Dummy Load confusion yet again......

[Richa]

Hi again EEVblog,,, got another beginner question,, I know it's been covered many times before,,, so i apologise for dragging it up again.
I'm going to build a dummy load for testing hi fi amps ,, (I'm only doing audio spectrum stuff) with my oscilloscope and function generator,,, but the confusion about wiring and voltage dividers has me once again baffled, I've lost count of the amount of vids,,, posts and tutorials that i've looked at on the subject... All very informative but still confusing,,. I intend to build a "stereo" version ,, 8 ohm,,, no pass through switching,,, please help with a definitive layout ,,
I understand why it should have 10:1 divider e.t.c but when looking at tutorials and circuit designs,,, many seem to conflict. Some don't even have any divider circuit.. I have attached a few,, whilst they make some sense they are all somewhat different,, from simple resistor for bnc output to much more complex circuits with switching ... I have even more design layouts on my computer ,,, that are different again.. I guess my simple questions are,
1. what would be a simple but effective 8ohm stereo circuit,,, with no switching? With BNC outputs for scope.
2. i want to use it as stereo channels... is this possible,?
3. which divider circuit do i actually need to protect my scope and for reasonably accurate measurements ..? is it the single resistor or is it necessary to use a more complex design.? what values ,, apart from 8ohm power resistors,, even these are different on many designs...
4. i intend to have fusing,,, as recommended by some,,, rating of 250ma,,, ?

I've got to thank everyone who takes the time to post their tutorials and share their designs,, (but I'm still confused),,, i must have serious brain fog.!!!!!!!
And thanks to you all who can guide me through this subject,, i really don't want to blow up my scope or equipment.
I have gathered together all the parts i need including insulated scope BNC's,,,

My scope is Uni-T 2052cex+ (2 channel , 50mhz)
Function generator is simple Feeltech FY2200S (with grounding mod done)
I have an isolation transformer, if this is needed.
My only reason for not building it yet is the circuit and protection ,,, there is so many conflicting areas to deal with...
I am perfectly ok with the practical work but the theory is sometimes quite daunting,,
xrayonyb has covered this thoroughly as have many others,, but some don't have such elaborate designs,, and say it's ok,,, some advise massive caution,,, ?! and don't do this because....... some say you definitely can't use it as stereo outputs.!!
sorry for rambling on with a subject already covered... and i hope i've covered everything.....
Please help.. cheers

[Vovk_Z]

1. what would be a simple but effective 8ohm stereo circuit,,, with no switching? With BNC outputs for scope.
2. i want to use it as stereo channels... is this possible,?
3. which divider circuit do i actually need to protect my scope and for reasonably accurate measurements ..? is it the single resistor or is it necessary to use a more complex design.? what values ,, apart from 8ohm power resistors,, even these are different on many designs...
4. i intend to have fusing,,, as recommended by some,,, rating of 250ma,,, ?

1. Do you really need BNC output? Scopes typically have to be used with their proper 10/1 divider probes, with 10/1 attenuation but not in 1/1 mode (if you want full bandwidth and don't want additional troubles). So, I mean a usual 10/1 scope probe is a very good divider itself. So you really don't need BNC and a BNC/BNC cable for a scope. You just connect a scope probe to a leg of your dummy load resistor and that's it. That's both simple and effective.
2. Obviously you need two dummy resistors. You can connect GND wires together, or they will be connected by a scope GND when you connect both channels simultaneously. You may use series-parallel connection to get needed rated power of your load. E.g. you may use four series-parallel connected 8,2 R 50W/100W resistors, or more, or other resistance value to get 8 R and needed rated power (> 100 W typically).
3. 10/1 usually is quite convenient, but look to n. 1. As for me I use 10/1 attenuator only to connect an amplifier output to my sound card (for THD investigation). Obviously, sound card cant withstand more than 2-3 Vrms at its input and really needs a divider. I use 1800 R / 200 R divider, it has exactly 10/1 attenuation (20 dB). But that's for sound card only, because scopes are typically connected with a proper 10/1 scope probe, so needed attenuator is inbuilt.
4. A typical dummy load for audio amp is just a dummy resistor so is quite a simple device and don't really needs a fuse until you know what you do. Otherwise, if you want to put a fuse then its rated current really depends on a schematic, where you want to place it (?).

My mostly used 8 R dummy load is on the attached photo. It's just a bunch of 82 R 10 W resistors in parallel. It is intended for single channel testing, because I mostly don't really care much about two channel tests. It has XT60 type connector but not banana because I use it multi-purpose, e.g. to load some power supply too.

[inse]

I would be careful connecting both amp GND/- outputs.
This will work for most audio amplifiers but will lead to trouble for full bridge configurations.
Also to completely switch off your speakers, the GND needs to be separated, otherwise you will always have the R-L difference signal across them.