# How To: Set gains via TrueRTA's Oscilloscope Function



## ErinH

Seriously. I don't understand how to set this up to test clipping on the amps.


I don't even know where to begin. I know a few others here have asked about this, too. Someone care to make a brief tutorial?


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## ErinH

*Re: Someone care to show us how to use TrueRTA's oscilloscope function?*

Anyone want to tackle this? Or at least give some ideas? As simple as it seems, I can't wrap my head around this.


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## rekd0514

*Re: Someone care to show us how to use TrueRTA's oscilloscope function?*

I am definitely interested as well particularly when using the mic mate. I am wondering how to properly set everything up and to properly test for db for level matching, phase, oscope for tweets, TA, frequency analysis for the different drivers, how to average correctly, etc. Would be great info that could be used by many on here.


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## braves6117

*Re: Someone care to show us how to use TrueRTA's oscilloscope function?*



rekd0514 said:


> I am definitely interested as well particularly when using the mic mate. I am wondering how to properly set everything up and to properly test for db for level matching, phase, oscope for tweets, TA, frequency analysis for the different drivers, how to average correctly, etc. Would be great info that could be used by many on here.


X a zillion!

Just recieved the setup, and I'm completely lost. 

A tutorial addition for db for level matching, phase, oscope for tweets, TA, frequency analysis for the different drivers, how to average correctly, etc would be great for this forum. Somebody please step up


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## dvsadvocate

*Re: Someone care to show us how to use TrueRTA's oscilloscope function?*

I tried the Osci function before on my Alpine 9887 to get its clipping point. Dead accurate as a real osci, if your laptop is up to speed. I was using a Macbook Pro 2.33GHz C2D. I simply attached the pre out RCAs to a male RCA to mini phono jack to the inputs of my lappies soundcard. Now I just dont know how to do it with the outputs of my amplifiers.


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## ErinH

*Re: Someone care to show us how to use TrueRTA's oscilloscope function?*

^ that's what I'd like to figure out; how to test for clipping at the amps. I can't for the life of me figure this out. I'm sure it has a simple answer, too.


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## chuyler1

*Re: Someone care to show us how to use TrueRTA's oscilloscope function?*



dvsadvocate said:


> I tried the Osci function before on my Alpine 9887 to get its clipping point. Dead accurate as a real osci, if your laptop is up to speed. I was using a Macbook Pro 2.33GHz C2D. I simply attached the pre out RCAs to a male RCA to mini phono jack to the inputs of my lappies soundcard. Now I just dont know how to do it with the outputs of my amplifiers.


It is accurate as an oscilicsope but I wouldn't trust it for finding your clipping point. After all how do you know it is the clipping point of your head unit output and not the clipping point of the line-input on your computer. Your computer may only accept 2v input while your head unit could put out 4v.


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## ErinH

*Re: Someone care to show us how to use TrueRTA's oscilloscope function?*

Woot! Think I got it thanks to Andy's help, and the below comes from the thread I made here:
http://diymobileaudio.com/forum/showthread.php?p=575603#post575603




I bought a 1/4" patch cable from radio shack earlier. I then took a spare run of wire and used the diagram below (top half) to go by. I took about 4' of speaker cable, attached the positive to the tip of the 1/4" connector, and the negative to area 2 (in diagram). Then I ran the other ends to the amplifier. 









*
Here's pictures of the setup:*
This is the 1/4" patch cable I picked up from RS. They didn't have a cheap one that had the ring (#2 in diagram above) so I had to pay $15 for this 6ft gold plated patch cord. 










This is the speaker wire wrapped around the appropriate points of the cable connector end using the diagram above:










Connection at the amp (note that I have quick disconnects... that's for quick disconnection when taking impedance measurements where a 1k resistor is used)










Connection of patch cable to the mobile pre:










Overall view:










*
Test:*
I turned all the speakers off except for the one in question so I would keep all other speakers safe while doing this. I put the volume at 28, started a test tone at 75hz and then started taking measurements via TrueRTA. I don't know if I did things right, but I did at least get values that made sense. They follow:
*
Gain set at lowest point:








*


*Gain set at halfway point:*









*Gain maxed out:*










Note how the wave gets more like a square as it progresses? Even at halfway it's not a true sine wave and is somewhat kilted.


