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lukeamdman

Luke's basic amplifier tests

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I think I might be having an issue with my scope...

 

Using a pair of multi-meters it all works fine.

 

Using the scope, a 10x probe on channel 1 to measure Vrms directly out of the amp also works fine.  However, as soon as I even connect probe #2, the readings for probe #1 are severely reduced. 

 

Capture_zpsrd1ne0vr.jpg

 

The two probes seem to interfere with each other, and somehow it's actually causing current to run through the probes and heat them up!?!?!

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I should have my resistors this week so I'll see what sort of results I get.  Thanks for the info on the resistor to use and the diagram.  Great stuff and appreciation.

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I should have my resistors this week so I'll see what sort of results I get.  Thanks for the info on the resistor to use and the diagram.  Great stuff and appreciation.

 

I think the issue is that all of the channels have the same common ground.  If that's the case, I doubt your scope is going to behave any differently unfortunately. 

 

I tried just using a single probe across the resistor and that also doesn't give accurate readings.  Additionally, once you approach ~0.75A it starts to severely distort the signal, so much so you can easily hear it in the sub. 

 

Next I used a cheater plug on the scope's power plug to bypass the ground.  That actually worked for measuring voltage across the resistor!

 

BUT...I still can't use a second probe, so I might have to stick with the multi-meters.  I emailed Rigol, so we'll see what they say. 

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I've never measured current draw like that personally, I'm assuming that you are measuring the voltage drop across the resistor to determine current draw?

I've always just run the MM probes in series, but I guess at the current we are talking about that doesn't work well(my Fluke limit is 10a I think).

I'm assuming it's a grounding or Coupling issue in the Scope,. and it seems like the scope is actually acting as it is inline/parallel because of the resistor.

It's been a LONG time since I've used any of that side of my brain, I threw out all my books/electronic parts years ago when I determined that I didn't need to fix stuff anymore(IE: didn't build RF amps or anything anymore- it still took a LOT of effort to throw boxes of parts and books away after years of building stock) cuz most things these days are throwaway..I'm wondering if you need some kind of coupling circuit/cap to keep the Scope out of the line, except to measure across the resistor.

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I'm definitely curious to hear how this is resolved.  I definitely would be wary of connecting anything that could create a voltage difference between those two grounds, given what you've observed.  I'm actually surprised that a fuse didn't blow or something.

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Yikes.  Luke and Gage, please be careful and re-check your methodology 20 times before using your new tools.  You are basically connecting 2 or more sources of high energy together that might all be at different potentials.  No joke fellas, this can and has killed people.  The only thing that I can advise in good conscience is to get a differential probe and stick to 1 input at a time or return your tools for hand held battery operated units.  If you are going to lift ground with a cheater plug or an isolation transformer on either ends of your rig, you are doing this at a high risk and you should wear thick gloves and isolate equipment with thick rubber.  I do not recommend that anyone do this unless you have a thorough understanding of the fundamentals of what you're doing and take precautions. 

 

All you need the scopes for is to find the point of clipping for a given frequency.  You can do this separately with only the load connected.  Use your DMM for all other measurements (once you confirm their accuracy using a safe method) if you're not going to take my above advise please.     

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Yikes.  Luke and Gage, please be careful and re-check your methodology 20 times before using your new tools.  You are basically connecting 2 or more sources of high energy together that might all be at different potentials.  No joke fellas, this can and has killed people.  The only thing that I can advise in good conscience is to get a differential probe and stick to 1 input at a time or return your tools for hand held battery operated units.  If you are going to lift ground with a cheater plug or an isolation transformer on either ends of your rig, you are doing this at a high risk and you should wear thick gloves and isolate equipment with thick rubber.  I do not recommend that anyone do this unless you have a thorough understanding of the fundamentals of what you're doing and take precautions. 

 

All you need the scopes for is to find the point of clipping for a given frequency.  You can do this separately with only the load connected.  Use your DMM for all other measurements (once you confirm their accuracy using a safe method) if you're not going to take my above advise please.     

 

I tested this using multi-meters before trying it on the scope, but I didn't anticipate the same ground from all the probes (a "duh!" moment after thinking about it for a few minutes).  Oh well, you live and you learn. 

 

I of course started slow with the scope and it was obvious there were issues before even reaching a single volt, so no harm done. 

 

I was really hoping to use the scope for this since it has nice graphs for the voltage history and it's easy to take a screen shot, but that won't be happening.  I suppose I can still use the scope to see how accurate the clip lights are on the amps. 

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My test enclosures now have external amp and sub connections so I can isolate them quickly with the pull of a banana plug jumper.  I am not experienced enough to ignore your warning so I will only use the DMM when taking amp connected readings.  Thanks for the save Shredhead.  So Luke, did you tell your electrical engineering friend that you planned to hook up an oscope where you were previously showing hooking up a DMM?

 

I'll go back to my original plan of only using the oscilloscope to view the incoming signal to the amp.  I also have a DATS that I will use to take sub measurements.  It will be interesting to see the T/S parameters for a cold sub, warm sub, and a Lone Survivor sub session :)

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My test enclosures now have external amp and sub connections so I can isolate them quickly with the pull of a banana plug jumper.  I am not experienced enough to ignore your warning so I will only use the DMM when taking amp connected readings.  Thanks for the save Shredhead.  So Luke, did you tell your electrical engineering friend that you planned to hook up an oscope where you were previously showing hooking up a DMM?

 

I'll go back to my original plan of only using the oscilloscope to view the incoming signal to the amp.  I also have a DATS that I will use to take sub measurements.  It will be interesting to see the T/S parameters for a cold sub, warm sub, and a Lone Survivor sub session :)

 

My buddy always used a pair of DMM for this and now I know why :D

 

I basically copied his setup for testing current, but he used a 1ohm resistor since he never needed to send more than a volt through the setup.  I'll  be sending more power through hence the much lower resistance of 0.01ohm. 

