Hamptone JFP trouble

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THE CURRENT THAT FLOWS THROUGH ONE JFET AMP MODULE [ONE 2N5457 FET, ONE MPSA14 AND ONE ZTX653] IS 40mA.
NOT 400mA !!!

A complete mic pre channel needs aprox 80mA at 24V.

chrissugar
 
two questions...
first after the LM7824 regulator i have 24v of regulated power with my wallwart. can i split the lead and send 24v to the jfet circuit and send 24v to a 1:2 power transformer to get 48v for phantom power? (im trying to do this simply but im pretty new at this)

second, can i add a 6volt pilot light after the voltage regulator? do i need to place a large resistor in front of it? i understand v=ir but i dont know if the regulator will ensure 24volts to the jfet board if i put the phantom power or pilot light in parallel.

thanks
 
No you can't because your wallwart is 24volt DC (you say is regulated).
Transformers can work only with AC.

chrissugar
 
so how do most wall wart devices ramp up the power to support phantom 48v? i know that the bellari mp105 uses a wallwart (12v i think) and they have phantom power.

i want to add phantom to my hamptone jfet from tapeop #37
thanks
 
Here's one way: http://www.uneeda-audio.com/phantom/phan01.gif

You might want to do a search of the forum. The question has come up before, I think.
 
Hi I'm a newbie and was trying to build one of these based off the one on this page


http://www.geocities.com/m_natsume/hamptone_jfp


I'm using a 24VDC 600mA wallwart every time I plug it in the 7824 voltage regulator smokes and starts to catch fire. :shock:


What am i doing wrong?
 
If your 24V wallwart is regulated (puts out 24V exactly), you don't have any headroom for the regulator to work.... and you don't need the regulator anyway. If your wallwart actually puts out a few extra volts, like Scott Hampton's does, then your circuit is pulling too much current through the regulator. That's a scary thought.

Use Ohms law to calculate how much power the regulator is dissipating. If it is dropping 6 volts at 165mA (as stated in the article), use the equation V x I = P which gives almost 1Watt.

That should be okay with the 7824, but chances are your circuit is drawing more current, due to the beta(hfe) of the ztx device being too high.

Try a different 7824 (the smoking one is likely no good anymore) on a big heatsink. If possible, use an hfe checker on the ztx transistor, so that your current source draws less currrent to begin with.

By the way, this design sounds great. It does take selecting and trimming to get it working right, though. Don't be discouraged!
 
Okay I think i've made progress. The circuit passes audio signal but no power has to be applied. I'm waiting for switches to come in from australia. Is there a way to jumper the switches to leave the phantom power on and the pad off? I've been having a doozy of a time trying to find the equivalent of a s1450 switch here.
 
[quote author="bigwillz24"] The circuit passes audio signal but no power has to be applied. [/quote]

Sound without power?! :shock:

What's going on? Voltage readings (particularly at the emitter of the ZTX, i.e. across the 100ohm R)?

To amend my last post, I just remembered that this design draws around 165mA for two complete channels, with two JFPs per channel. Only supposed to be about 40mA per JFP. The ones I built draw a little more.
 
grrr...

The switches came in & I put them in they are working properly i guess the leds light up when I push them in but now I don't get any audio out of it. :sad:

I think I was fooling myself earlier and the audio signal wasn't really going through the circuit.

So close yet so far. :oops:
 
Hi!

I'm ordering parts for Mnats version of Fetboy and would like to know how many ZTX653 and 2N5457 do i need to buy to select them.
And do i need to select both?

Thanks!
 
Based on my own and others experiences, buy ten times as many 2n5457 as preamps you are building. Also, based on comments in another thread, Scott Hampton uses ON-Semi brand, which are supposed to have a lower beta than the Fairchild ones. That would help them to bias correctly with the standard Drain and Source resistors.

The ZTX devices I can't comment on, because I didn't use that part in my builds. You could also try the BD139. I may try that on the next pair I build, the TO220 package is easier to heatsink.
 
Ok, thanks. Anyone knows about the ZTX? Lots of people said this one really have to be selected. Btw, i can get ON Semi from Farnell.
 
ok I've run into a problem with my fetboys.. I just threw them together to have a nice DI.

I get audio, but it's quickly distorted with any gain or from a loud guitar/bass input.

It's always humming badly, if I even try to connect the floating ground to a real ground the humming goes away but the signal drops maybe 15db and immediately goes distorted.

but here's the interesting/maddening part..

If I unplug the power from the unit while the guitar/bass is being played.. I get a few seconds of very loud, very good sounding guitar/bass.. I figure that this is how it is supposed to sound.

Any ideas where to look?
 
tried it. I removed the 47k Rs and tried the trims instead... no better.

I thought I read in one of the fetboy/hamptone threads about this problem but I can't find it anywhere now.
 

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