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Old 29th June 2010, 03:13 PM   #1
Tea-Bag is offline Tea-Bag  United States
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Default Firstwatt F1J

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The F1J is an update to the F1 where the two gain Mosfet transistors have been replaced by new state-of-the-art power Jfet devices.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
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Old 29th June 2010, 03:21 PM   #2
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not

ya didn't saw Papa's message : Happy Birthday Mr. Nelson Pass!

greedy boyz ....





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Old 29th June 2010, 03:37 PM   #3
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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
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Old 29th June 2010, 03:51 PM   #4
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Quote:
Originally Posted by Tea-Bag View Post
........ Greedy and dumb
welcome to the Club

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

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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

I'm just saying that there is certainly something magical with these jfets in output ...... small LU or bigger SS , whatever .....
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Old 29th June 2010, 03:57 PM   #5
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Maybe when one stop's tweaking, they are in Nirvana,
but maybe tweaking is Nirvana, or at least the path to Nirvana.
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Old 29th June 2010, 04:21 PM   #6
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Buda is everywhere





















under many different names

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Old 29th June 2010, 05:30 PM   #7
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woh...wait a min.

You mean to tell me that now that I joined here...PASS IS GOING TO RETIRE!!!

ZenMod...you're the next one in line I guess.
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Old 29th June 2010, 07:59 PM   #8
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.......PASS IS GOING TO RETIRE!!!

......
do not believe in everything you read at forums

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Old 1st July 2010, 01:21 AM   #9
eyoung is offline eyoung  United States
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Default F1j ???!!!

First I am going to change my name to ZenWood and then I am going to think about what I am going to do with all these IRFs I got... I know maybe I will build both. At any rate I want to see a proposed schematic for F1J maybe I can use my aging f1 clone boards...

Regards, ZenWood
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Old 7th July 2010, 06:22 PM   #10
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I would love to convert mine over! Anyone know how?
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