flaheu

Hamptone JFP based preamp is "motor boating" at hi
« on: December 17, 2007, 06:43:16 PM »
OK, I've built a 2 stages JFP preamp DI.
After days of fight to find the correct ground tha kills all the annoying noise I tried tu push the preamp to max but when I pass the 3/4 on the 10K log pot between the 2 stages it starts to "bo-bo-bo-bo-bo".

Im'm using OEP mic input TX wired 1/6.45.

This nasty preamp seems full of gain, maybe I've too much gain ?

EAch Jfp block is matched to have 12VDC at its output.

Guys, do I have to reduce gain ?

Exept that this preamp is really a nice one, plenty of "body" as a bass Di it's  wonderfull.

Thanks for your comments.
As MagnetoSound said:
"Sad lack of balls in today's bands, IMO"


rodabod

Hamptone JFP based preamp is "motor boating" at hi
« Reply #1 on: December 17, 2007, 07:27:55 PM »
Are you using the shielding cans over the transformers? (first things first...)?
Quote from: tv
punchy fat bastard chip

Vetsen

Hamptone JFP based preamp is "motor boating" at hi
« Reply #2 on: December 17, 2007, 11:29:13 PM »
I have built three channels of the HJFP.  Well two in the form of mic pres and another one in the form of an overdrive effect for EL GTR.  The PCBs were purchased from Scott Hamptone in the very early days when he offered partial kits.

The mic pres have OOdles of gain.  Too much for me.  I built them without a pad because I didn't think i would need it.  It was my first experience with a fixed gain preamp.  I find it useless without the pad.  I always reach for it too.  I'm just too lazy to open it up and solder a switch and a few resistors in there.  Oh well, someday.

As for the EL GTR overdrive, I had way too much output until I installed a resistor in the 1-5 meg range on the input of the second stage to GND.  It put everything into place.  I know that your application is a bit different, but I just wanted to let you know my experiences with this circuit.  

The two schematics are the same from my memory (preamp and overdrive) aside from the extra resistor I described, and a pot on the output.  I also used electros for C4 and C6 my schem.  (DC blocking caps on the output of each stage) bypassed with a small value polly.  One of my guitar playing freind/clients wouldn't give it back after playing through it.  

For a DI application, I would imagine that you would only need one stage.  I have never tried it though.  Give it a shot, see what happens.

Hope that helps.

Vetsen
"The smoke from the flux is filling my lungs..."

maxheadroom

Hamptone JFP based preamp is "motor boating" at hi
« Reply #3 on: December 18, 2007, 03:26:03 AM »
maybe a stubid question, but what power supply do you use?
i have the same problem with this. mine is a hamptone mic pre /instr pre.
nearly the same as the fetboy circuit. for the output i use a OEP transformer (input and output are with shielding cans). this has a lot of gain!
with guitar the loud crackle ( its the bo-bo-bo) sometimes occourred.
but with bass with a little bit higher gain its there.

for reducing the gain i put a resistor between output from the first stage and the pot on the input of the second stage.
i must fix some problems on the pre. my intrument input jack is not isolatet with the case, and the power supply is a cheap "switching power supply"   :shock:  
i used it for testing the schematic.
freddy get ready, it's time to rocksteady

flaheu

Hamptone JFP based preamp is "motor boating" at hi
« Reply #4 on: December 18, 2007, 03:54:14 AM »
Yes i use the shield can over the OEP but my output tranformer is an edcor 600:600 WITHOUT SHIELD.

Do I have to put a serie resistor after the pot between the two stage ?
My psu is linear 24V dc .
As MagnetoSound said:
"Sad lack of balls in today's bands, IMO"

maxheadroom

Hamptone JFP based preamp is "motor boating" at hi
« Reply #5 on: December 18, 2007, 04:23:25 AM »
i put the resistor before the pot.
my suspicion is that the crackle is a oszillating. i did my own layout, you can see it here:
http://gasman.lima-city.de/diy/fetboy_wired_rev1_picexport_.JPG
http://gasman.lima-city.de/diy/fetboy_wired_rev1_schematic.JPG
its a long time since a made my own layout :-)
i assemble the board without the pad and without the phantom. the resistor before the pot is hard wired and not in the schematic.
freddy get ready, it's time to rocksteady

clintrubber

Hamptone JFP based preamp is "motor boating" at hi
« Reply #6 on: December 18, 2007, 04:52:33 AM »
Motorboating can be stages influencing each other by means of a (common) power supply.

flaheu

Hamptone JFP based preamp is "motor boating" at hi
« Reply #7 on: December 18, 2007, 05:05:00 AM »
Clintrubber,

Is it possible to"isolate" the 24V between the  2 stages ?

The 2 stages are directly wired at the psu output.

Thanx
As MagnetoSound said:
"Sad lack of balls in today's bands, IMO"

clintrubber

Hamptone JFP based preamp is "motor boating" at hi
« Reply #8 on: December 18, 2007, 05:09:15 AM »
Quote from: "flaheu"
Clintrubber,

Is it possible to"isolate" the 24V between the  2 stages ?

