F4 power amplifier

Earlier in this thread it was mentioned that the equivalents to the input JFETs are 2SJ74 and 2SK170, which I checked in my Toshiba databook to be true.

To the contrary of many people who complain these cannot be found, there are at least a few suppliers selling these.

This supplier sells them in small quantities and reasonalble prices:
http://www.tech-diy.com/smallsignal.htm

They also have low international shipping charge and accept Paypal.
 
The Company Store

I'm not endorsing and have no financial connection to... simply clarifying and relating past experience...

tech-diy.com is jackinnj on this forum. I've always had great service on the few orders I've placed with him.

Hope that helps.

Ryan
 
Idss

What Idss range should we be looking for? Or should the current be a certain amount through the JFETS, then source resistors adjusted. Does the Idss even make a difference when used as a follower?. With a range of Idss JFETS the current could run from about 2 to 8 ma with 22 ohm source resistors. (If I read the data sheet correctly).

Would'nt it be nice to have all those resources, time, measurement equipment, Oh and talent.

Thanks Nelson for another way to use amplifiers.

BDP
 
The one and only
Joined 2001
Paid Member
The choices for the JFETs is pretty flexible, as are the decisions
about the Source resistance. Clearly you want to have a
voltage rating comparable to the supplies (in this case about 40V)
and the self-bias current should be set with an eye on the
maximum dissipation.

:cool:
 
Audiofanatic said:
Nelson Pass made and want to shear with us

Maybe i'll get to see that marines cut and shave after all.
We have been behaving badly, but it's all Jack's fault.
(i feel more for Priscilla, Q of the Desert II. Co-starring with Hugh Dean, Greggy B and Anthony H)

Still waiting for the one who desires to go four F4s bridged and in parallel.
The F4 should receive the 2007 easiest and most flexible amp award, imo.

I've been so busy reading and re-reading the SA/4e owners manual that i forgot to say thank you to Mr Nelson for the big effort to put up many pages for download to please a few.
Birthday feeling revisited.
 
> Now we just need somebody to desigen a differntial input for bringing up the gain and bridging two units X-ed ofcourse.

That is actually very easy.

Just remove the frontend JFETs and substitute that by the Blowtorch Clone circuit on the Solid State Forum. No need to X, because it has no global feedback.


Patrick
 
I think theres a diffence in the basic Blowtorch schematic and the F4 Schematic. In the blovtorch the signal is taken outside the diff input. Where the F4 modulates the centerpoint between the input pair.
Using the Blowtorch would result in something like the Ayre V3. Amplifer. (may still be good)

I may be wrong as i have no theroretical beckground so please bear with me if I confuse matters.
 
Nelson Pass said:
The choices for the JFETs is pretty flexible, as are the decisions
about the Source resistance. Clearly you want to have a
voltage rating comparable to the supplies (in this case about 40V)
and the self-bias current should be set with an eye on the
maximum dissipation.

:cool:

First of all THANKS!! for sharing this. I cant wait to hear it! :D

In the datasheet I have for 2sj108 it says 25V - is that a typo in the datasheet or are you referring to 40V across both the transistors?

What is the maximum voltage either one of the jfets will be exposed to?

This is my datasheet for 2sj108:
http://www.alldatasheet.com/datasheet-pdf/pdf/30516/TOSHIBA/2SJ108.html

What would the sonical impact be from using jfets with less transconductance in this configuration?
 
The one and only
Joined 2001
Paid Member
It appears to be a matter of meeting a gate leakage spec,
which is not of particular importance here.

On the Toshiba's I see 1 uA leakage at about 46 volts, which is
rail-rail on that amp.

If you use some other brand, you might want to test the parts -
or you could pick another part. If I run into issues, I'll change the
part or consider some sort of limiting.

:cool:
 
The one and only
Joined 2001
Paid Member
cviller said:
What would the sonical impact be from using jfets with less transconductance in this configuration?

I wouldn't expect much of an issue. We are mostly trying to
drive the capacitance of the Mosfets and the resistance of the
bias circuit, and the JFETs are there for that. Since I don't see
much increase in distortion at the top end, I'm sure you have
some latitude. You can increase those 10K resistors to 15K,
but when I tried that I didn't see much improvement.

When the amp has no load, the distortion drops a lot, so
clearly the Mosfets are the elephants on the dance floor. I
think you could sub in a lower transconductance.

:cool:
 
Thank you so much for these details!

There are plenty of cheap and more importantly "in-stock" jfets out there, if high transconductance is not too important. :D

Transconductance is probably more important in circuits like the babbelfish (improvised aleph j).
I'm planning on doing some listening experiments with different jfets in that circuit to find a nice sounding replacement for 2sj109.