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Old 20th March 2011, 12:23 PM   #21
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by swordfishy View Post
The PCB layout is attached ...
OK, now I'm lost. Can you post an updated schematic to compare?
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Old 20th March 2011, 12:29 PM   #22
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by swordfishy View Post
If I want more current I need to switch fets. I'm going to try a J203.
Or a couple of J202 in parallel in the meantime for testing.
Also, IMHO, the top fet should have at least the same Idss as the bottom one, otherwise it will have positive Vgs, causing gate current.
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Old 20th March 2011, 12:39 PM   #23
GregH2 is offline GregH2  Australia
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Quote:
OK, now I'm lost. Can you post an updated schematic to compare?
Oh dear.

It's pretty much the same as my first post except the speaker return is to ground and I'm only using 2 pairs of output fets rather than 3 (M1 + M3, M4 + M6):
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File Type: pdf ExpressSCH.pdf (29.3 KB, 29 views)
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Old 20th March 2011, 12:45 PM   #24
GregH2 is offline GregH2  Australia
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OK, I made it match what I am currently using for testing. Here it is:
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Old 20th March 2011, 12:51 PM   #25
Zen Mod is offline Zen Mod  Serbia
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transconductance of inputs is critical

use either J309 or J310 , with adequate source resistor for lower one

observe dissipation and don't use less than 5mA through them

in fact - use Jfet for upper one and any CCS you want , instead lower one
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Old 20th March 2011, 01:00 PM   #26
GregH2 is offline GregH2  Australia
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Quote:
Originally Posted by Zen Mod View Post
transconductance of inputs is critical

use either J309 or J310 , with adequate source resistor for lower one

observe dissipation and don't use less than 5mA through them

in fact - use Jfet for upper one and any CCS you want , instead lower one
Thanks ZM,

I will definitely up the current.

I used the J202 as they are made for audio use so I assumed they would be better. Is that incorrect? Could it really be that critical in this source follower application?

They also have a 40v rating which is necessary for this particular application as they see 35v across them at turn on.

The J203 would be good if I want more current too I think. As soon as I have some new fets I'll keep experimenting.

Last edited by GregH2; 20th March 2011 at 01:06 PM.
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Old 20th March 2011, 01:17 PM   #27
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally Posted by swordfishy View Post
Thanks ZM,

I will definitely up the current.

I used the J202 as they are made for audio use so I assumed they would be better. Is that incorrect? Could it really be that critical in this source follower application?

They also have a 40v rating which is necessary for this particular application as they see 35v across them at turn on.

The J203 would be good if I want more current too I think. As soon as I have some new fets I'll keep experimenting.
I'm using J309 or J310 in and I'm satisfied with them , comparing to 2SK170 or even 2SK369
I don't see powering up as problem , regarding voltage ;

thinking about CCS - you can use something as on attached schm
of course - you'll loose slightly to rail to rail drive , comparing to solution with one jfet CCS , but who cares ....

edit : play a little with mosfet gate resistors , if you want to alter leading edge of output oscillogram ; there is no use looking at gates
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File Type: jpg JFET input current amp.jpg (52.6 KB, 142 views)
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Last edited by Zen Mod; 20th March 2011 at 01:21 PM.
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Old 20th March 2011, 01:28 PM   #28
GregH2 is offline GregH2  Australia
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Hi Zen Mod,

Nice photoshop work with the schematic there! I had thought of using a bipolar ccs, but the jfet ccs is so neat! I really don't want to lose any voltage swing if possible as I already only have 30V P-P. It may turn out to be my only choice though. I need to do some simulations.

Thanks for your input on the JFETs. What p channel device do you recommend (for an F5).

Thanks again,

Greg.

Last edited by GregH2; 20th March 2011 at 01:32 PM.
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Old 20th March 2011, 02:16 PM   #29
tsz is offline tsz  Sweden
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Quote:
Originally Posted by swordfishy View Post
Hi tsz,

No I haven't tried that. Is there any reason to?

The speaker return is connected to ground (unlike the schematic).

The PCB layout is attached (but I'm only using 2 output pairs, not 3).
Nah, not if the speaker return is connected to ground. I just thought it looked odd with different references for input and output (very poor PSRR I imagine).

What made you abandon the constant current draw approach?
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Old 20th March 2011, 02:25 PM   #30
GregH2 is offline GregH2  Australia
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thanks tsz,

I haven't abandoned the approach, I'm just not using it for this initial testing phase. In fact when I get this circuit working smoothly I will be redesigning it with a negative rail and constant current as per godfrey's circuit here:

Constant Current Pros and Cons

Looks promising I think.
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