Normally it would go on the negative speaker line because normally, that is ground and perhaps even mains grounded, and the scope earth lead is also probably mains grounded.
If both scope and amp are mains grounded and you got it 'wrong', then the scope lead would short out the amplifier output.
If both scope and amp are mains grounded and you got it 'wrong', then the scope lead would short out the amplifier output.
You should measure the voltage with the DMM - much more accurate than with a scope. You can have the scope and the DMM on the load at the same time, of course.
Jan
Given the vast percentage of DMM's with poor frequency response > 400 Hz to 1 KHz, the greater accuracy is not a slam dunk.
Know your measurement gear! ;-)
Withavee,
I found this helpful, you can do your own published spec
for the amps that you've built. Then do some square wave
tests and it will show you how well your amps are doing.
I've done some of it myself by using the Tektronix
Cookbook as a guide. Here is a link to some of mine.
LINK
Over on the
slewmaster CFA threads, fine Still4given and you'll
find Terry does a lot of the square wave testing and
has many examples. Start at the end and work your
way forward. I'll post a link if I can find one. Link
scroll down to post 7972 and a few after that.
Yes it stresses the amp. The amp should not fry
being stressed, if it does, something is wrong and
that is what you want to know.
Here is the Tektronix Cookbook of Audio Tests LINK
If you fool around where the link is there is some good
info on the site for vintage information.
Someone here can help you figure out the amps gain
and go from there, it sounds like you've got a scope
and that's good.
You don't need the FFT for these measurements, scope is good.
Hopes this helps. These will give i you the info you want
without a lot of iffy stuff. Read carefully and if any of the
gentlemen here have reservations about the TEK Cookbook,
please post here so we all can learn.
MOOLY's point is
well heeded...about stress....but still these shouldn't
hurt a well designed and built amp. Heck I did them
with an Antique Amp.
Cheers,
I found this helpful, you can do your own published spec
for the amps that you've built. Then do some square wave
tests and it will show you how well your amps are doing.
I've done some of it myself by using the Tektronix
Cookbook as a guide. Here is a link to some of mine.
LINK
Over on the
slewmaster CFA threads, fine Still4given and you'll
find Terry does a lot of the square wave testing and
has many examples. Start at the end and work your
way forward. I'll post a link if I can find one. Link
scroll down to post 7972 and a few after that.
Yes it stresses the amp. The amp should not fry
being stressed, if it does, something is wrong and
that is what you want to know.
Here is the Tektronix Cookbook of Audio Tests LINK
If you fool around where the link is there is some good
info on the site for vintage information.
Someone here can help you figure out the amps gain
and go from there, it sounds like you've got a scope
and that's good.
You don't need the FFT for these measurements, scope is good.
Hopes this helps. These will give i you the info you want
without a lot of iffy stuff. Read carefully and if any of the
gentlemen here have reservations about the TEK Cookbook,
please post here so we all can learn.
MOOLY's point is
well heeded...about stress....but still these shouldn't
hurt a well designed and built amp. Heck I did them
with an Antique Amp.
Cheers,
When you are measuring power amps, the question of where the 'scope earth lead goes can be very non-trivial and even a little life threatening.
Depending on your scope and lab setup, and the amp you are testing, hooking up the 'scope earth lead can lead to equipment damage and perhaps personal harm.
The worst cases are amps with bridged outputs, or amps that have intentional or non-intentional connections from the chassis or electrical outputs to the power line.
Yes for bridged output amps, for which the load has no ground connection, you need to be carefull (can still measure to ground thought, and double the output voltage).
But in most cases, with a load to ground, connecting the measurement equipment ground lead directly to the 'bottom' of the load (and the 'hot' measurement lead to the 'top' of the load) takes care of all issue. You can also frighten people totally unnecessary 🙁
Jan
But in most cases, with a load to ground, connecting the measurement equipment ground lead directly to the 'bottom' of the load (and the 'hot' measurement lead to the 'top' of the load) takes care of all issue. You can also frighten people totally unnecessary 🙁
Jan
All true, but in 2015 the probability that a person asking the question has a PC-based FFT as their one and only measurement tool is very high.
I try to direct such people into a low cost DVM with decent frequency response such as the Unit-T UT-61E. Of course the Unit-T is not perfect and I've been lectured about that (as if I didn't know) but not a lot of people seem to want to put themselves into the shoes of an audio measrument newbie in 2015...
It turns out that a FFT has only peers for detecting clipping which is the essence of an amplifier power output measurement. IME a FFT is even better than the residual output of a classic THD analyzer which of course might be displayed with a 'scope at which time the 'scope now can compare with the FFT.
Of course, we then have to contend with the fact that modern power amp power ratings more likely than not involve a fair amount of clipping.
After the FFT and the good DVM, most newbies are probably going to start looking at 'scopes, and now the question of legacy scope versus USB scope is before us... We can keep our heads in the 1970s as long as we want to, but in the end we need to have an answer that relates to USB scopes.
..and that brings back memories. How old were you the first time you saw (or induced) a blue flash with a 'scope probe ground lead?
I can't even remember the exact order or date, but I know that I saw quite a few in Army Electronics school @ Fort Bliss and made at least one of them myself well before that. ;-)
I suspect that the post modern newbies don't always shrug these off as quickly as some old timers did.
There is much withavee can learn from all this but I still maintain you have to get practical and observe... you can not do this by theory alone.
