fet boy

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I would like to add an impedance balanced output to this pre as referenced here:
http://www.jensen-transformers.com/an/an003.pdf
Can I simply wire another 10uF cap from pin 3 to ground, or would I need to add another resistor as well.  I am unsure of the existing output impedance of the circuit, and what value, if any, resistor to add.  This is for a very compact portable build, and I just don't have room for an output transformer.  Thanks for the help!
 
Actually the fet boy its impedance balanced.
the hamptone needs an op transformer.
Do you have a look at fet boy schem?
 
Hey there, I built 4 fet boys a few years ago, and now 1 channel is intermittent.  Does anyone have a working link to the schematic and layout diagrams?

I love these pres. Been otherwise my workhorse go to pre's.
 
resurrecting this......

now that my 2 Calrec EQs are well under way I am interested in doing a couple of FETboys - but the project links are no longer operating on this thread.  I've googled a lot too.

anyone have the info for me to build using self etch pcbs ?

Cheers
Pete
 
Hi Pete,

here is a lot of information.

https://web.archive.org/web/20110501011353/http://www.diyfactory.com:80/projects/fetboy/fetboy.htm

https://web.archive.org/web/20060819043420/http://www.diyfactory.com/projects/fetboy/hamptone_fet_scan.jpg

8)
 
Hello guys, i finally build this preamp but i see the transistors are getting super hot

Heating could be solution or i made a mistake somewhere

I found several forums talking about it but no real explanation
 
Transistors on mine were really hot as well. If I remember correctly calibration should limit the heat.
 
I calibrated as described and indeed it works fine, just scared they won't last much maybe nowadays we can find other transistors holding better this curent ?

The ztx and 2n5457 are running very hot
 
Hello there, i finally had some time to check, still getting quite hot, and 100R burns out after some time

it does on both of my builds
 
Hello there, i finally had some time to check, still getting quite hot, and 100R burns out after some time

it does on both of my builds
It's been a while since I've built some Fetboys. It is very important that the FETs fit and thus the operating points are exactly right, otherwise the components behind them will boil off. This is actually a design flaw, because an amplifier circuit should be independent of this. Some have used therefore integrated bias trimmers, I have selected the correct FETs from a large handful.

I think it's worth it if you like that kind of thing. Awesome amp with lots of color and character.
 
i actualy bought the PCB from creative clown, it has trimmers but no guideline on the calibration procedure

is there any guide on what voltage should be at some specific points?
 
thank you very much, youre more than helpful
Ok, this is the luxury trimmer version ;-) I tried with my first batch 2N5457s also with trimmer and could not reach a stable result. Your first goal is, and this is probably not new for you, to have the drain voltage of the FET at half the operating voltage. If you have to get extreme with the source trimmer, the FET will not fit. That was my experience. The realization that some (most?) FETs are simply not suitable, even with trimmer.

For the second trimmer I would orient myself to the original.

25123-3ffed10c16d1c2a21283ea718884d6b6.jpg

As I said, I didn't get a good result with the trimmers*, so I selected the FETs for the original resistor values. With this I was successful, it is a warm to just acceptable hot affair, but stable. I may have replaced the 100R resistor with a 1 to 2 watt type or possibly two 0.6 watt 200R in paralell. I am not sure.

*I think the trimmers work if you only need to do light adjusting with it, if you get extreme with it you will pay the price elsewhere for it.

Hope that helps a little bit...
 
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We would certainly had Flunked our exam if we had delivered a schematic like this with no stabilisation of Q2 & Q4 in my youth ...!!!

A HUGE improvement could be had if there is another NPN added both places -> Emitter to GND, Base to the Emitter off Q2 and Collector to Base off Q2 and do the same with #2 connected to Q4. This turns a very unstable (and potentiel very Noisy) Q2 & Q4 into very stable ('Noise-Free') Constant-Current Sources (Sinks) about 6mA. To avoid other problems - change TR3 & TR6 to 22K fixed resistors.
If you really need to change the Current - change R9 & R16 .... But don't go Too Low.

Per
 

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