Using a Voltage Meter to set your amp gain? - diyAudio
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Old 18th January 2008, 12:26 PM   #1
Flyin11 is offline Flyin11  United States
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Default Using a Voltage Meter to set your amp gain?

I've seen from a few sites that you can use a voltage meter to measure how much power is coming out of amp to set your gain to the right output. I uploaded a file on one I found to my site so you can download it and take a look. It's a Excel file. http://www.flyin11spage.com/Pictures/Audio-Calc.zip If you go into Gain Control Adjustment you will see what I'm referring to. You are suppose to set the voltage meter to read AC voltage and insert the leads into the channels you are using. When I insert them into the speaker channel inputs I get nothing on the voltage meter. However, if I touch the power and ground it reads around the voltage on the chart. I'm not sure if that's what they meant how to do it or am I doing something wrong? I mean, I get no read out from the channels on the amp but I get the reading from the power and ground, that doesn't make too much sense to me. Wondering if anyone has ever tried this using a voltage meter and what were your results? How did you do it? I might just have to wing it and go the regular route and turn it little bit by little bit everyday till I get the right mark but that could take forever. This would be a faster way for me and I really don't like having to stop the car every so often and turn the dial and so on, all day long to try and find a good setting. Any help would be appreciated...Thanks
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Old 18th January 2008, 01:07 PM   #2
luka is offline luka  Slovenia
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Hi

Use test tone, 50 or 60Hz, must be 0dB, get them from ROE or where ever you can find and do like this, you must have dummy load or speaker connected to be 100% sure, if you don't output could be set, but will be lower when load will be connected

http://www.youtube.com/watch?v=zm4aaC87ARg
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Old 18th January 2008, 01:30 PM   #3
Flyin11 is offline Flyin11  United States
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Thanks...I'll have to check that out when I get home as my work blocks youtube stuff...
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Old 18th January 2008, 04:51 PM   #4
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It sounds like you had your meter set to DC volts if you read voltage across B+ and ground.
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Old 18th January 2008, 05:41 PM   #5
Flyin11 is offline Flyin11  United States
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I don't what is going on...I did like it said to do once again during lunch and I'm getting nothing. Again, if I have it set to AC and touch the capacitor or the power and ground of a amp I get like 31.8 but that doesn't change when you change the volume. I'm still getting nothin in the speaker terminals which is weird cause I have sound full blast and all. I might try to use another one or something and see if another meter will work. I don't get it. It didn't work for the Alpine either and I touched the gold plated screw down terminals and all. I stuck the red lead into the + speaker terminal I use and the black in the - ...zippo...
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Old 19th January 2008, 01:52 AM   #6
Flyin11 is offline Flyin11  United States
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Okay...After checking out Luka's post about the Youtube thing and watching it, I figured out what I did wrong LOL I didn't have the RCAs hooked up That might have helped. I didn't want it to play through the amp and have sound so I took them off and thought if I just touched the terminals it would show the output but I didn't think about the headunit volume and all that LOL

So, after going through the process, I have to say, I don't think I truly believe the readings on it. I mean, I checked and it said it was at like 40v coming out of it which according to that chart I had is equal to 800watts into a 2ohm load. Peak maybe but that is no way RMS. The amp was tested and is rated at 431rms max in a 2ohm load. Yep, that was with the bass boost off as well. My 2 JL's are rated at 50-150w RMS each and 300w max. I tried setting it according to the chart at 350w but it didn't sound and hit hard enough to me. So I ended up setting it to 400w and it sounds pretty clean and hard now. I think JL's are a little underrated anyways as I don't see any issues with it at 400w and 200 into each sub. Like I said though before, I don't think the numbers are exact as there is no way that it was pushing 800watts I might try and turn the bass boost down a little bit and that should clear it up a little but I think I have it where I want it now.
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Old 19th January 2008, 04:19 AM   #7
ppia600 is offline ppia600  United States
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If you are matching gain on two amps, just set the meter to the lowest AC range and connect it between the two amps using the same speaker terminal on each amp. For example, use either the positive or negative of the bridge terminal on both amps. Run the same signal mono (better to use a "y" adapter splitting one signal into two from the deck to be more accurate) into both amps. Set one amp at a good point that matches the deck's volume control with good headroom and low distortion. Now play the set frequency coming from the deck (I used 40hz and then tried 80) with a constant volume level (use the repeat function to stay on the same track) and adjust the second amp until you find a point where there is the least difference in voltage between the two amps. I adjusted mine where there was about a 20mv average difference between the two and it sounds great. Amazing how much difference there can be in the gain controls between two of the same exact model.


I connected the negative meter lead to the positive bridge terminal on one amp and the positive meter lead to the positive bridge lead on the other. I might try to draw something...
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Old 19th January 2008, 06:57 AM   #8
Flyin11 is offline Flyin11  United States
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Nah...Just setting the gain on 1 amp...The sub amp...
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Old 19th January 2008, 08:35 AM   #9
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Using this method is not 100 percent spot on...it is better than the doing it by ear method...using a multimeter simply gets you in the ballpark range.

Speaker outputs are ac voltage...power and ground are dc voltage
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Old 19th January 2008, 08:56 AM   #10
luka is offline luka  Slovenia
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Hi

Well true, but that is if you look at power only at one time, like now, now, now...

But if you look at average value of a sine wave, say 40v AC, and have this voltage on 4 ohme load you will get current that is also average.... Average, if only one freq. is same as RMS so...
40v^2/4 = 400w and this is rms value...
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