looking for recommendations: preamp a perfboard - EDIT: FETBOY

aviel

Well-known member
Joined
Feb 6, 2018
Messages
88
Hey,
i just completed my DIY 500 lunchbox, equipped with a diyre cp5. looking to add another one to the the arsenal.
i am looking for something that i can build on my own (perfboard, since i am not int etching yet), do you have any recommendations? something "colored" can be nice.
 

rock soderstrom

Well-known member
Joined
Oct 14, 2009
Messages
1,037
Location
Berlin
My advice is the Hamptone JFET Preamp aka Fetboy. Easy to build,  tons of colour as micpre and DI for instruments.
 

aviel

Well-known member
Joined
Feb 6, 2018
Messages
88
Thanks! Will have a look on this forum i assume there should be plenty of info? Did you build one?
 

aviel

Well-known member
Joined
Feb 6, 2018
Messages
88
Another thing is, i would like to use it with my current psu in the lunchbox. I have +-16, i assume i could convert it to a dual rail supply, what do you think?
 

rock soderstrom

Well-known member
Joined
Oct 14, 2009
Messages
1,037
Location
Berlin
You do not need a dual rail psu for the fetboy, it will work with a  16V single rail supply. 24V would be better though.  You asking for colour, the low voltage will give you more of them. 8)

Just build it up, only a few components a needed. You can start without the transformer, just use it as a DI box, then you can extend it with a cheap 1:5 to 1:10 input transformer and you will get everything you want.

It is a perfect project to build it up on perfboard, the result could be great in my opinion. The only challenge is to work with FETs, the need a lot of adjusting or selecting.

Have fun!
 

aviel

Well-known member
Joined
Feb 6, 2018
Messages
88
Awesome!
one thing though,
must of the links are offline, do you have some on your side that you might share? pictures/diagram/sound samples/ anything?

Thanks! ;D
 

rock soderstrom

Well-known member
Joined
Oct 14, 2009
Messages
1,037
Location
Berlin
aviel said:
Awesome!
one thing though,
must of the links are offline, do you have some on your side that you might share? pictures/diagram/sound samples/ anything?

Thanks! ;D

I know it is a bit PIA but it is all there. Check the several threads here in the forum, you will find a lot of useable infomation.
 

aviel

Well-known member
Joined
Feb 6, 2018
Messages
88
got it,
i am still thinking if i should i convert it to dual rail supply. +-16.
what psu did you use? just a regulated wallwart?
 

rock soderstrom

Well-known member
Joined
Oct 14, 2009
Messages
1,037
Location
Berlin
I have done it with a LM317 based PSU and it works with a unregulated wallwart, aswell. Null problemo! There is no need to convert it to a bipolar PSU, but its DIY you can do it that way if you want to.
 

aviel

Well-known member
Joined
Feb 6, 2018
Messages
88
the only reason i wanted to convert is to use it with the PSU i already have, which is +-16v for diyre CP5 preamp. i am assuming that 16V  supply only will reduce the headroom drastically.
 

rock soderstrom

Well-known member
Joined
Oct 14, 2009
Messages
1,037
Location
Berlin
AFAIK, +-16V = 32V with a floating earth. You can use that or regulate it down to 24V with a LM7824 or LM317. I have never done this before, but it should work, right?

I would start with a test setup based on the +16V rail, just to see if the headroom is sufficient for your application. My bass DI operates with 18V and I really like it.
 

aviel

Well-known member
Joined
Feb 6, 2018
Messages
88
Im not sure about converting it, the PSU earth is grounded to the chasis and to the wall, so i wouldn't say it is floating. the audio grnd is also grounded to the chasis, i am not sure if it iwll work.
 

rock soderstrom

Well-known member
Joined
Oct 14, 2009
Messages
1,037
Location
Berlin
My idea was to isolate the ground of the fetboy from chasis and PSU ground so that the Fetboy is floating. But I am not sure if it works.

Again: "I would start with a test setup based on the +16V rail, just to see if the headroom is sufficient for your application. My bass DI operates with 18V and I really like it."  ;)
 

aviel

Well-known member
Joined
Feb 6, 2018
Messages
88
yes, i will definitely try it first with 16 Vdc. will also add a input transfomer for mic input
 

rock soderstrom

Well-known member
Joined
Oct 14, 2009
Messages
1,037
Location
Berlin
2n5457.

https://www.banzaimusic.com/search.php?mode=search&page=1&keep_https=yes
or
https://www.musikding.de/2N5457
 

aviel

Well-known member
Joined
Feb 6, 2018
Messages
88
what about the BD139? did you use darlington pair as in Fabio's schematic? is it critical to get that hfe of 20.000?
there was some discussion regarding if you should match and bias them, but seems that the links are offline
 

rock soderstrom

Well-known member
Joined
Oct 14, 2009
Messages
1,037
Location
Berlin
I use always the MPSA14/ZTX653 version, they are cheap and easy to source. In my opinion it is critical to bias the FETs into the right position (~B+/2). You can do that with trimmers. I prefer to select my 2N5457s for the the right one. I got a little selfbuild testing device for that. The disadvantage of this method is, you need a lot of FETs, because their parameters are all over the place.

Another advice: I have made my source capacitors switchable, so I can control THD, gain and the overall sound of the preamp. Very useful in my opinion. Some have used potis for that, for me the three switching positions are enough. Choose the size of the caps according to your taste.
 

aviel

Well-known member
Joined
Feb 6, 2018
Messages
88
will a DMM that measure  hfe be enough for selecting the fets?

i have ~6x 2N5457 and ~20x J201, hopefully i can use J201 instead. do you select the 5457s according to hfe? or Voltage?
 

Latest posts

Top