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Old 17th January 2013, 07:47 PM   #481
OllBoll is offline OllBoll  Sweden
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Quote:
Originally Posted by Zen Mod View Post


I told you from where to start ..........

you'll need to fiddle with loading resistor group (output side of C3) , to tweak for your load

in any case - leave there at least 100R , to have discharge path for C3 - useful when connecting/disconnecting speaker
True, forgot that one... but a discharge resistor is in place =)

But why would I need a loading resistor, wasn't that just something Nelson put there to reduce output impedance?
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Old 28th July 2013, 10:56 AM   #482
OllBoll is offline OllBoll  Sweden
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Now I've gotten new ideas and this amp is one of them I want to revisit: as with the last time I want more power without complexity so I'd thought I'd swap the CCS with a Hammond 193V ( 150mH, 1 ohm, 3A ) like they used in the Simple SIT amp.

If I've understood correctly it'd be just as easy as throwing out the active CCS and chucking in the inductor like in my picture... but since all this knowledge exists here I might as well ask =)

What I wonder especially is if the idle current isn't by default at about 2A, how do I make it so?

And if I've understood correctly the output impedance will be mostly dependant on the CCS so with the inductor it will stay high, where as passive resistors would be comparatively low?
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Old 28th July 2013, 12:34 PM   #483
Zen Mod is offline Zen Mod  Serbia
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for Iq set - measure voltage across Rs group ;

I=U/R

set Iq with P1

output impedance will be , same as with CCS iteration , mostly dependent of Q1 ..... more precisely - slightly lower than with CCS on top
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Old 28th July 2013, 02:46 PM   #484
OllBoll is offline OllBoll  Sweden
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Quote:
Originally Posted by Zen Mod View Post
for Iq set - measure voltage across Rs group ;

I=U/R

set Iq with P1

output impedance will be , same as with CCS iteration , mostly dependent of Q1 ..... more precisely - slightly lower than with CCS on top
Nice.

Another question though this is more academical since I'm not sure I'll actually do it: Say I wanted to celebrate the non-use of one of my heat sinks in the amp by adding another (matched) output jfet, would this be the way of doing it? Or maybe I will. Will two jfets operating at say 25W each perform better than a single one at 50W?
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Last edited by OllBoll; 28th July 2013 at 02:50 PM.
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Old 28th July 2013, 03:44 PM   #485
Zen Mod is offline Zen Mod  Serbia
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in this case ... be sure that Pa found sweet spot for them

so , if you ask me ...... two with 100W will work better than one with 50W

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Old 28th July 2013, 04:07 PM   #486
OllBoll is offline OllBoll  Sweden
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Quote:
Originally Posted by Zen Mod View Post
in this case ... be sure that Pa found sweet spot for them

so , if you ask me ...... two with 100W will work better than one with 50W

ZM, your trying to corrupt me... and... it is... working...

Might as well try with 48V rails and 2 Jfets at the start since that is what I have configured already. Question remains if the parallel outputs schematic is decent enough to work or if there is some part missing. If I've understood it is common to add series resistance on the output to sync the output devices, is that needed in this case or is it OK since both are identical and matched?

And another thing, does the source degeneration contribute any stability to the circuit or is it only distortion reduction?
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Old 28th July 2013, 04:16 PM   #487
Zen Mod is offline Zen Mod  Serbia
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"series" resistance - you have exactly enough of that in source resistor groups

having dedicated group for each paralleled Jfet is a must .

however - when I wrote that Pa established sweet spot for them , I meant on Iq , Uds (you can alter it , but 48V will be too much ) and amount of source resistance

so - stay at no more than 24V PSU (+/-20%) and 50W max per Jfet ........ while choke will double your cards

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Old 28th July 2013, 04:19 PM   #488
OllBoll is offline OllBoll  Sweden
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Quote:
Originally Posted by Zen Mod View Post
"series" resistance - you have exactly enough of that in source resistor groups

having dedicated group for each paralleled Jfet is a must .

however - when I wrote that Pa established sweet spot for them , I meant on Iq , Uds (you can alter it , but 48V will be too much ) and amount of source resistance

so - stay at no more than 24V PSU (+/-20%) and 50W max per Jfet ........ while choke will double your cards

=)

In that case I think i'll have to stay at 1 Jfet for now since the choke can only handle 3A current... unless I get another one... in the future...
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Old 28th July 2013, 04:46 PM   #489
OllBoll is offline OllBoll  Sweden
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Ah, so the source resistances work in that way.

Say you only have 1 Jfet, are the source resistors still necessary or are they removable as in the F6 or simple SIT? Or hmm, I remember someone talking about that the SITs were especially well behaved when used with no degeneration, much more so than the R100 and other enhancement mode Jfets and the F6 does have a feedback loop.

Last edited by OllBoll; 28th July 2013 at 05:00 PM.
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Old 28th July 2013, 05:31 PM   #490
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feel free to try various values for Rs , same as output loading resistor value ( defining output impedance )

let your ears decide what suits you best
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