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Old 26th March 2007, 10:19 PM   #31
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Quote:
Originally posted by jupiterjune


Grey- Is there a particular problem with driving the gate negative -- actually, now that I think about it, I don't think I entirely understand the question -- the BOSOZ has n channel devices, and the J105's have Vgs off of app. -7 volts. So those gates are negative with respect to the jfet sources. But these are p-channel devices.......I am trying the understand the problem you were seeing.

JJ
My post was in reference to post #3, wherein the poster references another circuit. However, the same principles apply.
There's a reason that you don't see the "big guys" using circuits that drive devices past their limits: it's not a good idea.
Let's begin with the transfer curves that you can find in the datasheets. You'll note that for any device that we would describe as "linear," the chart stops when the control element--in this case, the Gate--hits the axis. True, you can find charts that go past the axis...and they're usually in textbooks as examples as to why you don't want to overdo it. In a nutshell, the behavior of the device becomes grotesquely nonlinear. Going a little bit over the line gives you distortion. Going a little bit further drives the device into catastrophic breakdown.
The whole point of John Curl's complementary JFET follower (which is what the poster was referring to, as I recall) is that each part--the N-ch and the P-ch--is matched against the other. The further you get from that ideal, the more you have to look at ways to rein in the stronger device. The typical way to do this is to add resistance under the Source of the part with the higher Idss in order to bring the pair back into balance.
To allow the Idss of one part to completely override the other is not a particularly good idea. Let's assume for the moment that you're not driving the underdog part so hard that it's drawing Gate current (this is bad), you're pretty much guaranteed that the JFET is in a very nonlinear state; not something I'd call a resistor.
This is DIY, you're pretty much free to do what you want. I choose not to do this particular thing.
Just for giggles, test the circuit for distortion, particularly with a strong input signal.
And yes, using the CCS to overdrive the gain devices in a differential gives you pretty much the same sort of problem. Depletion mode JFETs are not meant to be operated as though they are enhancement mode devices.

Grey
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Old 27th March 2007, 01:00 AM   #32
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Grey-
Thanks for the feedback.

I am going to spend some time playing around with the basic j-fet circuits to get a full understand of all the particulars of how they work.
Much appreciated.

JJ
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Old 27th March 2007, 03:02 AM   #33
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Quote:
Originally posted by GRollins


My post was in reference to post #3, wherein the poster references another circuit. However, the same principles apply.
There's a reason that you don't see the "big guys" using circuits that drive devices past their limits: it's not a good idea.
Let's begin with the transfer curves that you can find in the datasheets. You'll note that for any device that we would describe as "linear," the chart stops when the control element--in this case, the Gate--hits the axis. True, you can find charts that go past the axis...and they're usually in textbooks as examples as to why you don't want to overdo it. In a nutshell, the behavior of the device becomes grotesquely nonlinear. Going a little bit over the line gives you distortion. Going a little bit further drives the device into catastrophic breakdown.
The whole point of John Curl's complementary JFET follower (which is what the poster was referring to, as I recall) is that each part--the N-ch and the P-ch--is matched against the other. The further you get from that ideal, the more you have to look at ways to rein in the stronger device. The typical way to do this is to add resistance under the Source of the part with the higher Idss in order to bring the pair back into balance.
To allow the Idss of one part to completely override the other is not a particularly good idea. Let's assume for the moment that you're not driving the underdog part so hard that it's drawing Gate current (this is bad), you're pretty much guaranteed that the JFET is in a very nonlinear state; not something I'd call a resistor.
This is DIY, you're pretty much free to do what you want. I choose not to do this particular thing.
Just for giggles, test the circuit for distortion, particularly with a strong input signal.
And yes, using the CCS to overdrive the gain devices in a differential gives you pretty much the same sort of problem. Depletion mode JFETs are not meant to be operated as though they are enhancement mode devices.

