Semisouth JFET's: What are they good for?

[Nelson Pass]

The KGSS seems like a lot of work. I think you could manage
that with a couple of balanced SJEP170R550's.

 

[zhoufang]

Me too think that KGSS is a lot of work, and there is the need to collect the list of odd parts.

The fullsize STAX has around 100pF capacitance including cable.
So at 20kHz the min Impedance is about 80kohm.
The Stax amp with the highest voltage has :Maximum output voltage 450 V r.m.s. / 1 KHz
So if I over design and maintain 450Vrms to 20kHz, the maximum current draw is just 5.6mA rms or 8mA peak.

I'm thinking of a SE fully balanced amp, 2SK170 (parallel a few) input as common source, cascode with single R550 at 1000V Vds, bias current 25mA.
Simple single stage amp?
At least now I see some points of having 1+kV device for diy audio.

 

[Zen Mod]

you can make famous ZM's See-Saw amp - tweaked for high voltage



http://www.diyaudio.com/forums/pass-labs/133433-choky-susy-f3-lu1014s.html

( if you are interested - I can think about drawing exact schematic in next few weeks ) ....

 

[zhoufang]

Dear ZM,

That "Choky Susy F3 for LU1014's" definitely looks interesting and relevant.
What do you think about using paralleled 2SK170 in place of the LU1014, and SS R550 as the cascode FET? These are what I have too many at hand
I'm not sure R550 works well with just 20-40mA of current, but for CG operation maybe low transconductance does not matter.
The Stax SR-404 alone (without Stax amp) looks relatively affordable, cost ￥ 34,400 from amazon japan. With DIY amp I think it will look interesting.
Thanks for offering to drawing exact schematic! I will try to digest your work and draw a schematic by myself and ask for your opinion. More fun that way, right?

 

[Zen Mod]

Dear ZM,

......... More fun that way, right?

you can look here , too ( for comparison ) :

Donut a' la overkill - DIYAudio.rs

 

[Tea-Bag]

Well I ran my Mini-A with adjustable R8, did not lower drain resistor yet on IRF610s, using R100 for output transistor and IRF240 for CCS.
Last time I had it running, it was sounding a bit lean. This time it did not, as I removed the bias resistor, at 68k. Of course, it sounded much better, but with some lean sinks it was running 100c on the transistors. Oops.

 

[Jen-B]

The KGSS seems like a lot of work. I think you could manage that with a couple of balanced SJEP170R550's.

Well Nelson, if you ever have a spare moment and post that schematic you would have at least one avid reader!!!

...said Jen, avidly.

 

[Nelson Pass]

Ask and ye shall receive.

 

[Jen-B]

Ask and ye shall receive.

Wow that was quick! Thank you Nelson.

I'm quite new to the design side of things, but that looks like a long-tail pair differential mu-follower. Let's see if I can understand it right... it's going to take a typical 2V input and pump out 400+ volts for electrostatics so the gain will be high. I guess that the gain is first going to be derived by (source resistor R2 + internal rsistance of the fet Q1) divided into (high R formed by CCS Q3, R8). Then the gain is going to be reigned in and finalised by the feedback formed by R6 and R4. With equal and opposite signal on the other side of the pair. I hope that is right.

I guess that with a CCS in each upper leg there is no need for a CCS in the tail, R1. So perhaps this should be called a 'short tail pair'!

I thaink a pair of electrostatics don't take too much current so if the quiescent was reasonably high the percentage variation through Q1 and Q2 might be quite small which I presume is good for distortion. However if the quiescent were high then the power dissipation (hence heat) would be high because of the elevated voltages involved. No free lunch.

(Humm, well, at this point I think I better learn a bit more about fets before saying any more and sounding too dumb. So I'll hunt out a tutorial or two and get my reading hat on).

Thanks again.

 

[Nelson Pass]

Here's a revision. You'll need the extra gain.

 

[Zen Mod]

baaaad Papa , baaaad !



should we include tail CCS , toss input caps , then include input gain stage(s) , balance mech , toss output caps ,and make it SUSY ?

 

[masag1]

I received my 4 pcs from magura, and want to use them in my ZV9 as substitude for
cascode and CCS Mos-Fet.
Can I just change the Irfp240?

Any help is welcome.

Regards

Sam

 

[Tea-Bag]

I think they would do more harm than good as CCS. Keep the 240's.

 

[Tea-Bag]

As cascode, I think it will still take most of the sonic signature of the LU.

 

[HVfanatic]

Build an Aleph Jx, replace the lower fets with a Semisouth. Keep the upper mosfets... ;-)

 

[Tea-Bag]

yep. Have them running in a MiniA as lower Fet. Sounds nice. Working on a Aleph-JJ using one or two on the output.

 

[Nelson Pass]

Certainly you can save some money by using the Mosfets
on the top, but the best results I have had with similar
circuits were from a mu-follower and same parts. If you
match and/or tweak the values as the load line characteristics
will cancel pretty well. With dissimilar parts it's not quite as
good.

 

[masag1]

Thank's a lot for your advice

With kind regards

Sam

 

[knutn]

Semisouth depletion FET Amplifier

The Semisouth depletion FET SJDP120R085 seems to make it easy to make a simple output stage a la the First Watt J2 Mu Follower: http://www.firstwatt.com/pdf/prod_j2_man.pdf.


If Q4 should be replaced by a Semisouth Enhancement FET, the gain is lower due to the lower VGS.

 

[HVfanatic]

Hi Nelson,

your comment only applies to Aleph J, but in case of Aleph JX I would expect that the distortion would cancel. Am I wrong???

Dirk