation to prevent clipping will be the +13 dBu reference (with 3 dB additional attenuation that I will build into the headphone amp).
A certain SPL for a given digital signal - and to calibrate the level just below clipping the headphone amp at 0 dBFS - are requirements that exclude each other.
Obviously I would like to prevent someone (not me of course :-) accidentally setting the reference level to +19 dBu. That's why I would love to be able to "lock" the ADI-2 to the +13 setting in options.
This one‘s easy:
• Activate “Auto Ref Level”, even if you don’t use it.
• Then dial the Volume while watching the Ref Level display just above the Volume dBr.
The Ref-Level switch-down point is 0 dB digital attenuation.
• Now activate “Lock Volume”.
Without going to the menu and deactivating “Lock Volume”, no one can change the output level then.
Of course, in the menu “Lock Volume” and “Reference Level” can be changed.
Foolproof doesn’t exist, the fools are too inventive 
For 99% foolproofness, an external passive attenuation or even better, a fitting gain of the headphones amp, in conjunction with ADI-2 DAC’s highest Ref Level needs to be used.
For 100% security a password lock would be needed, that doesn’t exist.
Lock Volume here means the volume can not be changed at the ADI-2 anymore. If the headphone amp has its own volume control that one could be used.
The ADIs also have Lock Keys, which would prevent someone not familiar with the unit to change the RefLevel. On the other hand, aynone not familiar with the unit wouldn't find that setting in the menu...
Just wanted to say how great it is to see you here :-)
Thank your for all of your contribution in the audio world.
KaiS wrote:The following assumes you want to use ADI-2‘s Digital Volume Control:
In this case “Auto Ref Level” function is quasi mandatory for best signal quality.
“Auto Ref Level” works best when it can use it‘s full working range, all existing reference level steps.Therefore calibrating your estat amp‘s clipping point to ADI-2 DAC‘s full scale output would be the way to go.
ADI-2 DAC‘s full scale analog line output level (manual page 20):
XLR balanced:
(nominal Ref Level +13 dBu) + (6 dB balanced gain) + (3 dB Volume‘s headroom) = 22 dBuRCA unbalanced:
(nominal Ref Level +13 dBu) + (3 dB Volume‘s headroom) = 16 dBuIf you don‘t want to use ADI-2 DAC’s Digital Volume Control, the “Lock Volume” - function does the job.
Thanks for your response. I do not want to use auto reference level. First of all, as mentioned in the RME manual, any improvement in signal to noise ratio by automatically optimizing the analog output reference level will not be perceived. Second, I work with a calibrated level system called the K-System (which I invented) where a given dBr monitor attenuation will produce a predictable SPL with a given audio source. My main mastering loudspeaker system is calibrated in this way and makes the process of evaluating audio compression much easier.
For the external headphone amplifier, I would like to have at least a predictable SPL with a given source with a given dBr attenuation. Therefore I need a single, fixed analog output reference level from the ADI-2. I've already calculated that the external power amplifier will clip if I use the +19 dBu reference (balanced) from the ADI-2. The optimum reference in my situation to prevent clipping will be the +13 dBu reference (with 3 dB additional attenuation that I will build into the headphone amp). Obviously I would like to prevent someone (not me of course :-) accidentally setting the reference level to +19 dBu. That's why I would love to be able to "lock" the ADI-2 to the +13 setting in options.
I have your book and know K-System.
Great!
For the external headphone amplifier, I would like to have at least a predictable SPL with a given source with a given dBr attenuation.
…The optimum reference in my situation to prevent clipping will be the +13 dBu reference (with 3 dB additional attenuation that I will build into the headphone amp). A certain SPL for a given digital signal - and to calibrate the level just below clipping the headphone amp at 0 dBFS - are requirements that exclude each other.
Not necessarily, depends on the headroom of the amplifier. But I was trying to say that no clipping would be a secondary and probably unnecessary benefit, as long as the amplifier is below clipping at 0 dBr I would be happy.
