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Old 12th April 2007, 08:21 PM   #321
Jaac is offline Jaac  Netherlands
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Quote:
Originally posted by Nelson Pass


When in doubt, Plitron.

Or amplimo ?
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Old 12th April 2007, 09:00 PM   #322
Jaac is offline Jaac  Netherlands
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Quote:
Originally posted by Variac
OK, I searched, but I haven't seen a reference to what a reasonable VA rating would be for a single transformer to feed both channels of the F4 ....
maybe this one is a good option for you (stated in the USA)

toroid on ebay
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Old 13th April 2007, 04:47 AM   #323
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Thanks Choky, you did my homework
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Old 13th April 2007, 12:17 PM   #324
Darry is offline Darry  France
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Default F4 power amplifier

Mr. Pass,

I am very interested by your F4 Firstwatt amplifier , no feedback, no voltage gain. My tube préamplifier have an output level in extensively sufficient voltage (Unbalanced and balanced mode ) for a good working of the F4.

I have Martin Logan Vantage speakers. (sensitivity 92dB, nominal, impedance 4ohms)

The Vantage uses a 8" aluminum-cone woofer along with a 200W amplifier (active subwoofer) and a passive traditional electrostatic transducer. (Crosssover frequency 400Hz).

But this speakers have a minimal impedance of 1ohm to 20KHz.

I know that there is not a lot of energy to 20KHz, also, can you say to me if you made tests with a F4 on the low impedances. The working is it correct ?

What configuration of the F4 is preferable with my MartinLogan?

Must me to add other pairs of Mosfet?

Thank you for your answer

Darry
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Old 13th April 2007, 01:16 PM   #325
cviller is offline cviller  Denmark
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I have received some requests for my shortened layout, so I thought I could just as well upload it.

The space between transistors is 4cm and the whole board is 25cm long. I have only marked one mounting hole, but it should be possible to drill at least one more somewhere, so you can mount it as nice as steenoe did it!!

Component numbers are the same as papas, except for the prefix 1xx on one of the channels.

ps and pdf files along with a couple of notes:

http://viller.org/audio/2007apr_f4/
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Old 13th April 2007, 06:56 PM   #326
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Default Re: F4 power amplifier

Quote:
Originally posted by Darry
I know that there is not a lot of energy to 20KHz, also, can you say to me if you made tests with a F4 on the low impedances.

What configuration of the F4 is preferable with my MartinLogan?

Must me to add other pairs of Mosfet?
I have not tested the amplifier into 1 ohm, just 2 ohms.

For the ML's I recommend balanced drive with the stock
circuit.

If you need more current, perhaps increase the value of
the protection zener diodes.

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Old 13th April 2007, 08:14 PM   #327
Darry is offline Darry  France
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Default F4

M. Pass, thank you very much for your answer

Darry
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Old 13th April 2007, 10:05 PM   #328
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Quote:
I have almost the entire BOM for F4s, well, almost. The concern is that the input JFETs are nearly impossible to obtain in my country. I do have a bunch of BF244, BF256, 2N5461, 2N5486, a couple of 2N5459s and J111s and nothing else. Which of these can I use to get the F4 idea off the ground.
Anyone on this? I'd really appreciate inputs!
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Old 13th April 2007, 10:44 PM   #329
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally posted by Samuel Jayaraj


Anyone on this? I'd really appreciate inputs!

besides needed Vds (look at original used in F4) use ones with highest xconductance you can find.
even two in parallel if needed

hehe-if Babo can do with two certainly non complementary paired.........
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Old 14th April 2007, 12:27 AM   #330
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I confess that I'm still underwhelmed by the idea of using such different parts in the front end...but yes, music can be made to come out if you try hard enough.
I would suggest the use of a pot between the Sources of unmatched JFETs. Each end of the pot goes to a Source. The wiper becomes the output node. Set the pot such that the DC offset (relative to ground) is least. Use the smallest value pot you can get away with. If 10 Ohms will do it, then use 10...if it takes 50 Ohms or 100 Ohms, then so be it. If you're finding that you need something like 500 or 1000 Ohms, try different JFETs.
No, you won't get DC at the output--the DC blocking caps will see to that. But don't say that I didn't warn you if you test the distortion and find that it's much, much higher than you expected.
By all means, experiment, but do so intelligently.

Grey

P.S.: I'm surprised that no one has asked if you can use MOSFETs at the input. The answer is yes, but it will take a few more parts. If you want, I suppose you could call this Hint #3, although it's not necessarily headed down the same path as #1 and #2.
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