Setting the Amplifier Optimum Gain with Multimeter

[Lameassonwheels]

What is Gain setting in an amplifier?

The output voltage from the Head Unit is matched with the Amplifier input to get the best output from the setup. The common HU output voltages are 2V-4V.
(You can find some Pioneer HUs lists "Hi-Volt Preouts (4 V)" as a special feature). The higher gain setting not necessarily makes your setup louder.
If the output voltage of the HU is less, then the gain need to be set high in the amp, and if its is high (4V etc) the amp need a low gain setting. The higher the gain then the amp is more sensitive to what is fed from HU.

Similarly you can find information like below in the amplifier specification:
Maximum input signal: 6V
Maximum sensitivity (minimum input signal for rated output): 200mV
(Source: JBL GT X-646 Manual)
So the idea is to set the gain as optimum without pushing either amplifier or HU to its limits and deliver a good sound output at the same time. Unlike the crossover settings and bass boost it is not a great idea to set the gain just by sensing with 'ears'.
P.S: There is a crude method of increasing the gain till the speakers starts to distort.

Prerequisites & Notes:
• Digital multi meter which reads AC voltage.
• Screwdriver.
• 50Hz (For Subwoofer channels) and 1000 Hz (For front channels) sound files burnt in CDs or copied to USB drives.
(Download the files from the below post). Optionally you may generate the frequency tones as required specifically for your
setup using Audacity (an Open Source freeware).
• Double check the calculations before setting the gain voltage.
• Make sure you are not exceeding the rated values of speakers and amplifier.

Calculating the Gain

Basis of this calculation is derived from Ohm's law.

Voltage = Square root ( RMS power of amplifier channel x Connected Speaker Impedance)

I have used the 4 channel amp, JBL GT X646 here in the calculations.
2 channels are connected to front components and other 2 are bridged to connect a subwoofer.
The values will change specific to your amp make and model.

Amplifier channel-Front
No of channels=2
RMS output per channel of AMP=70W
Impedance of Speakers connected=4Ώ
A= (RMS out of amp* Speaker Imp.)=280
√A=AC Voltage (Corresponding to Gain)=16.74 VAC per channel

Amplifier channel-Rear/ Subwoofer
No of channels=1 Bridged
RMS output (with Bridge tied load)=200W
Impedance of subwoofer connected=4Ώ
B= (RMS out of amp* Speaker Imp.)=800
√B= AC Voltage (Corresponding to Gain)=28.29 VAC Bridged channel.

Make necessary modification in the calculation when 2Ώ speakers, rear component speakers, 3 preout HU, 2/5 channel amplifiers etc are used.

 

[Lameassonwheels]

Generating the test sound files using Audacity

Download the software from audacity.sourceforge.net. Go to "Generate" in the toolbars and select "tone" from the drop down menu. Select "Sine" as the waveform, type in the required frequency (Eg:100Hz), type in the amplitude as 0.5 to make it to reference level (0db). Enter the duration of the tone (Eg: 5 minutes). Go to "File" and "Export" the sound file. Refer the 'Equalizer spectrum' for standard frequency range of various instruments and audio components. Please find the 50 Hz and 1 KHz test sounds in the attached zip files.

 

[Lameassonwheels]

Setting the gain on the amplifier

• Gain setting is done for one channel at a time, so disconnect the other channel inputs and speaker connections during this.
• Turn off the Equalizer, crossover settings, bass boost on the HU. Turn up the volume till 3/4 th of the upper limit.
• On the amplifier, turn the gain screw anticlockwise fully, make the crossover filter settings to OFF/ FLAT. Turn off the bass boost.

We will start with the subwoofer bridged channel. Disconnect the RCA input cables from the front input(ONLY) and all speaker wires from the amplifier.

• Now Set the multimeter to 'AC Voltage' mode and ‘200V’ range. (IMPORTANT: NOT DC Voltage, otherwise it may damage the multimeter)
• Connect the red probe of the multimeter to the + terminal of the bridged channel and the black probe to the - terminal of the speaker connectors.
• Insert the 50Hz sound CD/ USB and play the sound file. Make sure the HU volume is turned up 3/4th or more.
• Now observe the reading in the Multimeter screen. The amplifier will have pots for adjusting the gain/ input level. Adjust the pot by turning it clockwise gradually with a screw driver till it reaches the 28.29V level. (As calculated above. May be different in your case).
• Repeat the procedure for front channel and play the 1 KHz sound file. Disconnect the RCA input cables from the rear/sub input (ONLY) and leave speaker wires as disconnected as before. Adjust the pot till 16.74 in front channels. Check the Voltage reading in both FR, FL channels.

Now put the RCA and speaker connections back. Set the Equalizer, crossover, bass boost on the HU and crossover, bass boost settings on the amplifier as it sound sweeter to your ears. Now We are good to go, enjoy your favourite music.

The HU used here is Sony XAV62 (3V preout) and 4Ώ JBL subwoofer and component speakers connected to the JBL GT X646 amplifier.

 

[Akash]

Article moved from TIP section. Thanks for sharing!