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Old 25th November 2011, 04:07 PM   #1
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Default JFET Phono Pre-amp clipping

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I built one of the above; I've started to notice a slight clipping/break-up of the sound at times when a powerfull vocal or horn-section kicks in (Otis Redding is the main offender).

I think it's the Phono-pre clipping.

I measured the drain voltage voltage of the two JFETS with no signal going in, they are sitting at about 7V.
I don't know much about design but I would have thought with a 24V rail you might want them near the halfway point to avoide clipping.

Does this make sense?. I used 2k7 instead of 2k4 Drain resistors I dont know if that might have caused the problem.

Can any of the more experienced tell me if I'm barking up the right tree?
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Old 25th November 2011, 08:15 PM   #2
mjf is offline mjf  Austria
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hello.
i would have a look at the input stage first..........
you can lower the drain res (r3), so the drain volts go up to 12v or so (with a 24v supply).........
or you can build in a source res (with only a few ohms, may say between 1........10 ohms or so) .
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Old 25th November 2011, 10:10 PM   #3
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I a test and a couple of sums and dropped the 2k4 (2k7) to 1k8 and got the drain voltage to float to 12v. It sounds excellent.
Otis is still breaking up on the high notes though. I wonder if it's somthing to do with the stlus alignment!
Thanks anyway

J
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Old 26th November 2011, 12:58 AM   #4
Irakli is offline Irakli  United States
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Insert R=50ohm resistor between the source of T3 (second transistor) and ground.
Also it will not hurt to insert 10-25ohm resistor between the source of T2(first transistor) and ground.

good luck.
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Old 26th November 2011, 07:23 AM   #5
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Hi Irakli
Thanks, but what would that achive that dropping the drain R hasn't?

Ta

J
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Old 26th November 2011, 01:34 PM   #6
DF96 is offline DF96  England
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Moves the gate voltage away from forward bias. These are junction FETs, so the gate should never go positive. The circuit runs them without bias, so it is sailing close to the wind.
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Old 26th November 2011, 06:42 PM   #7
mjf is offline mjf  Austria
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now i was curious about this minimalistic phono pre.
the Idss of my 2sk170 jfet on the sim was too high for that schematic, so i had to build in source res to come up with the drain volts into the half supply region......
and there is a second res (100k) at the input, because many mm-preamps used to have an input impedance around 50k.......
and a res connected to the gate against rf would be nice......
input cap c1 can be seen as a subsonic filter.......
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Last edited by mjf; 26th November 2011 at 06:47 PM.
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