Need a little advice on Dummy Load build for audio amplifier work

[frobroj]

So I breadboarded the attached schematic out and it seems to work just fine ie I can bypass to the speaker and switch between 4 ohm and 8ohm. The thing that concerns me is its all built on my very poor understanding and assumptions of path of least resistance. I have a few questions. Do you all think I need any diodes or anything to protect the Amp side? Also I only have these open/close switches. Should I just buy some more elaborate switches and do more there or is this adequate? I struggle some times but I try to keep things as simple as possible. Any advice is greatly appreciated! Oh one more quick question. Where would be best point to have oscilloscope measurement leads? Guessing as close to the input as possible (Also should those test leads be out of circuit completely or do you think I could just half tap on to the circuit?). I am sure this is a bit of a mess so I apologize. Again built on poor assumptions and youtube videos. But I did stay at a holiday inn last night so...



Thanks All!

[ledtester]

Do you all think I need any diodes or anything to protect the Amp side?

I don't think so.

Also I only have these open/close switches. Should I just buy some more elaborate switches and do more there or is this adequate?

Individual switches are fine. Just make sure you don't close both of them at the same time -- your amp will see a very low load impedance. The only other concern is that your switches can handle the current. What's the output power of your amp?

But I did stay at a holiday inn last night so...

Cost is a issue that is often raised on this forum. If that is a concern of yours may I suggest looking into a Motel 6 or Super 8.


Update: Now that I've taken a little closer look at your schematic, perhaps you want to do something like this:



The problem is that one of the 8 ohm dummy loads is always in your circuit. If you use a SPDT switch (SW1) then it decides whether to use the speaker or the dummy load and then SW2 decides if 8 ohms or 4 ohms of dummy load will be used.

[BrokenYugo]

I would think the speaker switch should be a DPDT with no center detent that disconnects the dummy load when the speaker is applied and vice versa.

Gonna take a big heatsink to dissipate 100 watts for long.

[frobroj]

Wow perfect! Yeah gonna mount them to a think chunk of aluminum scrap I have here. Don't plan on running for too long just long enough to put a meter on it. Really appreciate the help! I just wasnt finding the way to keep those resisters out of circuit. Not enough coffee today I suppose. Thanks a ton!!!! And luckily I wont be doing 100watts just upwards of 25watts max. Probably be around 15-20 for the most part.

[Ground_Loop]

Your top-shown 8 Ohm load resistor is always in circuit.  Might need to add another switch or change the Bypass switch to SPDT type to switch between resistors and Speaker.

[bobbydazzler]

Your top-shown 8 Ohm load resistor is always in circuit.  Might need to add another switch or change the Bypass switch to SPDT type to switch between resistors and Speaker.

It's a 4/8 ohm switch, the top 8ohm resistor needs to always be in circuit or it would just be an 8ohm/8ohm switch.

[Ground_Loop]

Your top-shown 8 Ohm load resistor is always in circuit.  Might need to add another switch or change the Bypass switch to SPDT type to switch between resistors and Speaker.

It's a 4/8 ohm switch, the top 8ohm resistor needs to always be in circuit or it would just be an 8ohm/8ohm switch.

I just noticed LedTester circuit.  I assume you are modifying to his.

[frobroj]

Yep moving to that solution. Appreciate all the help!

[Trader]

I have 2 dummy loads of 4.2 and 8.4 ohms (400W), can I use them for a "professional" amplifier load testing?

With this 5% extra resistance, what will be the quality difference in the test results comparing to a precise 4.0 and 8.0 Ohms? Thanks.

[ambrosia heart]

With this 5% extra resistance, what will be the quality difference in the test results comparing to a precise 4.0 and 8.0 Ohms? Thanks.

Impedance,z, of speakers changes from time to time. Due to
this reason,  z only affects the amount of current drawn
from output stage.  Of course, volume level cannot be
too high; clipping of sinewave appears, otherwise.

[mansaxel]

I have 2 dummy loads of 4.2 and 8.4 ohms (400W), can I use them for a "professional" amplifier load testing?

With this 5% extra resistance, what will be the quality difference in the test results comparing to a precise 4.0 and 8.0 Ohms? Thanks.

The value of the resistors is likely just fine. The rôle of the dummy load in amplifier testing is to verify that it can stably deliver power according to spec, over time, and 5% is not going to give issues unless you're trying to measure in competition. As has been hinted at, a loudspeaker is a much more complex load than a resistor, which means that the amp likely is going to have an easier time with the load than a speaker.

One thing I'm thinking of is that it would be nice to have a variable monitor output, like in the attached schematic. I'm just hand-waving the potentiometer value, and come to think of it, the pot probably would benefit from a series resistor of say 32Ω just to keep things tidy.

But, as always when heating resistors watch the temperature! It'll go very hot very quick.

[perieanuo]

hi,
i don't like the switches idea, but if you really want them, put some 12V relay with NO+NC contacts, you ain't 'forget' the both open or both closed.
the only problem remaining, switching relay time when you power the relay with amplifier on, i don't recommend this (power or not the relay as needed then power the amp after, or put in special relay with specific contact opening/closing order, such thing exists for special apps)
putting speaker+load simultaneously, why doing this?or no load at all, same question, why?
diodes-never!

[mansaxel]

hi,
i don't like the switches idea, but if you really want them, put some 12V relay with NO+NC contacts, you ain't 'forget' the both open or both closed.
the only problem remaining, switching relay time when you power the relay with amplifier on, i don't recommend this (power or not the relay as needed then power the amp after, or put in special relay with specific contact opening/closing order, such thing exists for special apps)
putting speaker+load simultaneously, why doing this?or no load at all, same question, why?
diodes-never!

Look at the schematic I posted, the speaker/load switches are a DPDT switch, so will always be in a sane mode. Either speaker or load (with speaker option, controlled via potentiometer.) .

[gbaddeley]

My dummy load for audio power amp testing is 2 strings of eight 1ohm 50w Dale metal case resistors mounted on a hefty heat sink. I use flying leads instead of switches. Fairly easy to hook up 4, 8 or 16 ohm loads. No switch or relay contact losses. No risk of flipping the wrong switches. Just need to check the wiring before powering up!

[ErwinsCat]

I believe others spotted the issue with the circuit, if not, the top resistor always will be parallel with the output and when it switches to the speaker, it will be in parallel with the speaker.
also using resistors for load in an audio amp is not %100 replacement for speaker. it is okay if for example, you lake to see how your heat dissipation or if your power transistors are stable and can handle the current but speakers are inductive and also dynamic load. imagine you are hammering metal on top of an Anvil installed on the concrete floor, now imaging you hammering the same metal on top of the anvil that is installed on top of some springs and shocks, the resistor is The anvil on the concrete and anvil and spring perhaps the speaker. even the size and design of the speaker cabinet have lots of influence on the speaker and amp response. to go with the same analogy imaging the anvil and shocks and spring floting on the top of a small box on top of the water and every time you hammer the metal the energy transfer from hammer to metal and with some delay to the floating box and when the box and everything else bobbing up and down water level changes and rush out and in like ocean tides. that's perhaps the analog to the dynamic of a speaker and its cabinet.