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Old 7th March 2012, 01:34 PM   #11961
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Quote:
Originally Posted by Zen Mod View Post
instead of 2 lag caps (one per leg ) you can put one - from source to source of Jfets
Or add more output FETs although the added open loop ampl. raises the closed loop freq.
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Old 7th March 2012, 01:35 PM   #11962
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Quote:
Originally Posted by Zen Mod View Post
just ignore that last post of mine

coffeeless brain is even emptier , than usual
. . . because they are in phase . .
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Old 7th March 2012, 02:17 PM   #11963
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In phase as : he ran out of coffee beans and brains.
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Old 7th March 2012, 05:36 PM   #11964
labjr is offline labjr  United States
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Does anyone know if there are current plans to make an F5 board with a P3 pot on it?
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Old 7th March 2012, 06:38 PM   #11965
JZatopa is offline JZatopa  United States
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Quote:
Originally Posted by lhquam View Post
The intrinsic square-law characteristics of the JFETs and MOSFETs cause nearly all of the distortions. Proper adjustment of P3 can reduce even order distortions to nearly zero. Increasing the bias currents helps, but increases idle power dissipation. Nothing much can be done to reduce odd order distortions other than increasing the amount of negative feedback, at the cost of increasing distortions at higher orders.
Could you tell me what the intrinsic square-law is or give me a link where I can learn about it?

It sounds like only the active devices contribute to the distortion of the amp. Are there other jfet/mosfets that we might want to consider for the F5 that might not have perviously been considered?
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Old 7th March 2012, 07:26 PM   #11966
labjr is offline labjr  United States
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Does Fairchild have any SiC outputs yet?

Fairchild Adds Silicon Carbide Technology
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Old 7th March 2012, 07:29 PM   #11967
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Quote:
Originally Posted by DJNUBZ View Post
Could you tell me what the intrinsic square-law is or give me a link where I can learn about it?
A FET is called a square-law device because of the relationship of ID to the square of a term containing VGS.

So the transfer characteristic is not a straight line but curved.

Last edited by bobodioulasso; 7th March 2012 at 07:34 PM.
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Old 8th March 2012, 12:06 AM   #11968
lhquam is offline lhquam  United States
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Quote:
Originally Posted by triode_al View Post
So if we make the solving a bit more complicated, we could say - instead of nulling second harmonics al together - for example set that at 10 dB higher than 3rd harmonics (which is a given and can be calculated, given the measures characteristics of the devices).
The picture below shows what I think happens, this picture is from a differential pair by the way (Héphaïstos, in L'Audiophile 2/1988).
Attachment 270412
With P3 we also move the operating point a bit up or down, and are content with the trifle of added second harmonics (as long as they are in phase, being natural).

albert

by the way, I did try to lower high frequency gain for a high roll-off, by adding capacitance across the feedback resistors R5-R8, even 2 nF matched to about 2% was not good enough and gave damped ringing (way above 100kHz), so at the end I left the bandwidth all open myself, and that sounded best.
I now see a simpler and more general way to express the problem. The exact Vout(Vin) function for the JFET+MOSFET chain can be a Taylor series expansion of the full equations, resulting in polynomials:

P(x) = sum(i, c[i]*x^i)

The output from pair of amplifier chains with feedback is then:

Vout = P1((1-a1*Gain)*Vin - Vt1) - P2((1-a2*Gain)*Vin - Vt2)

The harmonic analysis in now done in the Fourier domain. For a given input Vin(f) at frequency f, the output response at frequency i*f is c[i]*Vin(f). This approach allows the use of complex coefficients, representing phase information (and capacitors and inductors).

I haven't completed the details, but I expect the response for i-th harmonic something like:

Vin^i * (c1[i]*((1-a1*Gain)) + (-1)^i * c2[i]((1-a2*Gain))


Another interesting discovery:

Source resistor degeneration is known to be a type of local negative feedback. I have equations that show that source degeneration introduces higher order harmonic distortions in proportion to the amount of local feedback. This suggests higher harmonics might be reduced by reducing source degeneration. Unfortunately, that increases the open-loop gain, which increases either the amplifier Gain or the global feedback.
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Old 8th March 2012, 03:30 AM   #11969
lhquam is offline lhquam  United States
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Default Start an F5 theory thread?

Quote:
Originally Posted by triode_al View Post
I am mystified now . . . we try to maximize 2nd harmonics of the right phase, don't we? (see my avatar . . that shows a tube output).
...
Can you also find that point in the math, that would be cool.
albert

All of you who are interested in circuit theory as applied to the F5 (and related amplifiers) I suggest we start a new thread. I am very interested in thoroughly understanding why the F5 can be adjusted to work so well.
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Old 8th March 2012, 08:30 AM   #11970
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Quote:
Originally Posted by lhquam View Post
I have equations that show that source degeneration introduces higher order harmonic distortions in proportion to the amount of local feedback.
You need equations for that ?
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