One JFET poweramp looking for THE FET SOUND - diyAudio
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Old 29th August 2010, 12:18 PM   #1
lineup is offline lineup  Sweden
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Default One JFET poweramp looking for THE FET SOUND

JFET has a different sort of distortion than BJT.
Most poweramps have BJT input + Error correction.
And even those poweramps having JFET inputs
has got so much gain and negative feedback,
that they should not sound at all or sound just like BJT.


This idea of mine uses only 1 JFET to amplifiy and take care of feedback.
It will have some distortion, but of the type of dist that JFET has.

See diagram.
It is a very simplified presentation of my idea.

The signal comes across R3.
You need no input cap, but we usually attach one anyway.
2N4416 JFET was chosen because of VGS.
2SK170 is not very good, because here we need VGS 1-2 Volt for producing some milliAmperes.

On top is current mirror.
That mirror will not change or amplifiy. Just mirror.
On the bottom is corresponding CCS, current source,
to balance current coming from mirror.
This stage drives the output stage follower.

The feedback and gain is handled by R2/R4.
And feedback is taken to the JFET Source.

What is important is to add a good output stage.
One JFET (or other small signal device) will give more distortion
the heavier the load is.
So, to give suitable light load here I would say we need a
Triple Darlington or similar.
With very high input impdance.

I have a couple good working protypes in SPICE.
Where distortion is at very respectable level.
I will post one.
And, yes!
It gives what I was after: mostly 2nd order distortion.
THE FET SOUND is here ...
Attached Images
File Type: png fet_sound_idea.png (6.8 KB, 1040 views)
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Old 29th August 2010, 02:14 PM   #2
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Quote:
Originally Posted by lineup View Post
JFET has a different sort of distortion than BJT.
Most poweramps have BJT input + Error correction.
And even those poweramps having JFET inputs
has got so much gain and negative feedback,
that they should not sound at all or sound just like BJT.


This idea of mine uses only 1 JFET to amplifiy and take care of feedback.
It will have some distortion, but of the type of dist that JFET has.

See diagram.
It is a very simplified presentation of my idea.

The signal comes across R3.
You need no input cap, but we usually attach one anyway.
2N4416 JFET was chosen because of VGS.
2SK170 is not very good, because here we need VGS 1-2 Volt for producing some milliAmperes.

On top is current mirror.
That mirror will not change or amplifiy. Just mirror.
On the bottom is corresponding CCS, current source,
to balance current coming from mirror.
This stage drives the output stage follower.

The feedback and gain is handled by R2/R4.
And feedback is taken to the JFET Source.

What is important is to add a good output stage.
One JFET (or other small signal device) will give more distortion
the heavier the load is.
So, to give suitable light load here I would say we need a
Triple Darlington or similar.
With very high input impdance.

I have a couple good working protypes in SPICE.
Where distortion is at very respectable level.
I will post one.
And, yes!
It gives what I was after: mostly 2nd order distortion.
THE FET SOUND is here ...
Link p12 has FETs on it.

Distortion depends a bit on the accuracy of the SPICE model which may not match reality for that degree of analysis - but I do like FETs.

If you really just want 2nd and very little else, you need a triode front end like a nice 6N2P or 12AX7
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Old 29th August 2010, 04:12 PM   #3
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What was the point of quoting the entire post? It added nothing and wastes bandwidth.

w
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Old 29th August 2010, 05:53 PM   #4
AndrewT is offline AndrewT  Scotland
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not only bandwidth but had he included the sketch he would have broken Forum rules, which prohibit quoting entire posts.
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Old 17th March 2011, 08:00 PM   #5
lineup is offline lineup  Sweden
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I am sorry I forgot this topic.
I will post a working circuit
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Old 17th March 2011, 08:08 PM   #6
lineup is offline lineup  Sweden
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Default JFET with triple darlington

Triple darlington output works the best.
- Class AB
- Max 25 Watt into 8 OHM
- Less than 0.020 % THD at 25 Watt
- It is fast, 2 MHz -3dB at gain >20

Second Image shows 25 Watt into 8 ohm
Attached Images
File Type: png FETsound_1.png (24.0 KB, 780 views)
File Type: png FETsound_50watt.png (21.9 KB, 762 views)
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Last edited by lineup; 17th March 2011 at 08:16 PM.
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Old 17th March 2011, 11:27 PM   #7
MiiB is offline MiiB  Denmark
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would it be possible to adapt it for +- operation and fit a pair of lateral fet's for output devices...the triple seems to complex for my liking...

M
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Old 18th March 2011, 06:46 AM   #8
Bonsai is offline Bonsai  Taiwan
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Lineup, nice simple design - but I agree a lateral FET output is the way to go. Otherwise, what about a nice Locanthi 'T'?
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Old 18th March 2011, 08:00 AM   #9
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My vote also goes with lateral mos output
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Old 18th March 2011, 09:06 AM   #10
MiiB is offline MiiB  Denmark
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A little thinking...on your concept...Sure it'll sound good.. distortion is app -85 dB second order...then dropping to app -100 dB third and even lover 4.th...

Dual rail lateral FET output..here with 600 mA..app 30 W dissipation each...a little hot maybe..


DC component:0.00494159

Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+03 7.672e+00 1.000e+00 -0.18 0.00
2 2.000e+03 4.651e-04 6.062e-05 85.67 85.85
3 3.000e+03 4.593e-05 5.987e-06 49.85 50.03
4 4.000e+03 1.180e-06 1.537e-07 -117.73 -117.54
5 5.000e+03 3.060e-07 3.988e-08 -80.47 -80.28
6 6.000e+03 4.605e-08 6.002e-09 51.90 52.09
7 7.000e+03 5.001e-08 6.519e-09 -2.38 -2.19
8 8.000e+03 3.390e-08 4.418e-09 -46.01 -45.83
9 9.000e+03 7.551e-08 9.842e-09 167.27 167.46
Total Harmonic Distortion: 0.006091%
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File Type: pdf J-fet-amp-1.pdf (24.6 KB, 213 views)

Last edited by MiiB; 18th March 2011 at 09:33 AM.
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