John Curl's Blowtorch preamplifier

[dimitri]

May I ask you guys, if BT is this overpriced piece of crap in heavy useless chassis with ugly wiring and crooked Teflon plates inside, based on false principles and made from obsolete semiconductors, why there are more than 20 hundreds posts here?

 

[anatech]

Hi Dimitri,

May I ask you guys, if BT is this overpriced piece of **** in heavy useless chassis with ugly wiring and crooked Teflon plates inside, based on false principles and made from obsolete semiconductors, why there are more than 20 hundreds posts here?

Because it isn't. As John said, the chassis added greatly to the cost and customers in that price bracket expect exotic construction.

I'd have to say that John & Co. did target there buyer pretty well. I do have an idea how much time it takes to hand select parts, so by comparison of other quickly made units, the price is not out of line.

-Chris

 

[john curl]

You guys have little or no idea of thick aluminum shielding, whether it works or not. You are like young college students attacking the instructor.
Since I personally own and use a CTC Blowtorch, and know that the shielding works, why am I attacked for doing it? You don't have to do it. I may not do it again, myself. It costs too much, but it certainly works OK.

 

[PMA]

Some of us do
I tried several times here to explain principle of eddy currents shielding.

John, look at this picture to see input connector (BNC).

For Cu, frequencies and skin depth:

50 Hz 1 kHz 10 kHz
9,3 mm 2,1 mm 0,66 mm

Skin depth would be a thickness of material when mag. field component is decreased to 37% of original level.

 

[rdf]

Looks like a few uplink amps I've seen. Slick.

 

[PMA]

This is not an amp - or - there is an input amp section (with realys). Not audio.

 

[john curl]

Your shielding looks thick enough to me. My aluminum shielding is about 10mm thick on the front, top and sides. The back is only 6.5mm. Am I in trouble?

 

[PMA]

Mine is 8mm on all sides. I need Al because Fe, mumetal etc. saturate, as my instruments work in the vicinity of 40kA/50Hz test currents. Saturated shield makes troubles.

I am sure you are not in trouble. For 60Hz AL, the skin depth is 10.9mm. Your 6.5mm would decrease 60Hz to some 50% of original intensity. Faster mag. fields will be decreased much more.

For AL:
60Hz .... 10.9mm
1kHz .... 2.67mm
10kHz .... 0.845mm
1MHz ... 8,45172E-05 mm

 

[john curl]

Now would it be better to have used aluminum foil and plastic? Would have saved money, that's for sure. How about conductive nickel coatings?

 

[PMA]

Ni + Fe coating might be an excellent idea for you, because audio works in normal conditions. The mumetal has skin depth of 0.247417 mm at 60Hz


I cannot use it because of quite low value of saturation intensity of mag. field. The mumetal has mag. field saturation Bs = 0.6T

 

[john curl]

Mu metal is non-linear. I don't like to use it for that reason.

 

[PMA]

Yes it is. And it is not very good, as it can induce voltages when non-linear.

The Nickel itself might be good, as rel. permeability might be in hundreds. I will try to find out.

But the nickel has worse resistivity. It looks that skin depth of nickel might be something like 4mm at 60Hz. It is about 3x better than Al, but nothing to call home about. There are different nickels, though.

 

[john curl]

How about silver? Too expensive?

 

[PMA]

Regarding skin depth, just between Al and Cu.

 

[john curl]

I think that silver should be considered, with a gold plating. It would look good, and shield well. The gold plating has to be fairly thick in order to not have any pinholes. That would ruin the finish.

 

[Tazzz]

How about Zamak?

 

[bogdan_borko]

Kryptonite?

 

[Vogue]

