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Old 2nd December 2011, 11:12 AM   #1
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Default Luxman LV103u replacing output FET's

Dear friends,

I am currently repairing a Luxman LV-103 amplifier with shorted output FETS, so I am unfortunatelly forced to replace them along with some other broken conponents (emitter resistors etc.). But there is a small problem; the 2SJ115 / 2SK405 are almost nowere available, and the ones that are available are most probably fake.

So I sought to a nice replacement and I think I have found them; a pair of 2SJ200 / 2SK1529. Is this a proper replacement? I saw that there were some spec deviations between the 2SJ115 / 2SK405 and 2SJ200 / 2SK1529, namely different gate-source capacities and the idling current should be somewhat differend. Is this going to affect the soundquality? I was also wondering if I have to change the drivercircuit and how much lower or higher the idling current has to be? And I would like to ask if I need to match the FETS.
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Old 2nd December 2011, 11:56 AM   #2
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From what I can tell from the datasheet is that the 2SJ115 / 2SK405 are lateral mosfets so the answer is no, the 2SJ200 / 2SK1529 are not direct replacements. The latter are vertical mosfets.

I dont have schematics of that amp but if it happens to have a VBE multiplier it may be possible to modify it so 2SJ200 / 2SK1529 can be used.

A more suiteable replacement would be 2sj162 / 2sk1058.
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Old 2nd December 2011, 01:21 PM   #3
ilimzn is offline ilimzn  Croatia
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Originally Posted by homemodder View Post
From what I can tell from the datasheet is that the 2SJ115 / 2SK405 are lateral mosfets so the answer is no, the 2SJ200 / 2SK1529 are not direct replacements. The latter are vertical mosfets.

I dont have schematics of that amp but if it happens to have a VBE multiplier it may be possible to modify it so 2SJ200 / 2SK1529 can be used.

A more suiteable replacement would be 2sj162 / 2sk1058.
DEAD WRONG and you should have searched - this has been discussed at length during the years.
J115/K405 are NOT lateral, they are so called PI-channel devices which behave like vertical MOSFETs but have a low treshold voltage of about 1V or so.
J200/K1529 are their direct descendants, foa slightly higher Id max, but otherwise they can be used without changes in the LV103.
Even the J201/K1530 can be used there, regardless of slightly higher input capacitance because their increased transconductance effectively bootstraps this capacitance to the level of the originals.

Using laterals in these amps is possible but requires changes to the bias circuit AND changes to the PCB layout because lateral MOSFETs have a different pinout. Also using VMOS/HEXFET parts is possible but again, a completely different mod to the bias circuit is needed. Laterals will actually reduce the performance of this amplifier, so it would be a lot of effort to get a inferior end result.
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Old 2nd December 2011, 01:39 PM   #4
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Originally Posted by ilimzn View Post
DEAD WRONG and you should have searched - this has been discussed at length during the years.
J115/K405 are NOT lateral, they are so called PI-channel devices which behave like vertical MOSFETs but have a low treshold voltage of about 1V or so.
J200/K1529 are their direct descendants, foa slightly higher Id max, but otherwise they can be used without changes in the LV103.
Even the J201/K1530 can be used there, regardless of slightly higher input capacitance because their increased transconductance effectively bootstraps this capacitance to the level of the originals.

Using laterals in these amps is possible but requires changes to the bias circuit AND changes to the PCB layout because lateral MOSFETs have a different pinout. Also using VMOS/HEXFET parts is possible but again, a completely different mod to the bias circuit is needed. Laterals will actually reduce the performance of this amplifier, so it would be a lot of effort to get a inferior end result.
Maybe, Ill see after I ve gotten hold of proper datasheets.
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Old 2nd December 2011, 01:40 PM   #5
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Hi Homemodder and Ilimzn,

Thanks for your replies, I really appreciate that. So if I understand correct the FETS are suitable, and I dont have to change the idling current (bias) which corresponded to 65mv for the original devices (measured at the 0,22 ohm resistors).

Thats interesting, so a higher transconductance brings the capacitance a sort of down. Thats great to hear :-) Could you maybe explain how this precise works? (I am pretty new to mosfets and their terminology).

I wanted first to replace the FETS by a pair of IRFP240 / irfp9240, but I was scared this would influence the soundquality badly.

Last edited by Kaplaars; 2nd December 2011 at 01:53 PM.
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Old 2nd December 2011, 06:02 PM   #6
ilimzn is offline ilimzn  Croatia
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IRFP240/9240 are not very good electrical complements (transconductance is different by a factor of over 1.5), they are only process complements (N and P doping reversed, but electron mobility is different in N and P so you end up with a non-complementary part). They also have a much higher treshold (over 3V typically) so the biassing arrangement would have to be changed. You will not be able to reach the required treshold to get more than a fraction of a mA of idle current without modification.

