maxheadroom

Hamptone JFP based preamp is "motor boating" at hi
« Reply #20 on: February 14, 2008, 11:28:51 AM »
ok, i understand.
unfortunately i bypassed the 470µ Cap with a 100n one. but i can remove that.
freddy get ready, it's time to rocksteady


maxheadroom

Hamptone JFP based preamp is "motor boating" at hi
« Reply #21 on: February 14, 2008, 04:29:14 PM »
what about putting an low pass, for example a 33k - 220p combination at the
instrument input?
freddy get ready, it's time to rocksteady

skipwave

Hamptone JFP based preamp is "motor boating" at hi
« Reply #22 on: February 14, 2008, 07:33:41 PM »
Quote from: "maxheadroom"
ok, i understand.
unfortunately i bypassed the 470µ Cap with a 100n one. but i can remove that.


That's not a problem. You would simply take the low side of both capacitors (where they used to meet ground) and run a lead to your potentiometer/rheostat. The other side of the pot gets grounded. Now you have a means to lift the bypass capacitors to some potential above ground. Consider their function of reducing feedback by providing the AC audio signal with a direct path to ground. If you add resistance to that path, you reintroduce some amount of feedback.

ChrisSugar implemented it as a "harmonics" control:
http://www.groupdiy.com/index.php?topic=1184&postdays=0&postorder=asc&start=35
Quote from: PRR
Now, maybe you don't, or shouldn't, grab the ribbon for far-harpsichord, nor the hot condenser for snare-kissing... yet often we do.

flaheu

Hamptone JFP based preamp is "motor boating" at hi
« Reply #23 on: February 15, 2008, 07:16:46 AM »
skiptwave,

You mean that you tight the leads from both capacitor to a pot going to ground ?
As MagnetoSound said:
"Sad lack of balls in today's bands, IMO"

flaheu

Hamptone JFP based preamp is "motor boating" at hi
« Reply #24 on: February 15, 2008, 07:16:59 AM »
Sorry double post,

I thought you would connect each capacitor lead to its respective pot.
So I can connect both capacitor (each from each gain stage) to the same pot.

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

maxheadroom

Hamptone JFP based preamp is "motor boating" at hi
« Reply #25 on: February 16, 2008, 06:48:50 AM »
yesterday i checked the "motorboating at hi level" with a bass.
and its NOT gone.
remember, i made two power lines, with seperate voltage regulators.
each gainstage has a own power line. i thought i can eliminate the motorboating, but its not a solution for this problem.
Someone has an idea?
freddy get ready, it's time to rocksteady

guitarbill55

Hamptone JFP based preamp is "motor boating" at hi
« Reply #26 on: February 16, 2008, 04:58:37 PM »
I would try vetsons idea of reducing the gain of the first stage; maybe even a 1 meg pot to ground instead of R9 or floating the pot's ground above ground with a 10k.     High gain stages run into other high gain stages have always given me motorboating fits.   I would also try reducing the power supply voltage to the first stage as someone else here has suggested with a resistor and decoupler cap(s) to ground.

Let us know what you find!  I'd like to build one of those myself.

gb
"Eat your sockeye with wild blueberries" - Copper River Red

maxheadroom

Hamptone JFP based preamp is "motor boating" at hi
« Reply #27 on: February 16, 2008, 07:38:26 PM »
i forgot this to tell:
i put a trim pot between the first-stage-output and the volume pot.
so i trimmed the gain a little bit.

i tried out when the motorboating was there, to reduce the signal with the trim-pot. the motorboating was killed, but of course the volume was lower.
so i increased the volume-pot and the motorboating was there again.

maybe caused of to much voltage on the second-stage-input?

i willl try to reduce the first gain station with putting a resistor between the 470cap/100n and gnd.
freddy get ready, it's time to rocksteady

maxheadroom

Hamptone JFP based preamp is "motor boating" at hi
« Reply #28 on: February 19, 2008, 12:58:57 PM »
yesterday i did a test with the changes above. i reduced both gainstages, the fisrt more as the second.
the motorboating starts later, but is there. i make the test with a fender precision with the lowest tone and maximum output from the bass.
it is possible that the second stage have to much input from the first stage?
freddy get ready, it's time to rocksteady

pantsonfire

Hamptone JFP based preamp is "motor boating" at hi
« Reply #29 on: February 20, 2008, 05:00:24 PM »
did you oversize the output cap in the interest of better low end response? I'm asking because if you did, the "motorboating" might be from negative feedback in the lower frequencies that the original value output cap wouldn't have allowed through. I had this problem with the tube version.


maxheadroom

Hamptone JFP based preamp is "motor boating" at hi
« Reply #30 on: February 20, 2008, 05:11:30 PM »
my output cap is a wima 10µ poly. i don't changed this value.
how did you solve this problem?
freddy get ready, it's time to rocksteady

pantsonfire

Hamptone JFP based preamp is "motor boating" at hi
« Reply #31 on: February 20, 2008, 05:28:12 PM »
the schematic for the lcmp that i built calls for a 2.2uF output cap that i replaced with a 6.8uF poly. first, i replaced the oversized cap with the value called for in the schematic. the motorboating disappeared. then i adjusted the resistor on the output to compensate for the oversized cap. i'm sorry, but i don't remember the formula i used to find the correct r value. i'm a novice and i had the help of a local master in sorting this out. i just wanted to point this out as a potential cause of the problem since you seem to be having trouble getting it sorted. perhaps someone else here can assist with the appropriate formulas if it seems like a worthwhile investigation.
good luck.

maxheadroom

Hamptone JFP based preamp is "motor boating" at hi
« Reply #32 on: February 21, 2008, 03:09:55 PM »
thx patson for your info.
which output resistor did you adjust? the 2,2M resistor?
freddy get ready, it's time to rocksteady


 

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