Operating instructions
2
LA-610 Operating Instructions
The LA-610 is a vacuum-tube microphone/instrument/line preamplifier with basic EQ
and an optical program dependent compressor. The main preamp circuit has two gain
stages that utilize a dual-triode tube operating in a class A single-ended configuration.
Variable negative feedback is applied to both of these stages to control gain, distortion,
and frequency response. Balanced inputs and outputs are transformer coupled.
610 Front Panel (left side)
This panel has the Input Select, Gain, Level, EQ, Pad, Polarity and +48 volt Phantom
controls as well as a Hi-Z Input. Each control is discussed in the following sections.
LOW
HIGH
GAIN
LEVEL
Hi-Z
4.5K
7K
10K
-15 dB
PAD
0
-5
-10 +10
+5
Line
2.0K
500
2.2M
47K
Mic
Hi-Z
70
200
100
0
+1.5
+3
+4.5
+6
+9
-1.5
-3
-6
-9
-4.5
ø
IN
0
+1.5
+3
+4.5
+6
+9
-1.5
-3
-6
-9
-4.5
ø
OUT
+48V
OFF
10
9
8
7
6
5
4
3
2
1
0
Tube Mic Pre
Figure 1: 610 Front Panel
Input Select
The Input Select switch determines which input is active: Mic, Line, or Hi-Z. Within both
the Mic and Hi-Z areas, the switch includes two settings to select different input
impedances.
• Mic: Selects the signal from the rear-panel, balanced, MIC INPUT XLR connector.
The impedance for the MIC INPUT can be set to 500 or 2K . Switching between
these two positions while listening to a connected microphone may reveal a different
tonal quality and / or gain. A typical mic preamplifier should have an input
impedance equal to about 10 times the mic output impedance. For example, if your
mic has an output impedance between 150 and 200, set the switch to the 2K
position. However, since making music is not necessarily about adhering to technical
specifications, feel free to experiment with the settings to attain the desired sound.
You will not harm your microphone or the LA-610.
• Line: Selects the signal from the rear-panel, balanced, LINE INPUT XLR connector.
LINE INPUT has an input impedance of approximately 13 k ohms and is intended to
accommodate mixers, tape machines, other mic preamps or any device with a line