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| By: Victor on Sat, 24 Jan 2009 01:23:11
| | Tweaking an audio system is not as easy as one may think..a few tools, technical know how and definitely very very trained hearing are necessary for it... anyways.. here is the first step towards it.
Gain settings Simplified.
For the ease of it , I have considered a JBL GTO - 75.4 amplifier.
1 - Set everything to "0" - flat eq, hpf off, lpf off bass boost off,loudness off, all x-overs flat... every thing a huge 0 on the HU as well as the amp.
2 - Get the gains all the way down...before you proceed further..
3 - Take a DMM ( digital multi meter ) and a 1KHz test tone to set gains on a class AB amp for mids and highs
download it from here
http://www.box.net/shared/jznyirvzty
burn it on a cd to play ...
4 - Set the DMM to measure AC voltage ( not DC ) , remove ch 3 & 4 speakers as we are going to set only ch1& 2 right now.
5 - Play the 1KHz test tone and put the volume on the HU at 80% of the full volume...
6 - Connect the + of multimeter to +ve of speaker output terminal, and -ve to -ve ....
7 - If the amplifier is 75wrms x 4 ch, you should aim at a voltage reading of 17.3 volts ( this is calculated by ohms law ). So adjust your gain knob till you get a reading of 17.3 volts. this would be a precise technical way to set amplifier gains.
8 - Disconnect ch1&2 and connect 3 & 4 repeat the same process for gain settings of these channels.
9 - slight deviation in these readings would be ok.
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How to know what voltage reading to aim for...?
simple..
Volts = sqroot of {watts ( rms) x load ( ohms )}
say for example you got an amp that is rated at 50wmrs per channel @ 4ohms,
then
Volts = sqroot { 50wrms x 4 ohms }
= sqroot {50x4}
= sqroot {200}
= 14.14 volts
if you are setting gains on a sub woofer you can use the following 50 Hz test tone, rest of the procedure remains the same..
http://www.box.net/shared/j38l4psh2q
lets say you got a 600wrms @ 2 ohms rated amplifier, play the 50Hz track and aim for
Volts = sqroot {600 x 2}
= sqroot {1200}
= 34.64 volts.
If you are bridging 2 ch on an amp for powering a sub, simply double the voltage reading... how??
lets see.. say we got a 2 ch class A/B amp rated like this
a - 100wrms x 2 @ 4ohm stereo
b - 200wrms x 2 @ 2 ohms stereo
c - 400wrms x 1 @ 4 ohms bridged
Va = sqroot {100wrms x 4ohms} = sqroot {400}= 20 volts ( stereo )
Vb =sqroot {200wrms x 2ohms} = sqroot {400}= 20 volts ( stereo )
Vc = sqroot {400wrms x 4ohms} = sqroot {1600}= 40 volts ( bridged )
20volts setting in stereo mode, where as 40volts setting in bridged mode... most class A/B are stable only at 4 ohms in bridged mode.. so simply double the voltage you aim for...
a lot of people trust their ears, but it depends on how trained your ears are at differentiating between clean and clipped signals.
the above technique is a fool proof technical method to set the gains on your amplifier, JUST RIGHT....
Fenil Mehta
Sound & Acoustic Designs.
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