Thanks for the options, Silasmellor. I wasn't aware that the store sells it, but getting anything from the States is very expensive for me. So, I'll investigate the other options first.
If matching the 170's and 74's is important, I may order a dozen or so 74's and try matching it myself. But that depends on the pricing. Otherwise getting matched quads might be a good idea.
If matching the 170's and 74's is important, I may order a dozen or so 74's and try matching it myself. But that depends on the pricing. Otherwise getting matched quads might be a good idea.
This guy is also reliable.
FET Audio | Hi-End Audio Projects
For the M2, the higher IDSS ones can be used, but substitute the 10R JFET source resistors with 15-20R.
FET Audio | Hi-End Audio Projects
For the M2, the higher IDSS ones can be used, but substitute the 10R JFET source resistors with 15-20R.
This eBay seller is an acquaintance of Nelson's and his Toshiba parts are genuine.
http://stores.ebay.com/punkydawgs/
http://stores.ebay.com/punkydawgs/
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Thank you everyone for your suggestions and advice. Much appreciated. You've saved me from buying fake SJ74's. I will now certainly only buy from a recommended seller.
@Dennis Hui
It's a great idea to ask the sellers for 74's with similar Idss to what my 170's are. If that approach doesn't pan out, I may just get some matched 74's and and then buy more 170's locally and try to match the 74's. Fortunately, 170's are still freely available over here.
Come to think of it. Maybe I should buy a bunch of 170-BL's and offer some matches for sale.
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@Dennis Hui
It's a great idea to ask the sellers for 74's with similar Idss to what my 170's are. If that approach doesn't pan out, I may just get some matched 74's and and then buy more 170's locally and try to match the 74's. Fortunately, 170's are still freely available over here.
Come to think of it. Maybe I should buy a bunch of 170-BL's and offer some matches for sale.
Eric, I saw that but it hadn't dawned on me that I could use that for unmatched JFets. Thanks for pointing it out.
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This hack is only suitable for the last few mV’s. Don’t do this to close a large dc offset. ZM mentioned somewhere in this thread that much dc correction done by changing r3/r4 results in different transconductance between the jfets (or maybe he meant that transconductance difference is not resolved this way). So (near) perfect matching jfets remains preferable.
Steef.
Thanks for the warning, Steef. It was not my intention to use unmatched Jfets and then cheat by using the hack.
Groetjes
Christo
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I don't have specific knowledge here - I would think that this determination would require some analysis with a scope and/or a curve tracer to see what happens as the difference between the two jFets gets larger.
Personally, I would use this technique to even up differences of less than 2mA at most. I would feel better using it to even up differences that are less than 1mA.
Personally, I would use this technique to even up differences of less than 2mA at most. I would feel better using it to even up differences that are less than 1mA.
I meant 5mV in the previous message.
I have 2SK170's matched to within 1.5% for the M2 project. I suppose that will be fine.
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