# What can I use to measure amp clipping?



## z_accoustics (Sep 8, 2006)

I would like to test to see where my amp clips given certain 0db test tones and adjust the gains accordingly. What tool can I use to measure this?

I have seen audio interface devices and pro audio gear that have a red light that lights up when the signal is clipping. Something this simple would be great. Such as perhaps a voltmeter that had the added feature of detecting clipping. I've borrowed an oscilloscope to do this before but it seems unnecessary (and expensive to buy).

thanks


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## traceywatts (Jun 2, 2008)

search. i think there is a tutorial for this. try dfw brownieque clip as your terms.


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## dingaling (Apr 14, 2005)

oscillioscope, try a fluke 123 on ebay


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## Oliver (Jun 25, 2007)

Per Dingaling [ Fluke 123 20Mhz Scopemeter - eBay (item 320371749404 end time May-19-09 19:22:34 PDT) ]

7 hrs to go


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## metanium (Feb 2, 2007)

Oscilloscopes aren't necessary for this method....http://www.diymobileaudio.com/forum/diyma-tutorials/51435-setting-gains-w-o-oscilloscope.html

All you need is a voltage divider (easy to make - DIY) couple of cables and a Radio Shack mini amp ($20). Easy-to-use solution for 1/10th what that Fluke will run you.


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## dsh2009 (Apr 28, 2009)

can TrueRTA and a laptop be used to measure this? i haven't used my pc rig in a while but i seem to remember there is an oscilliscope feature in the software? i might need to check my levels as well soon just for ****s.

-d


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## MarkZ (Dec 5, 2005)

Yeah, trueRTA has an oscope function in it.

OP, you should have searched first.


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## qstarin (Mar 25, 2009)

I'm still quite unsure about the necessity of loading an amp while doing this.

Actually, what I'm unsure about is why more "gain setting tutorials" don't cover it.

I mean, the actual resistance between the output terminals of an amp play a part, don't they?

If I had set my gains exactly using a scope on an 8 ohm load, and then switched to a 4 ohm load, wouldn't my gain setting *have* to change?


Assuming yes, what then are people doing for a resistive load?

I sure as hell ain't using my actual driver for a full power test tone, and I don't see many resistors out there at 2 or 4 ohms capable of dissipating hundreds, or even thousands, of watts.

If I do the gain setting w/o scope mentioned above, I'm hooking some little amp up to the speaker outputs of my amp through a voltage divider.

Um, okay, that's straightforward enough. But I would *think* that a mini-amp through a voltage divider is a whole entirely different ball-game than a 4 ohm driver.

I mean, the amp isn't "seeing" 4 ohms on its terminals when its hooked through a voltage divider to a mini-amp, is it?

And if not, wouldn't the gain setting achieved be meaningless?


Just another in my many confusions over something so simple.




Oh, and TrueRTA may have a "oscope function", but that doesn't mean anything. You still have to physically connect the wires from whatever device your setting to the laptop in a way that TrueRTA would be able to read the voltages etc. for an oscope. I don't think it works that way.


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## MarkZ (Dec 5, 2005)

qstarin said:


> I'm still quite unsure about the necessity of loading an amp while doing this.
> 
> Actually, what I'm unsure about is why more "gain setting tutorials" don't cover it.
> 
> ...


You've just highlighted one of the (many) reasons why using a scope of any kind is a bad idea when setting gains. The amp's output voltage swing depends on how much current is being drawn from it, primarily due to losses in the power supply. But the current, as you've pointed out, depends on the resistance of the load. And this resistance CHANGES during musical playback -- as a function of frequency (impedance is frequency-dependent) and as a function of voice coil temperature (ie. "power compression").

So your question is valid... what impedance do you use? I dunno. Do you make the measurement with the speaker (voice coil) hot or cold? Do you make the measurement using pink noise, white noise, 1kHz sine wave? Changing any of these variables will give you a DIFFERENT result.

This is why the _precision_ that people are trying to achieve when using a scope to set gains is nothing more than a pipe dream. The system has too many variables to define a set clipping point.



> Oh, and TrueRTA may have a "oscope function", but that doesn't mean anything. You still have to physically connect the wires from whatever device your setting to the laptop in a way that TrueRTA would be able to read the voltages etc. for an oscope. I don't think it works that way.


TrueRTA has a calibration procedure so that it can read voltages. But absolute voltage values are unimportant here. The idea is to run the 0dB tone and view it on the scope, adjusting the gain until that sine wave begins to exhibit flat tops. There's your clipping point. But, as I mentioned above, your clipping point is going to change based on a number of different variables.

The issue with TrueRTA, however, is that you need to step down the voltage into something that your sound card can handle (usually +/-1V p-p). Rigging up a voltage divider circuit is a good start. I put together a little box from Radio Shack parts that's nothing more than an input and output connector (for leads), a potentiometer, and a resistor. You can also do something a little more elaborate to protect the sound card by incorporating a zener diode circuit to clamp high outputs, if the sound card doesn't already have such a thing built in.

