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Old 5th February 2009, 01:38 AM   #2691
EUVL is offline EUVL  Europe
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> you can try with them , but these have fairly low xconductance comparing to verticals ;and you need gain in F5 output stage ...... you'll have lower open loop gain , so less feedback ;

True. But you can lower the source resistor from 0R47 to 0R22 to compensate for that, as the source resistor is actually dominating in the original circuit.

> second - they also have pretty smaller Ugs , so drain resistors in first stage must be smaller , further decreasing that stage gain ...... further decreasing OLG

Not true. You can keep the drain resistor values by lowering the first stage bias by say 2mA or so. No problems for the JFETs.

> don't think that you're first with these thoughts

Of course not. Been there before, umteen pages back.
Jackinnj also have a layout with Hitachi laterals.




Patrick

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Old 5th February 2009, 01:49 AM   #2692
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally posted by EUVL
>.....

Not true. You can keep the drain resistor values by lowering the first stage bias by say 2mA or so. No problems for the JFETs.

.......

Patrick


true ...... but even in stock F5 current through first (Jfet) stage is already on lowish side ........

I know that Jackinnj tried them ( and like them ) ...... but 2mA is sooo low ..... and that wasn't F5 anymore ;

you can do that , Jack too ..... but not everyone , knowing what's needed to make it work .
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Old 5th February 2009, 01:59 AM   #2693
EUVL is offline EUVL  Europe
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> but 2mA is sooo low .....

I said, "You can keep the drain resistor values by lowering the first stage bias by say 2mA or so."

8mA - 2mA = 6mA, not 2mA, Sir.


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Old 5th February 2009, 02:01 AM   #2694
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally posted by EUVL
> but 2mA is sooo low .....

I said, "You can keep the drain resistor values by lowering the first stage bias by say 2mA or so."

8mA - 2mA = 6mA, not 2mA, Sir.


Patrick

thank you , Sir .

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Old 5th February 2009, 02:05 AM   #2695
EUVL is offline EUVL  Europe
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You're welcome.


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Old 5th February 2009, 02:32 AM   #2696
Peter Daniel is offline Peter Daniel  Canada
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Any particular reason that feedback resistors are listed as 3W, they seem to run rather cool?
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Old 5th February 2009, 02:36 AM   #2697
EUVL is offline EUVL  Europe
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> Any particular reason that feedback resistors are listed as 3W, they seem to run rather cool?

The theoretical max swing is say -4V from rail.
Even in my balanced version, that would be 12V. Which means 10V across the feedback resistor.

10*10/50/2 = 1W. Safety factor 3 -> 3W.

I actually use 4x PRP 1W in parallel.

Actual signal level is of course normally much lower.
So in theory you might get away with 0.5W or so.




Patrick
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Old 5th February 2009, 03:01 AM   #2698
jackinnj is offline jackinnj  United States
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Oh man, I am in trouble now.

I am using 4 F5's -- with Fairchild devices driving these Pioneer (TAD) S-1eX spxrs. I have a bunch of the Renesas (Hitachi) devices but haven't used them since last year.

On the bench is an NP/aX design from the AX article -- and we are are gonna look at the THD% figures for various combinations of JFET with Idss all over the lot. I know that THD% is not the ne plus ultra stat for the definition of quality, but it's the onlu completely reliable and reproducible stat that I can produce.
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Old 5th February 2009, 02:27 PM   #2699
woodturner-fran is offline woodturner-fran  Ireland
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Something odd is going on with my F5. When I push the volume a bit I get lots of breakup distortion.

Need to investigate this.....


Fran
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Old 5th February 2009, 03:17 PM   #2700
udailey is offline udailey  United States
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Fran,
Try a new pot and also reflow your solder connections to the pot on the board and on the pot. If that doesnt do it then try reflowing the rest of your soldering. I find that usually when I have crackling due to change in volume that there is a screwed up solder joint somewhere and I have also had that happen with junk pots or nice pots with bad solder joints. Never know, if you are using a stepped pot you could have a nasty joint somewhere on there as well.
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