Scientific approach to setting amplifier gains

[cab13367]

We have all seen on this forum the helpful posts by Phil, David, and Brian on how to properly set amplifier gains by ear. The issue with this is that these three have been around stereos most of their lives so they can easily hear what max output/borderline distortion sounds like. But for the rest of us, I don’t think it’s as easy.

Which brings me to the purpose of this post. The manual for my M6600 JL Audio amp describes a way to set the gains by measuring output voltage at the speaker wires (with the speakers NOT connected) and setting the amplifier gain to achieve a specific target voltage. This method requires the use of a test CD that contains sine waves at specific frequencies which JL sells for $10. You set the HU volume to about 3/4, insert the CD, choose the track with the appropriate sine wave frequency, then turn up the gain until the voltmeter reading equals the target voltage. The target voltage is calculated using the following formula:

Target Voltage = the square root of rms power per channel times resistance. For example, on my M6600, which is rated at 75 watts per channel at 4 ohms, the target voltage when connecting a pair of 4 ohms speakers to a pair of channels is 17.3 volts. This is calculated by taking the square root of [ 75(rms power) x 4(resistance in ohms) ]. 75 x 4 = 300. The square root of 300 = 17.3.

So you would be looking at a voltmeter instead of listening for distortion or clipping which at least in theory, would seem to be a more accurate and objective way to set the gain. This method assumes that your speakers are rated to handle the maximum rms output of your amp.

So has anyone used this method and if so, could you comment on how it worked for you? Or is everyone doing it by ear? I am an engineer by education and manage an engineering office so the technical method appeals to the way I think

Phil/David/Brian - any thoughts on this?

Thanks,

Al

[lewisb13]

So crazy I was JUST typing out a new post asking if you could use a multimeter to match the output voltage at each channel with the output voltage preout(s).

[cab13367]

So crazy I was JUST typing out a new post asking if you could use a multimeter to match the output voltage at each channel with the output voltage preout(s).

Brad,

If you are going to do this, please read "The Nine-Step Procedure" on pg 14 of the manual in the link below. Unless you own this amp, you'll need to calculate your target voltage based on the formula in my first post.

http://marine.jlaudio.com/pdfs/M-08/M6600.pdf

And here is the link to the CD with the test tones:

http://www.shopatron.com/products/pr...1.1.0.0.0.0.0?

Al

[lewisb13]

Rock on. Thank you so much. This should also help all of us that have the 6 channel amps with analog gain controls for the cabin speakers. Its impossible to match 6 speakers with a little screwdriver and your ears.

[EarmarkMarine]

The JL Audio method is dead on to find the pre-clipping point of the amplifier. But there are many variables beyond that in considering the particular speakers that you have and the type of system you have. We have a very defined prescription for tuning that uses a number of sonic and visual cues. However, as much as we believe that our methodology is the best way I must tell that your individual system objectives will change the way you tune even in a procedure that is as rigid as ours. Your equipment selection will have an impact. Your system expectations will have an impact. One person wants an honest and linear system that is the mirror image of the way it sounds on the studio monitors while another person has something entirely different in mind. We all have experiences and notions that shape our idea of what 'good' sound is. There is no absolute right and wrong. To give you tuning advice we have to ask alot of questions and gain a better understanding of what you want personally. You can give me a ring and I'll walk you through the process.

David
Earmark Marine

[cab13367]

Rock on. Thank you so much. This should also help all of us that have the 6 channel amps with analog gain controls for the cabin speakers. Its impossible to match 6 speakers with a little screwdriver and your ears.

One more thing - when calculating the target voltage, be sure to use the rms rating under the conditions you will set the gain. For example, if you look at the specs of the M6600, RMS output is given as 55W at 12.5V (engine not running) and 75W at 14.4V (engine running).

[cab13367]

The JL Audio method is dead on to find the pre-clipping point of the amplifier. But there are many variables beyond that in considering the particular speakers that you have and the type of system you have. We have a very defined prescription for tuning that uses a number of sonic and visual cues. However, as much as we believe that our methodology is the best way I must tell that your individual system objectives will change the way you tune even in a procedure that is as rigid as ours. Your equipment selection will have an impact. Your system expectations will have an impact. One person wants an honest and linear system that is the mirror image of the way it sounds on the studio monitors while another person has something entirely different in mind. We all have experiences and notions that shape our idea of what 'good' sound is. There is no absolute right and wrong. To give you tuning advice we have to ask alot of questions and gain a better understanding of what you want personally. You can give me a ring and I'll walk you through the process.

David
Earmark Marine

David,

Thanks for the response. It seems that you are talking about more than just setting amplifier gains though. Sure, there will be some tweaking afterwards such as setting the equalizer settings, crossover points, etc. but I am only addressing setting of amplifier gains. Or am I misunderstanding your post?

Thanks,

Al

[EarmarkMarine]

Al,
If we are talking about a simple fullrange system then its just as simple as JL Audio describes. But in a more complex high and lowpass system both the gain and crossovers are going to be very integral and can't be separated. These are interdependent adjustments. Determining your weakest component will also decide the tuning sequence. For now I think you are good.

David
Earmark Marine

[cab13367]

Al,
If we are talking about a simple fullrange system then its just as simple as JL Audio describes. But in a more complex high and lowpass system both the gain and crossovers are going to be very integral and can't be separated. These are interdependent adjustments. Determining your weakest component will also decide the tuning sequence. For now I think you are good.

David
Earmark Marine

Thanks David. I do have a more complex system with three amps and three amplification "zones" - sub, towers, and cabins. So I guess the JL method will not be good enough in and of itself. But at least it will tell me where the max gain setting is of each amplifier and then I can work down from that.

Al

[mmandley]

The JL Audio method is dead on to find the pre-clipping point of the amplifier. But there are many variables beyond that in considering the particular speakers that you have and the type of system you have. We have a very defined prescription for tuning that uses a number of sonic and visual cues. However, as much as we believe that our methodology is the best way I must tell that your individual system objectives will change the way you tune even in a procedure that is as rigid as ours. Your equipment selection will have an impact. Your system expectations will have an impact. One person wants an honest and linear system that is the mirror image of the way it sounds on the studio monitors while another person has something entirely different in mind. We all have experiences and notions that shape our idea of what 'good' sound is. There is no absolute right and wrong. To give you tuning advice we have to ask alot of questions and gain a better understanding of what you want personally. You can give me a ring and I'll walk you through the process.

David
Earmark Marine

I think this is the best response to a question Dave has ever given. Not all techy and easy to understand. Nice way to be straight forward and not elusive Dave. 2 thumbs up