JFET input, MOSFET VAS, LATERAL output = Perfect!! - Page 42 - diyAudio
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Old 27th May 2011, 12:13 PM   #411
lineup is offline lineup  Sweden
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thanks for good KSA1381 info, homemodder

no rush, swordfishy. we take it as it comes.
hope AKSA will guide. he is a cornerstone
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Old 27th May 2011, 01:59 PM   #412
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Quote:
Great circuit.
Different.
But in the spirit of this topic with FET, MOSFET etc.
Yes, sorry, it does not really fits to the theme.

Quote:
Will you or have you built it, moschfet?
No, at the moment I have an other project and I want to finish it first. It was more an idea.
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Old 27th May 2011, 04:16 PM   #413
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Smile board design

Anybody going to construct a PCB?
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Old 27th May 2011, 04:22 PM   #414
lineup is offline lineup  Sweden
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Originally Posted by moschfet View Post
Yes, sorry, it does not really fits to the theme.
No problem.
I think it is an interesting circuit with all mosfet.
And it fits very well here in this topic.
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Old 27th May 2011, 05:09 PM   #415
GregH2 is offline GregH2  Australia
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Hello all,

Good news. JFET version is built. Schematic is the same as MOSFET version, with some small differences made to the dc offset adjust to provide a slightly negative gate bias at the input fet. I will post a full schematic over the next day or two.

The amp seems as stable as before. The only difference is there is a bit more ringing on the square wave, but I think this is a layout issue - despite the fact that this is my 4th version of this amplifier it is my worst layout yet. Also I only had 390nf caps for PS bypassing.

As it is 2:00 in the morning and my other half is asleep, I have only been able to try some very low volume listening tests. Initial impressions suggest that the JFET has done everything they are known for, and sound quality has improved in every way possible. Voices have taken on a really "airy" character and it sounds like an amp you would pay many thousands of dollars for. I'm impressed to say the least. I will write more tomorrow after I have been able to have a proper listen, but I think it's easily going to live up to expectations. The JFET front end looks to be the icing on the cake.


Who would have thought you could get such a nice sound from only 5 active devices?

Going to start on a pcb design over the coming days.

Oh, by the way, thanks to everyone who posted options fo VAS transistors. I'm looking in to those now.

Greg.

Last edited by GregH2; 27th May 2011 at 05:22 PM.
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Old 27th May 2011, 05:39 PM   #416
lineup is offline lineup  Sweden
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swordfishy, Greg

Exciting to read.
JFETs should be an improvement.
Now, get some sleep. I standby for more later.
(In Sweden it is 6.38PM,18.38 now)
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Old 28th May 2011, 12:53 AM   #417
AKSA is offline AKSA  Australia
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SWF,

Gudonya!

Does this mean both input and VAS are both JFETs? What jfet are you using for the VAS?

For the input filter, a very necessary gizmo with extended FR, you need a 1K series resistor after the input point, then a 1nF to ground, then the blocking cap, then the input jfet gate. An amp without an input filter is a disaster waiting to happen....

Lineup, I too had my birthday a couple of weeks back, happy birthday to both of us!

Personally, I feel that people who design with more than 10 transistors for a simple power amp should be heavily fined to teach them the error of their ways.....

Hugh
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Last edited by AKSA; 28th May 2011 at 12:58 AM.
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Old 28th May 2011, 01:09 AM   #418
fab is offline fab  Canada
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Quote:
Originally Posted by AKSA View Post
SWF,
...
For the input filter, a very necessary gizmo with extended FR, you need a 1K series resistor after the input point, then a 1nF to ground, then the blocking cap, then the input jfet gate. An amp without an input filter is a disaster waiting to happen..........

Hugh
I have already said that in post #358:
"...Are you sure your input filter with C2 from post #336 circuit is really effective.... ...maybe if your input V3 has a hi output impedance."

Maybe the message was not so obvious after all...
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Old 28th May 2011, 01:28 AM   #419
AKSA is offline AKSA  Australia
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Fab,

Why do you think I said it again?

Don't be disappointed; most authors are never read......

Hugh
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Old 28th May 2011, 02:50 AM   #420
GregH2 is offline GregH2  Australia
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Fab,

I saw your post and I did reply in the very next post I made.

Unfortunately as you did not suggest an alternative and I was distracted by other aspects of the circuit I had not have not gotten around to trying anything different.

Hugh, thanks for your filter description. Are you suggesting I don't use a gate stopper or did you just assume I would use one anyway?

Oh, by the way, the vas is still the BJT version.
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