Mystery NEC J44/K163 JFETs in a Hafler XL-280

[HaflerFreak]

Hey everyone, I thought you might find this an interesting little story.
I am in the middle of refurbishing and, hopefully, repairing my old XL280 and have uncovered some mysteries concerning this amp's past. I bought this amp as a showroom model that had been 'repaired'. As a 19 year old kid, I never thought to check under the hood - it sounded great.

Well, now that it's apart I can tell you that this poor thing was used as some kind of 'junker for parts' before it was repaired and sold to me.
Most of the caps do not match the original spec parts except in capacitance, and the wiring was left a little sloppy. Resistors appear original (even most carbon films are within .1-.5% tolerance - right on Hafler!) and output MOSFETs are original.

By far, the biggest oddity of all was when I found 4 pairs of NEC K163/J44 JFETs in place of the original K147/J72 pairs. I don't know if the K147/J72's were stolen or blown up.

I have been planning on replacing the K163/J44 pairs with K170BL/J74BL's but I just read that the K163/J44 pairs were used in Krell amps and are considered to be high quality parts.

If it was your project, what would you do?
The amp has functioned for 17 years with these K163/J44 pairs, should I even bother to change them? I have absolutely never read anything about anything but K147/J72 or K170/J74 pairs in this amp, and I would like it to sound as 'good' as possible of course.

If I did pull these K163/J44's, I would be willing to send them to a member of this forum, free of charge, if they need them.

[mlloyd1]

i'm probably speaking for myself as well as others ... if the jfets currently in circuit now are working, leave 'em alone.

you will get bigger changes (improvements???) in sound by doing other things, like increasing output stage bias, etc.

you bring up a good point though. with the scarcity of toshiba complementary jfets continuing to get worse, how long before old gear gets snarfed up only to be used as a source for expensive jfets ...

mlloyd1

btw, forgot the link to the datasheet for the 2sj44 in case you were curious: http://www.necel.com/nesdis/image/TC-3344A.pdf

[HaflerFreak]

Thank you mlloyd1,
I knew you would probably have some info for me. It's great to know there is another XL-280 fan out there. It says something that I could love this amp this much after all these years with compromised guts.

As far as the 'bad side' saga goes, well...
I have that side in pieces again, resistance across D-S for Q1,Q2,Q3, and Q4 (the JFETs) is:
Q1=56 Ohms, Q2=76 Ohms, Q3=50 Ohms, and Q4=38 Ohms.
Sound Ok?

I have already ordered 20 and 20 of the 2N5550/2N5401 pairs from ON Semi in case the cascodes are an issue. I upgraded the 2N5550's to 2N5551's. Tougher and less noise to boot.

I have noticed that Hafler may have had a typo in his schematics concerning the 'NP2222A'. Everything I can find points to this actually being a 'PN2222A'.

I have been able to source some caps that should far outperform what was in here. For example, I found a 400V .01uF Metallized Polyester at C4. Terrible. Spec is a 160V PP.

Oh well, that's all for now, gotta go - maybe I'll post pics later.

[Stormrider]

Ahh yes... The NP2222A is actually a PN2222A. IIRC thats just the transistor in the biasing circuit and can be replaced with any decent npn with a high hfe. I used a 2n3904 in place of the 2222A in one of my amps without any issues.

Also, Hafler was known to use what parts they could find to meet price demands. I have found several mismatched parts in my amps. Usually its only capacitors and resistors, parts that do not make much of a differance to the operation of the amp. It is odd they would use a differant jfet, but it is possible.

[mlloyd1]

stormrider, i think you're right!.
the 2SK163 and 2SJ44 were made by NEC back when they used to make such parts. My understanding is they weren't too shabby parts compared to the Toshiba's. i've seen them in a number of other audio products over the years.

mlloyd1

[wrenchone]

Actually, it's Hitachi/Renesas that was the manufacturer for the lateral output FETs used in the Hafler (and other) amps.

[mlloyd1]

actually, i was talking about the jfets used at the input


mlloyd1

[Charles Hansen]

Originally posted by mlloyd1
stormrider, i think you're right!.
the 2SK163 and 2SJ44 were made by NEC back when they used to make such parts. My understanding is they weren't too shabby parts compared to the Toshiba's. i've seen them in a number of other audio products over the years.

M LLoyd is right. The NEC's were excellent transistors, and very comparable to the 2SK170/2SJ74 pairs. I wish they were still available, as I would be using them.

Both of the above devices have about half the transconductance of the original 2SK147/2SJ72 parts. So you may have to change the value of the source resistors to optimize things fully. But if the amp is working well, I would leave it alone.

[wrenchone]

sorry about that, Mloyd - I unnerstand now. NEC was a pretty low-key and underregarded manufacturer of small signal stuff back in the 80's. I have a bag of 2SJ45s from NEC that has been languishing for lack of an application. It would be a one-time thing, though, as I have no idea where I'd get replacements.

[HaflerFreak]

Both of the above devices have about half the transconductance of the original 2SK147/2SJ72 parts. So you may have to change the value of the source resistors to optimize things fully. But if the amp is working well, I would leave it alone.

I am assuming R1, R2, and R3 are the source resistors... Correct me if I am wrong.
R1=2210 Ohms, R2=47,500, and R3=110,000

All the resistors in this amp test remarkably close spec, including R1, R2, and R3. There are no 'mods' to the values of anything except the JFETs and capacitor voltage ratings.

I have some Dale RN60D 47.5K and 110K that test low (like 47.2K and 109K) - maybe worth a try while it's apart?

I must say, I consider myself an Ok technician, but I am not qualified to re-engineer this baby (nor do I want to), I would, however, throw in a couple off value premium resistors if it would likely improve my sound.

I will wait to hear what the forum has to say on this.