JFet Preamplifier - Help needed.. - diyAudio
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Old 25th November 2013, 02:21 PM   #1
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Default JFet Preamplifier - Help needed..

Hello everybody!

I the past i've heard a preamplifier with JFets.

The original schematic was designed for phono preamplifier with passive RIAA equalization between the two main gain pcb.

The passive RIAA equalization is provided by R24 - R25 & C18 - C19 network.

In the preamplifier that i've heard the line input was bypassed to the 2nd pcb after the RIAA network.

I'm interested of constructing a line preamplifier with this schematic.

The original JFets was the E231 from Siliconix ( Q1-Q2), the Q3-Q4 are the BC182, Q5-Q6 are the BC212, Q7-Q9-Q10-Q12 are MPSA56 and Q8-Q11-Q13 are the MPSA06.

I couldn't find the E231 but i've got the U231 dual Jfet and all the other transistors.
I made the schematic on Multisim, everything ok on the sine but there is a distortion on the square pulse of 1Khz..

Could someone help about the components needed to complete the schematic as line preamp? Input-output filter maybe ? Some mistake as it is now ?

All the components values are on the Multisim file..

Thanks a lot!!
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File Type: jpg Siliconix.jpg (162.1 KB, 317 views)
Attached Files
File Type: pdf U231.pdf (165.9 KB, 34 views)
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Old 25th November 2013, 03:04 PM   #2
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Sine and square 1Khz
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File Type: jpg square.jpg (121.8 KB, 266 views)
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Old 25th November 2013, 04:43 PM   #3
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Your amp is ok, it only shows oscillation becouse you switch from sine to triangle to square wave on-the-fly...start simulation with square wave...and put some load on output!
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Old 25th November 2013, 04:53 PM   #4
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Well thank you very much for your interest!!
I didn't knew that!

What about input filter ?
Line input directly to Q1 ?

Everything ok with the bandwidth?

If everything is ok i'll start the pcb layout..

Last edited by mot on/off; 25th November 2013 at 04:56 PM.
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Old 25th November 2013, 05:04 PM   #5
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Yes, you have to place an input filter (e.g. 1k/33k//100pF)...but gain in your amp is way to high for line preamp (32x) you should aim for something like 4-6x and compesate for reduced gain, otherwise amp will not be stable...put a small capacitor over R18 (like 10-33pF) and play with the C11 value.
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Old 25th November 2013, 05:13 PM   #6
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Yes the gain is that high because of the original Phono gain preamp..
It would be better that R29 is about 4~6K..
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Old 25th November 2013, 06:34 PM   #7
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My information, from a 1986 Siliconix data book, says the E231 crosses to a J231. Was the E231 a dual?
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Old 25th November 2013, 07:13 PM   #8
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Hello,

Your information are correct indeed!

From the Siliconix Data book the E231 cross the J231 single n-JFet.

I couldn't find either E231 or J231.

Linear Systems have a Cross Reference catalog that had both of them crossed to LS842 Dual Jfet. (page 9-10)

U231 dual Jfet also cross LS842 and it was the only one from Siliconix that i could found after a lot of effort.. (I took it for the matched pair)

E231/J231 could be replaced from BF244/BF245 and 2N5458 i think..
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File Type: pdf Linear Systems Cross Reference.pdf (322.2 KB, 25 views)
File Type: pdf Siliconix Cross Reference 2.pdf (108.4 KB, 29 views)
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Old 26th November 2013, 04:29 AM   #9
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You are correct, I use the BF245C FET in my guitar preamps - but in a much simpler design than yours.With a minimum drain current of 12 mA, you can replace Q3-7 with resistors and keep the current sources Q8 and Q11 - but scale the current requirements accordingly to drive the driver transistor Q10 - which will drive the push pull amplifier pair Q12 and Q13, using simple output transistors like the BD139, BD140 to drive headphones and the output amplifier.
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Old 26th November 2013, 12:26 PM   #10
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So in case that i choose the 2nd gain pcb for a line preamplifier, and adjust the R28-R29 network for a x4~x6 gain (1K-4K/6K) i still need help on the input/output filters..

Big thanks in advance!
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