Hi All - I want to build either an F5 or F6 (prefer to do an F6) but the store is out of JFETs - anyone got a matched set they are willing to sell?
For one thing, they don't have to be matched...you'll get very good results with un-matched.
You can pair up 2N5457 and 2N5460 to approach the transconductance of a single K170/J74 pair -- mind you that the pinout is different. Their transconductance is around 5,000mS vs 20,000mS or more for the K170/J74, noise figure is two orders of magnitude worse but input capacitance much better.
Ok, so it sounds like you are saying I can buy the unmatched packages of K170 and J74 and install randomly pick one of each and it won’t cause any problems? I honestly have no idea
It certainly won't blow up! In later versions of the F5 a 200 ohm trim-pot was used in an attempt to balance the gain of the P and N-channel devices.
The Linear Products variety are available from Trendsetter Electronics.
If you get a package of random devices from a third party, you can match for Idss quite simply -- short the gate and source to ground and measure the current which flows through the device using a 9V battery. This is Idss and matching Idss will start to get you close to ideal matching.
You would need to place in parallel about 4 each of 2N5457/5460 to get close to the gain of a single 2SJ74, but this isn't an insurmountable task.
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> parallel about 4 each of 2N5457/5460 to get close to the gain of a single 2SJ74
2N5457/5460 have a transconductance of about 3mS each and Idss of ~ 3mA.
4x in parallel gives 12mS at 12mA.
4x 2N5460 in parallel has an eq input noise of 30nV/sqrtHz
2SK170 has a Yfs of 45mS at 12mA, 2SJ74 close to 50mS.
Noise is about 1nV/sqrtHz for both.
I guess not quite close ??
Patrick
2N5457/5460 have a transconductance of about 3mS each and Idss of ~ 3mA.
4x in parallel gives 12mS at 12mA.
4x 2N5460 in parallel has an eq input noise of 30nV/sqrtHz
2SK170 has a Yfs of 45mS at 12mA, 2SJ74 close to 50mS.
Noise is about 1nV/sqrtHz for both.
I guess not quite close ??
Patrick
I guess not quite close ??
Patrick
I've done it, and it works.
The noise is almost irrelevant -- the MOSFETs themselves are at least 2 orders of magnitude noisier than the input stage -- and they are running "hot".
Hi All - I want to build either an F5 or F6 (prefer to do an F6) but the store is out of JFETs - anyone got a matched set they are willing to sell?
It looks like spencer still has NOS Toshibas for sale.
> I've done it, and it works.
Sure, but I would not call 12mS close to 45mS myself.
Cheers,
Patrick
Toshiba 2SK170 data sheet.
(I just matched 40 pieces BF862, fwiw. They are beasts.)
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I have a set (2 pairs) of LSK170/LSJ74 that I purchased from the DIYA store before. Never been used, apart from measuring them on a breadboard for noise and curve. Happy to sell them to you at the same price of the store (plus shipping and PayPal fees).
PM me if interested.
I’m selling them as I’ve now got a stash of Toshibas, and I prefer them to the LSK/LSJ.
PM me if interested.
I’m selling them as I’ve now got a stash of Toshibas, and I prefer them to the LSK/LSJ.
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