Pro Amp difference in channel volume? - AVS Forum
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post #1 of 19 Old 12-19-2009, 08:10 AM - Thread Starter
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Is it normal for pro amps to have one channel that is louder than the other?

I've had two amps and they both had this. I didn't measure the first one since I didn't have a meter then but my current amp has a difference of about 1-2 db between channels on a RS meter measured right at the speakers midrange driver.

I tried swapping inputs and swapping the outputs from the amp and its always the same channel that shows an approximate 1-2 db over the other channel at the same volume/gain knob positions on the amp.

So is this normal? Do all amps from any company suffer this?
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post #2 of 19 Old 12-19-2009, 09:15 AM
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The dozen or so amplifiers I've measured with volume knobs cranked all the way have always been within about 0.1dB or so. Intermediate volume positions will be all over the map unless it's got stepped attenuation.

Are you using the same speaker when comparing channels? The manufacturing tolerances of speakers are easily in the 1-2dB range.

A better way to do this test would be to put a voltmeter on the output of the amplifier and feed the same sine wave into both channels.

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post #3 of 19 Old 12-19-2009, 09:34 AM - Thread Starter
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"Are you using the same speaker when comparing channels?"

I'm using the same model of speakers but I have a pair of them that I'm using.

"The manufacturing tolerances of speakers are easily in the 1-2dB range."

You mean that different speakers of the same model might have a sensitivity difference of about 1-2 db due to manufacturing differences?

I don't have a voltmeter but its an idea. Maybe I can find an electrician and ask him to test it out...

I suspect something is wrong with the amp if this is not normal.

By the way how did you measure the amps to within 0.1db?
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post #4 of 19 Old 12-19-2009, 09:45 AM
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Try switching the speakers between channels, LtoR , RtoL and see if it make the same reading but now just opposite of the way it was. If not then it has to be something else than the amp. If it does then make sure everything you are useing for the test has the correct settings.
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post #5 of 19 Old 12-19-2009, 09:58 AM
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My EP2500 is like this. It has stepped attenuation, but the "steps" are not in the same spot so one channel (I don't remember which one now) is a tad louder than the other. At WOT, I don't know. I never ran it WOT.

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post #6 of 19 Old 12-19-2009, 10:13 AM - Thread Starter
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LtoR , RtoL? Do you mean switching the left and right speakers? I basically did that by swapping the outputs on the amp, but I did not physically move the speaker to the other speakers position. When I tried this it was still the speaker that was connected to channel 1 that was louder, it did not stay on the speaker that was previously connected to channel 1. The reading was not *exactly* opposite which is why I noted 1-2db rather than an exact number.

What is WOT? And what do you mean by the steps are not in the same spot?
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post #7 of 19 Old 12-19-2009, 10:35 AM
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Quote:
Originally Posted by Decadent_Spectre View Post

LtoR , RtoL? Do you mean switching the left and right speakers? I basically did that by swapping the outputs on the amp, but I did not physically move the speaker to the other speakers position. When I tried this it was still the speaker that was connected to channel 1 that was louder, it did not stay on the speaker that was previously connected to channel 1. The reading was not *exactly* opposite which is why I noted 1-2db rather than an exact number.

What is WOT? And what do you mean by the steps are not in the same spot?

WOT = Wide Open Throttle
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post #8 of 19 Old 12-20-2009, 04:49 PM - Thread Starter
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Could someone suggest an appropriate,accurate and safe voltmeter and how I would go about testing the output?
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post #9 of 19 Old 12-22-2009, 04:37 AM - Thread Starter
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I was able to find someone with a voltmeter, this is what I found -

At full gain the voltage was about 107 on each with a difference of 0.4, amp was clipping. I used WinISD to generate a sinewave.

At 24 gain (I tried to keep it as close to 24 as I could but there may be a very very minor difference as its not stepped) the voltage was 67 on channel 1 and 64.7 on channel 2 with a difference of 2.3V.

So is the amp faulty?
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post #10 of 19 Old 12-30-2009, 04:59 AM - Thread Starter
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I conducted tests with parallel input to rule out differences in stereo inputs, using WinISD generating a 1000Hz sinewave @ full (no attenuation) in WinISD and full at sound card volume with a flat frequency response in the sound card EQ, with parallel input enabled on the amp, hence using only one input source at any given time. Tests conducted by a voltmeter. When voltage is stable a single value is listed, otherwise a range is listed, like 130.3-5 meaning 130.3-130.5V to denote fluctuations in the voltage as tested. At max gain the amp clips. The red clip lights are more faint than the yellow signal lights and clip limiters are on. 30Hz filter is on.

