DIY Factory

FETboy Mic-pre
A project brought to you by Fabio Bauman and Matt Allison
and of course, Scott Hampton
Updated 26 February 2006

Background to the project

Born from Scott Hampton's FET based transformered input Mic-pre designed for Tape Op ... here is the project that somehow got named the FETboy.

A request came to Fabio from a member of the LAB (Group DIY) to build him a few boards based on Scott's design that was offered up in Tape Op. So Fabio decided to do a PCB layout and made a few boards. He sent a couple of left-overs to Matt who set about assembling them and documenting the process as he went along and hopefully with some pictures to give project some hands on experience. With only a few deviations from Scott's original offering, most of the core elements were retained but, with exclusion of an output transformer, the output
is now cap balanced and uses an OEP Transformer on the input.

Despite being a 'simple' design, it could end up being a not so simple build if you don't choose or BIAS your FETs correctly, none of which was
part of the original documented and there was no parts list. Fabio's PCB is a Double Sided board which makes it a little hard to home etch,
though I'm sure we could ask Fabio if he would make it available, I know he still has a couple left and he may he them go for just a few dollars.
Kev thinks home etching a Double Sided Board is possible

I think this is an awesome little project and for a couple bucks you have a cheap, coloured, flavouring that will find a home in any studio.

Matt

Some comments from Matt

I'm finally grabbing a moment to breath and decided to put down my thoughts on the FETboy.

As stated the inspiration for this pre was Scott Hampton's design that he offered up in Tape Op. Scott had previously offered up a Tube Pre but
decided to try and achieve a simple, great sounding solid state circuit. You can see the original article here:
hamptone_fet_scan.jpg
[link] = http://www.diyfactory.com/projects/fetboy/hamptone_fet_scan.jpg

The design revolves around a single ended Jfet 'gain block' called the JFP Gain Module, a building block that can be used as a DI, Line Mixer
and in our case, a Mic Pre.

The basic circuit comprises of an input transformer (1:10), a DI/Instrument input, 2 JFP Gain Modules connected to a 5-10K LOG pot
driving one stage into the next and an output isolation Transformer (1:1 600:600) which runs off of a +24V regulated Wall Wart. (I choice to
adapt one of Peter C's PSU and have a regulated +24V rail and + 48V rail for Phantom Power)

Simple enough right? So lets start by examining the break down of the JFP Gain Module.

Each block features a 2N5457 N channel Jfet with an output stage added. Signal is applied to the gate of the Jfet and the output is taken off the Drain (amplified and inverted in phase). A 50R resistor has been added that is not bypassed by the 470uF cap, which reduces the third harmonic by about 20dB. The NPN transistors act as a voltage follower providing the output drive, with near Unity gain. The DC voltage on the drain is slightly above the supply and directly biases the MPSA14 Darlington transistor. A Darlington is used to minimize the loading on the 2N5457 (Darlington transistors have a much higher input impedance). The ZTX653 is a transistor as an active load (beta = 100min).

Since the output also has roughly 1/2 the the voltage on it, a coupling cap on the output is required to block DC, but allow the audio path to
pass. This is a key component and worth a few extra bucks for a good quality poly.

The output impedance of the voltage follower is a function 47K resistor on the base of the ZTX653. If this resistor is too big, the output will
not have enough drive and will be clipped on one 1/2 of the waveform. On the opposite end of the spectrum, if you make it too small. More power
will be dissipated than needed in the output devices. The value shown provides adequate drive for typical line inputs, with or without the 1:1 output transformer.

Each module is capable of +-26db's of Gain.

So with that out of the way.... Lets dig in...

I, as with many others like to start with the lowest profile components and build up from there. I started with all the resistors, ceramic caps, Jfet sockets, trim pots and electrolytic caps. I was rather sacrilegious with my DC blocking caps, although it was recommended to use Polys I used Tantalums! Don't ask why... partly because it was all I had on hand, and partly because I wondered what it would sound like, and overall I'm pretty happy, but will prob. try a poly sub to A/B at some point.

