BF862 Preamp

[juma]

Originally Posted by wrenchone

The radio hack breadboard solution shown by jackinnj looks like a no-brainer. ...

Right. With a bit more effort you can transform it to TO-92 part:

diyAudio

diyAudio

[buzzforb]

Originally Posted by juma

Piece of cake...
This one was a bit tougher, i used just a simple 15W Weller soldering iron...

I have seen a guy on YouTube use a solder wick role as a way of spreading solder to tiny legs. i think it helps with heat as well. Impressive!

[jackinnj]

Originally Posted by buzzforb

Juma, I got my BF862's today. All I could do was laugh. Anybody who works with those little things regularly should be commended. Oh Yeah, I got 30, but i don't think that was enough.

You can get solder past from Kester -- I apply it with a really fine tweazer -- then you hold the part down with a probe and just touch the critter's legs with the soldering iron (I use a hako with a needle tip.)

If you have a lot to do you can adapt a toaster oven -- there's a specific protocol of heating and cooling that you're "supposed to follow".

The RatShack protoboards have enough space between the traces to use "0805" components. For me, this allows a lot more components on a board.

Lastly, the gates of the paralleled BF862s will fit on the long runners, but not all smt transistors are configured the same.

[crt]

Originally Posted by juma

I received a request for version of this preamp that uses 2sk170BL (Idss about 9mA). Right now k170 is easy to get and BF862 is a SMD component (not convenient for old hands and eyes ), so here is the schematic.
The gain is a bit higher (9 dB) and the PSU is a bit different (be exact with input DC voltages +28V and -8V).

How much current needed for positive and negative supply?

how many times gain actually for 9db?
Sorry if my question to basic

[juma]

Originally Posted by crt

How much current needed for positive and negative supply?

Per channel :
Q1 and Q2 will draw Id near to Idss (at max. swing) from pos. rail
Q1 is alone on the neg. rail and it will draw a bit less (source doesn't swing as hard as drain) but let us allow for Idss too.
For stereo multiply that by 2 and allow enough margin for loses in PSU (shunt reg eats a lot of current).
Then multiply everything by 2 to be in comfortable zone..

Originally Posted by crt

how many times gain actually for 9db?

Page 4 : http://www.live-audio.com/studyhall/dB_calculations.pdf

[wrenchone]

I finally got around to hacking a BF862 adapter together for my jfet sorting jig, using a 6-pin SOT-23 clamshell-type ZIF socket available reasonably cheaply on E-Pay. I had to do some rewiring, but I now have a jig that plugs into the "J170" socket on my tester, so I won't go insane trying to sort parts. I've had a strip of 100 parts waitng around for this development. I wont be doing this preamp, but instead have an RIAA amp that will use gain blocks consisting of a BF862 and a p-channel small signal mosfet. I'll be starting another thread for that project whaen I'm more sure of results.

Anyway, if you search on E-Pay for "SOT-23", you'll run across a lot of parts, and several Chinese/HK sellers offering the socket. It's originally intended as an adapter for some PIC-type device in a 6-pin SOT-23 package, but it will accomodate a 3-pin SOT-23 just fine.

[jameshillj]

That looks like the one from emcs2009 for $50 - SOT23-6 to DIP 8 PIC -

looks like the clamp presses the transistor down into a recess with the 6 (?) contacts underneath, yes? and connect the pins to external cctry for testing as per your photos.

Are there some extra transistor on the pcb? If I may ask, what do you use them for?

[wrenchone]

There's nothing extra on the PC board, though there are a few spots here and there for surface mount components. I added a 1k 805 resitor on the board (not visible) as a gate stopper, as the BF862 is very slippery. I've had problems with spurious readings in test jigs before due to oscillation with far less slippery devices - all my new test/sorting jig designs incorporate gate stoppers.

[wrenchone]

Just for clarification, a fair number of the wires you see labelled in the top pic are the gold-plated contacts coming from the socket - I labelled the ones I wanted to use and traced out where the PC board connections went with a VOM (I'm old, but I ain't stupid...). The red-based right angle header was added by me, and that's the bit that plugs into my jig. I cut traces and added jumpers to connect the chosen contacts in the socket to that header. I also removed a second PC board that was sandwiched on top of the one you see in the pictures using a hot-air pencil (a great tool for removing parts in plated-through holes where a fair number of pins are involved - the dragon's breath does it...). The header went into some of the holes left behind by that removal.

[EUVL]

I bought my socket at Farnell. Not so cheap.
Removed 3 out of the 6 pins.

Patrick