Now, here's where I left the gain at; a point which looks much more like a true sine wave:











*Conclusion:*
This definately passes the sanity check. Yesterday I did a lot of tinkering with the gains. When I had the gains to about the halfway point (straight up and down on the dial) things sounded compressed. I noticed that lower than that things sounded better. Against my better judgment I left them at the halfway point, but will definitely drop them down, now.

***For those who do not have the m-audio mobile pre, and are instead using laptop soundcards*, I think you can still just as easily do this same test. The ONLY difference you should have is that you'd use a 1/8" TRS type patch cord instead of the 1/4" like I used. Laptops have a 1/8" connection; not 1/4". My radio shack had a ton of these when I was there earlier. Just make sure to get the kind with the ring (top connector in diagram at the beginning of this post). 

If anyone has ANY comments on something I may not be doing right here, PLEASE let me know and speak up! This passed the sanity test but it still doesn't mean I'm exactly right.


**Caution!*
Someone in another thread called to attention the input voltage of the preamp and that doing this could blow the preamp. This preamp has what I call a gain on it, so I just watched that to make sure the light didn't turn read (clipping). I don't know the input voltage of the mobilepre so that's something that should be addressed before folks go plugging their cable to the line in on the preamp, and especially laptop soundcards.


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## chuyler1

*Re: Someone care to show us how to use TrueRTA's oscilloscope function?*

From the screenshots you posted, I'm not convinced you did this correctly. You have the resolution set wrong to truly see a square wave. You should configure the oscope so that you can only see 1-2 cycles. Then as you increase the gain at some point the curve at the top most portion of the wave should start to flatten. That is the point where the signal is being "clipped". What you are describing as clipping in your screenshot is simply how TrueRTA renders the wave.

Take this screenshot:









The wave on the left is a proper signal with no clipping. The wave on the right is what you should see after you pass the point of clipping. The top and bottom of the wave get cut off.

I also want to restate my last post. Once you properly see clipping, there is no way of knowing if it is the amplifier clipping or your line-input clipping. You can use your technique to find the clipping point of everything except the amplifier. To find out the clipping point of the amplifier you'll have to use the microphone with the gain set very low and monitor the output from your speakers.


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## ErinH

^ good points. Looks like I have a lot of work ahead of myself. Now I feel bummed. 


Edit: Chuyler, I actually zoomed in to where I was only see 2-3 cycles when I set the gains on the midrange/tweeter amp. I'll go back out and look at the midbass amp again, though. 

You do bring up a good argument on the line input clipping. Hopefully we can resolve that here.


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## chuyler1

I think if you can nail down your HU and processors with TrueRTA...it should give you plenty of leway to set the amp gains to wherever you want. So it's not a huge deal but always something to keeep in mind.


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## ErinH

True. I saw a 1/4" to RCA cable at RS today. That would be really good for headunit/preamp gain setting. I'll probably go pick one of those up tomorrow, too. I guess all that's really going to show me is where the h/u & preamp clip, though.


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## BMWturbo

Careful when using line-in on laptop.

I fried a Left channel in on my laptop testing an amplifiers output signal


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## 14642

OK. Sory for the delay guys. Here's the voltage divider. There's one diagram for a balanced connection (like the M-Audio soundcard) and one for an unbalanced connection (like the line input on a laptop or consumer-grade card--creative, for example). BTW, I use a Creative notebook card. It works great and it's cheap. For that one, the unblanced connection is appropriate. I've drawn a mono jack here, but it should be easy enough to apply this to a stereo jack. Making one for an unbalanced stereo jack would be like the top diagram but the DUT would be connected to EITHER of the top pins and the ground connection at the bottom of the symbol for the connector.

Here's the link for the soundcard:

http://www.soundblaster.com/products/product.asp?category=1&subcategory=205&product=10769










Kirchoff's Voltage Law states (I'm paraphrasing) that the sum of the voltage drops across resistors in a series circuit are equal to the source voltage. So, in the diagram above, about 1/30 of the applied voltage is dropped across the 330 ohm resistor and about 29/30ths of the voltage is dropped across the 10k ohm resistor. If your sound card can accept up to 1V (this is the case with the creative card I use), then this divider will allow you to safely test devices that output up to 30V--that's 225 watts at 4 ohms. If you want to test a device with a higher voltage, make R2 smaller, but try to leave the 10k resistor alone. 