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I of course started slow with the scope and it was obvious there were issues before even reaching a single volt, so no harm done. 

 

I'm glad you took it slow.  That may be the only reason your scope didn't go up in smoke.  I'm kind of surprised that those grounds are electrically connected.  I would think that would cause problems in a variety of scenarios in which you'd want to use both sets of terminals at the same time.  Out of curiosity, have you measured the pair of ground terminals with the DMM in resistance mode?  If the resistance is not very high, that would confirm there is an issue there.  If it is high, I might try running a low current impedance sweep on it to see if the two are isolated only at DC.

 

It's a bummer that the scope doesn't work here, and I agree that the history data would be very useful.  I'm thinking that most amps will sustain a high output for a relative short period of time before clipping sets in.  With the history recording, you could see how long it takes for that to happen.

 

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Out of curiosity. When you were having the strange issues with the test rig were you measuring an amplifier that was bridged or a bridged configuration?

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Nice setup man.  I cant wait till whenever you get back to testing amps with your new setup.... I've been waiting for someone to bench the Crown XLS models to look at the sub 20hz stuff and put an end to the subsonic filter "Is it or Isnt it" an issue question.

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Out of curiosity. When you were having the strange issues with the test rig were you measuring an amplifier that was bridged or a bridged configuration?

 

Non-bridged.  

 

For non-bridged amps I could have just moved the resistor to the ground side and it'd work just fine, however, since bridged amps don't have a "ground level" connection I'd run into the same issue again.

 

A solution for this grounding issue is a differential probe, which I did try and it works, BUT, the "noise floor" of the scope+differential probe were too high to get mV numbers I trust to be accurate.  

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Nice setup man.  I cant wait till whenever you get back to testing amps with your new setup.... I've been waiting for someone to bench the Crown XLS models to look at the sub 20hz stuff and put an end to the subsonic filter "Is it or Isnt it" an issue question.

 

I'll test the XLS 2500 first and try to get it done by this weekend.  

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I'll test the XLS 2500 first and try to get it done by this weekend.  

 

No worries mate, I'm in no rush on my end as I haven't made any purchases just yet (and it still may be a month or two), but hope to have solid proof one way or the other so the issue can be put to rest.

 

I hope it goes the way I want as the Crowns fans aren't as bad as the iNukes :).

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As for which frequencies to test, I'm thinking 10, 20, and 40hz?

 

Think 5hz is also worth while?

 

May as well, doubt the output will be that much, but then you wont get asked later on for that number.  I'd also do 60 and 80 just to be safe on the top end too.  Mainly so you've got a decent test of what most AVR's can crossover at for the LFE of a sub.  I'd guess output should be about what the spec's is give or take, but again I'd just bench it once with a decent set of frequencies to satisfy as many folks as you can.

 

If its a pain to test at all those frequencies then I'd not worry about it, I just haven't seen near the bench test spec's for the XLS stuff I've seen for the iNukes like the 6000 which is what this is mainly compared to/competing with...

 

Also not sure how much its worth testing it at 8, 4 & 2 ohms for each.  I'd maybe see what spec's are out there for the iNuke 6000 and try to get the specs of the 6000 at the same ohm/frequency so you can do as close to a direct compare as you can.

 

Personally I am not planning on running it at 2 ohms as I figure if I run 2 subs at 4 ohms, there will be sections it will dip below, and since its rated for 2, it means it should handle those dips without a issue, whereas a 2 ohm load dipping may trigger protection...

 

So not sure how worth it a 2 ohm load specs is v a 4 and 8... but I'm not nearly as smart as 99.9% of the guys here so I defer to you or others on that call.

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Non-bridged.  

 

For non-bridged amps I could have just moved the resistor to the ground side and it'd work just fine, however, since bridged amps don't have a "ground level" connection I'd run into the same issue again.

 

A solution for this grounding issue is a differential probe, which I did try and it works, BUT, the "noise floor" of the scope+differential probe were too high to get mV numbers I trust to be accurate.  

 

 

Ok just checking.

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Crown XLS 2500 (Bridged/Bypass mode with clip limiting turned off)

 

 

XLS%202500_zpsa9inbyji.jpg

 

 

Even though the clip limiters were off, once clipping started the voltage wouldn't increase even when turning up the main volume.

 

At 10hz and lower, rather than seeing clip lights, the subs would start "pulsing" rapidly.  I've seen similar behavior from my Lab Gruppen IPD 2400. 

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SpeakerPower SP2-12000 (1ch driven)

 

SP2-12K_zpsfntvbryl.jpg

 

Once the clip lights were on I didn't push any further, and at 5hz it wouldn't have made any difference.  However, I've read several things that indicate on this specific amp, once the clip lights are seen, there's still 1-2db left before actual clipping occurs.  It's possible there was some power left in the tank.  

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It looks to me like you're only -2 dB from rated power at 5 Hz.  That's a pretty solid result.  Although I suppose it might not do as well with both channels playing.  Out of curiosity, why do you say that pushing it harder at 5 Hz wouldn't have made any difference?

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It looks to me like you're only -2 dB from rated power at 5 Hz.  That's a pretty solid result.  Although I suppose it might not do as well with both channels playing.  Out of curiosity, why do you say that pushing it harder at 5 Hz wouldn't have made any difference?

 

In order for the amp to lose power, voltage from the mains has to drop below 200v.  In my testing, I haven't got the voltage to drop more than 2-3v, so it stays right at 240v or above.  Voltage output was the same whether two channels were driven or one.  

 

This amp is basically two of the 6kw torpedo's in a single chassis.  

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