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Old 6th June 2007, 07:15 PM   #41
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Can all resistors be 1/4w ?
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Old 6th June 2007, 07:18 PM   #42
Netlist is offline Netlist  Belgium
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I wired up a PtP version with 30V, a 6ohm source resistor and a 1.2k drain resistor.
I connected a 600ohm headphone, one channel and I'm currently building the other channel to be able to listen properly.
FET is the BL version and bias is 7.3mA.
The gain is more than adequate for normal listening levels.

/Hugo
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Old 6th June 2007, 07:24 PM   #43
lineup is offline lineup  Sweden
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Quote:
Originally posted by Nelson Pass
Since the question has come up and not answered, the
situation with the gain, bias, and resistor values is as
follows.

The device is self biased, and we want the highest value of
bias possible for lowest distortion which makes us want to
use a low value Source resistor to ground.

At the same time, it would be nice to not have too much gain
by virtue of that Source resistor value. The apparent resistance
of the JFET is about 50 ohms, so the gain into a 10 K ohm load is
about 1000 / 60 which is about 24 dB.

We can get lower gain, but the distortion will increase - it's
all a trade-off. I didn't make any effort to exactly optimize these
values - they were just thumbnail calculations.


thanks for information
... I was wondering.
But you know these JFET and MOS better than me, Nelson
... for the time being. (what is your personal interest, you will know about)


I have had only some practical experience of
when the JFET gain was in order of 0.5 x gain of one Bipolar TO-92.

In this case you tell the gain is like 16.67, where a bipolar would have ~90.00
which makes as low a coefficient as: ~ 0.2

I understand this is dependent on several things, and most of which actual JFET we would use.
Very much Like when we would use different TUBES, in same sort of setup.
-----------------------------------------


x16, +24dB, is quite alright gain for a preamplifier.
It will be more allround amp
,
especially when signal comes from Sources
with lower output voltage than CD-players.
Say IPOD, MP3 and whatever they use nowadays.


Sorry, if I confused anyone here.
I just want to be supportive.
Now, we know a bit more, in this special case.


Thanks
lineup
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Old 6th June 2007, 08:02 PM   #44
Manu is offline Manu  Europe
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Quote:
Originally posted by twitchie
Very nice Manuel

Can anyone please explain what the pot is for and how/what to adjust?

I thought it was just the input attenuator.

Stephen
Thx
You beat me.
I'am still at etching.

manu
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Old 6th June 2007, 08:21 PM   #45
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Quote:
Originally posted by Manu


za nas francuze ne postoji strah
he he he

sorta James Brown mode :
>now you got me <

off JB mode


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Old 6th June 2007, 08:26 PM   #46
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Quote:
Originally posted by Nelson Pass
.... The apparent resistance
of the JFET is about 50 ohms, so the gain into a 10 K ohm load is
about 1000 / 60 which is about 24 dB.
....
Ahem ... 1000/60 would be more like 16.6667

... but I got no clue about calculating audio circuits, so forgive me if my comment is stupid

cheers, LC
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Old 6th June 2007, 08:36 PM   #47
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Quote:
Originally posted by lovechild


Ahem ... 1000/60 would be more like 16.6667

... but I got no clue about calculating audio circuits, so forgive me if my comment is stupid

cheers, LC

20log(100/6)
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Old 6th June 2007, 08:43 PM   #48
lineup is offline lineup  Sweden
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Quote:
Originally posted by lovechild


Ahem ... 1000/60 would be more like 16.6667
... but I got no clue about calculating audio circuits, so forgive me if my comment is stupid

cheers, LC
You are 100% correct.

Now in these circuits, the actual values are never 100%
the '1000' bit maybe be +/- 5%
the '60' bit, may be +/- 10 %

so we should need to use, (in 99 cases of 100 in audio circuits),
~ xxx
Just about anything we can imagine or can not think of
could effect the circuit a bit, more or less

so mathematically correct would be to say:
~16
... or say:
Voltage gain from input to output will be like 16, give or take some 15%

On paper we could get some figures.
In reality we can measure the Actual Case

For example one transistor marked BC550C
can have a HFE Gain between 420-800
This is what the specification from the manufacturer tells!

is like +/-30 %

Same with one transistor JFET like 2SK170.
It is wide spread tolerances we talk about.

We can make a good approximation on paper, in schematic.
If we, like Nelson Pass, has got many years experience of Audio Reality we can make a fair guess. But still is a guess.


Regards
lineup
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Old 6th June 2007, 08:51 PM   #49
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Quote:
Originally posted by vdi_nenna
Can all resistors be 1/4w ?
It's an honor to answer the first member of Diyaudio.

Yes, 1/4W is plenty.

-=Hugo=-
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Old 6th June 2007, 08:54 PM   #50
Vix is offline Vix  Yugoslavia
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Hi,

I just want to thank Nelson once again for this precious gift. This preamp is just great. No,I didn't build it yet... Even though I'd like to do it this weekend, there are some priorities...(Secret revealed: I am getting married on 1st of July!!!). So I will consider this preamp as a wedding gift from Nelson!

Best Regards

Vix

p.s. Steen, where are you? Secretly building it?
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