Fabio decided to use a relatively inexpensive iron, but one that yields excellent sonic results, the choice, the OEP A262A3E, wired 1:6.45. I found the optional Mu-Metal can is not so optional and is need to reduce noise/RF/EMF etc. Remember to remove the sticker from the transformer before you solder the can closed, this will help with identification in years to come (and don't do what I did, stick it on upside down, it is confusing, I've since corrected mine, but not in the pic below). With the FETboy putting out a decent amount of gain no output transformer is used, though one could use a 1:1 isolation transformer, instead it is pseudo balanced.

One more thing to note on the OEP is the RL (Resistive Load), there are a few options here and my suggestion is that it is best to play with the values and tune to your ears. I started with a 25K, but after some listening test I settled on 100K, which see allows the amp to see a 600ohm load, giving a balanced tone, the 25K had a bit of a mid peak which I opted out of. You could always wire a variety of resistors to a toggle switch for some interesting results.

Since I needed to get my 2 FETboys side by side in a 1U rack with 2 other pres (Spectra Sonics 101's) I had to mount all the switches and instrument jack off board as the board is designed to either be mounted 2 in a 1U horizontally or 8 in 3U vertically.

I choose to leave all the Transistors and Jfet's till last and I'll explain why now.

Though the FETboy is based around the JFP Module, substitution of some of the transistors were in order...

Since the ZTX653 and the MPSA14 transistor's can be hard to find and are fairly expensive. Fabio designed his board to use readily available and relatively inexpensive transistors like BD139/BD550A and BD135.

One of the biggest hurdles was to overcome the fact that 'original' MPSA14 Darlington has an hFE of 20,000! The away around this was to use a pair of either BC550A or BD139's transistors that when, multiplied together would bring you close to the original value.

Since BD139's were easily available I bought a few to try and find a few pairs. What I found is the stock BD139's didn't give me high enough hFE and I had to switch to BD139-16's, which have a higher hFE.

How I went about doing it was testing the hFE values of each transistor and drew a series of circles on an an A4 piece of paper and placed each transistor into it's own circle and wrote down the hFE of each one. I found this method very useful and was then able to extract the values at created pairs that would when multiplied would give me close to the 20,000 hFE so for example a pair consisting of say, 130 x 150 which gave me 21,000 hFE.

Since the output the ZTX653, which I substituted with a BD139 acts as an active load I chose to not be picky about it beta value as it needed to
be around 100 and all the transistors I tried met this requirement or more.

Great, so now I was all wired up as you can see in the pic below... But still missing the key component, the Jfet.

[picture] - FETboy3.jpg

As mentioned I choose to socket the Jfet's, and here is why...

Since we are not using the 'original' transistors we need to Bias the substitutes correctly. While Fabio has included 47K resistors suggested in the original article (R4 + R9 on the FETboy), these are only a 'guestimate'. I opted to install the 100K trim pots, designated as BIAS and BIAS 2 LEAVEING OFF R4 and R9, which allows you to Bias the FET's more accurately.

Here is the set-up procedure I used for finding suitable FET's. I bought a bag of them and prepared to spent some time finding the best ones over
a nice cuppa of Earl Grey.

1. Power up the pre without the Jfets (Q1/Q5)
2. Measure the V across R5
3. Adjust bias to have 4V across it (the trimmer)
4. Now put a Jfet in (Q1)
5. Measure the V at the node before C2
6. Try all the Jfets you have and use the one that give you the nearest 12V (Most of mine were around 14-15V)
7. If you get 12.3 or 11.7 just adjust bias to have 12V
8. Do the same for Q5 checking the V across R10 and the Node of C4

Odds are most of your Jfets's will output a higher voltage, most of mine seemed to be around 15V, so be prepared to go through quite a few if you want to try and match you gain stages.

Once I had found the right Jfets and had completed the bias procedure I was ready to roll.

All that was needed now was to rack these and call the project done. I'll post a few pics shortly of the final rack.

