Benchmark LA4 - Graph 2: Stepped Gain Control - Output Noise vs. Gain

Instruction Manual for LA4 with 1.0.X Firmware – REV A Page 30
Graph 2 - Stepped Gain Control - Output Noise vs. Gain Setting
The stepped gain control has a gain range of -112.5 to +15 dB in 0.5 dB steps. This plot shows the
A-weighted output noise over a gain range of -100 dB to + 15 dB. The A-weighted output noise is
plotted relative to the maximum balanced output level of 28 dBu. The signal to noise ratio at any
gain setting is the difference between the two curves. At maximum gain, the SNR is 15 dB - (-133
db) = 148 dB. At a gain of -20 dB, the SNR is -20 dB - (-146 dB) = 126 dB. This test demonstrates
the wide operating range of the fully-balanced low-impedance stepped gain controls.
Below a gain of -20 dB, the noise is determined by the thermal noise (Johnson noise) produced by
the resistors in the stepped attenuator. Each side of the balanced attenuator has an impedance of
just 1210 Ohms. This means that the balanced output impedance of the attenuator is 2420 Ohms.
This resistance determines the output noise of the balanced attenuators. At any gain less than -20
dB, the A-weighted noise floor of the stepped gain control in combination with the balanced output
buffers is -146.5 dB relative to +28 dBu. In other words, the A-weighted output noise is -118.5
dBu. This is approximately equivalent to the thermal noise produced by a single 2.5 kOhm resistor.
This shows that the active balanced output buffers do not add any significant noise to the stepped
gain control. The active output buffers are important because they provide low impedance drive to
downstream devices (such as power amplifiers).
Audio Precision
07/17/18 10:41:20
Attenuator_SNR.at27
ColorSweep Trace Line Style
Thick Data Axis Comment Cursor1
Cursor2
1
1 Cyan Solid
4 Anlr.Level A
Left Gain -20.070 dBr A
+14.947 dBr A
1
2 Green Solid
4 Anlr.Ampl Right A-Weighted Output Noise
*-146.039 dBr *-133.045 dBr
-150
+10
-140
-130
-120
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
+0
-146.039
-133.045
d
B
r
B
-150
+10
-140
-130
-120
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
+0
d
B
r
A
-100
+10
-90
-80
-70
-60
-50
-40
-30
-20
-10
+0
-20.07 +14.947
dBr A
dx=+35.017 dB
dy=-12.994 dB