In my case I'm stuck between a rock and a hard place as the headphone amplifier I am building will either go into clipping at -3 dBr in one reference case or produce significantly lower output in another reference case. Unless I build a 3 dB or greater attenuator. I have to finish building the amp and I strongly suspect the SPL will be "enormous" and there will be no temptation to push dBr up too much. In that case I would like to make another feature request where a user can calibrate dBr to their own preference, for example, 0 dBr ----> 83 dB SPL with -20 dBFS pink noise or some other reference. I seem to have found with headphones that they have to be pushed to a higher spl for the same perceived loudness than loudspeakers.
To be honest, I would be annoyed by 6 dB jumps in volume at certain positions of the dBr control, which is the behavior of auto ref. I would be interested in hearing anyone's opinion if there is any PRACTICAL advantage to the auto ref. MC's intention was to offer an optimized SNR at any of the reference levels, which is exemplary, but admitting that the SNR is so good already that any of these settings are likely to produce imperceptible noise. Perhaps with a highly sensitive IEM???
You wrote:
This one‘s easy:
• Activate “Auto Ref Level”, even if you don’t use it.
• Then dial the Volume while watching the Ref Level display just above the Volume dBr.
The Ref-Level switch-down point is 0 dB digital attenuation.
• Now activate “Lock Volume”.
Without going to the menu and deactivating “Lock Volume”, no one can change the output level then.Of course, in the menu “Lock Volume” and “Reference Level” can be changed.
Foolproof doesn’t exist, the fools are too inventive
Sounds interesting. Thanks. I will try that. Hopefully without blowing my ears out as I experiment!
…In my case I'm stuck between a rock and a hard place as the headphone amplifier I am building will either go into clipping at -3 dBr in one reference case or produce significantly lower output in another reference case. Unless I build a 3 dB or greater attenuator.
A volume dial headroom of 3 dB exists in ADI-2.
Clipping takes place at +3.5 dB volume setting only (for a 0 dBFS input signal).
This extra range can be used to set a fitting ref. level.
The headroom is kind of meant to handle ISPs (Inter Sample Peaks), but the way I understand your style of working those don’t occur at yours.
…I would like to make another feature request where a user can calibrate dBr to their own preference, for example, 0 dBr ----> 83 dB SPL with -20 dBFS pink noise or some other reference.
Why not just ask for integration of the K-System?
RME already has it built into DigiCheck.
…To be honest, I would be annoyed by 6 dB jumps in volume at certain positions of the dBr control, which is the behavior of auto ref. I would be interested in hearing anyone's opinion if there is any PRACTICAL advantage to the auto ref. MC's intention was to offer an optimized SNR at any of the reference levels, which is exemplary, but admitting that the SNR is so good already that any of these settings are likely to produce imperceptible noise.
There are no level jumps in Auto Ref Level, check it out.
Analog switching and digital volume control work hand in hand for a continuous volume control in steps of 0.5 dB.
This way SNR and resolution is kept relatively constant (within 5.5 dB) over a volume dial range of ca. 23 dB.
Interestingly ADI-2’s digital volume control does not produce the expected artifacts like THD, see manual page 65, 66.
This makes it superior to traditional potentiometer controls which can cause significant distortions, even the best.
(Not to mention the gain staging problems that analog level control causes in amp design).
Using ADI-2 as a frontend can significantly lower the necessary analog gain of a power/headphone amp.
ADI-2 DAC’s full scale output is +22 dBu.
A STAX headphone can take up to ca. 54 dBu (400 V RMS, 1100 V p-p).
So just 54 – 22 = 32 dB of gain is necessary in the eStat amp.
…Sounds interesting. Thanks. I will try that. Hopefully without blowing my ears out as I experiment!
If in doubt something could go wrong, I place ‘phones behind my ears.
RME User Forum → ADI-2 & 2/4 Pro series, ADI-2 DAC series → Feature request: Lock Reference level
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