Replacing mosFETs with BJTs

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ginetto61

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Joined 2009
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#1
Hi and good morning
Please excuse my ignorance
This is the output stage of an amplifier that I have at hand

1719037324969.png


Having an even childish obsession with modifications, I'm wondering two things
The first
The output mosfets are screwed to the metal case and I don't like it at all. Are there other more elegant solutions ?

The second
I read this online about replacing mosfets with bjts

Can you replace MOSFET with BJT?
For many circuit applications, BJTs with low saturation voltages are not only an adequate substitute for MOSFETs but also provide a number of advantages, with a low on resistance, working with a control voltage of less than 1V, offering excellent temperature stability and being non- sensitive to ESD
Click to expand...

Can it be done without big problems ? which could be suitable replacement parts?
In general i tend to prefer the sound of amps with bjts They seems like more clear to me ?
Thanks to everyone and have a good day
gino
 
#4
Hi thank you very much for your kind and valuable advice
Mosfets require biasing, so will bjts. Do you know how to calculate and adjust that? Different voltage levels are required.
Click to expand...
No And your words have convinced me to cancel the idea
I guess that for the mounting mod i have to ask in the construction section
Personally i like so much those designs where boards/parts can be removed easily for maintenance or upgrading The modular approach
This screwing output devices directly to the case is very british and i do not like
I was thinking to use a piece of those thermal conductibe sheets that isolate electrically and push the mosfets down with a single small metal bar above them
I have seen the solution used in a car amp once Like the pic below i mean
Much more elegant imho What you think ?

1719039838925.png
 
#5
steveu said:
https://www.diyaudio.com/community/threads/amps-with-2sk413-2sj118.15368/

Many will consider what you have to be as good as it gets. Using BJTs will require a completely different amp with driver transistors and thermal compensation etc...
Click to expand...
Hi and thank you so much for the very useful link
I am afraid i have almost decided to mod it at the cost of destroying it
This is the amp inside
1719040070705.jpeg


This is what i would like to do
  • Move the transformer on the bottom to make room for 4 low esr caps like 4700uF soldered directly on each output device (I like to have energy close to the output devices for faster delivery of current I am sure it could be very benficial)
  • I will put a new integrated diodes bridge close to the transformer and the main black caps that can be seen in the lower part of the board
  • I am even thinking to extract the mains transformer and put it in a separate box with an AC umbilical going to the amp I like so much the idea of external transformers No vibrations no emi .... I could even use a higher VA one
Is it so difficult to build power amps like this one that in my mind is just perfect ?
1719040953785.png

ok ... i do not like caps mounted horizontally They must stay vertical
But this is how a power amp should be built The layout i mean

I do not need many watts ... 40-50W/channel would be enough I have a small place
 
#7
this would be the saner approach I agree The sound is easy on ears Not very detailed but nicely rounded
I am just puzzled about how some brands build their equipment Some chinese products are much better built 😳
I will only put a metal bar above the mosfet with some silicon pad between the mosftes body and the bar I bought some for a mini pc years ago
This should avoid to put stress on mosfets case
I have to remove the board for some solderings of dried joints i have spotted
I have an alloy that is a joy to use
 
#9
hi thank you very much for the very kind and valuable advice
And what about replacing them instead with other better mosfets ? maybe more powerful ?
these are 100W devices Maybe other have a more extended SOA ? or they can withstand better the temperature
Can that work without altering the bias points ?

By the way I start to hate the way they are mounted Deeply
I love neat works like this for instance with an aluminum bar supporting the devices
I am thinking about a sandwich of two bars with the lower one touching the cabinet and the upper keeing the mosftes in place


1719061360886.png
 
#11
Hi, thank you very much for your very kind and valuable response
So if I were to burn/ruin the mosfets can I also throw the whole amplifier in the garbage ? then I don't do anything
It's a shame because I had seen some nice 150W Vishay IRFP240s, really tough and beautiful. So I'll close everything and hope for providence.
Thanks again
 
#13
ginetto61 said:
So if I were to burn/ruin the mosfets can I also throw the whole amplifier in the garbage ? then I don't do anything
It's a shame because I had seen some nice 150W Vishay IRFP240s, really tough and beautiful. So I'll close everything and hope for providence.
Thanks again
Click to expand...

