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login721

Need help with JFET preamp and chossing transistor .
« on: August 03, 2012, 11:14:20 PM »
Hello .I'm having some some problems with my current project .Can you guys help me please !

1.I build the JFET preamp.It's works , but cutoff too much low frequency.How can i fix it ?


2.This is the schematic of MXR headphone amp for guitar .I don't have 2n4401 and 2n4403 , so i replace them with 2n3904 and 2n3906.The result is output is too low .2n4401 and 2n4403 (collector current = 600mA) ,2n3904 and 2n3906 (collector current = 200mA).If i replace the transistor with higher Ic i will have louder output ???


Thanks for advance!
Sorry for my bad English!


Samuel Groner

    Zürich, Switzerland
  • Posts: 2841
Re: Need help with JFET preamp and chossing transistor .
« Reply #1 on: August 04, 2012, 02:02:45 AM »
Quote
1. I built the JFET preamp. It works, but cutoff too much low frequency. How can I fix it?

As shown it will not work reliably, because the gate of the second JFET does not have a DC bias path. Add a 1M resistor from the gate of the second JFET to ground (just as you have for the first JFET).

The output impedance of your preamp is pretty high; what load do you expect it to drive?

I'm not sure I understand the actual question correctly; do you say that the low-frequency corner is too low (which should not usually be much of a problem) or too high..?

Quote
2. This is the schematic of MXR headphone amp for guitar. I don't have 2n4401 and 2n4403, so I replace them with 2n3904 and 2n3906. The result is output is too low. 2n4401 and 2n4403 (collector current = 600mA), 2n3904 and 2n3906 (collector current = 200mA). If i replace the transistor with higher Ic will I have louder output?

No, 2N3904/2N3906 are a good replacement for this application. With just a 9 V battery, this headphone amp will not give very high output levels into high-impedance headphones. What headphones are you using?

In any case you should add small emitter resistors to Q1/Q2 to stabilise their bias point. 4r7-10r should be fine.

Samuel

login721

Re: Need help with JFET preamp and chossing transistor .
« Reply #2 on: August 04, 2012, 04:30:08 AM »
The output impedance of your preamp is pretty high; what load do you expect it to drive?

I'm not sure I understand the actual question correctly; do you say that the low-frequency corner is too low (which should not usually be much of a problem) or too high..?
I just find out the problem .I forget to install a capacitor .After installed , it's works fine .Thanks!


No, 2N3904/2N3906 are a good replacement for this application. With just a 9 V battery, this headphone amp will not give very high output levels into high-impedance headphones. What headphones are you using?

In any case you should add small emitter resistors to Q1/Q2 to stabilise their bias point. 4r7-10r should be fine.

i just use a small inear headphone .Something like this .
http://www.anythingbutipod.com/archives/images/sony-headphones.jpg
« Last Edit: August 04, 2012, 07:40:25 AM by login721 »

Samuel Groner

    Zürich, Switzerland
  • Posts: 2841
Re: Need help with JFET preamp and chossing transistor .
« Reply #3 on: August 10, 2012, 03:23:12 PM »
The easiest possibility to increase the gain of your headphone amplifier is to make R7 larger. Try 150k for about 10 dB more gain. C6 should be scaled inversely with R7, so about 47 pF in this case.

Samuel


 

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