Then you only have to be careful that any potential amplifier clipping you see isn't really the clamping circuit doing the clipping.


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## qstarin (Mar 25, 2009)

MarkZ said:


> This is why the _precision_ that people are trying to achieve when using a scope to set gains is nothing more than a pipe dream. The system has too many variables to define a set clipping point.



And here I just thought I was missing something - or making it all too complicated in my head.





MarkZ said:


> The issue with TrueRTA, however, is that you need to step down the voltage into something that your sound card can handle (usually +/-1V p-p). Rigging up a voltage divider circuit is a good start. I put together a little box from Radio Shack parts that's nothing more than an input and output connector (for leads), a potentiometer, and a resistor. You can also do something a little more elaborate to protect the sound card by incorporating a zener diode circuit to clamp high outputs, if the sound card doesn't already have such a thing built in.
> 
> Then you only have to be careful that any potential amplifier clipping you see isn't really the clamping circuit doing the clipping.



I guess my high-school experience with electronics is getting pretty rusty. I can certainly figure out how to make a proper voltage divider, but I'm unsure of how that circuit would affect the impedance "seen" by the amp.

A voltage divider would have some effect on the load, wouldn't it? (suppose I could google that)

I was thinking to just use big power resistors, but even the 100-200 watt ones are not that cheap, nevermind if I thought I'd want one for the subs - then I'd be looking at several in parallel - doesn't really seem like the greatest solution.


Normally, by ear suffices. But I'm the obsessive type. If I even think something's not perfect (and nothing ever is), it drives me near nuts until I find a better way.


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## MarkZ (Dec 5, 2005)

qstarin said:


> I guess my high-school experience with electronics is getting pretty rusty. I can certainly figure out how to make a proper voltage divider, but I'm unsure of how that circuit would affect the impedance "seen" by the amp.
> 
> A voltage divider would have some effect on the load, wouldn't it? (suppose I could google that)
> 
> ...


Sorry, I wasn't clear about the voltage divider thing.

I didn't mean to use one as the amp's load. I meant as the input to the sound card (assuming that's what's being used for TrueRTA's input source). The sound card will probably have only a 1v peak to peak input. Anything beyond that will overdrive the sound card. The output of the amp can be dozens of volts peak to peak. So you need to step down the amp's output voltage to be within the range of 1v. That's where the voltage divider comes in.

For amp loads, you can get big power resistors. But, like I said, that isn't going to mimic a speaker's load all too well because the speaker's impedance is going to vary considerably based on the frequency of the signal and the temperature of the voice coil. Dummy loads are more useful for benching amplifiers, and that sort of thing.


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## z_accoustics (Sep 8, 2006)

The voltage divider + 6 and 4 ohm resistors sounds like a good bet. I've seen the ohm load plot of my tweeter and it's 6 across the board for the frequencies i'll be giving it.

I know this is a DIY board but isn't there a pre-fab voltage divider out there that works out of the box for this type of application?


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## Oliver (Jun 25, 2007)

These can take a few watts 


Hot Water Heater Spare Parts
(Heating Element, 110 Volt, 1400 Watt 

Connect to speaker terminals and crank your amp til it shows all it has 

ca90ss connects his in what ever combination { series, series/parallel } he has to [ to mimick 4, 8, etc... ohm loads ]


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## qstarin (Mar 25, 2009)

Hmm .. yeah .. I just went out to twiddle some more, and decided to throw 6kHz -5dB to the tweeters. I dialed the gains up until I was reading 14.1V - or 50W into 4 ohms - and poof. Tweeter = open circuit ... again.

I really ought to just stop playing tones with speakers connected. It never really seems to get me anywhere but smoked drivers.

I guess I was wrong thinking a driver rated for 80w rms "long term power handling" could handle 10 seconds at 50w of a tone. 


The only explanation otherwise would be that I was clipping the HU, but it didn't sound like it at lower volume and everything was flat - no eq/boosts/etc.


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## Oliver (Jun 25, 2007)

It cannot cool itself with a continuous power running through it 

Duty cycle is way to high with tones.

with music there is on and offs that keep the speaker cool, as you increase the on time and lessen the off time *it goes POOF !*


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## qstarin (Mar 25, 2009)

a$$hole said:


> It cannot cool itself with a continuous power running through it
> 
> Duty cycle is way to high with tones.
> 
> with music there is on and offs that keep the speaker cool, as you increase the on time and lessen the off time *it goes POOF !*



Yeah, I was aware of that, generally - but I considerably underestimated the difference.


I was just reading a post from Andy Wehmeyer that said since amplifiers are constant voltage devices that its perfectly acceptable, given some simplifications, to set the gains without a load connected at all. That really does make everything so much simpler.

