o-scoping amps can be dangerous...

[shizzzon 461]

I'd like some knowledgeable input on this topic.

I have been told to o-scope amps with no load but am wanting to know if any of the following i've been told is true or not.

Do not have a load as the amp output from the amp could fry the oscope.

Do not have a load as the reading may be off.

Well, i've always o-scoped amps with no load.

I've o-scoped class d and class ab amps like that with no problems.

Today, i attempted to o-scope this class d amp and it got damaged immediately.

I contacted the manufacturer and was told it MUST have a load on it or it will do what it did.

I ask why and it is due to the amp seeing 0 ohms...

Wouldnt the amp go into protect? I dont understand.

Also, is it possible to oscope an amp with a load then?

[johnecon2001 82]

What amp is this? And what Oscilloscope?

[JimJ 11]

Wouldnt the amp go into protect? I dont understand.

Not all amplifiers have protection circuits...

I ask why and it is due to the amp seeing 0 ohms...

A no load condition is an infinite impedance, zero ohms would be a direct short

I can't tell you what the actual mode of failure would be on a SS amp, I know on tube gear the output transformers saturate with no load connected, but will actually take damn near a direct short where most SS amps will fry. It's the opposite for most transistorized stuff, it seems.

[95Honda 486]

The only way you would damage the scope is to put to much voltage on the input, I think most scopes can handle the output voltage of an amp.

An amp shouldn't immediately fail if it is driving an open or high impedance load...

Here is what could have happened... Where you using a DC power supply to test the amp, or in car? If you were using a grounded DC supply and hooked the scope to the output of the amp, you may have inadvertantly grounded out the amp output (the clip or shield of the scope probe is tied to chassis ground) and most amps don't like that at all...

In car it isn't a big deal because the vehicle ground will be isolated from the scope ground.

[chrismcdonald12 0]

cant set gains without the load attached... case closed right there.

[Adrian_D 520]

Also, is it possible to oscope an amp with a load then?

That's the way you should do it.

Put the probe on x10 (so you don't risk frying the o-scope with a high voltage) and "wire" the probe on parallel with the subwoofer.

[shizzzon 461]

ok thanks guys.

yea it was in car, not a bench tester.

The amp was on for 6 seconds then it went out and noticed smoke.

[MikeS 0]

ok thanks guys.

yea it was in car, not a bench tester.

The amp was on for 6 seconds then it went out and noticed smoke.

My test gear knowledge is pretty limited at best, but maybe you shorted the amp into the inputs of the scope with no load?

From what I've been known to understand, if you want to check a amplifier for power/bandwidth/linearity or whatever, you need a resistive load across the amplifier outputs. With the scope connected to the outputs as well.

Doesn't a Class D amplifier use a switching type power supply? Maybe it went into high frequency oscillation and died with the scope as a load?

[shizzzon 461]

i wasnt specific with you guys.

The oscope wasnt even connected yet.

The car was turned on and i immediately started walking to the rear to grab the oscope to connect it to the terminals when the amp then started to smoke.

It had no load on it and was only on for about 6 full seconds before smoke. I've been told it was because it had no load but also lower powered class d amps may not get damaged like higher powered amps would with no load...

I do not know why but i also do not know why if that's what caused the smoke to get damaged via no load because i have been discussing this with others and can't figure out why the protection circuit doesnt kick on if it really can't handle a no load scenario.

It specifically states protects against speaker shorts... That would indicate a 0 ohm load to me unless there is another type of short that i am unaware of.

I'm really afraid now if that's what caused it to go up in smoke because what if there ever is a speaker short, there's a possibility that it will fry the amp.

Well, the conclusion to the problem will be found by the end of next week.

[Tirefryr 1,742]

I've never had a problem running an amp with no load before, so I don't know where this is coming from.

[mrogowski 6]

I'm sorry, but that doesn't make sense. What make of amp is it?

If that is indeed the case, that amp design should never have left the beta stage.

[johnecon2001 82]

I'd venture to guess that something in your installation is seriously fucked. Did you ever figure out what happened with that Audioque that you blew up when strapped??

[shizzzon 461]

that's what they are working on...

they were strapped, not allowing a signal to pass. I had it severely cranked and it was barely passing a signal when the slave amp went out...

Couple days later, the signaling issue was fixed, hooked everything back up and the Master amp has worked fine ever since...

Weeks later, the slave amp came in(this is what this discussion is about). I wanted to strap them back up but with no load because i've always o-scoped amps before with no load without problem.

Within 6 seconds, the slave amp went up again...

Now tell me why hasn't the Master amp went out the first or the second time?

The slave amp has been tested by itself after repair and is in perfect working condition.

This time, they want me to send both amps back to see if the Master amp's output has been damaged causing the slave to burn out.

I agree... if it has speaker short protection, it better not have burned up because of a no load scenario because that makes no sense.

The volume was on 5.... 5 when the rest of the equipment is setup for volume 27.

