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Old 14th February 2003, 08:42 AM   #1
nikosgr is offline nikosgr  Greece
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Default Time To Replace Output Mosfets

Hello all,

i have my amp for 8 years ,its a class A 150w which uses 5x2sk
1058 and 5x2sj162 in the current gain stage.
My questions are:
1) Is there a need to change the mosfets with new ones?
2) If i do so,do i have to match them ?

I read in another thread that there isnt any need to match the specific mosfets .Is it true?

regards
Nikos
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Old 14th February 2003, 10:02 AM   #2
Kay is offline Kay  Germany
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>1) Is there a need to change the mosfets with new ones?

never change a working device,

but clean your relais-contacts sometimes.

best regards
Kay
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Old 14th February 2003, 10:11 AM   #3
JBL is offline JBL  Canada
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You risk damaging the amplifier by changing the output device. Also pc board generally don't like to be soldered more than once(depend on board but still..)
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Old 14th February 2003, 10:13 AM   #4
halojoy is offline halojoy  Sweden
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Quote:
Originally posted by Kay
>1) Is there a need to change the mosfets with new ones?
never change a working device.
Best regards
Kay
I agree with Kay
Transistors work without problem "forever"
Almost Only if they break down they need change.

There is also no new really replacements
for SK1058 and SJ162
that would give you substancial better performance.
You could instead get into the problem
of having to alter other things in the circuit
to fit the new transistors.

Clean it up, and change old Electrolytic Capacitor
and Carbon Potentionmeters and bad wornout Switches.
Those are component that gets bad with age.

Transistors live forever, if not shortcut.

/halo - does not live forever - at least on this planet
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Old 14th February 2003, 10:53 AM   #5
Bobken is offline Bobken  United Kingdom
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Default Re: Time To Replace Output Mosfets

Quote:
Originally posted by nikosgr
Hello all,

i have my amp for 8 years ,its a class A 150w which uses 5x2sk
1058 and 5x2sj162 in the current gain stage.
My questions are:
1) Is there a need to change the mosfets with new ones?
2) If i do so,do i have to match them ?

I read in another thread that there isnt any need to match the specific mosfets .Is it true?

regards
Nikos
Hi,

Unlike some components (like valves, for example) mosfets will not (usually!) deteriorate over time, towards the end of their useful life.

They usually either work OK or they don't work at all. So, as the others have said, there should not only be no need for such a change, but there is no point in doing this, either.

Regards,
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Old 14th February 2003, 10:58 AM   #6
Electrons are yellow and more is better!
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Default Re: Time To Replace Output Mosfets

Quote:
Originally posted by nikosgr
Hello all,
i have my amp for 8 years ,its a class A 150w which uses 5x2sk
1058 and 5x2sj162 in the current gain stage.
8 years is no age for a amp in home environment. 15-20 years may be "old".

As a conclusion: If it ain't broken, don't fix it.

Your question about matching (output transistors), it can be wise to do some matching only if you have more than one pair of transistors.
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Old 14th February 2003, 11:30 AM   #7
Jay is offline Jay  Indonesia
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8 years for a class-A amplifier (or AB I guess). Don’t you think that there is a need to re-adjust the DC offset, Halojoy?

Nikos, the Hitachi are in complementary pairs, what’s more, in your amp they are configured in parallel. Of course they need to be matched.
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Old 14th February 2003, 12:37 PM   #8
nikosgr is offline nikosgr  Greece
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OK guys thanks for the replies

please let me give you some more details about my amp and what improvements i have done since i bought it.
concerning psu
2x200va toroids (2x40000mf) and separate regulated psu for each channel on the voltage gain stage.
2x1500va toroids (2x100000mf) separate unregulated psu for each channel on the current gain stage.
amplified circuits
all electrolytic/poly. capasitors and resistors have been changed and been upgrated(starget/solen/holco)
wiring inside the amp
i used van der hull
Anyway what i really want to do to the current gain stage which uses three pairs, is to add an extra two pair of mosfets .
The amp comes in two versions:1)150w and 200w same schematic.
The positions of the extra mosfets to be used are on the pcb.
Surely i will have to increase the size of the heatsinks and the size of the box, to get the extra 50watts
Thats why i was asking you about matching.............
Concerning the dc offset is measured from time to time (80mv on each mosfet) 1mv dc at output on one channel and 4mv dc on the other .
Thats why i asked you whether its needed to change the old mosfets with new ones.......because what i really want to do is to
add two more pairs of mosfets to the existing three old ones.


Waiting anxiously for your suggestions
regards
Nikos
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Old 14th February 2003, 01:47 PM   #9
Electrons are yellow and more is better!
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How much voltage do you have over the output transistors?

Normally one pair is the best and you add extra ONLY because the single pair can't handle the power. With 3 pairs I gather that you can take out 400-600 W at 4 ohms in home environment. When you add power think how many decibells it will result in! 1200 W is only 3 dB higher volume!
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Old 14th February 2003, 02:01 PM   #10
Bobken is offline Bobken  United Kingdom
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Quote:
Originally posted by nikosgr
OK guys thanks for the replies

please let me give you some more details about my amp and what improvements i have done since i bought it.
concerning psu
2x200va toroids (2x40000mf) and separate regulated psu for each channel on the voltage gain stage.
2x1500va toroids (2x100000mf) separate unregulated psu for each channel on the current gain stage.
amplified circuits
all electrolytic/poly. capasitors and resistors have been changed and been upgrated(starget/solen/holco)
wiring inside the amp
i used van der hull
Anyway what i really want to do to the current gain stage which uses three pairs, is to add an extra two pair of mosfets .
The amp comes in two versions:1)150w and 200w same schematic.
The positions of the extra mosfets to be used are on the pcb.
Surely i will have to increase the size of the heatsinks and the size of the box, to get the extra 50watts
Thats why i was asking you about matching.............
Concerning the dc offset is measured from time to time (80mv on each mosfet) 1mv dc at output on one channel and 4mv dc on the other .
Thats why i asked you whether its needed to change the old mosfets with new ones.......because what i really want to do is to
add two more pairs of mosfets to the existing three old ones.


Waiting anxiously for your suggestions
regards
Nikos
Hi,

This is, of course, a quite different query from the first one, because this is now about adding to, and not replacing, mosfets.

As you do not seem to be very experienced in this matter, I would be very cautious about adding mosfets in the way you suggested, as several problems can possibly ensue, including dissipating the extra heat which you have mentioned.

Mosfets in parallel, as is the case here, need to share the current equally between them, and this is not always very straightforward to achieve. Also, the quiescent current will need to be changed in accordance with the requirements of the added transistors, and any added devices will most likely need their own gate stopper resistors.

Also, any stabilising circuitry or frequency compensation, may be in need of revision, too.

There are likely to be more severe 'mismatches' here than usual (i.e. If you buy all devices at the same time) as you already have some existing devices which you wish to add to.

In this case it might be easier to start again and replace all the mosfets to get a better initial match, but there is no guarantee of this, and is it worth all this extra cost and trouble?

In *any* multiple mosfet situation like this, you do need to ideally match the devices and/or use some low value source resistors to ensure this eqaul current sharing, otherwise at high outputs (and this, of course, is the reason for increasing the number of mosfets in the first place) unequal current sharing between the mosfets will quite likely cause some of them to fail.

I don't wish to discourage you in trying this, but do be aware that it is not as simple as connecting them up and away you go.

I would strongly reccommend that you study some of the circuits and writings of designers who have done this job many times before, like Nelson Pass, John Linsley Hood, and Erno Borbely etc., before you get too heavily into this modification.

I hope this helps.

Regards,
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