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Old 07-30-2018, 10:41 PM
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Should amp output voltage vary between channels?

I just installed a new JL Audio M600/6 amp to power my cabin speakers and sub. When I was measuring the voltage on the amp output channels to set my gains, I noticed that channels 1 and 2 had different voltages even though they are controlled by the same gain knob. When I set channel 2 to 17.8 volts, channel 1 was 16.8 volts. So, channel 1 is 1.0 volts less which is about 6%. Is this much variation normal? I double checked with a 2nd multimeter and got the same result. I have a message into JL Audio tech support but haven't heard back. (You can't call them any longer.)

I also checked the voltages between channel pairs 3 & 4 and channel pairs 5 & 6. Those voltages were the same within 0.1 volts, which seems perfectly fine.
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Old 07-31-2018, 08:53 AM
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Lower output voltage on the RCAs from the headunit?
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Old 07-31-2018, 09:21 AM
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This was with no load on the chnls, right? Are you using a single RCA pair for all chnls or does 3/4 had its own dedicated pair? Did you swap the right and left RCA plugs and see if the voltage difference moved or stayed?
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Old 07-31-2018, 10:08 AM
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Originally Posted by blackhawk View Post
I just installed a new JL Audio M600/6 amp to power my cabin speakers and sub. When I was measuring the voltage on the amp output channels to set my gains, I noticed that channels 1 and 2 had different voltages even though they are controlled by the same gain knob. When I set channel 2 to 17.8 volts, channel 1 was 16.8 volts. So, channel 1 is 1.0 volts less which is about 6%. Is this much variation normal? I double checked with a 2nd multimeter and got the same result. I have a message into JL Audio tech support but haven't heard back. (You can't call them any longer.)

I also checked the voltages between channel pairs 3 & 4 and channel pairs 5 & 6. Those voltages were the same within 0.1 volts, which seems perfectly fine.
Try enabling the crossover (HP or LP- shouldn't matter) and return it to the normal position, to see if it changes. If it does, it could be a funky switch.

Without a load, this method is only approximate because amplifier measurements are only accurate with a load- this method just happens to be the best way to set the level controls without making a lot of noise.
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Old 07-31-2018, 10:23 PM
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All my measurements were with no load and I only have one pair of RCAs going into the amp.

I had sent a message to JL Audio about the 1.0 volt difference. They said that they have seen a difference in 1.0 volt between channels and it wonít be audible. The difference at full power is a little over 8 watts. If you do the math, it is only a 0.61db difference, so fairly insignificant. JL never really said if 1.0 volt of difference is within spec but I assume it is.

I decided to go over things with a better scope tonight so I borrowed a Tektronix scope from work. The output voltage from each of the RCA cables out of the head unit was dead even out to 2 decimal places. Then I checked channels 1&2 and they were within 0.4 volts!!!. I double checked this with a multi-meter and it also showed a 0.4 volt difference. No idea what changed from the other day, but it got better somehow????

If you go by the manual, you should set the gain to 17.4V output unbridged. That gives you 75 watts. On this M600/6, if you go past that point it will start clipping at around 17.6V. That leaves very little headroom but you get the rated 75W. The M400/4 that I put in a couple of years ago would get up to about 18.0V before it would clip. I mentioned this to JL also, and they were surprised that I couldnít get past 17.6V. They said that they see a lot of amps get up to 19.0V before they clip. I did all my measurements with the engine off since my boat is on the trailer. Does the lower battery voltage have any effect on this?

I included a pic of the waveform at 17.2V with a 1000Hz test tone. The peaks of the sine wave have a lot of noise in them. I saw the same thing with the Fluke oscilloscope that I used the other day. Is it normal to see that much noise at full voltage?
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Old 08-01-2018, 07:53 AM
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Originally Posted by blackhawk View Post
All my measurements were with no load and I only have one pair of RCAs going into the amp.

I had sent a message to JL Audio about the 1.0 volt difference. They said that they have seen a difference in 1.0 volt between channels and it won’t be audible. The difference at full power is a little over 8 watts. If you do the math, it is only a 0.61db difference, so fairly insignificant. JL never really said if 1.0 volt of difference is within spec but I assume it is.

I decided to go over things with a better scope tonight so I borrowed a Tektronix scope from work. The output voltage from each of the RCA cables out of the head unit was dead even out to 2 decimal places. Then I checked channels 1&2 and they were within 0.4 volts!!!. I double checked this with a multi-meter and it also showed a 0.4 volt difference. No idea what changed from the other day, but it got better somehow????