Today, I would like to talk about the 'Blowtorch' philosophy.
Even when this thread first appeared, many misunderstandings of the overall design were expressed, and many aspects of the design might have even been laughed at. I started inputs to this thread, after some misunderstandings were expressed by others, in order to get things right.
First, the Blowtorch is an expensive to make design. However, it is not that way because we wanted to use rare metals or do a particular metal sculpture piece that would look elegant in a beautiful listening room.
The parts that we used to make the unit were both expensive, relatively large, with real force needed to turn any of the controls.
We did not design the moving parts, but they were designed for professional/military applications where no mistakes in position or transmission could be tolerated.
These parts had been selected by my former business partner Bob Crump and put into a working chassis. However, in Bob's first prototype, only a j-fet follower was used after the TKD pots. It was essentially a buffered passive preamp, with minimal input switching and dual volume controls, in order to NOT need a balance control as well. Someone, somewhere, still has this preamp and is probably very happy with it.
However, Bob saw a 'limited' need for a preamp like this, but with a few additions:
First, it should supply balanced output drive, because many power amps were being designed with balanced input in mind. This would make it easier, as well to separate the preamp from the power amp and not pick up too much interference from the cable itself.
Second, it should supply some forward gain, so that the pot has more real range. More than 6dB, but less than 20dB was decided upon.
Third, we would custom make each unit to meet the customer's real needs. We could make more inputs, add a phono stage, buffer the outputs for long cable runs, or even provide completely balanced input-output operation of the line inputs, WITHOUT any extra stages or other compromise. I.E. no added IC op amps in the audio signal.
Fourth, we wanted a no compromise, absolute polarity switch, that added virtually nothing to the signal but inverted absolute polarity.
We, of course, wanted one for ourselves, and that was part of the deal.
My part of the deal was to make the line amp module, and I decided to try to make it without global feedback. This was a first for me, except for j-fet followers in various circuits.
I decided to emulate what Charles Hansen had originally done, but to NOT use the output stage. I was also using the example of the Audible Illusions line stage which uses a single open loop tube as a line stage. Just think, I had to use 8 fets to emulate what 1 tube can do! Still, this offered some advantages. More simplicity than Charles' design and no coupling caps like in the AI design.
Since we were going to make a 'world class' design like an F1 race car, we chose to use Teflon circuit boards, polystyrene and teflon caps, and the best connectors, wire, solder, whatever, so there would be little or no compromise.
I know that many of you still laugh at the very idea that wire, solder, etc can make any difference, but they do, and we have known this for decades. We were not going to compromise the design, just because some young engineer working in another field primarily, doesn't have the experience with this, and thinks it is silly. I was 'silly' too, once, even when I developed many of the topologies that are in use today. It is in fact possible to be a good engineer, yet be a little wet behind the ears, in hard experience.
When we went forward with the project, we found a machinist who could make the chassis for us. It had to be aluminum, copper or silver. These were the only acceptable metals that would heat sink properly and not add extra distortion. We chose aluminum for obvious reasons, it didn't easily tarnish, it was cheaper than silver, and it apparently is easier to work. We originally tried welding the aluminum chassis together, and one prototype was made, but we decided that just hogging out a solid aluminum block was really the better, more consistent way, and we knew that the kind of customers that we would have, outside of ourselves, would worry over small imperfections in the chassis, etc. Of course, this all added to the price.
Perhaps I will say more, later on this project.

Interesting ...

I 'm building a passive preamp inspired by Bob Crumps choise of parts for the Blowtorch. So its quite fun to know that the Blowtorch in fact started out of from a passive design.

I now have almost all the parts (from the Blowtorch) in place, and is starting on the chassis. The chassis will be made from aluminium sheets - between 6 and 40 mm. thickness (6 mm for the back panel, 20 mm for top and bottom and 40 mm for front and sides).

John do you have any info on the J-FET follower used in the passiv design of the Blowtorch - or any other info on the passive design. Could be fun.

Vogue

 

[GRollins]

At first blush, this might seem a bit off the wall, but what are the EMF blocking characteristics of a foamed metal? By analogy, foam rubber is a better absorber of sound than a solid, flat surface of the same material. Light is absorbed better by a rough, matte finish than a smooth one. Granted, foamed metals are not standard construction materials, but if they behave in a similar manner, it might be useful.
No, don't ask me where to get such a thing. The only thing we've got here in SC are endless miles of pine trees and vines.
In the same vein, there's a classic physics experiment where you take a hundred razor blades and bolt them together. The individual cutting edges are bright and shiny, but clamped together side-by-side, they are pure black. Light enters the group of edges and begins bouncing. Each individual reflection is 90-odd percent efficient, but after thirty or forty reflections, the light is completely absorbed. Clearly, this is related to wavelength, so RF wavetraps would have to use larger wedges and 60Hz would be difficult to justify, but if a more absorptive surface were used (the shiny razor blades are used only for the Gee Whiz factor of using a shiny surface to absorb) then the wedges would not have to be as deep to give high efficiency absorption.

Grey

 

[Vogue]

I was thinking about corian and other materials in that category, but never found any data on them. I used corian for my DIY turntable - where it worked well (both design and sound).

In any case a solid chassis is proberbly better than a welded chassis or a chassis of aluminium sheets bolted together. But still bare aluminiums sheets bolted hard together should be okay - I hope.

Vogue