John Curl's Blowtorch preamplifier part II

[syn08]

Originally Posted by john curl

Yes they are, my fets cost me 15 cents. Learn and grow.

Ok, you had me with this one. 15 cents for a 2SK170, that's a really good price.

You have a unique opportunity to share with your fellow DIYers something valuable: your source/dealer, I am sure everybody would appreciate buying their 2SK170 at this price. If it's in quantity, I'm also sure somebody will organize a group buy.

For the rest of the technical details/reasoning above, I suppose you need to learn and grow yourself.

[john curl]

Everyone, I hope I can learn something from this latest exchange with Syn08.

[john curl]

Nobody in their right mind would use a k170 for a current source for a Norton equivalent. Too much Gm.

[syn08]

Originally Posted by john curl

Nobody in their right mind would use a k170 for a current source for a Norton equivalent. Too much Gm.

I'm not sure what's gm having to do in a JFET constant current source, I thought it's about Idss Anyway, then your Norton source is going to be noisy as hell, intrinsically much noisier than LM329. You can decouple/filter, but so can you the LM329.

OTOH, LM329 will have under 1ohm dynamic impedance (current independent), while your Norton thing will have probably the output impedance in the kOhm range. And your tempco will be as worse as the JFET (that is, very poor). Not to mention the precision, as good as the JFET Idss, like 6-30mA. And how about your "no compromise" approach?

Game over, I've already mentioned why arguing with you is pointless.

[john curl]

In my design GmR=10 WITHOUT the 100uf cap. What is the gain with Xc with frequency? It's easy, just plot it.

[ikoflexer]

I like the j201; costs me a bit less than 15c a piece.

[ikoflexer]

There's a nice book by Linden T. Harrison "Current sources and voltage references." I quote:

"In summary, the lower the drain current and the lower the output conductance, then the tighter the regulation will be. The closer the source resistor load-line intersects the zero TC point, the less the current source will drift over the intended temperature range, and the more stable and reliable your circuit will be."

[john curl]

Good input, Ikoflexer. Let Syn08 get around that! I am just a tired, old man, trying to help others.

[syn08]

Originally Posted by john curl

Let Syn08 get around that! I am just a tired, old man, trying to help others.

You are attempting to teach here history rather then audio design. Audio history is very interesting, though.

LM329 has a temperature coefficient of 0.01%/°C and 0.002% long term stability. Beat that with a JFET current source.

Relying on the J201 zero tempco point is an error. There's no spec and no guarantee for that (I'm not even sure if there is any, for this particular model). To stay into the field of audio, there's an old AES paper about, which, among others, shows how difficult (or even impossible) is to use the zero Tc point in JFETs:

FET Temperature Characteristics

Author: Sinclair, John C.
Affiliation: Zenith Radio Corporation, Chicago, IL
AES Convention:34 (April 1968) Paper Number: 557

[john curl]

Well, that is good, IF I needed it. Usually, the analog circuits that I make are not very dependent on the precise working voltage. They will work happily between several volts of DC power supply change. Simplicity, low noise, and no overshoot, are my main goals. Has been for 30 years. Seems to be important, still.