And to test that, I did just that - dialed in rms wattage for the amps with a 60Hz 0dB tone with the volume set at "-10dB" (about 90%). Despite having a tweeter blown, it sounded better than it has in a couple of weeks - eq flat, basically no adjustments other than that. I really suck at tuning by ear, obviously. ha


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## MarkZ (Dec 5, 2005)

qstarin said:


> Yeah, I was aware of that, generally - but I considerably underestimated the difference.
> 
> 
> I was just reading a post from Andy Wehmeyer that said since amplifiers are constant voltage devices that its perfectly acceptable, given some simplifications, to set the gains without a load connected at all. That really does make everything so much simpler.
> ...


Amps are constant gain devices (voltage gain). No matter what the load is, they'll supply the same voltage to it. [There are some exceptions, where there's an output impedance on the output, but it's safe to ignore that for this.] The point is that the gain is SET. The output will be a fixed multiple of the input.

However, the voltage SWING is going to depend on load. That is, the point at which it clips is going to depend on whether or not there's a load present.

The reason is because the amplification section is governed by a feedback network that acts like a really fast thermostat. It makes sure that the output is what the gain setting tells it to be. The power supply is NOT. So when you yank a ton of current from the amp (by connecting a load), the feedback network makes sure that the output remains what it's supposed to be, but the power supply will exhibit losses. The power supply dictates where the amplifier will clip because it specifies the maximum voltage swing.


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## qstarin (Mar 25, 2009)

Can't we assume we have an accurate power rating, so that if we set the voltage we can be sure the amp will not clip making that rated power with a load, assuming we can maintain supply voltage and supply enough current to power the amps. Assuming we trust the source of the amps rating.


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## Brian Steele (Jun 1, 2007)

MarkZ said:


> The power supply dictates where the amplifier will clip because it specifies the maximum voltage swing.


If the amplifier is going to be operated at or above its rated output impedance (i.e. you're not planning to attach 0.5 ohms of impedance to an amp rated to drive 4 ohms), I thing going by the open-circuit voltage should be fine for the purpose of this exercise. I used an oscilloscope to set the gains for the amps in my last install, with the amps open-circuit. It worked out fine.


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## vjgli (Nov 4, 2007)

I agree with MarkZ, setting gains in a car is a pipe dream. Besides the variable load, in Pro we're dealing with constant voltage. So, once we set the limiters, we know that the amps will not clip no matter what. We know this because the amps are getting consistent, constant voltage/current. In car, your lucky if you can maintain +/- 15 tolerance in voltage. Also, when we observe the x-mas lights, we understand that the RED LED is within 3dB threshold of clip. 3dB is doubling power my friend.
When the LED just start to show signs of 'flash', that 1dB into clip. When the LED stays solid during music passages, that's 3dB clip.

So, it boils down to listening. Please, I'm not trying to discourage anyone from trying. I am simply sharing the other side of this test/adjustment method.
Ultimately, take it to a reputable professional lab to have your amps measured. Not some repair shop like _ _ _ _.


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## MarkZ (Dec 5, 2005)

Brian Steele said:


> If the amplifier is going to be operated at or above its rated output impedance (i.e. you're not planning to attach 0.5 ohms of impedance to an amp rated to drive 4 ohms), I thing going by the open-circuit voltage should be fine for the purpose of this exercise. I used an oscilloscope to set the gains for the amps in my last install, with the amps open-circuit. It worked out fine.


The loaded and unloaded voltages will be different. Try it!


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## 14642 (May 19, 2008)

The reason to set the sensitivity or gain controls with some view of clipping isn't to be sure that the amps won't clip. Rather, it's to help maximize the signal to noise ratio of the system and provide plenty of resolution in the volume control. Setting amps so they don't clip with sine waves is wasteful and unecessary. 

The performance at high volumes isn't what we're trying to optimize, it's the performance at low volume...


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## tinnitus (Apr 28, 2009)

z_accoustics said:


> I would like to test to see where my amp clips given certain 0db test tones and adjust the gains accordingly. What tool can I use to measure this?
> 
> I have seen audio interface devices and pro audio gear that have a red light that lights up when the signal is clipping. Something this simple would be great. Such as perhaps a voltmeter that had the added feature of detecting clipping. I've borrowed an oscilloscope to do this before but it seems unnecessary (and expensive to buy).
> 
> thanks


I got on ebay and ordered me a bell and howell oscilliscope used,,, got it cheap...

I use it to test all of my audio problems......

but to use it properly you would need to take the bottom off of the amp and test your rail voltage with a multimeter so that you can set the sensitivity of the o-scope properly to get accurate sine waves....

works awesome,,, 

mine will show me the input signal versus the outpus signal,, so I can tell how many db increase im getting 
and also tell when im clipping
and also see phasing problems

its awesome


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## annoyingrob (Aug 24, 2007)

I bought an old scope on ebay for like 40 bucks a few years back, and use it to set my amps unloaded. It's not perfect by any means, but it makes me happy.


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