John Curl's Blowtorch preamplifier

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jacco vermeulen

jacco vermeulen

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Joined 2004
Al,
the cost of an extrusion mold of this size is immense.
My bet is on aluminium sand casting, and CNC machining finish.
Those 3 boys do love teflon.
 

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Terry Demol

Terry Demol

Member
Joined 2002
pinkmouse said:
I'm with Pavel, really, really ugly wiring.

Oh, and that chassis looks like a U shaped extrusion with top, bottom, and back plates to me. Good idea though. :)
Click to expand...

The bottom is not a separate plate. If it was, it would not have the
milling cutter witness marks on it but a rolled or linished (brushed) plate finish.

jacco vermeulen said:
Al,
the cost of an extrusion mold of this size is immense.
My bet is on aluminium sand casting, and CNC machining finish.
Those 3 boys do love teflon.
Click to expand...

I'm not so sure. Sand cast aluminium is quite porous and does not
hold a really high finish as would be required on the front panel.

I wonder if JC is around for a comment?

Cheers,

Terry
 
C

Christer

Member
Joined 2002
Actually, when I think about it, there are plenty of similar cast alu boxes available from my local electronics supplier. The price gets a bit high for large boxes, but nothing extreme. The largest box I found was about $100, but is probably a bit smaller than the Blowtorch one.
 
J

jbaudiophile

Disabled Account
Joined 2004
CTC Blowtorch...

Hi, Jacco

Yes, the CTC Blowtorch is really a full machined aluminium block... with the back and top as exceptions.

Just see here :
http://www.tgaudio.com/pictures/BlowtorchSilverInterior.JPG

Machining marks are clearly visible on the left side panel, and the front inside corners are just "rounded". Ahem... The birdview just shows... the case bottom... Bob Crump emailed me that he wanted the maximum structural rigidity... and also to build a full Faraday cage....

The Teflon boards PCBs were also CNC routed... And "six nines" silver cables used... Sadly enough, the low pic resolution doesn't justice the CTC Blowtorch. But, as an evidence, only 40 Blowtorch were planned to build, so not any advertisment needed...

From the only "technical" papers I could read about it, the Teflon boards were mandatory because of a full open loop running, and the Line amplifier is (only ?) 350 Khz bandpass... Seems to me that it is complementary Jfets driving Mosfets in a folded cascode way... I read twice John Curl's early works with Matti Otala - about TIM distortion - as it seems to be a good starting point for understanding the whole design...

And guys : any paper (revues,...) or (preferably internal) pics to share with your European cousins on the Blowtorch ? Would be of interest for some of us (Darry, Jacco, and ... I). Thanks !
 
SY

SY

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Joined 2002
SYclotron.com
Just chatted with JC. Yes, the case is indeed milled from a solid billet of aluminum. Pantograph lettering. $3000 manufacturer's cost for the case alone. All FET, no feedback, servoed. First stage is 4 quadrant comp diff, second is folded cascode using MOSFETs run at high current. Servo with roughly 1 sec time constant.

Input FETs are unobtanium from Toshiba. Feeder resistor in folded cascode is 5W Holco at 200 ohm.

I've been fortunate enough to see, touch, and hear one. Very nice. But I'll stick to tubes.
 
SY

SY

Ex-Moderator
Joined 2002
SYclotron.com
Nope, when John described the casework process to me, I estimated $1500 per case based on similar stuff I've spec'd with carved aluminum molds. He said, nope, it's $3k, but there's two cases. I asked, isn't that $1500 per case? A second or two of quiet and a, yeah, I guess that's right.;)
 
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