Firstwatt F1J

Tea-Bag

Member
Paid Member
2006-02-08 2:32 am
Kennebunk
Nelson appears to have re-spun the original Firstwatt beast, designed as current amplifier, good for fullrange speakers. It got good reviews originally, but not often seen (IMO) as a diy project. I think it's worth discussing here.

The addition of the F1J with SiC JFet's replacing the IRFP240 output devices may breath new life into the circuit. A single stage amp I think as refered as 'the truth machine'.

Here is what Papa says.. (Paraphrased)
The F1J is an update to the F1 where the two gain Mosfet transistors have been replaced by new state-of-the-art power Jfet devices. In addition, there have been changes to several resistor values and removal of the input zener protection diodes.
You may notice that the F1J also has approximately 1.5 dB more gain. This is because the Jfets have a higher transconductance figure that the original Mosfets.

There is another factor as well, which is that the F1J has a lower input impedance than the F1- approximately 30 Kohm balanced and 15 Kohm single-ended. The modifications that help achieve this bandwidth take advantage of lower impedances, so the F1J is recommended for use with preamps with ordinary to low output resistance – tube preamps without cathode followers will work but will not reap as much benefit at high frequencies.

This caveat aside, the F1J delivers a dramatic improvement in current-source amplification. I have often ventured the opinion that improvements in measured performance in simple Class A circuits (particularly with no feedback) usually have the best correlation between what you measure and what you hear, and the F1J bears this out. I think it sounds about twice as good, and I feel certain that you will experience the same remarkable improvements.

Maybe Nelson will share some details for those with the amp, maybe not :p
 

Tea-Bag

Member
Paid Member
2006-02-08 2:32 am
Kennebunk
sounds like someone else's handwriting...
Nah, I figure it's a topic worth discussing. I'm not posting to sell Jfets (not many left). Or likely build myself, as my F2J is singing and somewhat happy. (and mini-A w/SicOUT)
But I like to advocate for further tweaking, and full range speakers in general.
I can't stop thinking and playing about with this stuff. But still following other's ideas at this point. Greedy and dumb :(
 
........ Greedy and dumb :(

welcome to the Club :rofl:

now you know why I'm happy all my life - perfect recipe

sounds like someone else's handwriting...
Nah, I figure it's a topic worth discussing. ......

certainly is .... but I couldn't resist ;)

dunno - I think that I'm going to check thing or two with SS Donuts .....

interesting - first Donut - Magura's ; he just refuse to even think about any sort of tweaking circuit itself ; maybe implementation of servo , or something .... but not even to try SS instead of cascoded LU's .....

who am I to dispute Metaldanske's impressions :clown:

I'm just saying that there is certainly something magical with these jfets in output ...... small LU or bigger SS , whatever .....
 

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
I would love to convert mine over! Anyone know how?

Step by step:

1) You match up a pair of SemiSouth R100's or equivalent and
drop them in for Q1 and Q2

2) You eliminate the Zener diodes

3) Raise the input cap values to 10 uF

4) Replace R1 through R4 with 22 ohms

5) Replace R5-R8 with 47K ohms

6) Set the bias pot to +14V on the Drains

:cool:
 

Tea-Bag

Member
Paid Member
2006-02-08 2:32 am
Kennebunk
Methinks not quite an Aleph if it does not get a turbo boost from an Aleph CS...:spin: very close though.

Papa sez that mu follower/SRPP/whatever works better with those hot critters ;

I have no objection , according to my own tests

anyway - who am I to dispute his words :clown:


I've tried R100's in Mini-A, BA-1 output with IRFP240 as CS. IRFP9240 cs with F2, and De-Lite running an En-Lite with R100 output. All hold a similar clean sonic signature. Very close IMO. The one with the most top 2 bottom spunk would be the F2j clone. The Other's sound a bit more laid back by comparison.