My amp is too sensitive, what can I do?

[FAQ Team]

Most line sensitive amps are designed to have in input sensitivity of around 200-500mV for FULL output so most tuners, disc preamps, CD players and the like will run into them without problems.

However some have an input sensitivity as low as 75mV ( such as WAD 5881 MkII ) and this can cause problems, particularly with CD players. The easiest way around this is to fit an input attenuator at the amps input phono sockets.

This can be fixed resistors or a variable control so you can adjust sensitivity to suit.

There are two important values to know 1) input resistance (impedance) and 2) input sensitivity usually in mV. From these two figures it is relatively simple to change the input sensitivity.

Suppose that the two figures are 100K and 75mV, if you place a resistor of 100K in series with the input i.e. from input socket to the PCB then effectively you have put a potential divider on the amplifier and immediately decreased its sensitivity to 150mV.

The way this works is as follows. Imagine two resistors of 100K joined together in series (daisy chain fashion) so their total resistance is 200K. If you place this chain across a 1-Volt battery and measure the voltage with respect to the negative pole of the battery, at the positive terminal of the battery it will read 1 Volt (naturally) and at the junction of the two resistors it will read 0.5 Volt. This is because the two resistors are equal. If you now change the resistor at the top end i.e. attached to the positive terminal to 200K then the voltage at the junction will be in the ratio of 200:100 i.e. 2:1 and will read 0.333Volts. And so on.

A simpler method is to replace the top resistor with a variable control and adjust for best results. Then measure the variable resistor and replace it with a fixed one of approx. the same value. It is not critical.

The other solution is to place a potentiometer, say 500K, across the input socket with the wiper taken to the PCB input and adjust for best results, then replace with fixed resistors if you wish.

In this case the lower half of the 500K pot is in parallel with the input impedance of the amplifier so the sums are as follows:

Upper section of pot =250K, lower section of pot = 500K in parallel with 100K.

Ohms law states for resistors in parallel 1/R1 = 1/R2 + 1/R3 + 1/R4 etc. For two resistors only this can be shown to equate to R1 x R2/R1 + R2 or "product over sum". Replacing with numbers gives 250K x 100K / 250K + 100K = 250,000,000/350,000 = approx. 71,000.

So the input voltage is being attenuated in the ratio of 250:71 approx. 3.5 so your input sensitivity has gone down to 75 x 3.5 = 260mV

These sums can be applied to any input that you wish to reduce the sensitivity of.