Hamptone JFET mic pre help! - diyAudio
Go Back   Home > Forums > Live Sound > Instruments and Amps

Instruments and Amps Everything that makes music, Especially including instrument amps.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 18th May 2005, 03:46 AM   #1
diyAudio Member
 
Join Date: May 2005
Location: St. Simons Island, GA
Default Hamptone JFET mic pre help!

Hello! I have recently built a single channel Microphone pre amp from a schematic by Scott Hampton (hamptone.com). The schem was printed in an old issue of TapeOp. I had never built anything electronic before that was not a kit, and I really need help in laying out the PCB (actually I used perf board). I want to re-build the first channel of pre amp and then build a second for a nice two channel box. My first channel is rather noisy (ground hum) above 75% gain. I did not use an output transformer. It seemed like the output tx was not necessary.

Could anyone help me lay out this circuit? I have a link to the schem at the bottom. I have changed the LM78M24 to a LM78M20 after reading some things on Scott's website (please disregard the other notes I have scribbled next to the PSU!). I had alot of fun building this pre!

Please help me figure out where to put the components to lower the noise!
Thanks alot!

here is the schematic from TapeOp, designed by Scott Hampton -
http://rockstudio.com/jpg/JFPsmall.jpg
  Reply With Quote
Old 25th May 2005, 08:59 PM   #2
diyAudio Member
 
Join Date: Apr 2002
Location: chicago
Hello Rockstudio--
First, let me ask how you've grounded everything here, and what sort of input transformer are you using?

Second, let me recommend the Edcor WSM600/600 as an output transformer--it's inexpensive (about $8), and actually sounds ridiculously good, especially at that price. I think that, at that level of gain, output impedance might be a bit high, although upon second glance it looks as though the two NPN's are acting as buffers...
  Reply With Quote
Old 25th May 2005, 10:05 PM   #3
diyAudio Member
 
Join Date: May 2005
Location: St. Simons Island, GA
Thanks for your help! I am using a jensen JT-115K-E input tx. The schematic recommends a 600:600 output tx. I couldn't afford to buy the jensen model I found.
As for grounding, I have the power supply on its own small board with its own grounding lug. The pre amp has two grounding lugs, and I basically put straight wires to ground (for every cluster of components). I had never built anything from a schematic only, so I just used my imagination as to how the grounds should be connected.
The input and output jacks have short leads to ground as does the gain pot.
Is there a rule to follow when placing the transformer on the board?
Thanks Again!
  Reply With Quote
Old 26th May 2005, 02:58 AM   #4
diyAudio Member
 
Join Date: Apr 2002
Location: chicago
I'm just going over simple things in my head, so forgive me if I suggest something you've already tried.

Are you using twisted pairs for your power/audio leads?

Is your AC line in the chassis near any audio lines?

Do you have LED's or power switches located near any of the audio stages?

Am I correct in my interpretation that the input, output, gain pot, and audio stages all have separate lines to ground? I was reading and interesting bit of information today about layout and grounding. It talked about how, if you're running several wires to ground within the audio stage, the circuit will see all of them as very small value resistors. By returning all of these stages to a common ground point, there is negligible resistance that can cause voltage drops that appear in series with the signal. Then, what happens is those voltage drops get amplified along with the audio signal, and it shows up as voltage noise. Try running the audio stage grounding along a single bus wire, and then run that to ground at one point with a thick grounding cable, such as a piece of solder wick.

The reason I asked about the type of input transformer is because mic input transformers are susceptible to RF buzz if not shielded properly. Did you connect the transformer's shield to ground as well?

Hopefully some of these things will help, and you'll learn some stuff that will benefit you the next time you build one.