This would be useful... set the output of the amp to say 3 vrms (that's as measured on your meter, it would be around 8.5 volts peak to peak on the scope). Applythe signal to one channel only. Do that with no load attached. Now connect the 8 ohm test resistor across the output. The voltage should be unchanged. It may go less, it may go more but it should be unchanged. If it rises you definitely have a grounding issue.
Now try the same again but this time measure the output from the channel not in use. It should be zero. No signal. You may get no signal when the load resistor isn't connected in the other channel but then see a signal when it is connected. That points to a wiring issue.
All these are tests that can highlight wiring errors and grounding problems in a build. If you want to learn you have to do these things.
Testing at full power. Probably the best and least stressful way for the amp is to set the output near to clipping (no load attached) and then quickly dab the load across the output. The output voltage should remain unchanged and visibly distortion free.
This would be useful... set the output of the amp to say 3 vrms (that's as measured on your meter, it would be around 8.5 volts peak to peak on the scope). Applythe signal to one channel only. Do that with no load attached. Now connect the 8 ohm test resistor across the output. The voltage should be unchanged. It may go less, it may go more but it should be unchanged. If it rises you definitely have a grounding issue.
Now try the same again but this time measure the output from the channel not in use. It should be zero. No signal. You may get no signal when the load resistor isn't connected in the other channel but then see a signal when it is connected. That points to a wiring issue.
All these are tests that can highlight wiring errors and grounding problems in a build. If you want to learn you have to do these things.
Testing at full power. Probably the best and least stressful way for the amp is to set the output near to clipping (no load attached) and then quickly dab the load across the output. The output voltage should remain unchanged and visibly distortion free.
He said he has a Fluke 87 DMM.
Ought to be good enough up to 1kHz.
One can always compare the 60Hz measurement to the 1kHz measurement from the
signal source - or assuming you "trust" the amp, at the output of the amp. Worst case
apply a correction factor.
_-_-
Ought to be good enough up to 1kHz.
One can always compare the 60Hz measurement to the 1kHz measurement from the
signal source - or assuming you "trust" the amp, at the output of the amp. Worst case
apply a correction factor.
_-_-
Whoa Mooly, I'd NEVER "dab" a load across the output.
If there is any instability, that's pretty close to a sure way to blow up an amplifier.
_-_-
If there is any instability, that's pretty close to a sure way to blow up an amplifier.
_-_-
Mooly,
Are you suggesting running the amp with no output connections
and at full power, then dabbing (shoving, trying to connect, temporarily
connecting, taking resistor leads and touching output connections
to them causing surge or instant resistance and release where their
wasn't some before?)
So you are a proponent of this and if I recall worried about stressing
the amp with a square wave?
And this won't harm the amp at 50W, 100W, 200W, 500W or more
watts? If there is a problem with the amp that may very well
highlight that problem.
I guess we all have ways of doing things. I personally would try to
take a more conservative approach.
Please understand I'm not attacking you personally, just the method
of testing.
Are you suggesting running the amp with no output connections
and at full power, then dabbing (shoving, trying to connect, temporarily
connecting, taking resistor leads and touching output connections
to them causing surge or instant resistance and release where their
wasn't some before?)
So you are a proponent of this and if I recall worried about stressing
the amp with a square wave?
And this won't harm the amp at 50W, 100W, 200W, 500W or more
watts? If there is a problem with the amp that may very well
highlight that problem.
I guess we all have ways of doing things. I personally would try to
take a more conservative approach.
Please understand I'm not attacking you personally, just the method
of testing.
Arny, I became aquainted with the blue flash in 7th grade physical science.
We'd take aluminium foil sheets and connect the ground lead to it.
Then crank up the voltage and touch the positive lead to the
foil arching it and essentially eating up the foil. wouldn't be
long before the fuse would blow. The teacher would scold us.
Eventually though, the power supplies were kept locked up
in a supply closet somewhere never to be seen again.
After learning about electricity we were left with Bunsen burners.... : )
We'd take aluminium foil sheets and connect the ground lead to it.
Then crank up the voltage and touch the positive lead to the
foil arching it and essentially eating up the foil. wouldn't be
long before the fuse would blow. The teacher would scold us.
Eventually though, the power supplies were kept locked up
in a supply closet somewhere never to be seen again.
After learning about electricity we were left with Bunsen burners.... : )
Squarewave testing at full power can be an amp killer but full sine output and connecting a normal load shouldn't cause any problems for a well designed amp. It would make sense to start at lower levels though if there are doubts, and if only to get a feel for what its all about.
After learning about electricity we were left with Bunsen burners.... : )
and sprinkling iron fillings into the flame 😛
After learning about electricity we were left with Bunsen burners.... : )
and sprinkling iron fillings into the flame 😛
Neatest tricks I ever saw done with aluminum foil were done with AN/MPQ 39 Hawk CW radars while I was in the Army. Each one had its own 45 KW diesel generator when used in the field. Take the aluminum foil, ball it up and pitch it up into the transmitting beam. Poof! Of course I only heard about such things... ;-)
Mooly, hope you have a few drawers full of output devices (and drivers)?
Or else I'd guess you're not repairing too many amps... or you'd be repairing a bunch. 😀
Or else I'd guess you're not repairing too many amps... or you'd be repairing a bunch. 😀
How so 🙂 Amps out in the field I don't test, the customers would do that 😀 Many commercial amps, to put it bluntly, just aren't tough enough and curl their toes up at the slightest hint of abuse.
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