R.G.'s improved JFET matcher - vero trouble

Frequently asked questions about tools and instruments used in stompbox development.
Post Reply
User avatar
grrrunge
Diode Debunker
Information
Posts: 797
Joined: 20 Mar 2010, 10:51
Completed builds: Loads!
Location: Copenhagen, Denmark
Has thanked: 50 times
Been thanked: 203 times
Contact:
Contact grrrunge

Post by grrrunge »

I couldn't find any vero layouts on the interwebs for R.G. Keen's JFET matcher, so i went ahead and made one. I didn't have any single op-amps, so i made a layout that uses half of a dual op-amp.

Problem is, it's not working, and i've been going over the layout like crazy for the last hour. I just can't see what i've done wrong.
Everytime i stick a JFET in there, the DMM readuot drops from V+ to Vmid (9.44V - 4.87V). I find it kind of hard to believe that all of my 2N5457's have a vgs of 4.87V :D

Do any of you guys see some mistakes here?
JFET matcher.png
A true believer in the magic of Sherwood Forest Pedal Pirates
---
New base of operations: http://www.knucklehead.dk
Top
User avatar
PokeyPete
Resistor Ronker
Information
Posts: 371
Joined: 31 Aug 2008, 01:45
Has thanked: 11 times
Been thanked: 43 times

Post by PokeyPete »

My first impression is that your layout looks good. With your DMM probe at pin 3 you should never read more than 1/2 V+
or 4.5V (4.7V with a 9.4V supply) no matter where you touched the other probe. If the 10k/10k voltage divider was not
grounded properly then that capacitor would charge up to 9V (9.4V in you case) and you could get 9V readings like you
apparently are. Power it up....place your negative probe on 0V and the positive probe on pin 3 and measure. If you read
4.5V (4.7V) then that part is fine. But, if it reads higher there's a problem. I suspect a simple problem like a solder bridge,
cold solder joint, or a trace cut that didn't quite completely sever the trace. If all else fails you can try grounding the unused
pins of the op amp.
“No man is so foolish but he may sometimes give another
good counsel, and no man so wise that he may not easily err
if he takes no other counsel than his own. He that is taught
only by himself has a fool for a master.”
–Hunter S. Thompson
Top
User avatar
deltafred
Opamp Operator
Information
Posts: 1652
Joined: 06 Apr 2010, 16:16
Location: England
Has thanked: 808 times
Been thanked: 299 times

Post by deltafred »

The unused half of the opamp could be oscillating and upsetting things, it has both inputs open circuit.

Try setting it as a unity gain buffer, -ve input to output, and connect the +ve input to the the used +ve input (pseudo 0v)
Politics is the art of so plucking the goose as to obtain the most feathers with the least squawking. - R.G. 2011
Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012
Top
User avatar
grrrunge
Diode Debunker
Information
Posts: 797
Joined: 20 Mar 2010, 10:51
Completed builds: Loads!
Location: Copenhagen, Denmark
Has thanked: 50 times
Been thanked: 203 times
Contact:
Contact grrrunge

Post by grrrunge »

Tsk! :D I had a jumper wrong in my build. The circuit works now, and my layout is verified ;) Thanks for the help guys!
JFET matcher.png
:horsey:
A true believer in the magic of Sherwood Forest Pedal Pirates
---
New base of operations: http://www.knucklehead.dk
Top
User avatar
roseblood11
Tube Twister
Information
Posts: 1880
Joined: 23 Aug 2008, 14:21
Has thanked: 359 times
Been thanked: 312 times

Post by roseblood11 »

But it's true what deltafred said: Never leave an unused opamp unconnected.
Connect it as shown here for "IC1b":
http://forum.musikding.de/cpg/albums/us ... c_v1-2.jpg
Pin 5 could go to gnd instead.
Top
Post Reply

Return to “Measuring & Matching”