Speaking of which, I never realized how incredibly simple this circuit is! I think I'll build one myself, just for the sake of experimentation!
  Reply With Quote
Old 27th May 2005, 11:21 PM   #5
diyAudio Member
 
Join Date: May 2005
Location: St. Simons Island, GA
mnewport, thanks for your tips. I will try all of the above suggestions before I build the second channel. I was careful not too put anything AC near the pre amp board, and all of my lines are twisted together. I really want to make sure that I don't cause (inductance?) if I put something too close to a JFET or transformer. Thanks again! I think you should build on of these yourself, I have been using mine in the studio and loving it. I have to add some batteries for phantom power now...
  Reply With Quote
Old 16th June 2005, 08:38 PM   #6
diyAudio Member
 
Join Date: Jul 2004
Location: Johannesburg
Hi Rockstudio

There was some discussion about this unit in The Lab (where else! ) & a PCB layout. Have a look here :

http://www.prodigy-pro.com/FORUM/vie...4579c6490391f6

Regards
Peter
  Reply With Quote
Old 2nd June 2006, 06:29 AM   #7
diyAudio Member
 
Join Date: Jun 2006
Default Feedback for the JFET preamp and question...

Hello,
I just built one channel (so far) including a power supply for this Hamptone JFP design from that TapeOp mag article, too.

I have feedback for you as well as question(s) for anyone reading this...

First off, the most important thing I implemented was a solid "star ground" configuration. Star grounding is when you run all of your grounds back to a common point. Keeping them as short as possible and as large a conductor as possible, too. See my pictures as an example (link at the end of this message).

I used solder wick as the main ground-buss area that all grounds are tied to, which is physically located very close to the power supply.

Other than the grounding, of course keeping your audio paths as short as possible and shielded is important. Your chassis should be earth grounded (the third prong on your AC line cord coming in to the unit, if your powere supply is internal, that is) and isolate your signal ground from your chassis with a switch to lift or connect sig. gnd. to earth gnd. This includes the 1/4" jack for post input transformer connection, aka "DI".

Also, I did not use a PCB for all audio paths. More reliable than a PCB, more room for the parts to breath, easier to modifier, fix, revise, etc.

Shield your signal wires that travel more than a few inches.

Only use high quality coupling capacitors. Try the Solen Fast caps found at Antique Electronics > http://www.tubesandmore.com/

The problem that I am having IS THIS > My bias for the JFET is a little off. For example, as per the original design that Hampton issued, the JFET is biased in a manner that sets the drain voltage to ~17.5 volts. I don't know why, but it is off enough to cause severe asyemmetrical clipping. This is obviously unacceptable.
Anyone have any ideas??

Thanks~
  Reply With Quote
Old 2nd June 2006, 06:29 AM   #8
diyAudio Member
 
Join Date: Jun 2006
Default here is a link to my point to point version of Hamptone JFET preamp

http://home.comcast.net/~counterboun...tructframe.htm
  Reply With Quote
Old 2nd June 2006, 02:09 PM   #9
diyAudio Member
 
Join Date: May 2005
Location: St. Simons Island, GA
I finished my two channels, and I have a blast using it, although it isn't perfect, one channel is louder than the other and there is a fair amount of noise. I didn't use an output tx, but I did use Solen caps.
I would like to know how to set the bias for the Jfet, and the proper voltage at which to set it.
  Reply With Quote
Old 2nd June 2006, 07:51 PM   #10
diyAudio Member
 
Join Date: Jun 2006
I'll let you know soon, as I'm working on it. I've taken out my old text books and am going to breadboard a new version using the same part, so that if I do come up with a good, new solution, most people with this Jfet Pre (kit or not) can consider the new implementation for biasing the JFET. ...to be continued.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
JFet BOZ Tyimo Pass Labs 14 30th April 2006 03:18 PM
Can I replace BF245C JFET with J201 JFET? bigmike216 Parts 2 7th December 2005 06:50 AM
JFET and ZEN Skorpio Pass Labs 1 6th December 2005 09:30 PM
How to use Jfet? lumanauw Pass Labs 5 7th April 2005 03:12 AM


New To Site? Need Help?

All times are GMT. The time now is 10:04 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2