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Old 7th November 2015, 03:44 AM   #1
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Default LM4702 Bridge

Hi
I have toroidal transformer 3000VA 2x33V from UPS ,and two bones LM4702.
I have a problem, how to select the output transistors, and how many pieces to work for the amplifier in the bridge at 4R ?
What will be the output power ?
sorry for my English.
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Old 7th November 2015, 05:52 AM   #2
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That should make around 800W-1kW amplifier.
Number of output devices depends on the type of selected device and what you actually are going to do with your amplifier...?

Anyhow, if you select the popular 2SC5200/A1943 pair for example, you would probably need around 10pairs /side /channel, or so...
So, total 40pairs for a stereo amplifier + driver stage...
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Old 7th November 2015, 10:50 AM   #3
sangram is offline sangram  India
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4R 775W with that transformer, maybe little less due to voltage sag into 4 ohm load.

48V rails (approx) into an effective 2 ohm load 10 pairs will be okay (dare say even 7-8 will do). You can scale up but the losses double for lower loads in bridge configuration. Even if your transformer can take it the rectifiers start to sag at high currents.

What you have to watch for is driver current, not the output stage. At the power stated the load will be sucking down close to 20A, so you have to ensure a driver capable of providing enough current to the output stage without hitting its own limits (or getting close). Also this driver should be easy enough for the 4702 to drive. That is the more critical part of a build like this. A 3EF is probably mandatory for this kind of a build, and those are not easy to design or tame.
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Old 7th November 2015, 11:58 AM   #4
AndrewT is offline AndrewT  Scotland
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a 33+33Vac transformer will get close to 48Vdc for the power supply rails.

A very well built current capable amplifier should get around 95W into 8r0, or almost 190W into 4r0.
If it is designed to deliver enormous current, then maybe you could get 380W into 2r0.

If you bridge two of this last 2r0 amplifier you could get 760W into 4r0.
That is actually possible, but getting the necessary current delivery will not be cheap.

If you feel competent, you rewind the secondaries of your 3kVA transformer. Being so big it will have very few turns per volts, possibly around 1T/V
Converting it to a 4 secondary transformer with 4 off 40Vac, or 4 off 50Vac secondary windings give you many more options on what amplifier types/sizes you can attach. You could even go to ClassG/H instead of the hotter running ClassAB.
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Last edited by AndrewT; 7th November 2015 at 12:01 PM.
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Old 8th December 2016, 04:48 PM   #5
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Is this degree of control will be ok ?
with supply 2x52V DC and 5 pair NJW0281/NJW0302 to work with 2R Load
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Old 8th December 2016, 04:54 PM   #6
AndrewT is offline AndrewT  Scotland
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max reliable output power into severe reactance speakers with adequate heatsinking is given by:
Max output ~ total device Pmax / 5 to 6
If you have 5pairs of 200W devices you have 2000W for total Pmax and maximum output is ~ 400W to 333W
A bridged pair adds the two channels together to give 800W to 667W

The NJW0281/0302 might be 180W devices.

BUT !!!!!
a 2ohms load.
That means each amplifier has to be rated to drive a 1ohm reactive speaker load. That is not possible with a 52Vdc supply.
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Last edited by AndrewT; 8th December 2016 at 04:56 PM.
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Old 8th December 2016, 05:03 PM   #7
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Quote:
Originally Posted by gothye1984 View Post
with supply 2x52V DC and 5 pair NJW0281/NJW0302 to work with 2R Load
My mistake, I'm sorry

In bridge load is min 4R
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Old 8th December 2016, 05:11 PM   #8
sangram is offline sangram  India
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You can use this to calculate it yourself:

Transistor Safe Operating Area
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Old 8th December 2016, 06:28 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by sangram View Post
You can use this to calculate it yourself:

Transistor Safe Operating Area
Does his method take account of reactive loading?
Or does it only consider resistive loading?

The difference between reactive loading and resistive loading is enormous.
Read David Eather to see how to use reactive phase angle.
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Old 9th December 2016, 05:47 AM   #10
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Im search and found this project and PCB :

http://ratmayor1.wixsite.com/en-us/s...W-Est%C3%A9reo

it is 4 pair and drive one pair MJE15033/32 .Author declare

Quote:
Power Supply: 75V / 15A
Power Out: 500W RMS @ 8Ω / 700W RMS @ 4Ω (1000W RMS @ 8Ω / 1400W RMS @ 4Ω in BTL Mode)
THD: 0,005% 1Khz
Frequency Range: 3Hz ~ 120Khz
Damping Factor: ~200
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