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Old 22nd April 2011, 02:47 PM   #1
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Default Need help with LM4702 Amp

I ordered some free samples of the LM4702 chips from national and I want to build a 100W per channel amp and i have some questions about building it.

Does any body have a working and stable one side pcb layout using darlingtons or bipolars?

how exactly do i set the bias pot in the amp?

could i use the circuit http://www.national.com/an/AN/AN-1490.pdf with different darlingtons like the ones from on-semi?

do i have to use darlingtons are can't i use a driver transistor and a bipolar.
what ones should i use for the best sound quality?
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Old 22nd April 2011, 04:33 PM   #2
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Hi

Darlingtons are not a requirement, this IC is the input stage and VAS for both channels and the outputs are external. The datasheet states Iout as ~5mA. The Vbe multiplier transistor (that being the transistor that sets the bias via the bias pot and is mounted to the heatsink with the output transistors to track temperature and compensate bias drift) would need to have similar temperature co-efficient as the output transistors. If you choose Darlington output devices, keep in mind the driver is on the same die as the output and is subject to the same temperature change. Since the driver and output are in cascade connection, the temperature co-efficient of the whole is the product of the two transistors. You may need to use a Darlington as the Vbe multiplier. Problem is, it can be difficult to source a Darlington of different type that has similar temp co-efficient, you may need to add an extra emitter resistor to adjust the characteristics of the Vbe multiplier.

IMO, you will be better off to use seperate driver and output stages. Place a single resistor (with paralell cap for AC coupling) from one driver emitter to the other to set Ibias for the driver stage. When the Vbe multiplier (Qmult) conducts, the output transistors are off. Start with the bias pot at 100% and gradually turn off Qmult until voltage is seen across the output emitter resistors. What output load Z are you planning to drive with 100W? From that, what output transistors do you plan to use?
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Last edited by CBS240; 22nd April 2011 at 04:35 PM.
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Old 22nd April 2011, 06:06 PM   #3
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can i just use mosfets like the 2sk1058 / 2sj162 like in this document, and then i won't need the temp. transistor.

in this circuit there are no emitter resistors how do i set the bias if i cant measure the voltage drop across the resistors?

http://www.national.com/an/AN/AN-1645.pdf

can somebody please make a single layer pcb layout of this circuit
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Old 22nd April 2011, 07:07 PM   #4
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For 100 Watt you probably need two pairs of 2SK1058/SJ162
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Old 24th April 2011, 12:47 PM   #5
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Can i build this circuit with these transistors with same/better sound quality?

http://www.connexelectronic.com/docu...ier_Module.pdf


Driver Transistors : MJE15030, MJE15031

Output Transistors : MJL21194, MJL21193

Bias Transistor : BD139
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Old 25th April 2011, 11:42 AM   #6
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The 2SC/2SA output pairs are used in some high-end Japanese amps. I prefer newer BJTs than 21194/93. Consider those with higher ft and "flat" beta vs current.
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Old 25th April 2011, 12:02 PM   #7
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Quote:
Driver Transistors : MJE15030, MJE15031

Output Transistors : MJL21194, MJL21193

Bias Transistor : BD139
15030/31 IF you have "lowish" rail voltage (45-55V)
15032/33 for 60+V

MJL21193/4 for subwoofer use (best with triple OPS) , njw/mjl 0281/0302 or 2sc5200/a1943 for general hi-fidelity. ON has a (NJW)21193/4 -to-3p with higher gain/Ft that works good with EF2's/sub use.

Bias (Vbe) can be bd-xxx , mje340 , 2sc/ksa3503.

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Old 25th April 2011, 01:07 PM   #8
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Thank you for your help, are there any other output transistors from on-semi that work good for high fidelity use.

why are the mjl21194/93's only good for subwoofers

Last edited by benproiii; 25th April 2011 at 01:10 PM.
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Old 25th April 2011, 02:21 PM   #9
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21193/94 were used in Bryston old models. They are good for high-performance full-range amps. But you can easily find better parts at ON. I compared these two to newer parts where better performance is obtained by using new devices.
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Old 25th April 2011, 03:01 PM   #10
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Quote:
why are the mjl21194/93's only good for subwoofers
Much higher SOA - better for 4R loads. 2 pair 21193/4 = 3 pair 0281/0302's current wise.
They can be used for anything , but are best suited for a sub. They DO have lower Ft , but we listen to audio , not ultrasonics.
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Last edited by ostripper; 25th April 2011 at 03:03 PM.
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