John Curl's Blowtorch preamplifier

[fab]

Originally posted by john curl
...use difficult to find or obsolete fet's. It seems that only Erno Borbely and I have any stock of these fets and Erno sells them to audiophiles at a high price. ...

Check also at Ampslab for low prices:
http://www.ampslab.com/components_fets.htm
The service is excellent!

Of course, the most difficult part is to have matched jfets. Erno Borbely can provide matched parts but the price is even higher. However, it you need only a few fets to build a super preamp or driver of power amp, the price for these fets compared to other parst to build a complete equipment is still low in comparision in my opinion.

Fab

[jbaudiophile]

I guess that John is refering to 2SJ72 and 2SK147 (both V type ?)... Toshiba developped them for its own AUREX range... The brand no longer exists, and this parts are no more in production.... But the stock lasted for a while !

And NO, Erno Borbely has no longer any of them, and his current claim that he still have some stock to sell is absolutely wrong... I said him a couple times, but it seems that at moments he just forget to delete them from his current parts price list, again and again... I just know that because... Erno unmounted parts (V type) from some of his early prototypes to send them to me...

And for other John's schematics, some parts are really lacking. So, cloning is not possible for most of his previous designs.

No more special TN and TP0606N5 around, nor 2SJ75 and 2SK240, nor original J110... Sadly enough ! Some of these parts had very good specs at the time and still are amongst the best ones I used. And I spared some of these good vintage Japanese parts all along with US Motorola MPSU series and some others...

IMO and nowadays, similar Japanese parts are no better that old fashioned ones, but they are mostly monolithic duals in lieu of separate parts... Hence far better matching, and far easier to match...

But Darry is right that great topologies from John and others and some local experiments lead to great results with little parts cost... It's a pity that some Taïwanese just commercially copy original but no more in production Nelson stuff ! Legally, it is no counterfeiting... But those guys din't rob only schematics but prevented some thousands to discuss them for a while... So, IMO, they are only morrons !

Jbaudiophile

[Justcallmedad]

I worked a little more on the PSU that I proposed a few posts preceding; the schematic is a little bit different.

I built it in a prototype form:

Vout is +15V /-15V, Zout around 2 Ohms, input noise rejection about 70/80 dB until 1.5 MHz beyond my measures are not very significant as you could see on the photos.
I don’t use the common mode choke yet.
Noise is 6uV in the range of 10 Hz. - 300 kHz.
Load Regulation is 0.05 % in the range 0-150 mA.
Regulators uses Fred Dieckmann circuit to improve noise and PSRR.
Some measures could be not very significant taking into account the wiring…

As you can see…

PSU_Test1

PSU_Board

Apassgear:

For the power current sources I prefer BJT’s over Mosfet’s’, bias is simpler and less noisy in this case, parasitic capacitances are also lower, for the shunt elements I prefer Mosfets, lower gate currents and higher “channel” separation and drift, higher capacitances are not important at this place, but I can be wrong. I would carry out complementary tests later, maybe with other devices, at the moment I am quite busy in my job.

Jfet’s are 2SK170, 2SJ74, current sources are mostly devices like J508 to J511.
The problem for some fets and current sources could be the Vcc max around 25V for some of them in the case of Vout upper to +/-20V Medium power BJT’s are 2SA968/2SC2238, because I have them on hand, probably more recent and more powerful devices would be better.
Mosfets are 2SJ334/ IRL9Z34 and IRLIZ44 also because I have them since some work on cap multipliers and followers, I guess that IRF530/9530 could be o.k as they are easier to get.

[Ultima Thule]

John,

did you guys ever consider relays?

Cheers michael

[Agisthos]

JC what sort of hook up wire are you using in that preamp? Anything weird and wonderful?

[john curl]

A few simple answers to earlier questions:
The Vendetta preamp now has C, D, and T mods, since B.
It is now almost impossible to get Toshiba fets in matched pair. I have used k240, K146, k389, and their complements in most of my designs. To specify these now would be a problem.
Many of my less expensive designs use relays. Relays are OK, but silver on silver switches are better.
You have to ask Bear about the teflon silver cable that we use in the CTC. It is VERY expensive.

[Justcallmedad]

Rectifications on my last post:
Load regulation is 0.5% (typing error)
(measures are done without output caps in // to the shunt element).

John, have you tried cotton insulated silver wire ?

[jacco vermeulen]

Originally posted by john curl
I have used k240, K146, k389, and their complements in most of my designs.

Mmm, better make that 2 with a small stock of fets, + J73/J74.
BL and GR versions of K240, K389, and J74 can still be found, V-versions are more difficult.

[Ultima Thule]

John,

actually, I didn't expect you to bring up the contact material thing if else I didn't got you wrong here, is that the main issue to not chose relays.. well, I'm not a relay expert but imagine relays can be obtained with any kind of contact material and fashions, I thought you would answer something else, what's your take?

Cheers Michael

[Upupa Epops]

Why, John ? I saw many quite " black " contact in switches...But ofcourse, silver is " astral " metal and silver bulet you can use against vampires...