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Although I don’t do car audio, I typically tune my amplifiers with a low impedance resistor as a dummy load on an oscilloscope.
What wattage your amplifier is capable of determines how much your load resistor needs to be capable of handling. Although, putting resistors in parallel halves their load and therefore you can use 2 50w resistors, however in parallel you also halve the load (2*4 ohm in parallel is 2ohm load), so you can either use higher ohm resistors, e.g. 2*8ohm, or use 4*4 ohm and have them set up so two lots of 2 resistors in series, and then put those 2 groups in parallel with each other. These are just two examples to explain the concept, many combinations are possible. It is also important to have the resistors rated higher at higher wattages than what you’ll be using them for so you don’t get inductance from heating which might make the results unreliable. This can be helped by screwing them into a heatsink with thermal paste as an interface.
You then supply the amplifier with a clean sine wave (typically at 1khz).
When analysing the oscilloscope you will know when gain is too high since the peaks will start to flatten, when that point is reached I usually back it off a bit to be sure.
It’s also useful for amplifier testing since the fuzziness of the sine wave will demonstrate visually which amplifier might have a better SNR at a given frequency.
Hopefully this is what you meant by amplifier tuning Andy, otherwise this will have been a bit of a waste of time writing out this essay🤣
Elliott Dyson – Elliott Designs (YouTube) – 3rd year MENG Student -
@livinloudwithandy with home theater I use almost exclusively the DSP built into the room correction software. I still do some basic changes, such as increasing the subwoofer output and might make some basic frequency changes. Typically nothing crazy.
For my subwoofers, I use my omnimic and measure them at the seating area. I also check my results in winISD so I have a good idea of what I want to use for a high pass. By doing that, I can make sure to protect my drivers. I almost always put a fourth order high pass, typically centered around 20 HZ. For the low pass I usually send her that between 80 and 120 HZ. A ’80 HZ I have less of a chance of voices coming through. I’ll do a fourth order on that as well. Then I’ll make some basic DSP adjustments to get a fairly linear response. Although I must admit, sometimes I like a little bass boost in the 50 hertz range.
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One way I brought up the other day to tune output wattage is with a current clamp, a trimmer pot, and you can either use the speaker or a high end resistor able to handle the wattage you would be pushing through it as if a speaker at volume.
Volts from pre-amp devices will effect how much the amp can put out. So, if your amp doesn’t have a gain or bias knob, while checking for clipping, you can put a resistor or speaker for the amp output, put the clamp on the wires to the speaker, then put a trimmer pot before the input into the amplifier. By doing this, the trimmer pot can control the max input voltage of the signal to the amplifier, thereby limiting the max output of the amplifier.
Now why would we do something like this? To protect the speaker, primarily. You can tune the output wattage of the amp to never exceed the wattage the speaker is spec’d for for RMS continuous operation. In that way, whenever you crank the head unit all the way up, you never have to worry because the amount of wattage to the speaker cannot exceed the RMS rating of the driver.
A bit convoluted, for sure, but it should work fairly well. If using a speaker, try to sneak up from below to reach that level. You then have to make sure the head unit doesn’t clip when turned up all the way and that you don’t see any clipping from the amp using an oscilloscope or a meter that tells you if there is an issue. But if you are well within the max settings of the amp’s output, and you have it tuned to the RMS of the driver, you shouldn’t have any problem.
With a DSP, if the DSP has to set a dB range or to put on an output limiter (other than of the type where you exceed an output and it reduces for a period of time until the condition is clear and returns to normal operation, so I am not using limiter here in the normal sense of what you would design for a DSP deployment), by capping its output in the chain, it should reduce the voltage sent to the amp, thereby preventing it from being driven passed the RMS of the driver.
Please, let me know if you find anything wrong with this line of thinking.
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Good one! A point worth noting, is if you are trying to calibrate your amp for all input sources, you’d want to assume an output voltage of +/- 3v as that is the maximum peak voltage you should ever get from an aux out. Typical RMS voltages from aux range from 0.3v to 2v usually (you many rarely find 2.5v RMS, although this is quite rare to have such a high output). If you only ever use one input device though, set it’s volume to max and make sure your sine wave is at 0dbfs peak. (That’s what I typically use anyways)
– I should mention this is what I use for audio in general since I don’t do car audio, can’t see why it doesn’t apply to that too though 😉
Elliott Dyson – Elliott Designs (YouTube) – 3rd year MENG Student
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In case you haven’t seen these, the wattage on these resistors can do the job for most car audio subs:
https://tinyurl.com/y6jywsy7 (4 and 8 Ohm 1000W resistors ebay)https://tinyurl.com/bdfbwuvj (variable resistors with max power of 1000W/2000W/3000W with selectable max ohm and it can be adjusted to whichever ohm you need for your setup)
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I wouldn’t trust their power ratings since they are braking resistors so would be designed for short bursts. Also, they are made without the inductance in mind so it would most likely act as a low pass filter depending on the technology used for the resistor. In terms of power though it should be fine if run for short periods and most about half of the rated power (my guess), and as with any resistive load, it’s resistance increases with temperature (same with voice coils), so just try to remember that. 👍
Elliott Dyson – Elliott Designs (YouTube) – 3rd year MENG Student -
The speakers I am currently working with are around 100-200W. Well under the 500W 50% load mark. I figure take the rating you want to hit, double it and that is what you need continuous. Double that again, and you have roughly what peak would likely be needed.
I agree it doesn’t take into account inductance, but this is why you have this to rough out the settings without having to send a sine wave to the drivers.
Many of the high wattage resistors out there actually have a heat sink rating and a free air rating, where the free air is usually between 1/4 to 1/2 of the top rated wattage that you get with a heat sink. With the first type I put on there, it is in an aluminum sink enclosure of the design used for elevator braking, etc. To be clear, that does not mean these specific ones are rated for that, only that they use this same type of enclosure for braking resistors.
For the second type, it takes the round mountable resistors and adds an ability to put a ring to short in a certain distance, thereby varying the resistance. Once again, I mention the high wattage so you can go way over. For example, doing the 1/4 load idea, 3000W/4=750W.
But, your points are well taken.
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