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Possible issues in replacing bjts with more powerful parts
Possible issues in replacing bjts with more powerful parts
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Old 15th September 2020, 07:03 AM   #1
ginetto61 is offline ginetto61  Italy
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Possible issues in replacing bjts with more powerful parts
Default Possible issues in replacing bjts with more powerful parts

Hi ! if i understand well speaking of bjts more powerful ones have a bigger SOA than less powerful ones. Is this confirmed ?
I wonder which kind of issues i can encounter if i replace let's say a NPN 80W bjt with a NPN 200W bjt.
I know already that higher gain bandwidth product can lead to instability
and this is my main concern
If i have to replace a 3 MHz part can a 10 MHz part be a safe option ?
The goal would be to have a more robust output stage (i.e. increase not the power but the soa)
Anyone who has done something similar ? is this a silly idea ?
If there is a more appropriate section please redirect my question there
Thanks a lot and kind regards, gino
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Last edited by ginetto61; 15th September 2020 at 07:18 AM.
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Old 15th September 2020, 07:15 AM   #2
dreamth is offline dreamth  Romania
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a transition from 3mhz to10mhz transistor won't be really noticeable...but one to 60 or 80mhz is...Here you have some very extreme case where you have paralleled 200mhz drivers for 80 mhz final transistors...not much compensation is used throughout all the amp but every single stage is optimized for speed.Usually higher FT transistors can oscillate, their Cbe is very low so a small base series resistor is advisable.
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Last edited by dreamth; 15th September 2020 at 07:19 AM.
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Old 15th September 2020, 07:30 AM   #3
ginetto61 is offline ginetto61  Italy
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Possible issues in replacing bjts with more powerful parts
Quote:
Originally Posted by dreamth View Post
a transition from 3mhz to10mhz transistor won;t be really noticeable...
Hi ! thank you very much indeed. This solves my doubts.
My goal is to replace parts without possible having to mess with the rest of the circuit ... not always a schematic is available.
But i have an amp already repaired ... and they have used in the same output stage tip33c and tip35c that are different in power
So my idea is to use tip35c and their complementary all over the output stages ( i am also tempted to use just one pair and i know already that you like the idea of a single output pair ... i will not be stressing the amp because i will be using very few watt from it )
Is it also true, as i understand, that more powerful devices increase the SOA of the output stage ?
I have very clear in my mind that cheap products are designed with a very limited budget for parts. So the whole idea is to replace parts in key positions with better speced ones in order to gain something ... like for instance reliability or less noise, less distortion and so on.
Often in cheap amps i see wires or copper traces very thin also for the high current paths ... like from output devices to speaker terminals or the ones carrying the supply voltage. It seems that Ohm's law is neglected sometimes.
I like thick wires and even copper bars for high current paths. Better flexible wires anyway ... much more convenient than bars.
Thanks a lot again.
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Old 15th September 2020, 07:31 AM   #4
dreamth is offline dreamth  Romania
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Here you have an even more extreme case with (RF like)common base stage driving 80mhz transistors...but there's a catch here.People would say: Sacrilege! Too long wires to such fast transistors...those wires are just making small capacitors parallel to the active junctions making the circuit a bit slower and more stable. The common base stage(DLD) have no problem driving even higher capacitance...The DLD stage itself is wired that way...many people who saw that picture were in complete disbelief...saying that that amp is complete heresy but it works and there's no known case of an l-02 that got burned or self oscillating although it has 150v/us slew rate...Sansui and Kenwood were high speed circuit champions for sure.
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Last edited by dreamth; 15th September 2020 at 07:42 AM.
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Old 15th September 2020, 07:38 AM   #5
dreamth is offline dreamth  Romania
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Welll...if your transistors are going to burn they will likely heat the pcb around them so from a later fixing perspective it's better to burn the pcb traces than the transistors and the PCB around them...
For about 20...25 years there was almost no commercial design with fast output transistors as their production seemed to cease for a while.I've seen for a few years that NAIM got back to those 50..60 mhz final transistors and they certainly made some good amplifiers with them.Digikey has them in stock from time to time too.
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Old 15th September 2020, 07:47 AM   #6
ginetto61 is offline ginetto61  Italy
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Possible issues in replacing bjts with more powerful parts
Thanks a lot again. I wonder what kind of improvement a higher gain bandwidth product device could provide for audio purposes at least.
And above all if this improvement is audible
Are they really popular in power amps ?
As i understand with at higher values stability becomes an issue and the last thing i really want is an unstable amp. I alread burnt my fingers on the hot heatsink of an unstable amp
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Old 15th September 2020, 08:02 AM   #7
dreamth is offline dreamth  Romania
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High speed is not for the faint hearted ones .

I doubt that there are many people that would trade an l-06m or l-02a or l-08m or Sansui AU-919 for anything else ...
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Old 15th September 2020, 08:04 AM   #8
ginetto61 is offline ginetto61  Italy
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Possible issues in replacing bjts with more powerful parts
pic of the badly repaired output stage attached
as you can see parts of different wattage have been used
in the weekend i will take complete pics of both channels
overall i like the amp ...
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Last edited by ginetto61; 15th September 2020 at 08:12 AM.
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Old 15th September 2020, 08:10 AM   #9
ginetto61 is offline ginetto61  Italy
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Possible issues in replacing bjts with more powerful parts
Quote:
Originally Posted by dreamth View Post
High speed is not for the faint hearted ones . ...
yes ... but what they really give in return in sonic terms ? is their contribution to sound fundamental ?
From what i understand the front end is more important for sound than the final end ...
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Old 15th September 2020, 08:12 AM   #10
dreamth is offline dreamth  Romania
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try old batches of bd249c/250c if you can find them.I have a realistic STA2250 which was to some guys that replaced the original 15Mhz trz with tip35 (3mhz) and the pcb got fried completely.After extensive cleaning i used 2sc5200/2sa1943(30mhz) and all was fine...
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