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Old 22nd February 2013, 05:45 PM   #1
meltman is offline meltman  United States
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Default Wattage claims from L7 eBay amplifier

Hello all, I'm looking at an amp kit on eBay based on a single pair of IRFP240, IRFP9240 MOSFETS.

Class AB MOSFET L7 Audio power amplifier boards KIT DUAL-CHANNEL 300-350W

Are these wattage claims reasonable at the supply voltage specified rails?
Output power of 150W 8R DC + - 56V
300W 4R DC + - 56V
350W 2R DC +-45V

Here's another reference thread. The design is by LJM who seems to have many designs floating around here.

Thanks for any input.
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Old 22nd February 2013, 06:56 PM   #2
jaycee is offline jaycee  United Kingdom
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300W into 4 ohms... on one pair of mosfets, yeah right... maybe for a very short time before their SOAR is exceeded and they explode.

2 ohms, forget it.
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Old 22nd February 2013, 07:17 PM   #3
meltman is offline meltman  United States
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Yeah, exactly what I thought.

hmm.. Now to find something in that power range that is fairly simple with a MOSFET output stage.

I'd prefer to find a nice kit of some sort, but a bare PCB would suffice.
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Old 22nd February 2013, 08:34 PM   #4
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the voltage is good for the claimed power. but one pair of transistors can no way take that abuse.
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Old 28th April 2013, 10:22 AM   #5
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Quote:
Originally Posted by jaycee View Post
300W into 4 ohms... on one pair of mosfets, yeah right... maybe for a very short time before their SOAR is exceeded and they explode.

2 ohms, forget it.

I made an amp with 6 off this L 7 It sounded very good. I sold it to a friend . He woulkd use it for a 3 unit system and digital filters. For a while he used only one amp in each channel and some very good old speakers ( 4 ohm ) He has used a lot off amps on the system with no problems. He used the L 7 for some months and suddenly the L7 burnt itself and one off the speakers. Voltage was just 47 Vdc. Now I have to take the amp back and pay him the 5500 dk.kr. back as well.

Take care !
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Old 28th April 2013, 10:43 AM   #6
godfrey is offline godfrey  South Africa
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It would probably make a nice 50W amp with lower supply voltages. Not bad for the price, IMHO - $40 for 2.
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Old 28th April 2013, 11:14 AM   #7
AndrewT is offline AndrewT  Scotland
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find the datasheet for the output device/s
calculate the total dissipation capacity of the whole output stage.
maximum output power ~ = 25% for mosfets and ~ = 17% to 20% for BJTs if your heatsink can keep Tc< 75°C

eg a two pair output stage using mj21193/4 can deliver ~ 170 to 200W for Tc~85°C (increased from 75 due to Tcmax = 200°C).

If you keep them cooler for your duty, then you can get (a lot) more power output.
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regards Andrew T.
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Old 28th April 2013, 03:08 PM   #8
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by meltman View Post
Hello all, I'm looking at an amp kit on eBay based on a single pair of IRFP240, IRFP9240 MOSFETS.

Class AB MOSFET L7 Audio power amplifier boards KIT DUAL-CHANNEL 300-350W

Are these wattage claims reasonable at the supply voltage specified rails?
Output power of 150W 8R DC + - 56V
300W 4R DC + - 56V
350W 2R DC +-45V

Here's another reference thread. The design is by LJM who seems to have many designs floating around here.

Thanks for any input.
Well, it's a LIE, pure ans simple.

As of "having many designs floating around", I beg to disagree.
All I found was schematic less commercial posts, where the best (still absolutely useless) was a screen capture from some simulation program (not a PCB design one) with no parts values and many actually covered by the audio voltmeter image, a bad joke.
He also does not send schematics with kits , uses non standard pinout parts on purpose and even brags about
Quote:
I know why you think like that.
But it's a little secret.

Indeed?
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