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Old 26th October 2011, 10:43 PM   #1191
a.wayne is offline a.wayne  United States
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Wicked Salas ............. ..
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Old 27th October 2011, 01:02 AM   #1192
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Quote:
Originally Posted by Salas View Post
Exploring the 31dB beast. Drifty cheeky rascal! LOL.
Congrats Salas, Impressive -->

Are You using 2 pairs of OPs or will You populate missing
parts to a Stereo ver. on a single heatsink?
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Old 27th October 2011, 11:03 AM   #1193
Salas is offline Salas  Greece
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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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Old 27th October 2011, 03:32 PM   #1194
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according to Simple Symetrical Amplifier
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 ?

Last edited by guitar89; 27th October 2011 at 03:36 PM.
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Old 27th October 2011, 04:15 PM   #1195
Sheldon is offline Sheldon  United States
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Originally Posted by guitar89 View Post
Simple Symetrical Amplifier
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.

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Originally Posted by guitar89 View Post
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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Old 28th October 2011, 12:46 AM   #1196
Salas is offline Salas  Greece
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Default 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?
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Old 28th October 2011, 01:13 AM   #1197
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Quote:
Originally Posted by Sheldon View Post
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 ?
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Old 28th October 2011, 05:17 AM   #1198
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Quote:
Originally Posted by Salas View Post
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.

Last edited by Lazy Cat; 28th October 2011 at 05:40 AM.
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Old 28th October 2011, 06:39 AM   #1199
suresh is offline suresh  India
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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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Old 28th October 2011, 09:02 AM   #1200
Salas is offline Salas  Greece
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Originally Posted by Lazy Cat View Post

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.
NPN and PNP with different Vbe and hfe, will track better if bonded?
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