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Old 10th March 2007, 07:06 AM   #71
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Quote:
Originally posted by Audiofanatic
Nelson Pass made and want to shear with us
Maybe i'll get to see that marines cut and shave after all.
We have been behaving badly, but it's all Jack's fault.
(i feel more for Priscilla, Q of the Desert II. Co-starring with Hugh Dean, Greggy B and Anthony H)

Still waiting for the one who desires to go four F4s bridged and in parallel.
The F4 should receive the 2007 easiest and most flexible amp award, imo.

I've been so busy reading and re-reading the SA/4e owners manual that i forgot to say thank you to Mr Nelson for the big effort to put up many pages for download to please a few.
Birthday feeling revisited.
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Old 10th March 2007, 09:42 AM   #72
MiiB is offline MiiB  Denmark
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Now we just need somebody to desigen a differntial input for bringing up the gain and bridging two units X-ed ofcourse.
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Old 10th March 2007, 09:55 AM   #73
EUVL is offline EUVL  Europe
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> Now we just need somebody to desigen a differntial input for bringing up the gain and bridging two units X-ed ofcourse.

That is actually very easy.

Just remove the frontend JFETs and substitute that by the Blowtorch Clone circuit on the Solid State Forum. No need to X, because it has no global feedback.


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Old 10th March 2007, 11:01 AM   #74
MiiB is offline MiiB  Denmark
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I think theres a diffence in the basic Blowtorch schematic and the F4 Schematic. In the blovtorch the signal is taken outside the diff input. Where the F4 modulates the centerpoint between the input pair.
Using the Blowtorch would result in something like the Ayre V3. Amplifer. (may still be good)

I may be wrong as i have no theroretical beckground so please bear with me if I confuse matters.
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Old 10th March 2007, 05:54 PM   #75
cviller is offline cviller  Denmark
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Quote:
Originally posted by Nelson Pass
The choices for the JFETs is pretty flexible, as are the decisions
about the Source resistance. Clearly you want to have a
voltage rating comparable to the supplies (in this case about 40V)
and the self-bias current should be set with an eye on the
maximum dissipation.

First of all THANKS!! for sharing this. I cant wait to hear it!

In the datasheet I have for 2sj108 it says 25V - is that a typo in the datasheet or are you referring to 40V across both the transistors?

What is the maximum voltage either one of the jfets will be exposed to?

This is my datasheet for 2sj108:
http://www.alldatasheet.com/datashee...BA/2SJ108.html

What would the sonical impact be from using jfets with less transconductance in this configuration?
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Old 11th March 2007, 12:42 AM   #76
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It appears to be a matter of meeting a gate leakage spec,
which is not of particular importance here.

On the Toshiba's I see 1 uA leakage at about 46 volts, which is
rail-rail on that amp.

If you use some other brand, you might want to test the parts -
or you could pick another part. If I run into issues, I'll change the
part or consider some sort of limiting.

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Old 11th March 2007, 12:49 AM   #77
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Quote:
Originally posted by cviller
What would the sonical impact be from using jfets with less transconductance in this configuration?
I wouldn't expect much of an issue. We are mostly trying to
drive the capacitance of the Mosfets and the resistance of the
bias circuit, and the JFETs are there for that. Since I don't see
much increase in distortion at the top end, I'm sure you have
some latitude. You can increase those 10K resistors to 15K,
but when I tried that I didn't see much improvement.

When the amp has no load, the distortion drops a lot, so
clearly the Mosfets are the elephants on the dance floor. I
think you could sub in a lower transconductance.

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Old 11th March 2007, 11:43 AM   #78
cviller is offline cviller  Denmark
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Thank you so much for these details!

There are plenty of cheap and more importantly "in-stock" jfets out there, if high transconductance is not too important.

Transconductance is probably more important in circuits like the babbelfish (improvised aleph j).
I'm planning on doing some listening experiments with different jfets in that circuit to find a nice sounding replacement for 2sj109.
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Old 11th March 2007, 08:12 PM   #79
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Quote:
Originally posted by cviller
Transconductance is probably more important in circuits like the babbelfish (improvised aleph j).
It is.


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Old 12th March 2007, 09:40 AM   #80
juma is offline juma  Germany
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2SK246 and 2SJ103 - compl. pair, low transcond.
Vgds=50 V

2SK170(Vgds=40V)/2SJ74 (Vgds=25v)- a compl. pair (declared by Toshiba), high transcond.

All of them can be found at (for about $1 a piece):
http://www.ampslab.com/components_fets.htm
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