John Curl's Blowtorch preamplifier

[john curl]

Vogue, you are mistaken. The TEFLON is on the INSIDE of the case, not the outside. Look at a RAW case. Just little holes for the RCA connectors to stick out.
The fets we used are the 2sK389, NOT the 2sj389. Just a typo.

 

[anatech]

Hi John,

I can't tell YOU why you hear a difference. I designed BOTH topologies in the original Levinson JC-2 pre amp in 1973. Just LOOK at the schematic.

I didn't ask you to explain this to me. I was curious if you heard any difference.

Usually, a single differential input can be slightly quieter for a given open loop capacitance, BUT the overall distortion is about twice as much and the dynamic input current available (important in power amps), is much less.

See, this is helpful. Your experience is beyond mine here. I am self taught as well. I am not sure what you mean by input current though. I'm thinking a gate has only leakage current where your statement would make more sense to me if it were a BJT stage. Higher current = higher DC offset unless they are balanced and complimentary. So this is what I am trying to understand with your reply.

I know what I heard, your hearing may vary.

This is what I was curious about. You have not stated what exactly you heard or didn't hear.

Please, I am not asking you to explain anything but pure circuit operation, not subjective reasons why I may hear something. I think I will let you rest a bit. You seem to be on edge and my goal is not to upset you, but to learn from your own experience.

-Chris

 

[lumanauw]

Hi, Mr. Curl,

Is the complementary differential input Jfet (with R self-biasing) sensitive to rail changes?
http://www.diyaudio.com/forums/showthread.php?postid=1278517#post1278517

 

[mlloyd1]

i was taught to do just what charles suggested in one of his hints: stimulate the supply line(s) with an ac signal and see what the output does. either by simulation or measuring a real circuit. you can ground the inputs to study in isolation (initially).

mlloyd1

Hi, Mr. Curl,

Is the complementary differential input Jfet (with R self-biasing) sensitive to rail changes?
http://www.diyaudio.com/forums/showthread.php?postid=1278517#post1278517

 

[Vogue]

Vogue, you are mistaken. The TEFLON is on the INSIDE of the case, not the outside. Look at a RAW case. Just little holes for the RCA connectors to stick out.
The fets we used are the 2sK389, NOT the 2sj389. Just a typo.

I can of cause see that the teflon sheet is inside the case.

I was just trying research the importance of a air tight case – as a Teflon sheet could create a more air tight closure around the input/output jacks.

Vogue

 

[jacco vermeulen]

a Teflon sheet could create a more airtight closure.

You want to go swimming ?

 

[Magura]

You want to go swimming ?

I'd be real quiet about it if I were him....around here you get some rather unpleasent attention from the social services when talking about "taking a swim with a boat anchor"


Magura

 

[john curl]

There are 2 ways to seal a chassis. RFI shielding and air tightness. Both are important in the Blowtorch. We don't want an air flow across open loop semiconductors. They make extra noise when air currents flow by. The TEFLON block is a spacer. We could have used cardboard, but it would have generated increased DA and would not have been as stable over the years.

 

[john curl]

Another reason why we like a sealed chassis is to keep the air impurities from the kitchen, etc, from getting into the silver switches. It tarnishes them. They don't really oxidize easily, they sulfide! Keeping kitchen odors out is good. My kitchen is only 15 ft away, so it is important to me, for sure.

 

[Vogue]

Hi John,

Thanks for the info on the sealed chassis. I will try to implement this on my passive preamp project.

To jacco vermeulen

And no - I’m not swimming ... some evil people from Japan always try to harpoon me!

Vogue

 

[john curl]

The Blowtorch is MUCH MORE than a sealed chassis. The basic stability of the chassis, along with the highest quality materials that we could find, makes even more of a difference.

 

[anatech]

Hi John,

The Blowtorch is MUCH MORE than a sealed chassis. The basic stability of the chassis, along with the highest quality materials that we could find, makes even more of a difference.

I guess one could make an economy version. Just take a Hammond aluminum chassis, install the circuitry and fill it with expanding foam. An adhesive plastic covering would do for the visual aspect.

Just kidding.

-Chris

 

[MJL21193]

I can't tell YOU why you hear a difference... Just LOOK at the schematic.

The Blowtorch is MUCH MORE than a sealed chassis.

Please, I am not asking you to explain anything but pure circuit operation, not subjective reasons why I may hear something. I think I will let you rest a bit. You seem to be on edge and my goal is not to upset you, but to learn from your own experience

With all due respect,
I would pretty despertely need information before I'd go looking for it from the above quoted source.

 

[Vogue]

The Blowtorch is MUCH MORE than a sealed chassis. The basic stability of the chassis, along with the highest quality materials that we could find, makes even more of a difference.

Well for me it's a great inspiration to find the best solution for my own - less complex project.

The Blowtorch is an icon, but way to complex for me to copy and even to comprehend . And I will rather build my own project - with the nice inspiration and info I get from people on this forum.

Vogue

 

[jbaudiophile]

Hi, Vogue

And you are right !

It is a lot of fun, discussions, guesses from bad pics around, tests (and fumes !...)... But all entertainment, with a bunch of obsolete parts to find, even Xformers and diodes... And just guess what Mosfets are, too, and how they are placed, due to a real symetrical parts placement!

