John Curl's Blowtorch preamplifier

[jacco vermeulen]

Forr,
have you tried anything in line with the ideas of Gerard Perrot ?
(as Dutch, we know Hephaistos too)

 

[john curl]

I agree that the best open loop DC stability is the best way to design preamp stages. My design will work without any servos, but it would have some output offset due to the input matching, which is seldom perfect in fets.
As far as exactly how I make my particular design, I would rather not publish it here, or anywhere else, for that matter.

 

[Christer]

As far as exactly how I make my particular design, I would rather not publish it here, or anywhere else, for that matter.

No need for apologies John. You already share genereously of your secrets, much more than could be expected.

 

[Justcallmedad]

Grey, (and others),
I have 2 separate servos in each line amp, because it has differential outputs. One servo tries to keep the average of the two outputs near ground, and the other compares each output and adjusts them to be close to each other. The servos must be very low in frequency cut-off in order not to effect any asymmetrical low frequency transient that might occur on program material...

OF COURSE !!! The two differentials outputs cancelled in AC.!!! Thus the audio signal at the input of the DC-Servo compound is already very small!!!

 

[forr]

Hi Jacco Vermeulen
--Have you tried anything in line with the ideas of Gerard Perrot ?--

I would say I am working in a direction inspired by Perrot, but slightly different, as I came to different conclusions : I am looking for an inherently DC stable input stage. I admit this may appear somewhat pretentious.

~~~~ Forr

§§§

 

[00940]

Speaking of servo, what about this one ? http://headwize.com/projects/showfile.php?file=gilmore3_prj.htm

It works by adjusting CCS but its range of operation is rather limited.

 

[wrenchone]

I would think you would want the servo operation range to be limited. It should only be used to trim out the last bits of offset/drift after a proper job of manual tweaking. If the servo operating range (and bandwdth) is limited, it should not able to cause much trouble.

 

[courage]

John,

First of all thanks for your valuable contribution to this forum.

Two small questions:

a. What power amp matches the Blowtorch best?

b. Have you ever considered a battery power supply for the Blowtorch and if so, why have you chosen not to use it?

 

[john curl]

We normally use the CTC with the Parasound JC-1 power amp, which is also designed by CTC. Batteries would be impractical. It would take 6 auto batteries.

 

[Terry Demol]

We normally use the CTC with the Parasound JC-1 power amp, which is also designed by CTC. Batteries would be impractical. It would take 6 auto batteries.

John,

2.2 or 7 AH 12V gel cells work very well for preamps. We have used
them for years for our own personal ref designs.

Cheers,

Terry

 

[carlosfm]

John,

2.2 or 7 AH 12V gel cells work very well for preamps. We have used
them for years for our own personal ref designs.

In the middle of that there's 4 A/h, I use these on my phono pre.

 

[john curl]

Folks, I need at at least 1/2 A at +/-36V. How long do you think that these puny little gel cells will last, both short term and how about long term? I already have some experience in this, so don't just guess.

 

[jacco vermeulen]

at least 1/2 A at +/-36V.

In short, i read that as that more can be achieved with the combination of such circuits with high current needs and regulation than modest currents and batteries ?
Or nothing beats cubic amps ?

 

[carlosfm]

Folks, I need at at least 1/2 A at +/-36V.

Ok, forget it.

 

[Justcallmedad]

Folks, I need at at least 1/2 A at +/-36V.

You need +/-36V to supply a line stage??? By the way which are the specs of the Blowtorch? Vin max, Vout max, THD, IMD, noise, Zin, Zout etc. I just remember some posts speaking about a 350 KHz open loop bandwidth, and no balanced inputs.

 

[john curl]

The specs are not very spectacular. Perhaps .01% distortion at 2V out or so. Output impedance is fixed at 1000 ohms. Input impedance is whatever the volume control pot is.

 

[GRollins]

I always find it amusing when people try to push batteries as though they're some new technology that John, Nelson Pass, Charles Hansen, et. al. are somehow ignorant of.
Perhaps I'm a little off the beaten path, but I don't find 36V rails all that remarkable. I've got a line stage here at my elbow that uses +32 and -10V rails. Come to think of it, the unregulated rails start at roughly 40V. If I had used a complementary differential--as I assume John has in the Blowtorch--then the rails would have been symmetrical at 32V each. It wouldn't be all that difficult for me to justify increasing the rails a couple of volts. I would be reluctant to decrease them, though. Particularly, if it was just to shoehorn the design into an arbitrary voltage that happened to be available from batteries. Surely there are better ways to arrive at a target rail voltage.

Grey

 

[Justcallmedad]

I have just seen the all the photographs indicated in the post 32!!!, I knew only those of the former posts. Wow !!! Dual mono power supply pcbs!, nice really nice, about the Vendetta or the line amp don’t you like CMS resistors, no good sounding?

 

[Peter Daniel]

I had a short period of fascination with batteries, however, presently I prefer well designed AC PS over the power from batteries. It simply sounds better.

 

[john curl]

Peter, I cannot say that batteries are in any way inferior to a properly designed AC supply, but batteries are problematic in their own reliability limitations. It just isn't worth it.