Grey



I fully agree that using the complementary matching against the other could be the best. But, this can not stop new way of experiment and fun. The overriding does not necessarily mean that it passes the Idss axis as you say. Id like to think that the overriding means vertically shifting the Q-point keeping the original transfer curves slope. The overriding might not improve its own narrow span of current swing though.

The narrow span of current swing is not a big deal in my specific case. For example, Babo Zen needs very small current swing there. And, I dont see any symptom of actual problem from my Babo Zen yet, except the very good sound. I will see further if I meet any problem, for your info.


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Old 27th March 2007, 04:21 AM   #34
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> did you compare the 1529 to the IRFP240s (Ive got plenty of the latter.....)

Yes. I have published the curve somewhere. They have similar transconductance. It is well known that I prefer 2SK1529s. But that is a matter of taste. Nelson does not.

One hint concerning the Aleph J could be the use of 2 output fets instead of 3 (6?)

How you tell !!!!!
I use 12 Power FETs per channel in total.
Don't be so Dutch. IRFPs cost nothing !!


PS I am not German, onl live here. But you can still hate me nevertheless. (Joke)



Patrick
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Old 27th March 2007, 11:43 AM   #35
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Default found some bass

Hi,

this morning I wanted to check the cold startup abs. dc offset after setting it to zero yesterday evening.
Itaround 2,75 Volts wich means that I can easily raise the McMillan resistors to 20k or so.
I also checked the rel. dc offset wich was 4mV on the right and around 5V on the left......
Somehow while carrying the amp downstairs a source resistor had lost contact and there was a slight imbalance
This probably put some tension on the left woofer causing it not to woof very nice anymore.
So after resoldering and reseting the missing bass found its way back into my living room

The sound is quite nice now.

When Im back from holiday I will first raise the McMillans to 20k, this had quite some influence on the sound with the 9610s and probably will with the 2sj74s

William
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Old 27th March 2007, 11:57 AM   #36
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Default Re: found some bass

Quote:
Originally posted by wuffwaff
Hi,

this morning I wanted to check the cold startup abs. dc offset after setting it to zero yesterday evening.
Itaround 2,75 Volts wich means that I can easily raise the McMillan resistors to 20k or so.
I also checked the rel. dc offset wich was 4mV on the right and around 5V on the left......
Somehow while carrying the amp downstairs a source resistor had lost contact and there was a slight imbalance
This probably put some tension on the left woofer causing it not to woof very nice anymore.
So after resoldering and reseting the missing bass found its way back into my living room

The sound is quite nice now.

When Im back from holiday I will first raise the McMillans to 20k, this had quite some influence on the sound with the 9610s and probably will with the 2sj74s

William
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Old 27th March 2007, 02:54 PM   #37
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Glad that it was not the JFETs at fault.

But I would still encourage you to try 910R/no Rsource, and perhaps 12 FETs per channel at the same total bias.

I know it is a lot of work though.


Patrick
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Old 27th March 2007, 03:31 PM   #38
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Hi Patrick,

yes I will. I will probably use some 5R source resistors with a 50R trimmer in parallel though to avoid having to buy another 50 2sj74s (yes I know Im dutch)

I will also change the McMillan resistors to a value as high as possible.

Im already using 12Fets per channnel (IRFP240). Changing them to 2sk1529 would really be a bit of work as they are all hard wired and not so easy to change. It is probably better to sell mine and build some new ones.....this way I could make them a bit wider to fit a non humming transformer.

William
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Old 27th March 2007, 03:47 PM   #39
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If you have problems getting 2SK1529, let me know.

Also you would need to experiment with the source resistor for 2SK1529s. I have used up to 1k for 4 FETs per channel, but no compensation caps at the feedback resistor. You need to experiment.

Patrick
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Old 27th March 2007, 03:59 PM   #40
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Hi Patrick,

what do mean with source resistor? 1k seems a bit high for the power fet source resistors. Im using 0,33R at the moment (3 x 1R 1%).

William
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