Why a 10k resistor? Because we want the impedance of this circuit to be high so that it doesn't affect the rest of the circuit by drawing a bunch of current and so that we can use 1/4-watt resistors. The values I've suggested are standard values and should be pretty easy to find. The current through this circuit at 30V would be about .003A. You can leave this connected to the amp's output all day with no problems. 

You'll only need a divider that big for setting gains on amplifiers, when you have to drive the amp to full output. If you just want to test the frequency response of a device, you can use a much smaller one. For soundcards that accept 1V, it's nice to feed it about half of that, and that's the reason to have several voltage dividers of various ratios handy. 

Alternately, you could make an adjustable one out of a 10k potentiometer.


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## ErinH

So, to make sure I understand this, the DUT end is going to the amplifier. The opposite end, J1, would go to the 1/4" jack (1/8" jack for laptop) in the same fashion I've described above (wires wrapped around jack at tip and ring)? 
Essentially, the only thing that changes from the test I described in my post is to add a 10k ohm resistor and a 320 ohm resistor in the fashion you've described?


Thanks for your help.


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## 14642

Yup.


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## ErinH

Got it! Thanks for your help.


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## braves6117

So once rigged appropriately, this allows a safe measurment for setting the gains via analysis of the sine waves produced at a certain gain level?


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## ErinH

With Andy's extra advice, yes.

I had absolutely zero problems using my m-audio mobile pre, but I would think it has a higher input voltage tolerance. I don't know for sure, though. 

It worked great for me.


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## t3sn4f2

Bikin how do you plan to put this to practical use. You know where the amp clips at with a full scale tone, but how does that relate to music that might not even hit that peak and to _your_ ears that might not be sensitive to that clipping level. Once you change your head units volume to make up for those thing you loose your reference point and are blind again. 

Isn't this why setting them with your ear is best? Since it inherently takes those things all into account.

Also isn't the over all volume going to be low if you set things conservatively like this.

Although I can see it being useful for someone running many time the power to a speaker where clipping would be dangerous and where there is enough dynamic range to have good volume at low volume setting.


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## ErinH

t3sn4f2 said:


> Bikin how do you plan to put this to practical use. You know where the amp clips at with a full scale tone, but how does that relate to music that might not even hit that peak and to _your_ ears that might not be sensitive to that clipping level. Once you change your head units volume to make up for those thing you loose your reference point and are blind again.
> 
> Isn't this why setting them with your ear is best? Since it inherently takes those things all into account.
> 
> Also isn't the over all volume going to be low if you set things conservatively like this.
> 
> Although I can see it being useful for someone running many time the power to a speaker where clipping would be dangerous and where there is enough dynamic range to have good volume at low volume setting.


I posed this question to myself (in a shorter paragraph, ) so I’m glad you’ve asked. This really all comes down to opinion and preference so I’ll just share mine with you. My brain is scattered with this so hopefully you can follow my train of thought…

I am very aware that using test tones in a sense, prohibits the true musical output of your system. I wanted to set the gain level on the amplifiers at a volume at which I typically put the headunit at, or at least will now. I used 0db tones thinking that if I went with -3db tones I would essentially be losing dynamic headroom. My rationale is that by setting the gain a tad lower than it probably could be, you give yourself some ‘wiggle room’ when it comes down to the music. Using your ear is fine, but IMO it’s a bit rusty. I would be concerned that you could set the gain too high and result in compressing music… in that sense you’d lose dynamics. Is that logic flawed?

The ‘ear test’ definitely is what swayed my decision to stay with the gains as they are, and also confirm that this method does work. I know that typically it’s best to do a ‘blind test’, but as much as I’ve fiddled with my system I’d say I’ve gotten to know it _very_ well like I'm sure you know yours well, too. Even further, I had actually spent most of my Saturday going back and forth between music and test tones, trying to set the gain by ear. What I wound up with was this:
Tweeter = gain too low
Midrange = about right… maybe just a bit too low
Midbass = too high. 