The 2 stages are directly wired at the psu output.

Thanx

I don't recall the exact circuit-details, but it'll never hurt to isolate stages from each other by simple and adequately dimensioned R-C sections.
And it might probably even solve the motorboating, but that's hard to tell beforehand.
So giving each separate gain-stage an R-C power supply smoothing section obviously filters the supply further and also isolates stages (w.r.t. their power supplies) from each other at the same time.
The R inserted in the positive supply line is of course not a cure for imperfect grounding schemes, these can still influence (mess up) things.


Bye,

  Peter

flaheu

Hamptone JFP based preamp is "motor boating" at hi
« Reply #9 on: December 18, 2007, 05:26:53 AM »
thanks, I will try this out this evening
As MagnetoSound said:
"Sad lack of balls in today's bands, IMO"


clintrubber

Hamptone JFP based preamp is "motor boating" at hi
« Reply #10 on: December 18, 2007, 05:33:51 AM »
Quote from: "flaheu"
thanks, I will try this out this evening

I hope it solves your problem. If not then it'll at least filter your supply a bit further. You may need to play a bit with R & C values.

pantsonfire

Hamptone JFP based preamp is "motor boating" at hi
« Reply #11 on: December 28, 2007, 06:32:28 PM »
Did you ever get this sorted out? If so, what worked?
Cheers.

flaheu

Hamptone JFP based preamp is "motor boating" at hi
« Reply #12 on: January 02, 2008, 09:02:23 AM »
first happy new year everybody

Hello there, sorry to reply so late, I didn't have time for DIY these last weeks

finally, the "separate" 24v on each stage helped a little, but I must say that my wiring was not that right and not very tight, but finally with proper grounding it's OK.

This little beast is f***ing awesome for DI, that's for sure and for bass drum miking it's just wonderfull (fat, punchy & dirty lows).

But with a active bass the 1st stage distort much too quickly,

I would like to add a pad to the DI, but I don't know how to do it without "disturbing" the impedance and all.

Anybody can help me with the math ?

Thanks
As MagnetoSound said:
"Sad lack of balls in today's bands, IMO"

maxheadroom

Hamptone JFP based preamp is "motor boating" at hi
« Reply #13 on: January 02, 2008, 09:15:09 AM »
maybe you can use the pad from the mic input for all two inputs.
instead of the Gate Resistor of the first stage?

btw, did you put a series resistor between the two stages?
freddy get ready, it's time to rocksteady

flaheu

Hamptone JFP based preamp is "motor boating" at hi
« Reply #14 on: January 02, 2008, 04:58:57 PM »
maxheadroom,

No I didn't put the serie resistor between the stage, but I think I'll try.
because I can not turn the gain knob to full clock wise without having weird noises.

I'll post when i'm done
As MagnetoSound said:
"Sad lack of balls in today's bands, IMO"

maxheadroom

Hamptone JFP based preamp is "motor boating" at hi
« Reply #15 on: January 03, 2008, 05:27:15 AM »
i will try to connect a other power supply. in the past i had the bad switching supply (for trying out). now i will try a universal power supply that have 2 different pos. and 2 different neg. voltages (from 2 to 37V), plus the phantom.
frank from nrgrecording gave me that layout. so, i can supply the stages with two different supplys.
the other thing that i am thinking is to put a peak indication led on the output stage, to handle the level. but first of all i must search a useful circuit for this application.
freddy get ready, it's time to rocksteady

maxheadroom

Hamptone JFP based preamp is "motor boating" at hi
« Reply #16 on: February 12, 2008, 05:56:48 PM »
today i finished my jfp with two different +24V power supplys.
i must check the "mototboating" with hi level later when i'm in studio.
also i want to reduce the noise. for mic input its sure to terminate it with 200 ohms, like the impedance of a dynamic mic.
but to test the DI input (HiZ), which terminate resistor should i use to simulate an pickup? is there a recommended value?
the input resistor of the circuit is 10M in DI condition.
in this case, a noise is there. the lower the terminate-resistor I choose, the lower the noise.
freddy get ready, it's time to rocksteady

tv

Hamptone JFP based preamp is "motor boating" at hi
« Reply #17 on: February 13, 2008, 02:57:57 PM »
How about putting the 1k ~ 2k pot in series with fet's source elco as a "gain trim"?

for more inspiration peep here:
http://www.runoffgroove.com/fetzervalve.html
If you sprinkle when you tinkle, please be neat and wipe the seat.

maxheadroom

Hamptone JFP based preamp is "motor boating" at hi
« Reply #18 on: February 14, 2008, 06:02:29 AM »
sorry, i can't follow you... replace which resistor with a 1~2kPot ?
freddy get ready, it's time to rocksteady

tv

Hamptone JFP based preamp is "motor boating" at hi
« Reply #19 on: February 14, 2008, 07:09:48 AM »
No resistor-replacing!

Put a said potentiometer (as a rheostat) in series with 470uF capacitor, f.e. between the - leg of said cap and ground.

This way you could trim the circuit's gain.
If you sprinkle when you tinkle, please be neat and wipe the seat.


 

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