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Old 24th May 2013, 09:32 AM   #521
AndrewT is offline AndrewT  Scotland
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5r is the value difference that Euvl adopted.
i.e. 10r in the k170 source and 15r in the j74 source, if my memory is still right.
The sliding bias offered by the pot//fixed resistors, achieves the same thing by letting the two N & P see different r values to audio ground.
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Old 24th May 2013, 10:18 AM   #522
NicMac is offline NicMac  Italy
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Quote:
Originally Posted by AndrewT View Post
5r is the value difference that Euvl adopted.
i.e. 10r in the k170 source and 15r in the j74 source, if my memory is still right.
The sliding bias offered by the pot//fixed resistors, achieves the same thing by letting the two N & P see different r values to audio ground.
Your memory is correct about the resistor values used by EUVL. I actually employ a set of values as this increases the yield of K170/J74 that can be matched to trace perfectly.
Of course, if Nelson's "P3" pot//fixed resistors achieves exactly the same thing in adjustable way then I will rest my case.
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Old 24th May 2013, 10:42 AM   #523
AndrewT is offline AndrewT  Scotland
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10r + 10r and then a parallel 100r with the wiper connected to the 10r junction is exactly equivalent to 10r//50r in the source of each jFET, when the pot is set to exactly half way.
10r//50r = 8r3333.
Now move the wiper up towards the NjFET.
The upper resistance becomes 10r//40r = 8r0 and the lower resistance becomes 10r//60r = 8r571
Go a bit further:
10r//30r and 10r//70r gives upper and lower Rs = 7r5 & 8r75
and further 10r//20r with 10r//80r gives Rs = 6r667 & 8r889
and further 10r//7r with 10r//93r gives Rs = 4r118 & 9r029 (getting very close to that 5r difference).

Personally I would adopt 12r+12r and parallel that with 5r+200VR+5r giving 12r//105r (=10r8 at the midway setting). This avoids setting Rs to zero and avoids setting the Rs of either jFET rather low (which increases the Id and thus heat).
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Old 24th May 2013, 10:42 AM   #524
NicMac is offline NicMac  Italy
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Ok - so either a pot over R3/R4 (like permaneder had) or 0805 SMD solder pads right on the J74 source leg (i.e. in series with R4, but outside of the feedback loop). I personally prefer the latter. Of course the inductor and fan regulator provisions is of no importance to me...
I'm not the designer so obviously not my choice

P.S. The mosfet source resistors I would use are Caddock MP930 series. Not cheap but also non-inductive.
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Old 24th May 2013, 10:58 AM   #525
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Andrew, Nicolai, if one wants to achive the 5R difference why not start with different R3/4 in the first place and use the "stock" 200R P3 to finetune?

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Old 24th May 2013, 11:00 AM   #526
Anchan is offline Anchan  United States
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Nick- take my BOM that I posted a few posts back. Make whatever changes to it you would like. If you can find me a part # that I can get here in the US, I'd be happy to make the changes. For my BOM, I cobbled that together from some other standard F5 BOM's I found circulating, so its only slightly better than arbitrary. It seems you have put more thought into your selections, so I'm happy to improve things based on your recommendations.

I think I'd prefer to add the extra smd resistor rather than a pot, since I am not going that extra step to match the curves on the jfets. I ordered matched pairs, so I'm taking the leap of faith that they are matched enough for me


Quote:
Originally Posted by NicMac View Post
Your memory is correct about the resistor values used by EUVL. I actually employ a set of values as this increases the yield of K170/J74 that can be matched to trace perfectly.
Of course, if Nelson's "P3" pot//fixed resistors achieves exactly the same thing in adjustable way then I will rest my case.
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Old 24th May 2013, 11:03 AM   #527
AndrewT is offline AndrewT  Scotland
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do you know that k170/j74 will always need any Rs difference to equalise their gm?
do you know if the builder will substitute another pair of jFETs?
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Old 24th May 2013, 11:13 AM   #528
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Quote:
Originally Posted by AndrewT View Post
do you know that k170/j74 will always need any Rs difference to equalise their gm?
Not for all k170/j74, no. But may be for the ones I use.
Quote:
do you know if the builder will substitute another pair of jFETs?
Yes - he won't

In any case, I get your point. Thanks.

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Old 24th May 2013, 11:30 AM   #529
NicMac is offline NicMac  Italy
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If you have Idss-matched K170/J74 (which is what I think Spencer sells) they will never have the same gm. If the J74 are degenerated they may achieve equal gm and even equal Idss if well selected as shown in the figure I have attached (blue are the 5R1 degenerated J74).
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File Type: png 2SK170_2SJ74.png (48.8 KB, 232 views)
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Old 24th May 2013, 12:40 PM   #530
Anchan is offline Anchan  United States
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Quote:
Originally Posted by NicMac View Post
Ok - so either a pot over R3/R4 (like permaneder had) or 0805 SMD solder pads right on the J74 source leg (i.e. in series with R4, but outside of the feedback loop). I personally prefer the latter. Of course the inductor and fan regulator provisions is of no importance to me...
I'm not the designer so obviously not my choice

P.S. The mosfet source resistors I would use are Caddock MP930 series. Not cheap but also non-inductive.
Do you need to heatsink those Caddocks? (ie. would they have to be placed along the edge of the board, for that purpose?) I may or may not choose to use the Caddocks as well (pricey!), but I could at least add another trace with the appropriate footprint, so at least there is choice.

Back to the 10R R3, and R4 + degenerative resistor. The particular resistors I used there are rather jumbo 1W through holes. Really, I didn't think too much about them- rather just copied from a stock F5 BOM I found. But in thinking about it, that seems to be way overkill in power rating. How about I change both of those to 1/8th watt 0805 smd's (+ the degenerate in series)?
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