24db gain

ch 1 in ch 1 (First one referring to the DAC output channel, the second referring to the amplifiers input channel/XLR connection, same for all cases below. ch1/2 readings correspond to the ch1/2 outputs of the amp.)

ch 1 - 63.4V
ch 2 - 61.8V

ch2 in ch2

ch 1 - 63.9V
ch 2 - 62.7V

switched input -

ch2 in ch1

ch 1 - 64.1V
ch 2- 62.8V

ch1 in ch2

ch 1 - 64.2V
ch 2 - 62.8V


36db gain (max gain)

ch1 in ch1

ch 1- 136.6V
ch 2- 135.0V

ch2 in ch2

ch 1- 132.9-133.0V
ch 2- 133.7-8V

switched input -

ch2 in ch1

ch 1- 133.8V
ch 2- 134.9 - 135.0V

ch1 in ch2

ch 1- 134.3-5V
ch 2- 135.3V

11/12db gain

ch1 in ch1

ch 1- 0.88V
ch 2- 0.97V

Observations -
In the 11/12db case ch1's signal lights (yellow) were *brighter/lit up a lot more* than ch2 even though it has *less* output voltage than ch2. Sometimes switching the amplifier on and off in the exact same configuration of settings would yield differences of upto 1.2V. There is faint music coming from both speakers with the gain set to 0/minimum. The faint sound on ch1 is audibly louder than the sound from ch2 at 0 gain/minimum for both channels.

Would really appreciate any thoughts on if the amp is OK or not?
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post #11 of 19 Old 12-30-2009, 06:13 AM
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Your amp is behaving normally, this a very minor difference nothing to jump off a bridge, Bob already answered your questions over QSC forum http://media.qscaudio.com/forum/view...php?f=2&t=3217, Basically manufacture tolerance on every components could be 1%,5 or even10% from specific spec, so yes you will have a little difference between the 2 channels. From what you measured the are extremely close together.

Warning to prevent risk of injuries, you should always be smarter than the equipment you are about to use.
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post #12 of 19 Old 12-30-2009, 07:05 AM - Thread Starter
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Quote:
Originally Posted by kouack View Post

Your amp is behaving normally, this a very minor difference nothing to jump off a bridge, Bob already answered your questions over QSC forum http://media.qscaudio.com/forum/view...php?f=2&t=3217, Basically manufacture tolerance on every components could be 1%,5 or even10% from specific spec, so yes you will have a little difference between the 2 channels. From what you measured the are extremely close together.

I was worried something was wrong and just wanted an opinion from someone who did not work at QSC, just to be sure. Thanks for the vote of confidence. I am a bit new to measurements and what is the "norm" for amps so I had no idea.

I don't understand why whatever I connect to channel 1 is louder by 1-2db, must be something else I guess. I might need to investigate this further but I am out of ideas as to what to test. I guess there is always running one channel a little lower on gain than the other.
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post #13 of 19 Old 12-30-2009, 08:06 AM
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Quote:
Originally Posted by Decadent_Spectre View Post

I was worried something was wrong and just wanted an opinion from someone who did not work at QSC, just to be sure. Thanks for the vote of confidence. I am a bit new to measurements and what is the "norm" for amps so I had no idea.

I don't understand why whatever I connect to channel 1 is louder by 1-2db, must be something else I guess. I might need to investigate this further but I am out of ideas as to what to test. I guess there is always running one channel a little lower on gain than the other.

To me you are closer to 0.3 db difference, why investigate??????????? my father use to tell me do not fix what isn't broken yet. As far as i know all my amps are not exactly putting the same power, because you did set them at 24 db or what ever if that bother you just turn up the quieter channel or turn down the louder one a bit.

If you are worry of the huge difference (extremely small to me) a 0.88 vs 0.97 volts output at 12 db, why you bought a QSC RMX5050 to play only 1/2 watt ????. headroom is good but this is on the overkill side

You are saying the red lights are dimmer than the yellow i really hope so because the other way around you would have a major problem, the yellow lights (signal lights) if i remember ok will start to lite at 0,1% of max output so again not a real exact science, again components tolerance can give you a small difference.

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post #14 of 19 Old 12-30-2009, 08:26 AM - Thread Starter
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Quote:
Originally Posted by kouack View Post

To me you are closer to 0.3 db difference, why investigate??????????? my father use to tell me do not fix what isn't broken yet. As far as i know all my amps are not exactly putting the same power, because you did set them at 24 db or what ever if that bother you just turn up the quieter channel or turn down the louder one a bit.

If you are worry of the huge difference (extremely small to me) a 0.88 vs 0.97 volts output at 12 db, why you bought a QSC RMX5050 to play only 1/2 watt ????. headroom is good but this is on the overkill side

You are saying the red lights are dimmer than the yellow i really hope so because the other way around you would have a major problem, the yellow lights (signal lights) if i remember ok will start to lite at 0,1% of max output so again not a real exact science, again components tolerance can give you a small difference.

I agree that you should not fix whats not broken but I am getting a 1-2 db difference as noted on a SPL meter. Judging by what you and others have said its not the amp, in which case I don't know what it could be.

I did not buy the QSC for 1/2 W, lol. It was just testing. I currently use it to drive my CLS-215s (500W peak) to their limits with clean power and in the future will be using them for more power hungry speakers once I have the money. This was more of a "permanent" buy so that I am never short on power.