Something else I have yet to look into is the effect of the different frequencies (test tones) on the clipping point. If nothing else, I can always go back to using my ears.  I could also try using -3dB tones to see what the results are. Once I learn something I beat it like a dead horse. So, rest assured that I’ll chime in later with thoughts on that. 


But, for now, I’ll say that this method of setting my gains really helped me do wonders to my system. My wife is ultimately my ‘test’. I had her listen to it after it was all done and said that it sounded more ‘lively’. This was without her knowing that I had done gain work to it because she had slept the entire day. I played “Because the Night” (a favorite of hers) and she loved it. So, at least it just wasn’t me.


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## t3sn4f2

bikinpunk said:


> I posed this question to myself (in a shorter paragraph, ) so I’m glad you’ve asked. This really all comes down to opinion and preference so I’ll just share mine with you. My brain is scattered with this so hopefully you can follow my train of thought…
> 
> I am very aware that using test tones in a sense, prohibits the true musical output of your system. I wanted to set the gain level on the amplifiers at a volume at which I typically put the headunit at, or at least will now. I used 0db tones thinking that if I went with -3db tones I would essentially be losing dynamic headroom. My rationale is that by setting the gain a tad lower than it probably could be, you give yourself some ‘wiggle room’ when it comes down to the music. Using your ear is fine, but IMO it’s a bit rusty. I would be concerned that you could set the gain too high and result in compressing music… in that sense you’d lose dynamics. Is that logic flawed?
> 
> The ‘ear test’ definitely is what swayed my decision to stay with the gains as they are, and also confirm that this method does work. I know that typically it’s best to do a ‘blind test’, but as much as I’ve fiddled with my system I’d say I’ve gotten to know it _very_ well like I'm sure you know yours well, too. Even further, I had actually spent most of my Saturday going back and forth between music and test tones, trying to set the gain by ear. What I wound up with was this:
> Tweeter = gain too low
> Midrange = about right… maybe just a bit too low
> Midbass = too high.
> 
> Something else I have yet to look into is the effect of the different frequencies (test tones) on the clipping point. If nothing else, I can always go back to using my ears.  I could also try using -3dB tones to see what the results are. Once I learn something I beat it like a dead horse. So, rest assured that I’ll chime in later with thoughts on that.
> 
> 
> But, for now, I’ll say that this method of setting my gains really helped me do wonders to my system. My wife is ultimately my ‘test’. I had her listen to it after it was all done and said that it sounded more ‘lively’. This was without her knowing that I had done gain work to it because she had slept the entire day. I played “Because the Night” (a favorite of hers) and she loved it. So, at least it just wasn’t me.


Cool, I just thought there would be a concern with having too much clean dynamic headroom power that when you had a low level passage in a song, it would not be loud enough while driving or something. This would be the upside to compressing the the peaks with the amp like you said, where you raise the overall song volume at the expense of cleaner peaks.


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## 14642

Burn a CD with test tones that have a level of -10dB and 0dB. IF you have a signal processor, set it so the output clips at the top of the volume control (or 3/4 if you wish) with the 0dB track. DO the same for all the processors in your system. Then, go to the amp that runs the high frequencies. Set it so it clips at the same voolume control rotation with the -10dB track. Then, set the other amps so that they are proportionate to the high frequency amp, so the system sounds good. Now, you have a good signal-to-noise ratio and 10dB of headroom.


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## ErinH

Whoa! Nice!

So, set the processor with 0dB at whatever volume it takes to get to clipping (say 28 for example). Then go back and use the -10dB tones to set the gain at the same point at which the volume on the headunit was set (28). Then set the midrange & midbass by ear at 28?


Edit: Here's a link to some 15 second test tones at 0dB & -10dB at the following frequencies:
47
100
140
500
1000
4000
8000

http://rapidshare.com/files/154232933/Test_Tones.zip.html


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## ErinH

Andy Wehmeyer said:


>


Went to RS and picked up some 330ohm resistors.


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## HNaga

Is there software for tuning without hardware?