They think something in the install is messed up but i 100% disagree.

IF something was, the master amp would be effected somehow.

The only difference the slave have vs the master is it's power and ground connection.

I know exactly where they go and nothing is wrong with them. The first time the amp was connected, it used those cables of course with no problem.

Everything else, remote line, speaker lines... those are shared with the Master amp.

And the daisy chained rca jacks have been tested to work fine so it's not that either.

Can someone tell me this-

I do not feel like it's fair if the amp got damaged this time because there was no load and it clearly stated it protects from speaker shorts...

If that is what caused it, what would you guys say because i'm just going by what the manual says. IF it says it's protected from it, then i don't understand how it could be my fault?

You guys say that you've never heard of this before. I have nothing really to base my feeling or point on the subject... if this is why it happened other than it states in the manual it's protected from it.

[95Honda 486]

No load = infinity ohms = Open circuit

Full load = as little as 0 ohms = Short circuit

I think you have this backwards...

Short protection DOESN'T do anything for a no load connected condition. It is designed to shut the amplifier off when there is a short circuit across the output presenting a load close to 0 ohms to the amplifier. Most amplifiers DO NOT have circuit protection for no load or infinite ohms, this is because solid state amplifiers should have no trouble driving this load (or lack of load), unless they are defective.

Some (poor) designs may oscillate if there is no load load connected because of output dampening, but I am 99.9% sure that is not your case, or any case with even the cheapest car audio amps.

If you had your amplifier hooked up right it should have never smoke from no load connected, even if you were clipping the amp. Something was wrong with the amp or the install, double check again....

[Adrian_D 520]

How much time did it spend turned on before it crapped out ?

Did it make any strange noises ?

[shizzzon 461]

it took 6 seconds of power on time, no noises.

I'm not here to guess what happened because they will find out within a week or so.

It's just odd that the master amp always works and the slave always fails.

The master amp had no load either but it never has a problem.

That's why they are going to inspect the master amp to determine if it's causing the slave to malfunction.

95honda - thanks for info. Now i understand.

[mrogowski 6]

Hope it works out for you.

[sqguy 0]

I'm not real into strapping amps, but they typically have a strapped only input connection via RCA or data cable. This is so the feedback is connected in the two amps; then one does not get confused seeing the other amp's output. If that connection failed it could end in smoke but all depends on the design of the amp itself. I have never heard of an amp not being able to run with no load (no speaker connections), or 'idle' as most call it. DC offset is measured that way and is a standard measure of any amp far as I know. I would certainly think this issue is to do with strapping, either incorrect wiring or a fault in the amp. I am not 100% sure if you can run strapped amps idle, but one would think so. If you had a sub wiring fault or blew a sub it would instantly blow the amp every time if that is the case....and that is way stupid for any manufacturer to do. However mtx shows here they do not use a feedback connection: http://www.mtx.com/caraudio/products/manua...sure-Manual.pdf

[shizzzon 461]

Well, i'm gonna ask them all of these questions rather than assume the worst will happen after they diagnose the problem(s).

Now, when i get these back, if something gets damaged again, then it's clearly a wiring issue because they are going to test them strapped after they fix the slave amp again.

[Adrian_D 520]

Re-check your wiring while the amps are in testing. That way you're not wasting time and reducing the risk of more damage once they get back

Everyone can make mistakes.

[audio-neon 34]

I would check your power wiring from battery to amps for any tears in the insulation at all.

Also your rca's for tears.

When you get your amps back, try and make sure there isn't any metal touching the amp other than the screws, I have heard of amp cases grounding before and causing problems (although rare).

[shizzzon 461]

Well, i know the amps arent touching metal because they are mounted together on the box.

The entire car is still gutted and i dont see any tears anywhere.

The verdict of why the problem happened again will come later next week.

[sqguy 0]

Did you ever get anywhere with this?

[shizzzon 461]

Yea, it's your fault! hehe

Again, this problem had nothing to do with me.

A Toroid had shorted out inside the amp as soon as it was powered on.

They've had the amps for 1 month now. They have blown again since they've been in their possession.

They do NOT know what the root cause of the problem is that started all of this but are guaranteeing that they will not ship these out until the entire unit it inspected, repaired, inspected and tested for 1 whole day before shipping back out.

They blew while in their possession because an rca connector broke off inside the slave amp(the one that always messes up) and caused a signal issue which fried both amps.

Talked to them today, they should have been done either last Friday or today but still wasn't.

They said the slave amp, simply put, is being an ass and is giving them troubles. IT hasn't damaged itself again but they are having complications over somethin, just dont know what. They expect it to be fully working by late wednesday... This is like the 4th projected repair date.

[sqguy 0]

Ouch, this is getting to be a sad amp story. Well good thing they are working on it, hope they figure it out. Nothing can be out of whack when you strap...well guess that is obvious here isn't it. I have a couple amps here with broken RCA, many of them are really cheap plastic things. I wont even use an RCA cable if it goes on hard.