I have a number of Class D amps with BTL and PBTL configurations and want to use my Digital scope to do measurement. A connects balanced probe is very expensive so I was thinking of using a transformer to isolate the amp from the ‘scope.
Has anyone else done this and if not, can anyone see a reason why this would not work?
The transformer is taken from a high quality 100V line amp, designed to offer a 4 ohm load to the amp and feed either a 100 or 70v line system. It is rated at 240 watts although Judging by the size, that seems conservative.
Has anyone else done this and if not, can anyone see a reason why this would not work?
The transformer is taken from a high quality 100V line amp, designed to offer a 4 ohm load to the amp and feed either a 100 or 70v line system. It is rated at 240 watts although Judging by the size, that seems conservative.
Personally I don´t bother to measure across the load, which brings problems, but simply scope one output to ground, and if necessary the other, same thing.
Waveforms are straight visible which is the important job, as well as oscillations, clipping,any nastiness, and I can easily add both output voltages, even in my mind 🙂
Even without a differential probe, some scopes allow to dedicate one channel to each (balanced) speaker out and display the sum.
I MUCH prefer such direct measurements to going through a clunky transformer.
Waveforms are straight visible which is the important job, as well as oscillations, clipping,any nastiness, and I can easily add both output voltages, even in my mind 🙂
Even without a differential probe, some scopes allow to dedicate one channel to each (balanced) speaker out and display the sum.
I MUCH prefer such direct measurements to going through a clunky transformer.
100V line is for PA grade systems (voice or musak), and their quality is not outstanding, certainly not adequate for metrology. Your best bet is to use a good line or mic transformer, from Jensen or similar, combined with a zero-flux interface on the secondary (relatively simple and cheap)
But this is a class D amp so each output has massive switching noise.
You are pretty much forced to measure differentially or use massive filtering to get anything useful.
Jan
This Tx is from a Cloud amplifier, Made in the UK and has an extremely good Spec. The main reason for using this is that it is designed to offer a 4 ohm load.
I do have some Sowter line transformers but am concerned that without the correct 4/6/8 ohm loading the frequency response will be wrong.
Member
Joined 2018
If you want to measure with PC. You can use the USB-Audio interface which has the XLR Phantom Power supplied Input feature.
It must have a capability of balanced input and DC offset tolerances.
Just you need to prepare passive attenuator cable something like three resistors to divide the voltages.
The STPS software might be useful...
ARTA Software
CyberPit
It must have a capability of balanced input and DC offset tolerances.
Just you need to prepare passive attenuator cable something like three resistors to divide the voltages.
The STPS software might be useful...
ARTA Software
CyberPit
Member
Joined 2018
Transformers have a lot of parasitic capacitances, leakage inductance, magnetic distortion etc. A very good transformer is good for audio barely, if and only if it is driven and loaded exactly correctly. It is unlikely your transformer is better than your amplifier. To measure a balanced output, use two probes (two channels) and set the scope for differential mode.
Good call, but in that case I can use a simple cheap RC filter between any amp out and scope input.
Remember I want signal (voltage) into a 1M or higher scope input, not driving any kind of speaker.
And not "massive" either.
Standard Class D output filter is 6dB/oct LR, an inductor in series with speaker; conceptually same as 6dB/oct RC
As long as turnover frequency is the same (easy peasy), measurement is valid.
EDIT: and of course there is the trivial solution: measure not straight from amplifier out (before the inductor) but at the inductor-speaker junction .
Even if Class D, remember OP wanted to measure across load, no extra filtering added, even less a "massive" one.
If that was not his requirement, why should I add one?
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BTL amplifiers, especially class D rely on the symmetry to reject various nasties common to both outputs: PSU noise, etc.
Measuiring one output at a time will result in a very pessimistic measurement.
Even a crap transformer can be sublimated with a zero-field technique.
Here is a basic intro:
https://www.diyaudio.com/community/...transformer-based-circuit.356221/post-6247241
I post this one because I created it and know where to find it, but the net is full of resources, and there are probably better suited examples, but you get the idea
Measuiring one output at a time will result in a very pessimistic measurement.
Even a crap transformer can be sublimated with a zero-field technique.
Here is a basic intro:
https://www.diyaudio.com/community/...transformer-based-circuit.356221/post-6247241
I post this one because I created it and know where to find it, but the net is full of resources, and there are probably better suited examples, but you get the idea
I did try just with an output transformer and the frequency response measurements were virtually identical to this OTA Ken from the preamp output.