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Old 1st November 2011, 01:02 AM   #1221
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Some little cosmetic modifications

Borad is standart size : 160x100mm
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Old 1st November 2011, 09:49 AM   #1222
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Quote:
Originally Posted by Sheldon View Post
You can replace the front end transistors with BC550,560, with no changes to the front end components. BD139/140 can replace the 2SA1478/2SC3788, with no change. The larger part is used because they can dissipate more heat than the front end TO99 parts, and you can run more current through them to drive the outputs.

Sheldon
hm.... i'm thinking is there any other replacement for them at the input ? I personally favor 'On Semiconductor' 's transistor, good if any recommendation ?

I was thinking, if I were to put other transistor same as output transistor, is there any disadvantage (other than money aspect) and what is the advantage ?
Example : all transistor using NJL3281/1302 pair where NPN & PNP are correct position

Last edited by guitar89; 1st November 2011 at 09:52 AM.
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Old 1st November 2011, 10:20 AM   #1223
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Marc very good PCB you've made here.

Quote:
Originally Posted by Idefixes View Post
What about choice between NJL3281/1302 and NJL4281/4302?
If you didn't order them yet I suggest you to take NJL4281-NJL4302 because of their higher current gain (hFE) noted in datasheet.

In a real case of a 5A output (speaker) current VAS transistors (T1, T2) would have to provide 20mA signal current to the bases of the output transistors (at hFE=250). So for an amplifier design simple as this all transistors have to be at their best performance level (max hFE) to get lowest distortions possible.

I hope you will use this amp to drive speakers with impedance curve of more than 4 ohm.

P.S. VR2 should be 100 ohm trimmer. It will be set to 8,8 ohm at Iq(VAS)=5mA and Iq(OUT)=100mA (hFE=250).

Last edited by Lazy Cat; 1st November 2011 at 10:33 AM.
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Old 1st November 2011, 10:38 AM   #1224
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Quote:
Originally Posted by Lazy Cat View Post
Marc very good PCB you've made here.



If you didn't order them yet I suggest you to take NJL4281-NJL4302 because of their higher current gain (hFE) noted in datasheet.

In a real case of a 5A output (speaker) current VAS transistors (T1, T2) would have to provide 20mA signal current to the bases of the output transistors (at hFE=250). So for an amplifier design simple as this all transistors have to be at their best performance level (max hFE) to get lowest distortions possible.

I hope you will use this amp to drive speakers with impedance curve of more than 4 ohm.

P.S. VR2 should be 100 ohm trimmer. It will be set to 8,8 ohm at Iq(VAS)=5mA and Iq(OUT)=100mA (hFE=250).
Ok thanks Lasy Cat. I will have 5 couple of each. It will be eventually for Home theater 5Ch in small room (15m). These bord will make a compact size 5ch amp. The speakers doesn't normally goes under 5Ohms. Thanks for value advices. 5k on biasing pot is a device copy mistake.
Marc
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Old 1st November 2011, 10:59 AM   #1225
AndrewT is offline AndrewT  Scotland
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there is no way the ONsemi nor any other BJT can maintain 250hFE @5A of Ic.

Look at the typical graphs that ONsemi show in the datasheet. Then allow for the variation of hFE that is quoted in the table as min, typ, max.

Expect a range of hFE @ 5A Ic of about 80 to 120 resulting in a base current of 42mA to 62mA.
Now add on the driver's emitter resistor current.
That is what the driver must supply just to feed a resistive load with 5Apk output current.
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Old 1st November 2011, 11:38 AM   #1226
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AndrewT thanks for your calculations and support. Of course an output transistor with hFE=250 is hard or even impossible to find, but my calculations at given data were correct completely. I intentionally presented top level results to show what conditions we are coping in ideal case, as a top reference of what is possible to achieve. Some would also use outputs with hFE=40 but as I already stated, that is not recommended in this simple design.
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Old 1st November 2011, 11:48 AM   #1227
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Quote:
Originally Posted by guitar89 View Post
hm.... i'm thinking is there any other replacement for them at the input ? I personally favor 'On Semiconductor' 's transistor, good if any recommendation ?

I was thinking, if I were to put other transistor same as output transistor, is there any disadvantage (other than money aspect) and what is the advantage ?
Example : all transistor using NJL3281/1302 pair where NPN & PNP are correct position
One suggestion for you guitar89, try to built one of the simple SSA amp design and than play with different types of BJT's to get the performance you like. In general it is very important to chose BJT's with high current gain (hFE) value and you will be rewarded with great SSA sound.
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Old 1st November 2011, 11:56 AM   #1228
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Quote:
Originally Posted by Lazy Cat View Post
AndrewT thanks for your calculations and support. Of course an output transistor with hFE=250 is hard or even impossible to find, but my calculations at given data were correct completely. I intentionally presented top level results to show what conditions we are coping in ideal case, as a top reference of what is possible to achieve. Some would also use outputs with hFE=40 but as I already stated, that is not recommended in this simple design.
I am looking forward to your future work on even simpler front end such as attached. My darlington transistors have hFE 15K-30K, fT 60MH, Co 55pF. But why would this topology not suitable for ordinary output transistor?
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Old 1st November 2011, 12:16 PM   #1229
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LC, thanks for the advise, i also personally felt that is the best way to try out different transistor.
Just to verify, replacement for any transistor will not kill any components, as long it is not too weak ? (pardon for lack of knowledge about this)
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Old 1st November 2011, 12:21 PM   #1230
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Jay View Post
darlington transistors have hFE 15K-30K,
hFE at what current?
How does the effective hFE vary with current?
Integrated Darlingtons in my little experience do not behave as linearly as discrete, driver & output pair when well designed. You must not omit the driver emitter resistor and preferably the driver current must be decades higher than in an integrated Darlington.
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