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Old 11th September 2012, 08:15 PM   #1701
Zen Mod is offline Zen Mod  Serbia
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which cap ?

easy ....... calc for 0.5Hz with adjacent impedance
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Old 11th September 2012, 08:35 PM   #1702
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I have not directly measured the JFET transconductance, but have measured the output impedance of the Teaser-6 under the following conditions:

Feedback network: 50R/10R.
Closed loop gain approx 4.7
Rs=0, ie. AC coupling directly to JFET source
Rsens=0R12
Ibias=1.3A

Zo = 0R334

Measured open-loop-gain: 153
(Based on AC voltage measurement across transformer primary at 60Hz.
Not sure about accuracy.)


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Originally Posted by Antoinel View Post
Thank you Alex and lhquam. What is the value of S and thus Zo for Teaser-6 operating in Class A?
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Old 11th September 2012, 10:25 PM   #1703
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Quote:
Originally Posted by lhquam View Post
I have not directly measured the JFET transconductance, but have measured the output impedance of the Teaser-6 under the following conditions:

Feedback network: 50R/10R.
Closed loop gain approx 4.7
Rs=0, ie. AC coupling directly to JFET source
Rsens=0R12
Ibias=1.3A

Zo = 0R334

Measured open-loop-gain: 153
(Based on AC voltage measurement across transformer primary at 60Hz.
Not sure about accuracy.)
Thank you lhquam. Actual Zo = 0.334 Ohms = 1/2S. Solve for S= 1.5 mho. Possible right answer?
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Old 11th September 2012, 11:18 PM   #1704
lhquam is offline lhquam  United States
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I am not sure what you are trying to calculate. The feedback loop must be considered.

The actual transconductance of the JFETs is something like OLG/(2*Rload).
Rload is 8R || 60R approx 7R. We get S=10.77mho. That is consistent with the R100 datasheet.

Quote:
Originally Posted by Antoinel View Post
Thank you lhquam. Actual Zo = 0.334 Ohms = 1/2S. Solve for S= 1.5 mho. Possible right answer?

Last edited by lhquam; 11th September 2012 at 11:41 PM.
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Old 11th September 2012, 11:30 PM   #1705
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Quote:
Originally Posted by lhquam View Post
I am not sure what you are trying to calculate. The effective transconductance of the amplifier is CLG/Rload = 4.7/8 = .5875. The feedback loop must be considered.

The actual transconductance of the JFETs is something like OLG/(2*Rload).
Rload is 8R || 60R approx 7R. We get S=10.77mho. That is consistent with the R100 datasheet.
Just playing with numbers. Thanks for your calcs. They are robust.
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Old 11th September 2012, 11:31 PM   #1706
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Switched Xformers and changed caps to Elna Silmics II 470uF. Stereo running now. Clean and sweet midrange. Definitely need more gain from Pre IMO. I think or me personally, I would like a little more bloom. Don't know yet for sure. What if top cap is connected below fet and bottom cap is connected on bottom of Rs or attached to pot across Rs? I have an itch to play with harmonics. Also. If i were to run FE off Salas Shunt, how do i take care of the gronuding issue considering two psu's present and two sides of xformer?
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Old 12th September 2012, 12:00 AM   #1707
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naah

just cascode them ; they're acting as umbrella , no need for special reg

you can play with yay/ney Rs in modulation part

try first upper one to change
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Old 12th September 2012, 11:47 AM   #1708
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Default JBL inspired biasing for the F6

Having thought a little bit more, I suggest the following schema as a possible option for the bias.

Pas F6v2.jpg

It is in line with the JBL schema except that where they use a reference diode/zener I think the bias reference (I use 1k2 with an 2SK30 of Idss of 3,1 mA) should suffice.
The bootstrap ensures the current source sees the same potential all the time.
I have a bootstrap both sides just for the sake of symmetry.

JBL Energizer:

JBL Energizer.pdf
  • The diode is a special diode, and as far as I understand it zeners at 1,0V.
  • Else the bias voltage is generated across the 6,2 Ω resistor and the diode just acts as some kind of safety device.
The feedback is local to the driver amp and global, to the input (via a filter that compensates for special JBL loudspeakers).
Attached Files
File Type: pdf F6 ak v2.pdf (50.2 KB, 42 views)
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Old 12th September 2012, 12:13 PM   #1709
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It's symetrical by sight, though i think supplying the bottom voltage source from ground instead of output should be preferable.
(And no bootstrap at bottom)

Last edited by bobodioulasso; 12th September 2012 at 12:15 PM.
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Old 12th September 2012, 12:18 PM   #1710
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Quote:
Originally Posted by bobodioulasso View Post
It's symetrical by sight, though i think supplying the bottom voltage source from ground instead of output should be preferable.
(And no bootstrap at bottom)
Agree totally - I am just trying to pathethic over a small detail - the compression factor of the CCS, that will influence the operating point. I have no idea if there is a difference; simulation could show that.
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