Welcome, Guest. Please login or register.

April 04, 2020, 10:33:33 PM

Login with username, password and session length

Recent Posts

Author Topic: How to bias a JFET preamp?  (Read 6101 times)

Conguito

  • Regular
  • **
  • Posts: 13
  • Chip Points: 0
    • View Profile
How to bias a JFET preamp?
« on: November 28, 2006, 09:53:06 PM »
Hello, at first place sorry if my english is not good but I hope that anyone can understand me well.......
I have seen many schematics of JFET preamps here in the forum and I am new at building preamps, that's cause I do this question. I don´t know what to do to bias correctly a preamp. Anyone can help me?
Thanks.
Salu2.

syndromet

  • Elite
  • ****
  • Posts: 197
  • Chip Points: 38
    • View Profile
Re: How to bias a JFET preamp?
« Reply #1 on: November 29, 2006, 06:09:32 AM »
To bias a JFET, you simply turn the trimpot connected to the power supply of the JFET. In most cases you would want to adjust it to the half of the supply voltage. If your preamp runs on 18v, you would, in most cases, adjust the trimot so that the supply voltage to the Jfet is 9V. I like to adjust the trimpots by ear. Simply plug in the guitar, get a string to vibrate, and adjust the trimpot untill you have the correct sound. There is usualy just a tiny part of the rotation that give you a nice sound without gating.

Conguito

  • Regular
  • **
  • Posts: 13
  • Chip Points: 0
    • View Profile
Re: How to bias a JFET preamp?
« Reply #2 on: November 29, 2006, 10:11:15 AM »
So the best option is to do the bias "with the ear"? Or it is the easy way option? It's better to measure with the Voltmeter till I have in the Jfet the half of the power supply voltage? I mean if it's better for a long life of the preamp or this is not critical?
Salu2.

tubebass

  • Chipper
  • *
  • Posts: 4
  • Chip Points: 1
    • View Profile
Re: How to bias a JFET preamp?
« Reply #3 on: November 29, 2006, 09:57:36 PM »
I usually scope the drain of the fet and set the bias so the signal clips symetrically.

syndromet

  • Elite
  • ****
  • Posts: 197
  • Chip Points: 38
    • View Profile
Re: How to bias a JFET preamp?
« Reply #4 on: November 30, 2006, 06:19:52 AM »
as long as the jfet is rated to withstand the supply voltage, setting the bias wont have anything to say on the lifetime of the pedal. i suggest you adjust the pot by ear, since the bias voltage affects the quality of the sound. just leave the pot in the possition where you think the preamp sounds the best. that is if you dont have a scope like mentioned above.

Conguito

  • Regular
  • **
  • Posts: 13
  • Chip Points: 0
    • View Profile
Re: How to bias a JFET preamp?
« Reply #5 on: November 30, 2006, 12:56:26 PM »
Well, in my workplace we have scopemeters and I could probe to bias my preamps with this method, it's a great idea.
There's the option to do it by ear.......finally I think that what we all want is that our preamp sounds good for our own ears and everyone has his own taste, so if it isn't critical to made it by ear I think that's the best method.
Thanks all for your help and your ideas :tu:
Salu2 from Spain.


ildar

  • Guest
Re: How to bias a JFET preamp?
« Reply #6 on: November 30, 2006, 01:42:26 PM »
What I usually do is get close to 1/2 supply voltage using a DMM, then finish adjusting by ear. If you have more than one or two jfets in a circuit, this method saves some time, for me anyway.

joecool85

  • SSGuitar Admin
  • Legendary
  • ******
  • Posts: 3230
  • Chip Points: 988
  • SSG Creator
    • View Profile
    • thatraymond.com
Re: How to bias a JFET preamp?
« Reply #7 on: November 30, 2006, 03:02:10 PM »
I would agree, by ear is the way to go.  Who cars if it looks good on a scope, unless it sounds good it doesn't matter.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

Conguito

  • Regular
  • **
  • Posts: 13
  • Chip Points: 0
    • View Profile
Re: How to bias a JFET preamp?
« Reply #8 on: November 30, 2006, 03:11:46 PM »
Who cars if it looks good on a scope, unless it sounds good it doesn't matter.
Yes, that's really what I mean in my last post, but I wasn't sure if this could be bad for the preamp, I know which is the sound who I like and if I can Bias the preamp like my ear likes it for me it's better so as if I do it with the scope. Now that I know that there are no risk to bias the preamp like I want.......my problem is resolved.
Salu2.