The red clip lights are dimmer than the yellow signal lights at max gain with the clip limiters on, I have not tried without clip limiters.

The strange part is if the amp is in spec then oddly why is ch1 signal light lighting up more even though ch2 has more output voltage at 11/12db. That struck me as rather odd.
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post #15 of 19 Old 12-30-2009, 03:23 PM
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How you do measure the SPL? at sitting position or let say 3 feet away and a 2 feet above ground? because if from sitting position you can have a cancellation , also speakers are not made equally one speaker from a set can be more sensitive than the opposite side.

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post #16 of 19 Old 12-30-2009, 03:43 PM
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Quote:
Originally Posted by Decadent_Spectre View Post

"Are you using the same speaker when comparing channels?"

I'm using the same model of speakers but I have a pair of them that I'm using.

"The manufacturing tolerances of speakers are easily in the 1-2dB range."

You mean that different speakers of the same model might have a sensitivity difference of about 1-2 db due to manufacturing differences?

Different loudspeakers of the same model will have differences in level-especially at a single freq. This is due to variations in the drivers themselves and variations in crossover components.

A varation of only 1-2dB (as a window of acceptance-which would be +-1dB)would be considered VERY good. It is common for loudspeakers to be a good bit more than that.

When measuring something-you can only have ONE variable.

In this case you have several. The amp in question-the variations form different loudspeakers and probably one of the biggest-the physical distance from the loudspeaker to the mic.

When you are up close a very slight difference can make a huge difference.

For example, lets say you are 1/4" away on one cabinet. If the cabinets were identical-and you moved the mic to say 1/2" away- you could see a level difference of up to 6dB.

About the only position the gain of the different channels of the amp would be the same is full on. Anything less and all sorts of tolerances come into play.

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post #17 of 19 Old 12-30-2009, 05:33 PM - Thread Starter
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I measured it by pressing the mic against the speaker grill, I figured this would minimize distances between the driver and the mic and keep it relatively uniform although I am not sure if there can be any extra noise picked up by pressing the mic against the grill.

Ivan the amp isn't exactly the same at full on either as the voltages posted indicate, but as mentioned its close enough to "normal".

Everything makes a lot more sense now but could you explain the difference between the lights and the output? It does not affect the output but I am curious as to why it is occurring, faulty wiring of the lights?

Learning new things, thanks kouack/Ivan!
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post #18 of 19 Old 12-30-2009, 05:56 PM
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Quote:
Originally Posted by Decadent_Spectre View Post

I measured it by pressing the mic against the speaker grill, I figured this would minimize distances between the driver and the mic and keep it relatively uniform although I am not sure if there can be any extra noise picked up by pressing the mic against the grill.

Ivan the amp isn't exactly the same at full on either as the voltages posted indicate, but as mentioned its close enough to "normal".

Everything makes a lot more sense now but could you explain the difference between the lights and the output? It does not affect the output but I am curious as to why it is occurring, faulty wiring of the lights?

Learning new things, thanks kouack/Ivan!

The figures you gave are in the 0.1dB range of difference. I wouldn't worry about it. No way you are going to hear that. There are tolerances in all kinds of gear.

Regarding the mic placement-were you running the same signal to both loudspeakers? If it was music-there are different signals going to the two loudspeakers-that is what makes stereo.

If you were running some sort of tone-or test signal-was it the same actual signal? If I am reading your measurements correctly you have a different output form the same side of the amp, depending on what source you put in from the DAC.

And since you were on the grill cloth-the actual position of the mic-in relation to the physical loudspeaker driver could also account for some differences.

I wouldn't car about the brightness of the lights either-different colors have different brightness and different models of LED's have different light output.

I have never even thought of lights in that way.

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post #19 of 19 Old 12-31-2009, 05:06 AM - Thread Starter
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Quote:
Originally Posted by Ivan Beaver View Post

The figures you gave are in the 0.1dB range of difference. I wouldn't worry about it. No way you are going to hear that. There are tolerances in all kinds of gear.

Regarding the mic placement-were you running the same signal to both loudspeakers? If it was music-there are different signals going to the two loudspeakers-that is what makes stereo.

If you were running some sort of tone-or test signal-was it the same actual signal? If I am reading your measurements correctly you have a different output form the same side of the amp, depending on what source you put in from the DAC.

And since you were on the grill cloth-the actual position of the mic-in relation to the physical loudspeaker driver could also account for some differences.

I wouldn't car about the brightness of the lights either-different colors have different brightness and different models of LED's have different light output.

I have never even thought of lights in that way.

During the SPL tests I used music,100/500Hz sinewaves and white noise (WinISD). I used the same signal for both speakers with no changes to the volume/frequency etc. Yes depending on the the DAC output (ch1/2) and the amp input (ch1/2) that it is connected to there is a difference in the voltage output, they create four variables (as listed in my measurements) and they all seem to be different.

The grill was steel, would it have been better to measure further away?

I thought that the yellow signal lights indicated the power output, I guess that is not the case?
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