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## handy

it is very risky to use line input of your soundcard to check output of amp, ordinary line input of soundcard can only handle at about 2V. the output of the amp can produce at about 50V++.
Just my opinion only.

i only use the ossiloscope of True Rta to check if the output of the head unit is clipping or not, or input and output of external accesories like preamp, active xover, equalizer.

Unclipping signal​









Clipping signal​









My setup
in is from output off headunit, eq, xo , preamp
out is to input of aux, eq, xo , peamp









and if u are want to check the output of an amp, better use bigger Resistor for your safety and try not to clipp the input of your soundcard



















just my experience....


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## cacin

handy said:


> it is very risky to use line input of your soundcard to check output of amp, ordinary line input of soundcard can only handle at about 2V. the output of the amp can produce at about 50V++.
> Just my opinion only.
> 
> i only use the ossiloscope of True Rta to check if the output of the head unit is clipping or not, or input and output of external accesories like preamp, active xover, equalizer.
> 
> Unclipping signal​
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> Clipping signal​
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> My setup
> in is from output off headunit, eq, xo , preamp
> out is to input of aux, eq, xo , peamp
> 
> 
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> 
> 
> 
> 
> and if u are want to check the output of an amp, better use bigger Resistor for your safety and try not to clipp the input of your soundcard
> 
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> just my experience....


Hi,
I am wondering why your output goes to input of aux, eq, xo , peamp?
Isn't this supposed to go to the input of the sound card (if planning to use TrueRTA as Oscilloscope?)
Can you please also share what's the circuit inside the box?
What's the L PAD for? When do you need to use it?


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## 14642

That's what the voltage divider is for. A 50:1 divider will make it possible to measure the output of an amplifier that makes 50V. The resistor values should be chosen based on the current that will flow through them and the required ration to prevent over driving the sound card input.


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## handy

cacin said:


> Hi,
> I am wondering why your output goes to input of aux, eq, xo , peamp?
> Isn't this supposed to go to the input of the sound card (if planning to use TrueRTA as Oscilloscope?)
> Can you please also share what's the circuit inside the box?
> What's the L PAD for? When do you need to use it?


this is my BBmeasurement. it can be use as soundcard same as clio.
http://bb-measurement.tripod.com/
for L-pad = Voltage divider



Andy Wehmeyer said:


> That's what the voltage divider is for. A 50:1 divider will make it possible to measure the output of an amplifier that makes 50V. The resistor values should be chosen based on the current that will flow through them and the required ration to prevent over driving the sound card input.


thanks again Andy.


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## cacin

thanks 
For the voltage divider, can the 1/4 W resistor be used for the 330ohm and 10Kohm? Or do I need higher watt resistors?


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## cacin

sorry, double post


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## cacin

Hi all, sorry for not noticing that it was already mentioned that 1/4watt resistors can be used.

So far, I've seen that the voltage divider is for 1 channel only, if I plan to use the left and right channel, can I just do the circuit like this?


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## Brian Steele

Andy Wehmeyer said:


> OK. Sory for the delay guys. Here's the voltage divider. There's one diagram for a balanced connection (like the M-Audio soundcard) and one for an unbalanced connection (like the line input on a laptop or consumer-grade card--creative, for example). BTW, I use a Creative notebook card. It works great and it's cheap. For that one, the unblanced connection is appropriate. I've drawn a mono jack here, but it should be easy enough to apply this to a stereo jack. Making one for an unbalanced stereo jack would be like the top diagram but the DUT would be connected to EITHER of the top pins and the ground connection at the bottom of the symbol for the connector.
> 
> Here's the link for the soundcard:
> 
> Creative Sound Blaster Audigy 2 ZS Notebook Sound Card - THX-certified quality for your notebook
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Kirchoff's Voltage Law states (I'm paraphrasing) that the sum of the voltage drops across resistors in a series circuit are equal to the source voltage. So, in the diagram above, about 1/30 of the applied voltage is dropped across the 330 ohm resistor and about 29/30ths of the voltage is dropped across the 10k ohm resistor. If your sound card can accept up to 1V (this is the case with the creative card I use), then this divider will allow you to safely test devices that output up to 30V--that's 225 watts at 4 ohms. If you want to test a device with a higher voltage, make R2 smaller, but try to leave the 10k resistor alone.


