John Curl's Blowtorch preamplifier

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#683
I imagine that Zinsula's schematic is actually closer for several reasons, not least of which being that a straight cascode will relieve the input JFETs of having to deal directly with the rail voltage. This will limit the output voltage swing. There are other points, but I'm pressed for time and that's enough to be getting on with.

Grey
 
#684
Justcallmedad said:


JFets running at Idss, good! 🙂

But why "double" cascode ??? :bigeyes:
Click to expand...
Grey is correct, if you want say a voltage swing of 10V you need to set the sources of Q9-Q11 at >10V. If you assume an Idss of 10mA, the JFEt without the Q%-Q8 cascodes must dissipate >100mW (a double JFET like the 2SK389 will get >200mW) and that is quite a lot.
Of course the voltage swing could be limited....

Tino
 
#685
zinsula said:

Grey is correct, if you want say a voltage swing of 10V you need to set the sources of Q9-Q11 at >10V. If you assume an Idss of 10mA, the JFEt without the Q%-Q8 cascodes must dissipate >100mW (a double JFET like the 2SK389 will get >200mW) and that is quite a lot.
Of course the voltage swing could be limited....

Tino
Click to expand...


it is really a problem? Moreover the SK389/SJ109 are infact doubles SK170/SJ74 rated at 400mW...
 
#686
Justcallmedad said:
it is really a problem? Moreover the SK389/SJ109 are infact doubles SK170/SJ74 rated at 400mW...
Click to expand...
Maybe I got the Datasheet wrong, as it is stated 200mW "Drain Power Dissipation", so that means per 2 ?
Assuming 150°K/W (my guess, did not find it in the datasheet) junction to ambient, with 400mW we would get a junction temperature of 85°C at 25°C ambient, which is 40°C below the rated 125°C.
So, you may be right.

Anyone?

Tino
 
#688
Getting there

I have been reading this post with high interest. The pages devoted to volttage reg design made a good read.
Now this looking at the circuit has me thinking. It sure looks a lot like the X-BOSOZ.
Got set of unused TP boards in the closet. This looks it should be easy to convert. I suspect the actual BT runs close to the X-BOSOZ, 80 volts top to bottom, at 160 ma per channel. This looks like it should scale down in gain, output swing, bias current, and rail voltage more sucessfully.
Got to like any design using 2SK170/2SJ74, or 2SK246/2SJ103 as input devices.
Looking foreward to see what ends up here.


George
 
#689
Dear all,

If I’m not wrong, in the thread X2_ugs3,4 (http://www.diyaudio.com/forums/showthread.php?s=&threadid=54826) in the Pass forum a topologies quiet similar has been discussed. This means that 3 of our top designers have rather common view on the subject.

In this particular case I have the feeling that using (sharing) the same power supply for both + and – input side of the circuit will be better because the current drawn should be constant.

Newbe thought!

Best regards.
Philippe
 
#690
plep said:
In this particular case I have the feeling that using (sharing) the same power supply for both + and – input side of the circuit will be better because the current drawn should be constant.
Click to expand...
In my eyes one of the big advantages of such a balanced design. So you can even get away with regulators like open loop emitter followers, which have a highish output z. Those are very flat in frequency response up to high frequencies, they do not show this "inductive" behaviour of all the feedback regulator stuff.
 
#693
john curl said:
It is OK to publish this schematic on this website, as it is only a generic description, which I stole from Charles Hansen, anyway. This might make things more clear, as to what I was trying to achieve with the Blowtorch preamp. Thanks Charles.
Click to expand...

Hey, no problem, John! All of my ideas are just variations of your ground-breaking work 30 years ago, so turn-about is fair play.

When you published the schematic for the JC-2 in The Audio Amateur, it was like manna from heaven.

http://www.marklev.com/JC2/jc2schematics.jpg

Then a few years later you gave us another dose of brilliance:

http://www.marklev.com/JC3/jc3schematics.jpg

Thanks for everything -- you rock!
 
#694
Charles,

Good to see you back again and thanks for the schematics, but the remark John made was about a schematic of the BT line stage I sent him ta ask for his approval to post it here.

Nevertheless, and since your this generous, it would be great to see YOUR schematic John based his BT line stage on. Not asking too much, am I 😉

Regards
 
#695
The basic topology we used in all of our products for many years was similar to this:

http://www.diyaudio.com/forums/attachment.php?s=&postid=334963&stamp=1077770119

As you can see, it is a variation of John's circuits from 30 years ago. The main differences are that it is fully balanced and also zero-feedback. I believe that John was inspired by the knowledge that a zero-feedback design could work well, both measurements-wise and sound-quality-wise.
 
#698
If you look closely you will note that the posted circuit has DC feedback. So in this case, DC offset was not a problem.

To avoid the audible degradation caused even by DC feedback, all of our later products avoided this. Getting the offset to stay within reasonable values is somewhat tricky. Most of the issues were explained by Erno Borbely in some articles in Audio Amateur in (I think) the 1980s. There were two 100 watt MOSFET amplifier projects, one with a servo and one with DC coupling. I think they were called the Servo 100 and the DC 100.

The DC offset (over time and temperature) of those second generation preamps was usually within about 10 - 15 mV, while the power amps were usually within 50 mV. This is quite good, but I am always looking for ways to improve things. Our newer products have a modified topology that allows the DC offset to be kept within a few mV (over time and temperature) quite easily. This is all done with true DC amplifiers. There are no servos and no DC feedback.
 
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