How to scope a BTL amplifier?

[Bassblaster]

as title says, im playing around with some BTL amps and im trying to pick which one i want for a project down the road. how so i scope a BTL output to do a power test? i of course know how to scope a single ended amplifier, but not BTL.

 

[House de Kris]

Use a differential probe. Or, two everyday run-of-the-mill probes on two channels in subtract mode.

 

[Bassblaster]

Well, guess i cant do that then. i only have non diff probes and a single channel scope. oh well. could i scope just one end and compare the 2? i know it wont read the correct power, but really all i need is to see which one has a higher waveform. im looking at a difference of only 3-4W in a low voltage system so i want to get the most power i possibly can. STA540 VS TDA7377 8 ohm bridge 16V. STA540 has power chart, TDA does not.

 

[House de Kris]

Yes, you could always make ground referenced measurements of each polarity of the output, one at a time. A bit more tedious, but do-able. Or, you could buy a differential probe. But, since they are active you'll need a compatible scope, or an additional power supply box - which drives up the cost even more.

 

[OMGCat!]

You can get a fairly inexpensive, low distortion differential probe from Quantasylum. They made it for use with their original QA400 but it works great with a scope as well.

 

[Ray Gianelli]

Use a differential probe. Or, two everyday run-of-the-mill probes on two channels in subtract mode.

+1
This is how I've done it. A 2 channel scope is handy for a variety of things. And since the digital scopes have gotten popular and affordable the price of analog scopes had plummeted. I picked up my daily driver, a Tek 2445A 150 Mhz 4 channel scope for $100. I know you don't have one, but think about putting one on your wish list.

Here's a link to a thread I created showing the technique for scoping balanced output amps without damage:

http://audiokarma.org/forums/index....r-scope-to-amps-with-floating-outputs.750987/

 

[whoaru99]

If the scope was not grounded would it matter?

 

[Ray Gianelli]

If the scope was not grounded would it matter?

No, it wouldn't. That's the way I was taught to make these kind of measurements. That said, it's considered to be an unsafe practice, at least according to Tektronix:

WARNING
While the subject of this technical note is about Floating Measurements, some definitions of terms and general precautions
must be understood before proceeding. Historically, floating measurements have been made by knowingly defeating the built-in safety
ground features of oscilloscopes or measurement instruments in various manners.
THIS IS AN UNSAFE AND DANGEROUS PRACTICE AND SHOULD NEVER BE DONE!
Instead, this technical note describes instruments, accessories, and practices which can make these measurements
safely as long as standard safety practices and precautions are observed.

http://www.tek.com/document/technic...lloscope-measurements-and-operator-protection

 

[whoaru99]

@Ray Gianelli

I'm well aware of the risk. It means the chassis of the scope is at whatever potential the ground clip is at.

How much danger is there in a 16V amplifier?

This is the reason I eluded to that method.

 

[Ray Gianelli]

@Ray Gianelli

I'm well aware of the risk. It means the chassis of the scope is at whatever potential the ground clip is at.

How much danger is there in a 16V amplifier?

This is the reason I eluded to that method.

From previous post of yours that I've read, you appear to know your stuff. I'm just pointing out that it's considered an unsafe practice, and I've stopped doing it. You can do whatever you like. But the OP should know what's up so he/she can make their own decision as far as testing goes. Having only a 1 channel scope it's probably their only option anyway.

BTW, it's alluded, not eluded. One letter, 2 completely different meanings.

 

[whoaru99]

...

BTW, it's alluded, not eluded. One letter, 2 completely different meanings.

Yeah, I'm quite familar with that difference too. Someone else called me out on that just the other day. Grammar police are in full force. LOL.

Apparently eluding something is stuck in my brain.

 

[BinaryMike]

... all i need is to see which one has a higher waveform.

You could use some sort of isolation transformer to convert differential to single-ended. A 70V or 25V speaker line matching xfmr, for instance, or even a power line isolation xfmr. Just stick to frequencies around 50Hz to 1KHz. The turns ratio isn't critical, but you do want to avoid stepping the voltage up so much that it blasts your scope's vertical amplifier into oblivion.

 

[restorer-john]

All my test gear is earthed, effectively, and I have an ELCB (RCD) on my bench feed, as well as master switches etc, just in case something goes wrong. My partner knows the big red switch will kill everything instantly and she doesn't need to get near me to operate it. (from my Uni lab days- the big red button near the door)

Lifting earth on a DSO or CRO is not a good idea. DSOs have SMPS supplies and CROs have big biteys should anything ever go wrong.

I have an isolation transformer (240-240) but I don't use it, as I don't work on tube gear or live chassis tube stuff often. I have these theoretical arguments with myself over safety (or lack thereof) when using it and an ELCB (RCD). Technically, there are situations where the safety switch wouldn't trip when using an isolation transformer, compromising your safety.

Funny, this post had me continuity beep-checking every BNC sleeve and PSU ground in front of me and I found one where I'd swapped around + and - on my AudioLab analyzer, luckily only for passive component and speaker impedance sweep checking.

BTW, bridged amplifiers and soldering iron earthed tips don't play well together either- ask me how I know..

Mike's suggestion is a good one. A PA multi- tapped line transformer.

 

[Ray Gianelli]

Yeah, I'm quite familar with that difference too. Someone else called me out on that just the other day. Grammar police are in full force. LOL.

Apparently eluding something is stuck in my brain.

Sorry, I was just deputized by the grammar police and I'm drunk with power!