Danger, Will Robinson - I think you've got the inputs and outputs reversed in that diagram!!

Sanity check - the source ("device under test") should always see the TOTAL impedance of the divider circuit (R1 +R2) across its terminals. The input to the test device should see only see R2 across its terminals. Then Vin*R2/(R1+R2) = Vout. In your diagram, the DUT (the amplifier) is only seeing R2, not R1+R2.

See the following link for more details.

Voltage divider - Wikipedia, the free encyclopedia


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## DT053

Some of you may find after setting your gains using this method, that your system my not be as loud as it was before or as loud as you like it to be. 

Your results may vary 

Thats why some of the old AutoSound 2000 Cd's had tracks for settings gains that produced different distortion levels.... This is when the compromises begin.

dT


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## crosspug

Thanks for the inspiration people, we tested this function for the first time today. We used the voltage divider as mentioned by Andy^^ above. It seemed to work well.... 

Although I do have questions regarding the use of the M-Audio "gain" (we had it on minimum.....) when you are using just the normal stereo plug connection on the rear of the unit. When the gain is turned up (on the m-audio) a little it begins to clip the signal, is this normal? Any hints on what level other people are using?

It's just that it took ALOT of effort for the amps we tested to show clipping (unless using "bassboost" etc!!). Amps were Earthquake PA4030, Orion Extreme 800 and PPI Art 200.......

I just want to make sure that I'm utilising this correctly, does this require a form of gain setting before testing?

Thanks,

Jono


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## Brian Steele

crosspug said:


> Thanks for the inspiration people, we tested this function for the first time today. We used the voltage divider as mentioned by Andy^^ above. It seemed to work well....


As mentioned by Andy, or subsequently corrected? 

Just remember, for a voltage divider, the device that you are measuring (e.g. the amplifier) should always see the FULL impedance of the voltage divider connected across its output terminals. The measuring device (scope, DMM, etc) should only see R2. Then the voltage it sees will be R2/(R1+R2).


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## cacin

Hello all,
Anyone can tell me why the wave form that I got for a -10dB 4Khz looks like this?











I assume this is what we're aiming for right?


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## ErinH

oscilloscope clipping - Google Search


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## Brian Steele

cacin said:


> Hello all,
> Anyone can tell me why the wave form that I got for a -10dB 4Khz looks like this?
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> I assume this is what we're aiming for right?


It looks like some strange form of clipping.

The signal level doesn't seem high enough though to overload a typical soundcard's input channels.

Have you turned off all processing (BBE, Loudness, etc) at the source? If you doing gain settings, EQ should also be set flat


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## crosspug

Brian Steele said:


> As mentioned by Andy, or subsequently corrected?
> 
> Just remember, for a voltage divider, the device that you are measuring (e.g. the amplifier) should always see the FULL impedance of the voltage divider connected across its output terminals. The measuring device (scope, DMM, etc) should only see R2. Then the voltage it sees will be R2/(R1+R2).


Ok being nay but a simple boy, I used the diagram for 2 channels as posted by Cacin............ Now I have read what you've said soooo many times yet I still (even with the wikipedia diagram) yet to comprehend the way I *should* setup the voltage divider.

Please be aware I am totally inexperienced with electronics so please forgive me. 

Jono

PS. As per my last post, any hints from people on how to correctly set the M-Audio "Gain" for testing, it seems to create a situation that you can either *create* clipping (by raising the maudio gain) or remove clipping (by lowering gain on maudio) without touching the amp gain if thats what you are testing. Therefore when can you tell if it actually is the Amplifier/Headunit clipping not the input of the mobile pre?

Or have I just confused myself?


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## Brian Steele

crosspug said:


> Ok being nay but a simple boy, I used the diagram for 2 channels as posted by Cacin............ Now I have read what you've said soooo many times yet I still (even with the wikipedia diagram) yet to comprehend the way I *should* setup the voltage divider.
> 
> Please be aware I am totally inexperienced with electronics so please forgive me.


Easy: 
1. Correct R1 to R2
2. Connect source +ve to R1's other lead
3. Connect source -ve to R2's other lead
4. Connect measuring device -ve to junction between R2 and source -ve
5. Connect measuring device +ve to junction between R1 and R2


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## cacin

hello,
the current 2 ch circuit that I draw still needs confirmation from experts here 

correction on the drawing based on inputs by Brian would be to swap the line in and amp probe points.


