John Curl's Blowtorch preamplifier - Page 9 - diyAudio
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Old 18th January 2006, 07:42 AM   #81
Netlist is offline Netlist  Belgium
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Quote:
Originally posted by jacco vermeulen

Only trouble is how to tickle it off his lips.
Showing some respect for both the designers and the design is primordial.
This seeming masterpiece has surely undergone more research, trial and error and cooperative brainwork than we ever can imagine.
The 350khz bandwidth and carefully selected components sure must have been part of the fun and challenge to build the Blowtorch.
I see the end result of tree talented people who have pushed themselves to the limits of their knowledge to achieve this goal. Every single component including the wiring scheme must have a reason to be where, how and why it is there.
Stunning controversial beauty. All too much criticized with envy and the un-will to look and step beyond boundaries.
For a moment, I very much would have liked to be the Cabernet that has been drunk when it played his first notes of music.

Also, its not because a design is discussed here that the author is obliged to release all the info, schematics and building plans to the community.

/Hugo
 
Old 18th January 2006, 11:33 AM   #82
Darry is offline Darry  France
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Hello,

To use DC servo or not ?

A DC servo for many designers limits itself simply to the use of an OP (with some caps and resistances) connected between input and output.

John Curl, him, uses, for his DC servo, various very complex and very cunning configurations of several OP as in PLD1500 and PLD-2000 Parasound preamplifiers. Already, in the 1980s, the schematic of Dennesen JC-80 showed a very particular DC servo with 1 single OP and 1 dual OP .

There is and there will be always, good and bad audio conceptions, and good and bad DC servos.

For me, John Curl, with Nelson Pass, Charles Hansen and some the others, belongs to the elite of the audio designers.

Darry
 
Old 18th January 2006, 12:35 PM   #83
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Bob Crump of T.G. Audio visited. Bob showed the CTC Blowtorch preamp, deserving of a meeting by itself. CTC stands for Curl-Thompson-Crump, indicating the design is a collaboration with John Curl and Carl Thompson.
The case of the unit (actually two, one being a power supply) is milled from a solid billet of aluminum to minimize resonances and can be built to the customer's needs, with options such as a phase switch. A phono stage similar to the Vendetta Research (designed by Curl and Thompson) will be available soon as a built in option. It is sold direct only. It should offer serious competition to other no compromise designs.

www.chicagoaudio.org/newsletters/cas0499.html

John Curl, a well known electronic designer, notes in "L. A. Audio File", July 1998.
"........We finally get into the 90's. Digital is everywhere, analogue seems to be retiring into the backwash of obsolete technology, and computers are getting more interesting. The early 90's are hard for phono stage designers, and I gave up making Vendetta Research phono stages. There was little wrong with the Vendetta and we had an ongoing operation, but nobody was buying them! My techs went to Clearcom (to make professional intercoms) and I quietly closed up shop. It is now 1997, and everybody wants a Vendetta phono stage. It is ironic, but this is the way audio world sometimes works.... This is a sad situation, but many of these manufacturers are more interested in making money than giving the audio community a shot at making better audio products in general..."
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Old 18th January 2006, 02:18 PM   #84
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Quote:
Originally posted by SY
Hi, John, glad the forum issue is resolved.

Thanks for giving me the circuit info. Can you say anything about the power supply?

Guess it's elaborated Masao Noro/Kubota style (cascoded?) super-shunt regulator.
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Old 18th January 2006, 06:04 PM   #85
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Now, folks! Let's not start 'second-guessing' the power supply as well!
I think of the power supply as doing several functions. First, it must separate the channels from each other. IF you don't use a separate regulator for each channel, you will have extra crosstalk on this design. Therefore, each circuit board and each stage (in my design) has a series open loop regulator. There are 8 series regulators in the CTC preamp in the picture.
The other chassis contains 4 regulators Two series, and two shunt. It also has a passive PI network to block RFI and keep it off the ground return.
Now, after almost 40 years, why have I done things this way?
Well, 40 years ago, usually the ONLY regulator would be a single series type for the entire preamp. This was true with AMPEX audio pro recorders, or Dynakits.
Mark Levinson introduced the first +/-15V potted module active linear regulator to audio in the LMP-2 preamp. I used this approach, BUT I had to quiet the inherent noise of the regulator (zener stabilized remember) with an active low noise cap multiplier. This seemed to work great for years, BUT I realized that it introduced considerable (xtalk) between the two channels, because of the relatively high output impedance of the open loop regulator, especially because Mark added a 2 ohm resistor to the output for current protection.
The JC-2 had a problem with imaging. It was great for mono, but stereo was not as good as some other products. I traced this partially to the power supply buffer.
For Vendetta Research, I had to use a more sophisticated approach, in order to make it extremely low noise, and not contribute a sonic 'character' to the circuit. I found that ANY aluminum or tantalum cap that was used directly across the input circuit power supply would change the sound. Therefore, I had to find a quality film cap that would work OK. The caps are prominent in the picture of the CTC. This also forced me to use all fet followers, and remove any bipolar devices. Does anyone know why?
 
Old 18th January 2006, 06:18 PM   #86
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On an audio DIY forum, it is natural that there are quite many intervening electronicians, passionately involved in audio, who constantly scrutinize any piece of schematics, ideas, topologies, circuits, components, etc... When they are told that, for a product, some options were choosen for technical good reasons (this is the basic rule af any design, isn't it) they expect these reasons should be made clear.
If the so-called good reasons stay mysterious - it is the prefect right of the engineer not to unveil all his secrets - these too curious people have a very sceptical reaction, and their unkindness can be on the par with the one treating them as having silly ideas : they wonder why such claims are made in the forum, suspect some bluff and consider them just as free advertisements for a commercial product.

~~~~~~~ Forr

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Old 18th January 2006, 06:38 PM   #87
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Originally posted by john curl
This also forced me to use all fet followers, and remove any bipolar devices. Does anyone know why?
Yes, but I know that you will kill me if I tell....


 
Old 18th January 2006, 06:45 PM   #88
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Therefore, I had to find a quality film cap that would work OK. The caps are prominent in the picture of the CTC. This also forced me to use all fet followers, and remove any bipolar devices. Does anyone know why?
At that time was not easy to find higher value film caps.
FET can be biased with higher value resistors and
these can make an effective filtering together with lower value film caps.
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Old 18th January 2006, 06:48 PM   #89
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Originally posted by Charles Hansen
Yes, but I know that you will kill me if I tell....
So what are you waiting for? TELL!

se
 
Old 18th January 2006, 06:55 PM   #90
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Not sure I got the regulator topology right, but could it be that a JFET can be biased from the (lower voltage) regulated side, where the lower ripple makes a smaller cap sufficient in the biasing?

What's the price, by the way? A Blowtorch?
 

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