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Old 23rd October 2012, 03:18 PM   #571
cwier60 is offline cwier60  United States
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Quote:
Originally Posted by DarpMalone View Post
Can anyone tell me if I'm voltage limited with the L15d Pro @ +/-42v? I'm hoping to get 300w into 4R
Yes, you're voltage limited unless you're operating BTL (which I'm not sure whether the L15D is capable of, and certainly not into 4 Ohms). The math is pretty simple.

Pout = Vrms^2/R = Vpeak^2/2R

Vrms = Vpeak/sqrt(2)

At clipping (perhaps 10% THD), Vpeak might be ~95% of the supply, so let's assume Vpeak = 40V

Pout = 40^2/(2 x 4) = 200W
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Old 23rd October 2012, 04:32 PM   #572
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Thanks for the reply cwier60. Just trying to get this str8.

I was thinking (hoping) that because I'm using dual voltage (+/-42v) I would see 200W/rail. How does the second rail come into play, power-wise then?

I've always been confused about this

Thanks
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Old 23rd October 2012, 04:39 PM   #573
alkasar is offline alkasar  France
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voltage is one end of the question. The other is current.
P=R*IČ
300Wrms in 4 ohms means current is 8.66Arms, ie 12A peak

I wonder why L15D is advertised for up to 350W in 4ohms as the mosfet IRFI4019H absolute max rating is 8.7Amp (down to 6A when temperature goes up).



@darpmalone : look a the definition of RMS value for a sinusoidal. You'll get it
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Old 23rd October 2012, 05:42 PM   #574
cwier60 is offline cwier60  United States
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Quote:
Originally Posted by DarpMalone View Post
Thanks for the reply cwier60. Just trying to get this str8.

I was thinking (hoping) that because I'm using dual voltage (+/-42v) I would see 200W/rail. How does the second rail come into play, power-wise then?

I've always been confused about this
If you were delivering DC power then you would be closer to correct. With ±42V supplies, you can deliver a 40V peak AC sinewave, so for 1/2 cycle, the power comes from the +42V supply, and for the other 1/2 cycle, the power comes from the -42V supply. Each supply has to be able to deliver 420W peak to deliver 400W peak/200Wrms to the load, but the RMS or average power from the supply is much less. The fact that each supply rail delivers power for only 1/2 the cycle is a reason that charge pumping can be such a problem. Operating dual outputs in BTL mode or out of phase for stereo channels keeps the loading on both supplies more constant for the full phase. If course, I am basing my comment on a single frequency sinewave, but it's generally applicable for any AC waveform except that the peak to average ratio becomes even higher than sqrt(2). This is why it's advantageous to buy a power supply that has been designed for audio applications. The thermal capability of the supply needs to be based on the RMS power delivered, but the peak capability should be much higher. Otherwise, you're overdesigning a supply to handle the peak, adding cost, size, and weight. Cristi appears to understand this, and his Connexelectronic supplies do this well. It's not such an issue with an unregulated linear (transformer) supply, since the solution is generally just to add more capacitance to handle the peaks without excessive undervoltage.

BTW, if you had a single 42V supply, then your peak-to-peak, as opposed to peak, AC voltage would be 40V, so your output power would be 1/4.
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Old 23rd October 2012, 05:54 PM   #575
cwier60 is offline cwier60  United States
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Originally Posted by alkasar View Post
I wonder why L15D is advertised for up to 350W in 4ohms as the mosfet IRFI4019H absolute max rating is 8.7Amp (down to 6A when temperature goes up).
I have no idea what the IRFI4019H devices in the L15D can safely handle, but the 8.7A rating is for continuous DC operation. Any single device in the bridge sees much less than the rated output current. The power rating has more to do with the thermal design than the current rating, since for most MOSFETs utilized in switching applications, only the pulsed current rating means much, and that is still very dependent upon the application.
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Old 23rd October 2012, 06:26 PM   #576
alkasar is offline alkasar  France
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Quote:
Originally Posted by cwier60 View Post
I have no idea what the IRFI4019H devices in the L15D can safely handle, but the 8.7A rating is for continuous DC operation. Any single device in the bridge sees much less than the rated output current. The power rating has more to do with the thermal design than the current rating, since for most MOSFETs utilized in switching applications, only the pulsed current rating means much, and that is still very dependent upon the application.
agreed. Pulse rating is 34A for this mosfet.

It is difficult to estimate the pulse current really used, at least to me.
Maybe I overlook this : In my mind, even for switched applications, RMS current needed to deliver power in a load is an average which can be considered as equivalent to the constant current the mosfet should be able to deliver safely. Does it make sense or is this a wrong assumption ?

Music has less energy than a sinusoidal with same peak. Maybe would be ok for music.
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Old 23rd October 2012, 06:33 PM   #577
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Quote:
Originally Posted by cwier60 View Post
If you were delivering DC power then you would be closer to correct. With ±42V supplies, you can deliver a 40V peak AC sinewave, so for 1/2 cycle, the power comes from the +42V supply, and for the other 1/2 cycle, the power comes from the -42V supply. Each supply has to be able to deliver 420W peak to deliver 400W peak/200Wrms to the load, but the RMS or average power from the supply is much less. The fact that each supply rail delivers power for only 1/2 the cycle is a reason that charge pumping can be such a problem. Operating dual outputs in BTL mode or out of phase for stereo channels keeps the loading on both supplies more constant for the full phase. If course, I am basing my comment on a single frequency sinewave, but it's generally applicable for any AC waveform except that the peak to average ratio becomes even higher than sqrt(2). This is why it's advantageous to buy a power supply that has been designed for audio applications. The thermal capability of the supply needs to be based on the RMS power delivered, but the peak capability should be much higher. Otherwise, you're overdesigning a supply to handle the peak, adding cost, size, and weight. Cristi appears to understand this, and his Connexelectronic supplies do this well. It's not such an issue with an unregulated linear (transformer) supply, since the solution is generally just to add more capacitance to handle the peaks without excessive undervoltage.

BTW, if you had a single 42V supply, then your peak-to-peak, as opposed to peak, AC voltage would be 40V, so your output power would be 1/4.
Perhaps I'll explain what I'm doing... I'm building dual mono amps using 2 L15D Pro boards. I've built two linear PS's consisting of two 60VCT/450VA Paradigm transformers & 20,000uF/rail. So I've got +/-42VDC rails. (LJM recommends +-35V to +-50V for this board)

I figured +-42V would be a great compromise between 4R and 8R speakers. In-fact I thought I was over-designing the power supply. If I'm voltage limited with +-42V @ 4R, how is LJM recommending +-35V to +-50V?

Thanks again
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Old 23rd October 2012, 08:07 PM   #578
alkasar is offline alkasar  France
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300Wrms in 4ohms can be reached if power supply is +-50Vdc. (and if power supply can deliver the current and if mosfets can sustain such a current)

interesting also is that the pro board, same as previous one with onboard speaker protection, has a larger heatsink. He indicates that this is to be able to reach 300W in 4 ohms. The previous board, with smaller heatsink, was supposed to reach 350W in 4 ohms. Suspicious ?
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Old 23rd October 2012, 08:36 PM   #579
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Curious, yes... I will say though, that I fired the L15D Pro up last night and I'm amazed @ how cool it runs. The heatsink actually feels cool running @ +-42v. About as cool as the unit that wasn't powered up... The L25D boards that I have get quite hot (Running @ 70v, mind you)
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Old 31st October 2012, 02:27 PM   #580
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very curious if you like the sound ?
What speakers do you use, what kind of source and compared to what u used before .

Cheers ,

Rens
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