Step 1. Define / Identify the problem you are attempting to fix.

I dont see what swapping the vishay mosfets would accomplish, you would most likely will have to start modifying other parts of the circuit to deal with the characteristics of the irfp's.

The amplifier looks to be reasonably designed and manufactured to a price point and to fit within a particular case / chassis.
Given that the mosfets are mounted to the case and there is a pair per channel and the size of the transformer, I would estimate the amplifier is rated somewhere around 50Watts per channel into 8 Ohms.
ginetto61 said:
I do not need many watts ... 40-50W/channel would be enough I have a small place
Click to expand...
 
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#14
hi thank you for words that give me hope Actually i would intend to keep the original parts only rearranging them a little
The main issues are
  1. i am not an expert, I have just recapped some vintage amps in the past
  2. i do not have the service manual
  3. i do not know if these newer parts can be just dropped in place of the originals. I have been already advised the bias could be not right for the new devices and i do not even know exactly where to measure to check it
But it is very very good to hear that alternatives are actually available considering that the original are obsolete parts
I would like to take this amp to its limits I will not need more than its 40W/8 ohm ?
but i would like to increase the uF in the power supply and maybe even replace the mains transformer
The stock one should be a 150VA unit Better double that at least
I remember some comments about how delicate mosfets can be towards electrostatic discharges ?
i wonder if i could damage them while desoldering them
But again the fact that alternatives are available give me courage to go on
Thank you !
 
#15
Indiglo said:
Step 1. Define / Identify the problem you are attempting to fix.
Click to expand...
Hi there is nothing to fix I want to upgrade it For sure it is built with some money saving maybe in critical point
The capacitance for instance is low and far away from the output devices I do not like that
The output devices are screwed to the bottom case This is very not elegant
Indiglo said:
I dont see what swapping the vishay mosfets would accomplish, you would most likely will have to start modifying other parts of the circuit to deal with the characteristics of the irfp's.
Click to expand...
as a start i could measure the voltages on the mosfets ? that should give some indications ? that is easy to do
Maybe knowing the 3 voltages on drain gate and source at idle will provide some information about the possible replacements ?
Indiglo said:
The amplifier looks to be reasonably designed and manufactured to a price point and to fit within a particular case / chassis.
Given that the mosfets are mounted to the case and there is a pair per channel and the size of the transformer, I would estimate the amplifier is rated somewhere around 50watts per channel into 8 Ohms.
Click to expand...
You are perfectly right and i guess that is its power more or less But i would like to tell one story
I listened in the past to some amps from the brand Albarry It was an interesting test
There were two monos and one integrated The designs were exactly the same The main difference is that the mono have each more or less the same power supply (trans plus caps) of the integrated
With a full range speaker the reaction was to kick the integrated away The sound from the monos was so much more convincing
There was simply no comparison with the monos providing a completely different and so much more satisfying experience No comparison at all
SO the idea is to put a 300VA in and some more caps here and there Maybe even across the mosfet legs
The very fundamental issue is that as you say it appears to be
reasonably designed and manufactured to a price point and to fit within a particular case / chassis.
Click to expand...
i have no problem at keeping the mains transformer out of the chassis like i have seen in other amps
There will be also less vibrations And fill the chassis with high grade caps Like i have seen some designers do

i would like to add that the board is designed in a strange way The two channel are rightly place one on the side of the other Instead the output power devices are grouped in a way that makes the heath concentrated in a small area
This is exceedingly weird They could have kept the output pairs more spaced
In my mind the main problem is to design and optimize one channel Then it will be just a matter of doing a copy
Stereo means two channels Actually i prefer the definition of two channel amp Because it makes clear that the two channel should be as identical as possible For this i hate amps where one channel mirrors the other They are not identical And i love mono amps immensely Very very much They are just right
 
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#16
Looking at the PCB picture, it looks like you could double up the caps in the power supply. There are also other caps 'missing' (not installed) on the PCB. If you had a service manual, you could see what their function would be, if installed (power supply decoupling, perhaps). I would investigate this route first, as power supply upgrades can enhance an amplifier.
 
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