NAD C3050 meters and levels

[dougi]

After finally getting my NADC3050LE delivered, there were a couple of things I wanted to understand in relation to the analog input and preamp outputs, as I need to use both: run a separate phono preamp into the line in and drive a separate amp from the preout. Consequently I wanted to verify the overload points of both and see how the built in meters related to them, to see if they could be trusted.

SUMMARY (with DIRAC calibrated but set to off). Measurements done with a RME ADI-2 PRO FS, input full scale +24dBu

1. Line-in clips at 2.15V rms, confirming Soundstage! measurements.
2. Line-in clipping is at about +3 on the input meter
3. For the digital input/s, 0dBFS is about +3 on the input meter
4. For line-in, preout gain equals the volume level in dB
5. Pre-out clips at 4.2V rms, which does not tally with Soundstage! result of 2.6V. 4.2V does match up with the spec for the differential output of the TI PCM5242 DAC used in the C3050.
6. At volume levels of +6 or less, digital inputs will not clip the preamp output
7. 0 on the output meter is about 0.7V rms for pre-out, or around 50W into 8 ohms, assuming the amp stage has 29dB gain, so a bit less than the meter indication of 75W for 0.
8. If using the output meter for pre-out monitoring to a seperate amp, you can go well over +3 before 2V RMS output is reached (say +5 eyeball estimate).
9. Meter behaviour is not linear, exhibit overshoot with short bursts and have some variation with frequency.

OTHER NOTES (EDITED)

1. While turning DIRAC on subjectively reduces volume, for my calibration at least, only a dB or so is attenuated at 1kHz.
2. Overshoot of meters for short bursts is up to +1.5 on the meter, getting more as the bursts are shorter (10ms to 250ms bursts of 1kHz tested). About +0.5 for 250ms burst. +1.5 50ms or less.
3. Compared with 1kHz indication, meters indicate up to +1.0 higher for higher frequencies and 1dB lower for lower frequencies
4. The meters have a definate "knee" in the response around -20dBFS.
5. For digital inputs, the input meter is post DSP, including DIRAC but not volume. For the line input, it appears line in directly, no DSP. Which makes sense if the intent is to check on overload of relevant systems.

CONCLUSIONS

1. The meters are certainly not "VU" and some form of non-standard quasi-peak
2. Input meter can be usefully used for line-in or digital inputs to keep below clipping and the meters would probably be conservative for music signals due to the overshoot.
3. The pre-out has a good voltage capability and cannot be clipped at a +6dB max volume setting or below

SUPPORTING GRAPHS

LINE-IN THD vs LEVEL

PREAMP OUTPUT: THD vs OUTPUT LEVEL: line-in and various volume levels for digital in
There seems to be a glitch in the sweep for the volume= 9 setting

PREAMP OUTPUT: THD vs digital input level

APPROXIMATE INPUT METER LEVEL VS INPUT VOLTAGE

APPROX INPUT METER LEVEL VS DIGITAL INPUT LEVEL

PREAMP OUTPUT METER LEVEL VS OUTPUT VOLTS AND DIGITAL INPUT LEVEL (VOL=0)

 

[restorer-john]

Meters on amplifiers are traditionally 'calibrated' for rated power output into 8 ohms being the 0dB point on the 'watt' meter.

The NAD has its 0dB at 75W@8R which should be 24.5V RMS at the speaker terminals when the rear switch is set to 'speakers out'. But the amplifier is rated at 100W into 4 and 8 R which is 28.3V RMS for 8R.

The meters are clearly designed by the marketing department, not the actual engineering section as the markings are all over the shop and do not correlate dB with power output. -3dB with respect to 75W/0dB is 37.5W and yet the -3dB mark is lined up with about 13W! -6dB should be ~19W but it's lined up with 5W. It gets worse.

When switched 'signal in' it would be very easy to have set the unit to indicate correctly too.

I pointed out this meter fiasco when the unit was first announced a year or so ago. I figured it must have been a rough prototype I saw the picture of, but sadly the meter may look pretty but will tell you little to nothing useful. Hopefully they will fix the markings for V2.

 

[dougi]

I note that the latest BLUOS firmware update (4.4.12) has increased the volume a lot. probably at least 12dB. I daresay the preamp output will clip at lower input levels now. I may try and remeasure and confirm this. BLUOS/NAD have not confirmed whether this was intentional or not.....