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## cacin

Hi Bikinpunk and Brian,
Thanks for the inputs and comments.

I'll look around for similar clipping behaviour. My current thoughts is that it should be flat and not some sharp edges.

the signal was actually obtained from an Xtant amp.
I have tried 2 amps, a 202m and a A2002. Both showing same symptoms. I had tried a different RCA cables and different HU as well. the output is still having this kind of wave form.

the proper signal was from a MTX TA3404. with 1KHz -10dB signal.

I believe I had turned off all the processing and EQ on both HU before doing the measurement. I will try again when I have time this week.


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## crosspug

Ok, now I think/hope I understand! Thanks!

Is this what we're after?










Thanks,

Jono


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## cacin

hello crosspug,
That what I draw based on Brian's post too.


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## crosspug

Excellent..... More testing shall now occur!

Also I received some "strange" curves when testing, similar to what you have seen Cacin. I'll redo the tests to be sure that it wasn't the equipment (voltage divider), but instead of the flat spot occuring on the "up" of the curve but on the "down". We'll see if this reoccurs.

Thanks,

Jono


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## cacin

Just an update on the issue about the sharp waves.

I had tried it on the MTX amp and the same results occur.
However, when I actually send 1kHz sine waves to the xtant amps, it seems that a proper sine wave was seen.

Question would be, why is this only happening on the 4kHz test tone?
Is my HU clipping at 4kHz? Or, was is the test tones?


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## blacksvtf03

I checked out the Black Box Pro website. Is that product a complete replacement for TrueRTA/Behringer/Mobile Pre USB???? How much is it?


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## cacin

Hello all,
I had checked the test tones using a wave editor. Base on what I can see, there's a 2 second silence on the track. Could this be the culprit? But, all the test track that I burned had that 2 sec silence and on the 1kHz, it seems fine when I probe using osciloscope of TrueRTA.
Nonetheless, I have made another copy of the test tones with no 2sec silence after the track. Will try again and see what's the outcome.




blacksvtf03 said:


> I checked out the Black Box Pro website. Is that product a complete replacement for TrueRTA/Behringer/Mobile Pre USB???? How much is it?


Hello,
I had asked the manufacturer and based on my location (Malaysia) I was quoted USD500....


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## blacksvtf03

Where are the manufacturers located? The US?


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## HCWLSU101

So if I want to use the Oscilloscope function to test the outputs of my headunit what cables should I use? Should I use RCA's to 1/4 cable and hook both RCA's (L&R) to the headunit and then 1/4 to input of Mobile Pre or should I by a single RCA to 1/4 cable and do one speaker at a time?


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## jbholsters

Hey guys, I'm new hear, but saw this thread and thought I'd ad my $.02

When using an oscilloscope to set the gain on your amp, you don't really want to set them to the point before clipping. You will not have any dynamic head room if you do so. When I did this for a living we would set the gain on each amp to the point of clipping and then multiple the output voltage times 3. So if the amp is showing 10 volts right before the point at which it clips you would turn the gain until the voltage reads 30. This would then be the max gain for each amp. So if you have 4 amps, most likely you will be cutting the gain on 3 of the amps and the fourth would stay at (as per example)30 when tuning the system. Since you never listen to music that is at max saturation for more than a second or two, 3X works pretty well. Otherwise you would need huge amps on every speaker. Also keep in mind that if you are using an EQ, do not boost any frequencies only cut them. Especially on any of the amps that are set at 3X of clipping.

I think there is an old thread on David Navone's forum on the carsound forum. It may even be on his website.

Unfortunately if you listen to a lot of music downloads in the car, you may experience clipping in the music. I know iTunes downloads are all over the place when it comes to levels.


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## JayinMI

OK, I never saw any confirmation that Brian Steele's voltage divider configuration was any more (or less) "right" then Andy's.
Brian refers to R1 and R2, and in the diagram he quoted, R1 and R2 only apply to the balanced setup. In the unbalanced setup the diagram uses R3 and R4.

