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Old 21st December 2008, 10:44 AM   #61
juma is offline juma  Germany
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Quote:
Originally posted by apassgear

Why should you need an input cap?
If you look at the pic of the preamp you'll see no input cap - I don't use it, it's a matter of personal decision.
But I published the schematic and I feel responsible for well-being of less experienced member's woofers - people use diifferent sound sources - my TV, for example, puts out 3V DC
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Old 21st December 2008, 03:43 PM   #62
npapp is offline npapp  United States
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Hi Juma
Can you post a pic of the pcb traces? I would like to give your circuit a try but my attempts at laying out pcbs often resulting in loud humming
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Old 21st December 2008, 06:28 PM   #63
juma is offline juma  Germany
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Quote:
Originally posted by npapp

Can you post a pic of the pcb traces?
I used pcb layout from post #3 of this thread, just changed component values according to schematic in post #53 and output pair is BJT, not JFET (S=E, G=B, D=C)
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Old 21st December 2008, 08:09 PM   #64
lineup is offline lineup  Sweden
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Quote:
Originally posted by juma
After few months of experimenting here is the circuit that sounds best to me - it's the same topology as the first one in this thread but with BJTs in output and higher Id through JFETs (6ma).
It takes the +/- 24V from F5 (they are both in the same case - F5 as integrated amp)
juma,
Isnt 24 Volt a bit high for the 2SJ74 fet.

Had any issues?
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Old 21st December 2008, 08:16 PM   #65
juma is offline juma  Germany
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Lineup, if you look closely at the sch. in post #53 you'll see that JFETs are supplied with +/- 12 V
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Old 21st December 2008, 08:35 PM   #66
lineup is offline lineup  Sweden
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Yes, you are right.
= No problems
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Old 22nd December 2008, 09:23 AM   #67
estman is offline estman  Estonia
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Quote:
Originally posted by lineup

Isnt 24 Volt a bit high for the 2SJ74 fet.
Datasheet says Vgd=25V. Why are you concerned? I have reed here in forums, that this 25V is very conservatively stated.
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Old 22nd December 2008, 11:05 AM   #68
lineup is offline lineup  Sweden
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Good to know.
tanks
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Old 27th December 2008, 01:17 PM   #69
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juma-- do you think this preamp would work well built into an F4? and i have a B1 for volume control duty, could i just leave off the 10k pot? how would i implement that ? thanks bubba177
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Old 27th December 2008, 03:55 PM   #70
juma is offline juma  Germany
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Quote:
Originally posted by bubba177
juma-- do you think this preamp would work well built into an F4? and i have a B1 for volume control duty, could i just leave off the 10k pot? how would i implement that ? thanks bubba177
Not as it is because F4 needs ~22V peak input for full output. F4 already has built in JFET buffer so it doesn't need B1.
In fact, with circuit below you can lose input JFETs from F4 and connect the output of this preamp directly to point where C1 and C2 (F4 sch.) are connected.
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