Quite fundamental question I assume: what causes a class D amp to distort (more) at high output? Is it a current limitation or a voltage limitation (or something else)?
When you look at typical class D amp kit performance specs like these PartsExpress 500W at 2/3 of the max output the THD is given at 1%, whereas at full output this increases to 10%. Typically these high outputs are given at low impedance (3 ohm in this case) because of power supply voltage limitations. This particular specimen runs at 50 V.
Since you obviously want to stay away from 10% THD, the usable power is a lot less than the max specified.
Long intro... 😱
Reason I am asking is that I am considering to use this, or a similar amp on an 8 ohm speaker (with an impedance peak of 50 ohm at the resonance frequency at 50 Hz).
The 8 ohm impedance limits the max output considerably, I assume 3 / 8 * 350 = 130 W? because of the voltage limitation.
Now, if reaching the voltage headroom causes the increase in distortion, then an 8 ohm load will not allow me to use the remaining power in the amp, but if reaching the max current causes the distortion, then I could push the amp to its limits, because the current will be much lower than with a 3 ohm load.
When you look at typical class D amp kit performance specs like these PartsExpress 500W at 2/3 of the max output the THD is given at 1%, whereas at full output this increases to 10%. Typically these high outputs are given at low impedance (3 ohm in this case) because of power supply voltage limitations. This particular specimen runs at 50 V.
Since you obviously want to stay away from 10% THD, the usable power is a lot less than the max specified.
Long intro... 😱
Reason I am asking is that I am considering to use this, or a similar amp on an 8 ohm speaker (with an impedance peak of 50 ohm at the resonance frequency at 50 Hz).
The 8 ohm impedance limits the max output considerably, I assume 3 / 8 * 350 = 130 W? because of the voltage limitation.
Now, if reaching the voltage headroom causes the increase in distortion, then an 8 ohm load will not allow me to use the remaining power in the amp, but if reaching the max current causes the distortion, then I could push the amp to its limits, because the current will be much lower than with a 3 ohm load.
The increase in distortion is generally voltage clipping.
So yes with your 8ohm load you'll get nowhere near the 350W you'd get (at 1%) into 3ohms.
So yes with your 8ohm load you'll get nowhere near the 350W you'd get (at 1%) into 3ohms.
Yes, power supply voltage is always a limiting factor. 50x50/16=156 W. Or a little less...
This is basic electrotechnics, not ClassD specific.
This is basic electrotechnics, not ClassD specific.
Last edited:
Power at resonance frequency
Thanks guys. Not the answer I hoped for, but instructive non the less.
For my further education: given that the output is governed by the voltage and hence the impedance of the speaker, I assume that the maximum power output to the speaker at the resonance frequency, where the impedance rises (see chart below), is even significantly lower.
So that would mean a serious drop in bass output in this area. Is this the same phenomenon as the common bass drop off below the resonance frequency? Or will it be an additional effect as one is a voltage limitation and the other a physical speaker (in cabinet) limitation?
Adding a bass boost equalization won't fix this peak if voltage is the limiting factor, but perhaps it might help (a bit) at frequencies below the resonance peak?
Or is the resonance frequency where the THD starts to rise first because of voltage limitation? That would mean that, to stay within 1% THD also at low frequencies, the available power output is even less...
Thanks guys. Not the answer I hoped for, but instructive non the less.
For my further education: given that the output is governed by the voltage and hence the impedance of the speaker, I assume that the maximum power output to the speaker at the resonance frequency, where the impedance rises (see chart below), is even significantly lower.
So that would mean a serious drop in bass output in this area. Is this the same phenomenon as the common bass drop off below the resonance frequency? Or will it be an additional effect as one is a voltage limitation and the other a physical speaker (in cabinet) limitation?
Adding a bass boost equalization won't fix this peak if voltage is the limiting factor, but perhaps it might help (a bit) at frequencies below the resonance peak?
Or is the resonance frequency where the THD starts to rise first because of voltage limitation? That would mean that, to stay within 1% THD also at low frequencies, the available power output is even less...
Attachments
The resonance frequency is located at the pole frequency of the speaker's transfer function (assuming closed-box, i.e. 2nd order). The Qtc value is the determinating factor of the SPL drop at that frequency. With a Qtc of 0.5 you will actually have a 6dB drop at that frequency and with a Qtc of 1 you will have no drop at all.
Efficiency-wise the resonance frequency is not your enemy at all: There is usually less drop in radiated power than there is in consumed power. I.e. you do even have a little increase in efficiency at that frequency.
The reason why many class-d amps have 10% THD at nominal output is exactly because those manufactueres simply define their output power with 10% THD. They could as well state the output power at 1% THD - which would be more honest actually. But then the max output power would look less beefy.
But there is indeed some rise in THD for some class-d topologies long before the amp starts clipping. The reaon for this is found in the so-called ripple aliasing error: There is a signal-dependant shifting in time of the modulator sampling point taking place.
Regards
Charles.
Efficiency-wise the resonance frequency is not your enemy at all: There is usually less drop in radiated power than there is in consumed power. I.e. you do even have a little increase in efficiency at that frequency.
The reason why many class-d amps have 10% THD at nominal output is exactly because those manufactueres simply define their output power with 10% THD. They could as well state the output power at 1% THD - which would be more honest actually. But then the max output power would look less beefy.
But there is indeed some rise in THD for some class-d topologies long before the amp starts clipping. The reaon for this is found in the so-called ripple aliasing error: There is a signal-dependant shifting in time of the modulator sampling point taking place.
Regards
Charles.
- Status
- Not open for further replies.