Still to find the Vertical Mosfets John refered about, but perhaps NOT for the PSUs... Seem to me MT100 cased, and here, most of them are Japanese and lateral... But I still dig !

Anyway, we don't really want to compete with John Curl, eh ? But the idea to go further up his own road, but far behind him... Just makes things interesting when he turns back and show us all the traps around, and also a direction...

Cheers, John & Vogue !

----------------------------------------------------------------------------
"Sic gorgiamus allos subjectatos nunc" - Charles Addams

 

[john curl]

Comrade audio builders, please do not try to exactly make the CTC Blowtorch. It is a waste of your time. Do what you can do, best. However, the Blowtorch is not a bad product, and it will stand the test of time. Just watch!

 

[anatech]

Hi John,
I agree with you on both counts.

Use the ideas presented here to improve your own designs, do not rest on a design that isn't yours and that is in the past.

The Blowtorch will be a joy to use for their owners for many years to come. But this design has come and time should bring forward some newer designs. With any luck, they will be better or suit a different goal.

How's that?

This thread should be used for discussion of the Blowtorch ideas and possible improvements or changes to suit newer components.

-Chris

 

[jbaudiophile]

Hi, Anatech,

For sure I disagree with you here ! On most points BUT ONE.

Can you imagine seriously that audio hobbyist that engaged the Blowtorch way would leave half way ??? For sure not !!!...

Would you still use a ceramic cap in the audio path ? For sure not. Or use a crimped ends carbon resistor in your best designs ??? Still not... Some big guys in audio wrote thousands lines about nasty phenomenon, and we all WOULDN'T learn from them ???...

More and more audio guys are just MIXING those audio big guys SHARINGS. Thanks to them all ! Pass over Curl over Hansen over lots of audio comrades.... How is the mix??? Depends on so much factors (test facilities, parts availability, hobby time, WAF...).

All of these "big guys" have favorite parts (Charles Hansen Lateral Mosfets, Nelson Pass Vertical Mosfets, and John Curl... Maybe a mix.... (LOL !)), but the more I LEARN from him, and the more I think that he experiments OTHERS WAYS (obsolete GREAT parts (like VN and VP...), UHC mosfets (as Nelson explained, if you put "Vertical" in the middle, "Lateral" are on one side, and UHC on the other side...).
Just my "2 cents" opinion...

I just love this "times" technologies mix. And John does it better that competiitors... Old parts weren't so bad (stocked here MPSUs), and new ones ball better... Just my 2 cents still...

But I FULLY AGREE with you that some competition is possible. I am quite sure that NO DIYselfer doesn't rank to his "preferred" audio engineer last schemes or direction idea... Or to be equal or better his "teacher" would have NO SENSE.

So, the Blowtorch is not a full goal, but the way John's traced is... Designs variations are around... I chose to "stick" as far as possible to Carl Thompson one...

But I have to leave here... Some white dressed men are around the house, for sure to ask me WHY I am trying my best to "stick" to some designs...

Jbaudiophile

 

[anatech]

Hi jbaudiophile,
Some of us prefer to strike out on our own sometimes. We design our own thing that may be similar to many different existing designs. It's very hard not to build one thing similar to many. We look to the experience of others to solve our own problems.

Favorite designers? I don't know about that. I'd have to look at all of John's designs, then Nelson's and so on. This before I could make up my mind. No, just comfortable design ideas from many.

A favorite designer sounds like a fan club to me. I'm not so much into fan clubs. It makes the object of the club uncomfortable (on average). I don't know that there is a "best designer" out there.

Do my germanium transistors count in with good old parts? I could use ceramic caps and carbon composition resistors to get that authentic 60's horrible noise.

-Chris

 

[jbaudiophile]

Hello, Anatech,

This time, I fully agree with you...

Just what I said. Many forumers mix down inputs from "big guys schemes" to get an original scheme, then try their best to find the right part to achieve it..

And yes, I am quite a Curl fan here... Simply because I started with Nelson Pass for amps, and John Curl for preamps... But very often their ideas just cross (both physicists, eh ? ). And you know that I have not been in a position to get in touch with other forumers for a while (still !). So, my own "reference engineers" list is rather on the low side !

For sure your germanium transistors count... Just happen to use some as low voltage diodes... And also carbon big resistors (Allen Bradley kind of)... Yeah ! They are NOISY if any voltage is developped across them .

But in my experience, they are valuable still... I just put them in Mosfet gate connection to prevent oscillations (10... 100 Ohms)... Have been working very fine for me... For amps final stage, and contrary to John's experience with low level.

I suspect that a lot of forumers have stocked "old" parts (vintage) that are still far over the top nowadays. Just try to find a low Vgs(th)
vertical mosfet (say, 1V...). Almost impossible to get (.but with lateral Mosfets). I tried those old thingies and they do quite audio miracles... I know that John stocked some now unavailable parts too, that even no "modern" parts equal...

Design is design. Can be inspired, or borrow from "audio comrades". Or just strict copy (clone). Uninteresting at most, and rather boring... The Blowtorch PSU is still a hobby work here. So much EMI/RFI traps. And ultra-low impedance too. Well... Funny to "double guess" John, as most parts are gone, and the Blowtorch just a design line. Hope to rank equal with the PSU soon (about 3 months to get CMCs, would you trust me ????).

All the best, Anatech

Jbaudiophile

PS : if you are still using ceramic caps, you can get a free bunch from here free ! LOL !