If you go by the manual, you should set the gain to 17.4V output unbridged. That gives you 75 watts. On this M600/6, if you go past that point it will start clipping at around 17.6V. That leaves very little headroom but you get the rated 75W. The M400/4 that I put in a couple of years ago would get up to about 18.0V before it would clip. I mentioned this to JL also, and they were surprised that I couldn’t get past 17.6V. They said that they see a lot of amps get up to 19.0V before they clip. I did all my measurements with the engine off since my boat is on the trailer. Does the lower battery voltage have any effect on this?

I included a pic of the waveform at 17.2V with a 1000Hz test tone. The peaks of the sine wave have a lot of noise in them. I saw the same thing with the Fluke oscilloscope that I used the other day. Is it normal to see that much noise at full voltage?
The resolution of the display looks like it's set lower than its highest level- kind of surprised by that. As far as the noise, is the function generator in the scope, or is it separate? At the upper right of the display, it shows 'Noise Filter Off'.

IIRC, the output spec from JL shows a voltage range of 11.7VDC- 14VDC, right? Did you check the battery voltage while the amp was being tested? It may have dipped below the minimum in their spec.
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Old 08-01-2018, 10:37 AM
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The voltage will make a difference if the boat is running vs not. If I’m out of the water like you are I make sure to have my battery charger on so that I can maintain my 12 volts while I’m setting everything up. JL’s site shows that your amp would see a 20 ohm drop on 12.5 volts if your bridged. So 180 ohms per bridged channel instead of the 200 ohms that you would get on 14.2 volts. I wouldn’t worry so much about your voltage readings on the channel since your scoping the channels. Just make sure you have a clean wave peak to peak. Also, use a 40hz wave when you setup your sub channel.
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Old 08-01-2018, 11:48 AM
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Originally Posted by cbreck0318 View Post
The voltage will make a difference if the boat is running vs not. If Iím out of the water like you are I make sure to have my battery charger on so that I can maintain my 12 volts while Iím setting everything up. JLís site shows that your amp would see a 20 ohm drop on 12.5 volts if your bridged. So 180 ohms per bridged channel instead of the 200 ohms that you would get on 14.2 volts. I wouldnít worry so much about your voltage readings on the channel since your scoping the channels. Just make sure you have a clean wave peak to peak. Also, use a 40hz wave when you setup your sub channel.
You mean 20/180 Watts, I assume.

The JL setup procedure specifies 1KHz- the sub can be set independently, based on user preference and system capability. Obviously, if the sub output can't compete with the rest, it either needs to be enhanced, or the main level needs to be backed off.

I was wrong- they don't state power output as the same with 12VDC-14.4VDC- they actually show the output separately at 12.5VDC and 14.4VDC- the M600/6 shows [email protected] and [email protected]
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Old 08-01-2018, 01:18 PM
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cbreck0318 cbreck0318 is offline
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You do set the sub channel independently of the other speakers you are correct. You still want to use a 40hz or I believe JL uses a 50hz tone to set that channel. Back your gain off accordingly if the sub can’t handle the power. Which sub are you using?

Last edited by cbreck0318; 08-01-2018 at 02:16 PM.
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Old 08-01-2018, 04:55 PM
LDA6339 LDA6339 is offline
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Quote:
Originally Posted by blackhawk View Post
All my measurements were with no load and I only have one pair of RCAs going into the amp.

I had sent a message to JL Audio about the 1.0 volt difference. They said that they have seen a difference in 1.0 volt between channels and it wonít be audible. The difference at full power is a little over 8 watts. If you do the math, it is only a 0.61db difference, so fairly insignificant. JL never really said if 1.0 volt of difference is within spec but I assume it is.

I decided to go over things with a better scope tonight so I borrowed a Tektronix scope from work. The output voltage from each of the RCA cables out of the head unit was dead even out to 2 decimal places. Then I checked channels 1&2 and they were within 0.4 volts!!!. I double checked this with a multi-meter and it also showed a 0.4 volt difference. No idea what changed from the other day, but it got better somehow????

If you go by the manual, you should set the gain to 17.4V output unbridged. That gives you 75 watts. On this M600/6, if you go past that point it will start clipping at around 17.6V. That leaves very little headroom but you get the rated 75W. The M400/4 that I put in a couple of years ago would get up to about 18.0V before it would clip. I mentioned this to JL also, and they were surprised that I couldnít get past 17.6V. They said that they see a lot of amps get up to 19.0V before they clip. I did all my measurements with the engine off since my boat is on the trailer. Does the lower battery voltage have any effect on this?

I included a pic of the waveform at 17.2V with a 1000Hz test tone. The peaks of the sine wave have a lot of noise in them. I saw the same thing with the Fluke oscilloscope that I used the other day. Is it normal to see that much noise at full voltage?
This post could be written in Chinese and I'd understand no more or less of it
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