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Ebay kit looks interesting. 300w @ 8 ohms dual channel
Ebay kit looks interesting. 300w @ 8 ohms dual channel
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Old 5th June 2017, 02:27 AM   #1
K-amps is offline K-amps  United States
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Default Ebay kit looks interesting. 300w @ 8 ohms dual channel

Hi guys,

I am looking for a quick build power amp with little effort. RTR kits seem to fit the bill to drive a pair of DIY , 3 way inefficient bookshelf speakers that are a bit power hungry. I was initially looking at class D kits to power them, but since I have zero experience with class d builds, this kit caught my eye. Looks like a great price, its dual channel and quite versatile in operating voltages. Seems to have speaker protection built in and uses sanken outputs similar to the Denon/ Luxman designs of the late 80's.

Here is the link: https://www.ebay.com/itm/222529950811

Has anyone used it or know what amplifier its based on?

With much appreciation

K-Amps

Last edited by K-amps; 5th June 2017 at 02:59 AM.
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Old 5th June 2017, 10:47 AM   #2
AndrewT is offline AndrewT  Scotland
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the 2sa1494 are 200W devices that are down to 125W @ 60Vce and down to 80W @ 80Vce
I would not use +-80Vdc supply rails for these devices.

If you use +-60Vdc supplies, then you might get 160W into 8ohms (50.6Vpk) from the two pair output stage. (2pr @ 200W = 800W/5 = 160W of maximum output into severe reactance speakers.)
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Old 5th June 2017, 11:07 AM   #3
K-amps is offline K-amps  United States
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Thanks Andrew. Having interacted with you for over a decade, I knew I could count on you, to ensure the OP stage was adequately sized, specifically with thermal deration. :-)

I was planning to replace the 2 Sanken pairs with 8 pairs of njw0281/0302's and running it with 92v supplies. What I cant say is if the drivers supplied will be able to drive the mod.
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Old 5th June 2017, 11:24 AM   #4
AndrewT is offline AndrewT  Scotland
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That's a big change from 2pr to 8pr.
@92Vce the njw0302 is down to ~74W. That is well into the reduced voltage range above the knee in the SOA curve.
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Old 5th June 2017, 12:11 PM   #5
K-amps is offline K-amps  United States
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The question I have is (assuming all devices are running under their SOA): Is it easier or harder for a driver to drive 2 pairs of OP vs. 8 pairs? Assuming in both scenarios the load is 4 ohm resistive. My question is more around beta droop of the driver.
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Old 5th June 2017, 12:17 PM   #6
AndrewT is offline AndrewT  Scotland
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Beta droop needs to be examined when peak transient currents into the load are passing.
4ohm resistive sounds like a test load for an 8ohms capable amplifier.
I suggest that you analyse the output current requirement for speakers that can demand transient currents approaching and even exceeding three times the peak current into a resistive equivalent load.

that would require your 92Vdc supply rail to push current around the load circuit with a 2r67 resistor in there.
Assuming the maximum output voltage were ~80Vpk, then the peak curent would be 30Apk
If the drivers see an output transistor hFE of 80 at that loading, then they have to pass a transient of ~ 0.375Apk and each output is passing ~3.8Apk.
Many driver transistors of around 1A to 2A rating can manage that and their droop should not be too bad to become a severe loading on the pre-driver or VAS stage.

What driver quiescent current is required to bias the 8pr output stage? and how big would their heatsink/s need to be?
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Last edited by AndrewT; 5th June 2017 at 12:21 PM.
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Old 5th June 2017, 12:20 PM   #7
K-amps is offline K-amps  United States
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And thats where lies the rub, I don't know what bias current requirements are for the driver. Adding more output pairs reduces beta droop, but since the driver needs to supply bias to all OP devices, it will get loaded a bit, so under what conditions will it be better or worse for the driver stage?
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Old 5th June 2017, 12:24 PM   #8
AndrewT is offline AndrewT  Scotland
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analyse it.
try the 2pair
then look at 4pair and finally look at 8pair.
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Old 5th June 2017, 12:37 PM   #9
mchambin is offline mchambin  France
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I wonder how they can make a profit selling this at 30 bucks.
Looks too good to be honest.
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Old 5th June 2017, 12:59 PM   #10
K-amps is offline K-amps  United States
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I think they made many kits and now selling at cost. I see several sellers including Amazon/ Aliexpress/ fleabay etc
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