fet boy

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fucanay

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abby normal said:
fucanay said:
See anything in the schematic that could be a place to look? Usually it would be a cap that would filter a frequency, right? And to me it looks like there are only 2 caps in the signal path, C2 and C4.

Matt

Are you using those 220nf's off those 47k resistors? If so and just for shits and grins could you disconnect those to see if your issue is resolved? Or at best could you tell me the difference between them being in the circuit or not?

I don't want to start yanking parts out until there is a reason expectation that it may fix the problem. These boards are hard to de-solder parts from. I'm going to open it up tonight and check everything again.
 

abby normal

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fucanay said:
I don't want to start yanking parts out until there is a reason expectation that it may fix the problem. These boards are hard to de-solder parts from. I'm going to open it up tonight and check everything again.

I hear you. Especially if it's all bolted down. But there seems to be a lot of people who find these to be amazing preamps. I'm not a preamp guru but I think a 10uf cap should provide a wide frequency range. My next thoughts would be is it the transistors? Chris built these without that 220nf cap and wound up building something like 20 plus units because he felt they performed better than many other units he has used in the past. It could be other factors but how hard would it be to desolder two leads to find out if that is the issue?
 

Svart

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I find my fetboys very good for bass guitar when you let them lightly distort.

I find that they have lots of low end.  I would think that you have some kind of circuit issues.
 

Silvas

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Svart said:
I find my fetboys very good for bass guitar when you let them lightly distort.

I find that they have lots of low end.  I would think that you have some kind of circuit issues.

+1

Incredible on bass, DI or Mic´ed.

I think component selection and biasing is the key here.

As i said before on another topic, i have many high-end (and not so high-end) preamps at my disposal, and this pre is definitely one of my favorites. Clients love it, too !
 

fucanay

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should it be my understanding that adjusting the bias trimmer until I get 12v at c2 and c4 withou regard for volts at r5 and r10 will make q2, q3, q4, q6, q7, and q8 get hot? If I adjust it like that I'm getting around 8v at r5 and r10, which is about double what the bias procedure specified.
 

rotation

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slovenia
Hi guys,

i made several channels of this preamp on Mnats board and i'm wondering if it would be good to add both, RL and zobel filter (RC CC), as marked on Fetboy (Fabio's) schematic. I couldn't find any comments about this..
Also, how much attenuation gives combination of RPD1,2,3?


Miha
 

Silvas

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fucanay said:
should it be my understanding that adjusting the bias trimmer until I get 12v at c2 and c4 withou regard for volts at r5 and r10 will make q2, q3, q4, q6, q7, and q8 get hot? If I adjust it like that I'm getting around 8v at r5 and r10, which is about double what the bias procedure specified.

Probably q4 and q8 have too high hfe? what are you using here? i use BC550A in this position, with no problems. You should select something with a hfe around 100 or so.  But my biasing is done in the source resistor, then i adjust the 47K resistor (paralleled with trimmer on fabio´s fetboy) for the current draw i need for each stage. On the second stage, i aim for approx  6,3v (about 60mA, to drive heavy loads) and on the first stage i aim for about 25mA or so, to keep the temperature low even with no heatsinks on the MPSA14 and the BC550A, but more than enough current (i think) to drive the pot.

 

fucanay

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Yeah, using TREX. Works great. Star ground is from each Input and output jack and each fetboy board, and also 0V on the TREX. I think that is all. TREX mounted to the rear of the case (aluminum). I have no noise anymore. I have tons of gain and it's really super clean. But it's still brighter than what I was expecting. I'm waiting on 100 JFETs to find some more in the 12V range. Mine are closer to 13.5V right now.

I have to say, this thing is the Shure SM7's best friend.

Matt
 

fucanay

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All of the JFETs I just ordered were over 14V, so I put a trimmer in for R3 on one channel and adjusted it until I got in a better range and the measured it out of circuit and got a new value for R3 and R8, which ended up being ~560 ohms. I put that in both channels and re-biased all of them and I'm sitting at 12V on everything now. It seems the BD's are all running a bit cooler than before also. Seems like I'm all done with these. Finally. What a learning experience.

Matt
 

fucanay

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Yeah, it's pretty warm. I have a rear panel that I had to make out of scrap aluminum because the one I had from the case I recycled was screwed up, so I just mounted the TREX straight to that, isolated of course. I stick my hand on that spot on the rear panel once in a while to check how hot it's getting, and I don't think it's excessive. The TREX is pretty much designed to use this way from what I understand. Not the fetboy really, but a Neves run the same rails.

It may be better to have a 24V power supply instead of the 48V. But then you'd have to have another way to make the +48 for phantom.

EDIT: Maybe using this one and adding a voltage doubler for phantom would run cooler. But I don't know if it's needed. By the way, unless JLM is modifying those switching PSU's, these are the same brand and model if you buy the 48V version.

Matt
 

3nity

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Hey Mat.
That power supply will work fine but we cant double a DC.
What i was thinking and seriously its to use a DC-DC converter 24V to 48V.
the other best solution its a 24VAC and use a simple psu.

On my side i have my Trex but i just notice a small hum when holding a microphone but goes away when no mic is holded.

I'm sure its a groundloop but hey i have pin 1 on the in and out joined and from there to the 0V...its that righ like you did it?

Thanks
 

3nity

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heres what i did ..
Did exactly like you did (grounding)
T-rex was screwed to the back panel i positioned elsewhere with his own heatsink..
Works like a charm i was having some ground loops but my cable was wrong that was all.

It works wonderfully!!

I'm more and more in love with this forum!!
 

fucanay

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I wish I could say I loved the fetboy. After all the time I spent trouble shooting it, I'm not sure I like it much better than the pres that are in my RME Fireface 800. I just built a G9 too and I think it stomps all over the fetboy. This project was pretty anti climatic for me. The trouble shooting experience was worth it, because I now feel I can follow schematics better than I used to, so it wasn't all for nothing.

Matt
 

RuudNL

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I took a look at the schematic, but there are some things that are not clear to me:

- All amplification is done by the FET. The two following transistors are a unity gain buffer, dissipating something like 1 Watt.
What would be the disadvantage to replace those two transistors by a simple (1 transistor) emitter follower?
A transistor with an average Hfe of 150 and an emitter resistor of 2200 ohms would give a base impedance of something like 330 K.ohms, not noticably loading the FET amplifier stage. Even a unity gain OpAmp buffer would be possible, giving a zero ohms output impedance with good drive capabilities.

- IMHO there are no objections to replace the 1.82 K.ohms resistor by an variable resistor.
This resistor is completely decoupled by the 470 uF capacitor.
In this case it would be a lot easier to set the correct bias for the FET. Also it would no longer be necessary to select a FET that works with the 1.82 K. source resistor.
The AC amplification is set by the 15K / 50 ohms resistor.
 

MikeClev

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I built 4 little JFP gain modules (the JFET circuit the FETBoy is based on) all on a mini DOA style footprint. I replaced the 1.82k bias resistor with a 2k trimmer and was able to correctly bias every FET I tried in a matter of seconds. Its definitely the way to go... unless it detracts from the "sound" of the circuit, but I don't see how it would. I didn't do extensive testing on it, just enough to get sound through it (sounded fine!). I used MPSA14 and BD139 in the other positions. One of my previous posts details the exact circuit and also a way of using +/-15v power supply.
(EDIT: Here's the thread: http://groupdiy.com/diy/index.php?topic=40106 )

Cheers!
 

kooma

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About to finish my cheapo version:)
rack is from some old ethernet-thingy, external psu, no phantom, switchable output trafos..
 

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