Please feel to ask questions if things are unclear, or you wish to add/correct anything.

cheers

Matt

PDF Files

hamptone_fet_scan.jpg - original Scott Hampton article

fetboy_sch_rev1.pdf - schemnatic drawn by Fabio

BD139 - Fairchild Semiconductor

2N5457/8/9 - Fairchild Semiconductor

BC549_550 - Philips Semiconductor

OEP - info

Current Data
hamptone_fet_scan.jpg - original Scott Hampton article
http://www.diyfactory.com/projects/fetboy/hamptone_fet_scan.jpg
fetboy_sch_rev1.pdf - schemnatic drawn by Fabio
http://www.diyfactory.com/projects/fetboy/fetboy_sch_rev1.pdf
BD139 - Fairchild Semiconductor
http://www.diyfactory.com/projects/fetboy/data/BD139.pdf
2N5457/8/9 - Fairchild Semiconductor
http://www.diyfactory.com/projects/fetboy/data/2N5458.pdf
BC549_550 - Philips Semiconductor
http://www.diyfactory.com/projects/fetboy/data/BC549_550_philips.pdf

Parts List

Parts list for:
fetboy_parts_list.txt

an expanded parts list and suppliers numbers is in the works
and is a work in progress
list of Jaycar, Altronics and Farnell parts numbers
DIYF_parts_list.xls

FABIO FETboy
JFET PREAM BOM (Scott Hampton Design)

Designator Part

BIAS 100K
BIAS2 100K

C1 470u25
C1b 220nF
C2 10uPOLY
C3 470u25
C3b 220nF
C4 10uPOLY
CC 800pF
CPP1 470u63
CS1 470u35
CS2 470u35

DCIN 4 Way PCB Mount Terminal
HIZ "1/4"" PCB Mount Socket"
MIC_IN 3 Way PCB Mount Terminal
MIC_OUT 3 Way PCB Mount Terminal

Q1 2N5457
Q2 BC550A or BD139
Q3 BC550A or BD139
Q4 BD135 or BD139
Q5 2N5457
Q6 BC550A or BD139
Q7 BC550A or BD139 Q8 BD135 or BD139

R1 15K
R2 47R
R3 1K82
R4 *47K (Don't use if BIAS is used)
R5 100R
R6 15K
R7 47R
R8 1K82
R9 *47K (Don't use if BIAS is used)
R10 100R
RC 22k
RIN 10M
RL 27k/50k/100k
Ro 50R
RPD1 470R
RPD2 470R
RPD3 100R
RPP1 6K81/.1%
RPP2 6K81/.1%
RPP3 470R
RS1 33R
RS2 33R

SW_48V SPST Switch
SW_PAD DPDT Switch
TX_IN2 OEP A262A3E
VOLUME 10K LOG Pot

Pictures

Matt's first board

Fabio's own completed board

Fabio's unpopulated boards

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Some thoughts

from Matt
After a couple weeks of having these pre's around I am VERY happy with the outcome. I found the sound to be warm and harmonically rich, which saturates the more you drive it. Used as a DI it adds a clean, warm tone, with a slightly compressed sound, LOVELY as a Bass DI.

from Kev
It may be possible to do a design with a single stage with less overall gain and with an auto bias circuit would make assembly for beginers even easier.
Along the style of JLM's little animals Mic-pres.

More suggested parts and future enhancements

this section may not be needed

Some other variants from respected DIY'ers

the PimpTone from Neeno - a simply sensational looking unit

Neeno shares his latest project.
http://www.prodigy-pro.com/forum/viewtopic.php?t=13772
It is a pair of Fabio's FETboy Jfet Mic Preamps (Hampton Design). I kind of got creative and took it somewhere funky and fresh, and probably the first pink panel to be seen out there... Here is... the Pimptone!
The panel was anodized and then laser engraved with a white ink fill. I'll take some inside shots once I'm done with the wiring.
The plan for the 3 way switch will be to have various switchable transformer load resistors, creating a variety of colored tones.

A big thanks goes to Fabio and Matta for the help, support and above all, for their friendship !

Neeno has uploaded a few other pics at
http://www.studio21.ch/diy/neeno/gallery/pimptone_temp/

and from mNats
he doesn't know this is here yet ... please let him know if you are in contact with him
and please do vist his site
http://www.geocities.com/m_natsume/hamptone_jfp

Nat's parts list
JFPBOXG4.txt

Checkplot that Nat used
jfpboxchek.pdf

Copper only layer
jfpboxbott.gif

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brought to you by Fabio Bauman and Matt Allison and other Friends and the DIY Team at :
www.diyfactory.com