2SJ74 subs

[rafafredd]

So, as I could not live anymore without trying those JFETs, I´ve bought 150 pieces of 2sk170 to start macthing, but as I could not find the 2sJ74, I bought the recommended substitution, the 2sj103. Do you think it´s a nice sub?

I have been thinking about the forssell opamp version 1. On the paper Fred states that he only use +24v because the 2sj74 is 25v maximum, so if I can use the 2sj103, that is a higher voltage device (50v), maybe I can run the opamp hoter...

 

[chrissugar]

Hi Rafael

I was in the same situation two years ago when I bought 2SJ103 because i couldn't find 2SJ74. Although every source said that the 103 is the substitute for the 74 it is not very good. I think it is because of the transconductance but I don't have the datasheets here to verify. That is why i abandoned the jfet opamp project untill I could buy the 2SJ74. Keep in mind that it is even harder to find the high Idss version [2SJ74V] and you will need in every opamp a high Idss and a mid Idss one. Do not try with GR because at the output stage you need 10 or more mA to drive 600 ohms.

chrissugar

P.S. How much you payed for the 2SK170 ? I found them for 0.6 euros for large quantities. The best source for the 2SJ74 was 1.5 euros per piece for more than a hundred.
It is still cheaper than if you buy from Borbey, if I remember the 2SJ74 is 3.5 euros and if he measures them you pay more.

 

[rafafredd]

It sucks. I´ve just bought 50 pieces of those 103...

I payed 0.70 for the 170´s and I found only the GR and the BL versions. No V´s yet, but maybe I can sort some high Idss BL´s to use on the output...

So, now I need the 170v´s, the 74bl and the 74v. Well, back to hunting...

 

[tk@halmi]

Rafa,

I wonder if you could use two BL devices connected in parallel for the output followers. You would need the 10 ohm source resistors for both prallel devices. Added advantage may be having double the driving capability. Just a thought.

Tamas

 

[rafafredd]

Thanks, Tamas. I´m still searching for a got price for the V series JFETs but I I cannot find any, I´ll try your idea and see (I mean, listen :grin: ) how it turns out.

 

[chrissugar]

Hey Rafa, you can select the BL series and if you have some with Idss over 10 mA use them at the output. I tried long ago to do what Tamas sugested ,to parallel fets and was not satisfied with the waveform and distortion at the output. I looked to the datasheets and the admitance for the 2SJ103 is 4mS and for the 2SJ74 is 22mS and this is a very relevant parameter.

chrissugar

P.S.
I can't promise anything but if I'm lucky to find more 2SJ74 and 2SK170 are you interested to buy?
Anyone who is interested leave a message here about how many you would need of each. I supose if it is the same source the price will be around 0.6euros for the 2SK170 and 1.5euros for 2SJ74. I have to buy hundreds to have this price.

 

[tk@halmi]

[quote author="chrissugar"] ...was not satisfied with the waveform and distortion at the output.[/quote]

That is odd. Did you match them for Vgs and add separate source resistors?

Thanks,
Tamas

 

[Bauman]

Sad thing, that Forssell opamp looks very tasty but no luck with 74 also.

Tamas, maybe Linear Tech. can have some good substitute? Looks like P type fets are not as diverse as the N types. Very few out there...

cheers!
Fabio

 

[tk@halmi]

I have been able to get 2SK170BL and 2SJ74GR devices for cheap at MCM. So I decided to do something with them. I am testing a derivative of the Forssell opamp that looks similar up to the LVA (the first 2SJ74), but the output is based on pairs of BC639/BC640 BJTs. I am sure it will not sound the same as the original, but it should be something easier to get parts for.

Tamas

 

[rafafredd]

sounds like a nice alternative. When it´s done, let us know more, tamas.

 

[BradAvenson]

www.bdent.com has the 2sj74 series, (all flavors) they can do small quantities, but aren't the best prices.

 

[spacewig]

Hi, all.

I have heard of the 2N5021 as being the appropriate substitute for the 2SJ74. Could anyone tell me if this would be okay to use, please? Also, is there a PCB layout for the Forssell opamp? I have bought all the required pieces (except for 2SJ74) and would like to build some.

As always, thanks in advance for your help,
rob

 

[tk@halmi]

There are two major parameters where the 2N5021 differs from the 2SJ74.

Idss Transconductance
2SJ74BL 6-12ma 8-22mS
2SJ74V 10-20ma 8-22mS
2N5021 1-3.5ma 1.5-6mS

Unfortunately, the 2N5021 does not come close to either 2SJ74 device.

 

[soundguy]

do you guys know if the 2sj74 or 2sk170 could be used in an 1176?

dave

 

[tk@halmi]

The 2SJ74 is a P-channel device and you need and N-channel like the 2SK170 for drop in replacement. What is wrong with the 2N5457 or the BF245A? They are sold at every major component store at very low prices.

 

[soundguy]

Im currently using the 2n5457, nothing wrong with it at all just curious to see if different fets act differently in the circuit, Im building a few so variation could work for me.

dave

 

[rafafredd]

Found interesting dual fets at farnell but it looks like transconductance would be a problem also...

http://www.farnell.com/datasheets/13673.pdf

 

[tk@halmi]

I think you could cobble together a JFET opamp with that, but it would not be the same as Fred's.

There is a FET that could work in the VAS and the OP, the 2N5462, if you selected a high enough Idss (10ma), maximum transconductance (6mS) and lowered the drain resistor (330 ohm) to something less like 100 ohm.

 

[rafafredd]

would 2sj109 be an option for the 2sj74 in the Forssell opamp?

I could use two 2sj109 for a machted pair of opamps in the same PCB...

 

[tk@halmi]

Sure, the 2SJ109 works if you can get at least the BL rating.
I was just breadboarding one of the these opams using sorted BL parts throughout.
If you don't need to drive a transformer on the output you can use devices that can handle about 10ma. If you go with that into another gear that has a 10k input impedance it will be just fine. (Well, you may not be able to drive long cables, but 10 meters should not be a problem.)
Optionally, you could use one half of the 2SJ109 in the LVA and one half in the output.
If you can get an LT1010CT hang it off the output, include it in the feedback loop and you will be able to drive transformers down to 75 ohms.

Tamas