J200/K1529 are almost direct replacements. The output stage of the LV103 (and it's cousins) is a follower, so the larger Cgs capacitance of the MOSFET is 'bootstrapped', because the source follows the gate. Because the transconductance (a form of gain) is larger by the same factor as the maximum current with respect to the original MOSFETs, less Vgs is needed for the same current to the speaker, compared to the original MOSFETs. So, even though the capacitance is larger, it has to be charged to a smaller voltage to perform like the originals, making the larger capacitance of the replacements 'look' smaller - about the same as the originals. However, there is also Cdg, which indeed is larger by about 20%. However, Cdg is only a small fraction of Cgs, so these extra tens of pF are no problem for the driver stage.
An example of how this works is looking at paralleling the same kind of MOSFETs. Suppose Cdg is 100pF and Cgs is 1000pF, and transconductance is 2A/V - meaning, Vgs of 1V means the MOSFET becomes a 2A current source.
If you want 6A through your load, you would need to charge Cgs with 3V, for a single MOSFET. Also, you would need to change the voltage on Cds by the same voltage as 6A makes on the load.
Now, immagine you paralleled 2 same MOSFETs to do the same work - this would be like putting in one MOSFET that can handle 2x the maximum current, because MOSFETs are internally cell-based and larger ones are simply made by paralleling more cells.
Now, to get 6A, it means you need 3A per MOSFET. To do this, you now need to charge Cgs to 1.5V instead of 3V. If you had limited current to charge the capacitances, now it charges half as slow, because the total capacitance of paralleled MOSFETs is now 2x 1000pF, but needs to be only charged to half the voltage, so the net result is, as far as Cgs is concerned, the same.
However, the combined Cdg is also 2x 100pF, so if you are looking at total charge, you have an extra 100pF Cdg to charge. Fortunately, in the case of the LV103, the difference is not double but about 25%, which actually easily falls inside the tolerance margins of the design.

J200/K1529 are actually 'the next size up' J115/K405, and as close as you can get with respect to spec, tresholds, thermal characteristics etc. They are slightly larger (10A Idmax vs 8A) so offer a small measure of extra robustness. I have worked on several of the LV hybrid amp series, and these, as well as J201/K1530 are really the best and IMHO only option to use, in the absence of the originals.

It should be noted that the originals are out of production for close to 20 years now. There are small numbers around for a high price, but I have also seen fakes. IF you want to keep your LV103 absolutely original, the J115/K405 CAN be found but they will cost a lot and you may end up with fakes. Or, you can easily get genuine J200/K1529 and have an amp that will perform at least as well as the original setup, but it will also be more robust and reliable.

While you are under the hood of the LV-103, it would be wise to check all the soldering on the driver board and especially on the tube sockets (both are on the front panel on and behind the tubes). The best practice here is to remove the old solder and flux residue (you might be dissapointed by obvious thermal damage to the PCB ), and resolder with lead-free solder. The reason for this is thermal stress there, lead-free solder has a higher melting temperature and is less prone to forming cold/dry joints under thermal stress, but it's also less easily solderable as such.
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Old 2nd December 2011, 07:19 PM   #7
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perhaps the alfets from Semelab/Magnatec are a good choice - ask there:
Contact -TT electronics Semelab
Magnatec. ALFET Lateral MOSFETs
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Old 2nd December 2011, 09:11 PM   #8
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Ilimzn: thank you thank you thank you!!! Aha so that is how it works, I've learned a lot today :-) There are thus a lot of parameters which influence the behaviour of the FET. I think the best option for me is to go for the J200/K1529 FETS, these are well available here in Holland and not too expensive (about 10 euro's per pair).

You're absolutely right about that Ilimzn, there were a lot of bad solder joints. There was also another problem: the connectors from the driver board to the tube board were totally dried out. They had been literally crumbled to little pieces, so I had to solder new wires which replaced the connectors directly to the driverboard. I think the heat of the tubes played a role in this proces. To be fair dissapoints the quality of this Luxman me a bit, it's not the quality I am used to. Brands like Denon or Rotel are in that aspect much better over the years, while this amplifier was, if I am right, pretty expensive in the day.

Thats a clever tip, I've never thought of using lead free solder! I will certainly resolder the tubesocket contacts with lead free solder to the PCB :-)
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Old 3rd December 2011, 02:14 AM   #9
ilimzn is offline ilimzn  Croatia
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Quote:
Originally Posted by tiefbassuebertr View Post
perhaps the alfets from Semelab/Magnatec are a good choice - ask there:
Contact -TT electronics Semelab
Magnatec. ALFET Lateral MOSFETs
These are close to original Hitachi/Renesas parts, and not directly suitable. They can be used and the mod is fairly simple but due to the peculiarities of the LV103 design you get less output swing unless you use the big multiple-die parts, which tend to be well on the expensive side - also AFAIK the different pinout problem remains.
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Old 3rd December 2011, 02:19 AM   #10
ilimzn is offline ilimzn  Croatia
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Originally Posted by Kaplaars View Post
Ilimzn: thank you thank you thank you!!! Aha so that is how it works, I've learned a lot today :-) There are thus a lot of parameters which influence the behaviour of the FET. I think the best option for me is to go for the J200/K1529 FETS, these are well available here in Holland and not too expensive (about 10 euro's per pair).

You're absolutely right about that Ilimzn, there were a lot of bad solder joints. There was also another problem: the connectors from the driver board to the tube board were totally dried out. They had been literally crumbled to little pieces, so I had to solder new wires which replaced the connectors directly to the driverboard. I think the heat of the tubes played a role in this proces. To be fair dissapoints the quality of this Luxman me a bit, it's not the quality I am used to. Brands like Denon or Rotel are in that aspect much better over the years, while this amplifier was, if I am right, pretty expensive in the day.

Thats a clever tip, I've never thought of using lead free solder! I will certainly resolder the tubesocket contacts with lead free solder to the PCB :-)
I've never had the connectors actually disintegrate but unfortunately, the design of many Luxman units of that era (even much more expensive ones) shows a distinct lack of concern for thermal problems, mostly to do with using cheap FR2 PCB material and lack of heatsinks combined with dense spacing of heating parts. They would operate to spec well after the waranty runs out but it's not the longevity one would see in previous late 70s units... cutting costs and insuring repeat customers after a few years when repairs would turn out more expensive than replacing the unit? Rather risky strategy if you ask me...
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