Simple Symetrical Amplifier

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Hi guys :)

Finally, I am proud to announce that the SSA Low Impedance HP design project is prepared. It was a DIY community mutual project since there are ideas, knowledge, effort and of course time & energy of many involved, to offer free SSA amplifier project to all interested. ;)

Happy DIY, enjoy, regards Andrej :cheers:

P.S. Special thanks to Alex, you're the Man. :worship:

Thanks LC, Alex MM, :cheers: a bit different from the original SSA V12, LC, why the inductor on the output..?

regards,
 
Hi a.wayne

There were only minor non vital changes made, like capacitors from the rails to GND, LED's for fuses & +/-50V indication and an output inductor for HF output impedance matching. There is a direct output pin so you can omit the inductor if you don't like it, but for universal amp purposes it has to be included. :)
 
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...now I am in the phase to test high performance version and pics will follow. What I am after is an amp with no more than four outputs, capable of delivering some 100W and to have the best sound as possible (to mine subjective taste). I am not in a rush to make it immediately, now, that is even better since I love fiddling around the electronics stuff. The goal is always important but the road getting there is offering much more fun. :cheers: ... ;)

Andrej, did I miss this high performance 4 output version? I tried browsing the thread but can't find it.:scratch2:
 
There are voltage drops in the amplifier that prevents the front end from being able to drive the output to the full capability of the main power rails. By increasing the front end voltage this problem is solved. And it forces you to have a separate clean supply for the front end which improves performance.
 
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Exploring the 31dB beast.:D Drifty cheeky rascal! LOL.
 

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Yes stereo, its not rising serious heat though it runs ~210mA per 30V rail. Its a 0.28C/W sink. Mono as it is now face down, barely warm on the business side. Trying to establish some safe bias point in the drivers for now. Its prone to run there. The led helps a lot.
 

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LC,
according to http://www.diyaudio.com/forums/solid-state/193923-simple-symetrical-amplifier-20.html#post2671820
It was one of the simplest version of BJT, I personally like it, but transistor are hard to get (purchase) except NJL3281 (I'm going to use MJL3281/1302 pair instead).

What transistor could I replace ? (I have spare BD139,BD140 pair)
what value should be matching or better ? Is it require to change resistor/capacitor value with different transistor ?

Extra question : what is the function of 2SA1478,2SC3788 pair ?
 
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http://www.diyaudio.com/forums/solid-state/193923-simple-symetrical-amplifier-20.html#post2671820
It was one of the simplest version of BJT, I personally like it, but transistor are hard to get (purchase) except NJL3281 (I'm going to use MJL3281/1302 pair instead).

You can't use MJL parts with that schematic, as it doesn't have the internal diode for thermal compensation. You'd need to replace the diode bias string with a common VBE multiplier.

What transistor could I replace ? (I have spare BD139,BD140 pair)
what value should be matching or better ? Is it require to change resistor/capacitor value with different transistor ?

Extra question : what is the function of 2SA1478,2SC3788 pair ?

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
 
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Offset

I saw that the output offset trimming mostly works counter input offset, since there is full gain at 0Hz. That changes conditions when the input impedance is connected or not to a source. Can spell disaster when an RCA is accidentally pulled breaking earth first. What about a second trimmer for the NPN input transistor's Re? That could nearly null the base currents difference creating offset over the input resistor?
 
You can't use MJL parts with that schematic, as it doesn't have the internal diode for thermal compensation. You'd need to replace the diode bias string with a common VBE multiplier.



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
thanks sheldon for the information, but as in the schematic, I didn't see where does the diode connection of thermal track (NJL have extra 2 legs than MJL)
If I just replace the NJL with MJL, what will degrade or fails ?
 
I saw that the output offset trimming mostly works counter input offset, since there is full gain at 0Hz. That changes conditions when the input impedance is connected or not to a source. Can spell disaster when an RCA is accidentally pulled breaking earth first. What about a second trimmer for the NPN input transistor's Re? That could nearly null the base currents difference creating offset over the input resistor?

Basically SSA input is DC coupled, so normally there is a full gain at 0Hz. Preamp output DC must be exactly 0V at the SSA input pin, therefore correct DC offset trimming is performed with input pin shorted to the GND. I don't suggest you to have a trimmer in Re position because these resistors are in the signal gain chain and the difference between two of them would cause gain unsymmetry.
Better solution is to inject very small current from a second trimmer (+/-15V) to serial resistor to the input pin. This could enable SSA to have 0V input/0V output conditions with opened RCA input connector and no change would happen when no DC source will be connected.

BTW very nice proto layout Salas, only that I suggest you to glue input BJT pair face to face. It is mandatory because of the thermal conditions compensation. ;)
 
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i bought ksc1845 / ksa992 50 pairs with f grade hfe from mouser . using a multimeter in built hfe tester i measured the hfe . i found ksc1845 around 400 and ksa992 around 500
what should i do now for input transistor matched pair ? also can i use 2sa1145/2sc2705 o grade instead of bf471/472

BTW if the input is coupled with capacitor then the offset can be adjusted or maintained easily.
 
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