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...some notes on dBu and maximum preamp input levels

Posted by PeAK (A) on March 5, 2005 at 08:16:56

In Reply to: Benchmark Dac 1 output levels? posted by gdg on March 4, 2005 at 17:34:58:

Hi,

Not a Benchmark owner but this question arose for another DAC product with the option of higher output.

The output levels are mostly set by standards aimed at defining what input is needed to meet full rated power (or equivalent voltage) for a given input. For an amplifier than can, a 1V rms input will generated 28.2V rms (or a 100W). If average listening levels are average out to about 0.1W leaving about 60 dB (x1000) of dynamic headroom for peaks.

In turn, preamps are defined relative to the 1V rms level required by the amplifier. Almost all traditional preamps have a gain of about 4 (12 dB) and thus the sensitivity (for full power output) will be 250mV (i.e. the input needed to generate 1 V output).

The dbU is something from the studios and microphone days when things where viewed from the perspective of power or power gain. A arbitrarily low levevl of power (input) was chosen to be 1 mW into 600 ohms (microphone output and load impedance for optimum power transfer). This signal works out to be 0.775V rms and is called 0 dBu . It is the unit that the professional arena seems to used to discuss input/output levels.

The table below highlights the above information using commonly available input levels and some Benchmark specific numbers as a reference.

Input Level (V rms)	dBu	Notes	Required peak Preamp Level (V)
0.250	-9.8	Input sensitivity level	1.41
0.78	0.0	Reference Level	4.38
2.00	8.2	Nominal CDP level	11.31
2.45	10.0	Min Benchmark Preset	13.86
7.80	20.0	DI/O output level	44.12
21.84	29.0	Max Benchmark Preset	123.32

The trick of boosting signal levels for CD to 2V RMS allows the signal to noise ratio to improve for an preamp/amplifier with a given level of noise at its input stage. The penalty is that the maximum peak preamp output level increases and must be accomodated by appropriately larger supply voltages. The minimum preset output level from the Benchmark of 10dBU (13.86V peak) means the preamp better have supply rails on the order of 15 volts. You can find out more about this by reading about "overload margin". A bigger deal in the phono preamp days with rising input levels at higher frequencies. The advantage of analog ?

The suggestion earlier of calibrating the outputs using a 0 dbFS signal is a good one. Something around like 10 dBU should be safe for most active preamps while slightly larger levels should help improve the SNR for systems dominated by input state noise.

Note: Passive preamps only attenuate and by definition cannot clip an input waveform. The maximum input level in this case shifts to the power amp. The maximum preset level for the Benchmark could be accomodated by having a pssive preamp set to attenuate the signal by 26 dB (-26 dB) to pass a 0 dbFS signal without clipping.

Hope this helps,

PeAK

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