Hi, I use the Linkwitz closed-box Excel sheet to design a bass speaker. Thing is, for that sheet I need an estimate of Imax for the Dayton SPA300-D plate amplifier I’m looking at. I wonder if the rough estimate below is correct.
SPA300-D, Class D, 300W into 4 ohms, no info available on rail voltages, current limits etc.
Given 300W into 4 ohms I get 34V (P=V^2/R)
This in turn give me about 24V RMS (34V*0.7)
Given 300W and 24V RMS I get about 12A (P=I*V)
Is 12A the Imax I should enter in the closed-box sheet? I remember Linkwitz saying somewhere that Imax is higher than the continuous power limit, perhaps as much as twice that.
SPA300-D, Class D, 300W into 4 ohms, no info available on rail voltages, current limits etc.
Given 300W into 4 ohms I get 34V (P=V^2/R)
This in turn give me about 24V RMS (34V*0.7)
Given 300W and 24V RMS I get about 12A (P=I*V)
Is 12A the Imax I should enter in the closed-box sheet? I remember Linkwitz saying somewhere that Imax is higher than the continuous power limit, perhaps as much as twice that.
Those are already 34V RMS
No, you already estimated 34V RMS
You have 34 V RMS into 4 ohm so 8.5 A RMS
Or in another way, 300 W RMS into 4 ohms means 8.8 A RMS (slight differences because of rounding up numbers, your actual RMS Voltage was 34.64 V)
Never used that Excel sheet but anyway IF they want RMS Amperes, it's 8.8 A ; if they want peak, then it's 12.45A
Guess spreadsheet instructions make that clear.
That said, I am guessing they want to calculate whether the power amplifier can drive that speaker or not.
Good question since all speakers are reactive, one way or the other, and regularly drop below nominal impedance, at least 20% ... or worse.
Again, only guessing here.
Do you have impedance vs. frequency curves for your speakers?
Which in tuned cabinets will also be modified.
Thanks! See the error of my ways now.
Yes, I want to estimate whether a specific power amplifier can drive a specific speaker driver in a sealed enclosure of a certain volume. I get graphs like the one below. In this case the amplifier cannot drive that woofer to xmax. If I e.g. make the box a little bigger it can, up to a point near the resonance where the amplifier can no longer deliver enough power. At least that's my current (no pun intended ...) understanding.
Yes, I want to estimate whether a specific power amplifier can drive a specific speaker driver in a sealed enclosure of a certain volume. I get graphs like the one below. In this case the amplifier cannot drive that woofer to xmax. If I e.g. make the box a little bigger it can, up to a point near the resonance where the amplifier can no longer deliver enough power. At least that's my current (no pun intended ...) understanding.
Not sure about your curves, do they mean cabinet is tuned to 49Hz (at which point impedance and cone movement would drop) or it's closed box resonance which means the opposite?
In any case, reaching X Max is not dangerous, it only increases distortion beyond that.
Now avoid X Damage at all costs, it only takes seconds of voice coil slapping against back plate to destroy it.
In any case, reaching X Max is not dangerous, it only increases distortion beyond that.
Now avoid X Damage at all costs, it only takes seconds of voice coil slapping against back plate to destroy it.
If power consumption is going down impedance is going up. Looks like closed box resonance to me. And the slope is 12 dB per octave not 24. SPL @ Xmax falls at the same rate for a fixed # of mm of excursion.
Whether or not going beyond Xmax is damaging depends on the woofer. Big ones with strong motors and a lot of linear throw are relatively easy to get beyond x-max and bottom out given enough power. Cheaper ones with more limited throw may not have enough coil left in the gap beyond x-max to ever get to bottom out regardless of how much you throw at them. They will fail thermally first.
Whether or not going beyond Xmax is damaging depends on the woofer. Big ones with strong motors and a lot of linear throw are relatively easy to get beyond x-max and bottom out given enough power. Cheaper ones with more limited throw may not have enough coil left in the gap beyond x-max to ever get to bottom out regardless of how much you throw at them. They will fail thermally first.
It may be easier to think of the amplifier as delivering voltage gain, rather than "power amplification".
At four ohms (and lower), the amplifier's power supply will limit both voltage and current.
At higher impedance, the power supply's maximum voltage determines the available power.
This type of amplifier is not a perfect voltage source, although limited to ~34V RMS at 4 ohms, it can deliver ~40 volts into an 8 ohm load, and probably a bit more at the sealed speaker's 49Hz impedance peak where current draw is quite low.
Although that still may not be enough voltage to push the driver past Xmax at the impedance peak, the amp will run out of voltage and current at a lesser SPL below 49Hz.