## Gain Meter Purpose: The measurement and display of relative strength of two voltages (AC or DC) across or inside a circuit in dBs (decibels).

Signals often differ by great magnitudes. The miniscule output voltage from a moving-coil phono cartridge (.1mV) becomes amplified to 1000 volts peak-to-peak at the tube-amplifier’s output tube’s plate. The ratio between these two voltage is 10,000,000. But expressed in dB (decibels), this ratio becomes only 140 dB, a figure that is much more manageable. Asides from compressing large ranges, dBs allow fro easier math. For example, given three identical gain stages that produce a voltage gain of 31.6 times the input, placed in series, what is the total gain? It’s time to find the calculator. But a ratio of 31.6 in dB is 30 dB, which can directly be multiplied by 3 to yield 90 dB of gain; much easier.

 0.00001 -100dB 0.0001 -80dB 0.001 -60dB 0.01 -40dB 0.1 -20dB 1 0dB 10 +20dB 100 +40dB 1,000 +60dB 10,000 +80dB 100,000 +100dB

Alternatively, a linear display of relative signal strength can be used by pressing the “Linear” button. The display will then show the second signal divided by the first signal.

To measure the ratio between to signals in or through a circuit, the Gain Meter must be attached to two nodes and their references (usually, ground). The gain meter displays its measurements in two ways: text and a moving bar graph. The bar’s length expands and contracts with the ratios/gain being measured, which makes it an analog readout. When the measured signal gain exceeds the gain meter’s range, the bar changes color from its normal blue to red. Pressing the “Auto” button overrides the fixed range and auto adjusts the range to twice the highest distortion reading it sees.

The Functionality tab allows you to select between AC (mediated-coupled), where the meter functions as if it were coupled via a capacitor) or DC (direct coupling), where the meter reads the instantaneous voltage directly without processing. If the signal is AC, then choose whether the RMS (Root Mean Square) value, Average, Absolute Average, Peak-to-Peak, or the peak value should be used. Then select a sample period for the ammeter.