So, is it perhaps that the balanced diagram is incorrect and unbalanced is fine?

Jay


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## accelerator

cacin said:


> Just an update on the issue about the sharp waves.
> 
> I had tried it on the MTX amp and the same results occur.
> However, when I actually send 1kHz sine waves to the xtant amps, it seems that a proper sine wave was seen.
> 
> Question would be, why is this only happening on the 4kHz test tone?
> Is my HU clipping at 4kHz? Or, was is the test tones?


Most sound cards have a sampling rate of 44.1 kHz. So with a 4 kHz sine wave, you are only sampling ~11 points per cycle, which is why you get a jagged appearance. At 1 kHz, you get 44 points per cycle, so the waveform looks smoother.


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## Brian Steele

accelerator said:


> Most sound cards have a sampling rate of 44.1 kHz. So with a 4 kHz sine wave, you are only sampling ~11 points per cycle, which is why you get a jagged appearance. At 1 kHz, you get 44 points per cycle, so the waveform looks smoother.


Nope - that's more than enough points to recreate a smooth 4kHz cycle, if the correct filtering is used to recreate the analog signal from the samples.


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## t3sn4f2

Brian Steele said:


> Nope - that's more than enough points to recreate a smooth 4kHz cycle, if the correct filtering is used to recreate the analog signal from the samples.


x2.

Here's a soundcard output measurment of a *10*KHz sine wave at 16/4*8*.


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## jbholsters

You should really check for clipping with the amp under load.


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## cacin

jbholsters said:


> You should really check for clipping with the amp under load.


by using the voltage divider, wouldn't the 10k ohm considered to be as load?
or we need to connect a 'dummy load'?


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## jbholsters

I've always connected a dummy load. You can't get an impedence reading from the voltage divider as you would with a true load on the amp. But to be honest, just running a 1k sign wave doesn't show everything, but it's good enough. Just remember to multiply the voltage right before clipping 3X and turn the gain to that voltage (this is then the max that gain can be set, but you can always turn it down)


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## cacin

t3sn4f2 said:


> x2.
> 
> Here's a soundcard output measurment of a *10*KHz sine wave at 16/4*8*.



thanks for this. but, last night, when I use soundforge to create a 4Khz tone. I was getting a different shape depending on the setting.

with 96kHz, I manage to get a smooth tone displayed on the Soundforge window.

but, with 44.1kHz, the 'sharp' edges shows up.

when I load the test tone I have, it is really showing a sharp edge like the one that was probed...
Is it because of the 'level' of zoom (x-axis) that was used?

I'm getting more confused


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## chauss

Brian Steele said:


> It looks like some strange form of clipping.
> 
> The signal level doesn't seem high enough though to overload a typical soundcard's input channels.
> 
> Have you turned off all processing (BBE, Loudness, etc) at the source? If you doing gain settings, EQ should also be set flat


I believe the flatting of the sign wave at top of peak is caused from not enough headroom in the amplifier for that gain setting .

This is all great - but I thought gains were adjusted via voltage outputs for given frequencies which should come with the amp or be available thru dealer network- at least JL Audio amps do? The engineers that created the amps put all the work in while designing the product- (at least I would hope so...)

If you are running a single frequency through the amp to check for clipping I would think each frequency range would have to be checked. Different amps behave differently depending on the electronics within- don't they?
Some amps are better at producing say 20Hz waves at high power than others that may be better at 17KHz signals at full power. Does this make sense? (ie: amps with big toroidal transformers wound with heavy copper wire are better at supplying high current required for low frequencies- while the older E type transformers are better at the high range frequencies. The compromise is made in the wire size and number of turns on the magnet).

If you are running active 3 way setup- ignore the above paragraph since each amp runs a smaller range of frequencies and can be set independently.....


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## jbholsters

chauss said:


> If you are running a single frequency through the amp to check for clipping I would think each frequency range would have to be checked.



Ideally, but in practice you get close enough with 1. Plus, you are going to triple that voltage on the gain anyway (if you want any volume). Your ears won't be able to detect clipping @ 3X the voltage unless you are Jaime Sommers


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## chauss

Good point.
I was just trying to help deduce why the variance from one frequency to another on the scope.


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