IC double clipper design, what have i missed here ?

[MartyMart]

I have a circuit that's just doesn't have any output and I'm a little stumped as to what I've done wrong.
I'll have to describe it as I have not sketched up a schem as yet, so here's the setup :
Standard dual opamp JRC4580
Powered by 9v pin 8
VB is 4.4v - standard resistor pair/22uf cap to ground
Input via 1m to ground, 22n cap to + input and VB to + input also
Across feedback loop pins 1&2 is 120pf cap, pair of symetrical diodes and 1M linear pot
wired as variable resistor ( gain pot )
- input ( pin 2 ) has 220n cap and 3k3r to ground
Output ( pin 1 ) 2k2 and 47n cap to pair of diodes to ground, also symetrical with small 1n cap in parallel.
From here I enter IC + input ( pin 5 ) which has a 470k from VB
Pins 6&7 connected with jumper ( buffer ) and output from 7 via 100n cap to volume pot, 100kA
Voltage readings :
1 - 4.22
2 - 4.41
3 - 3.89
4 - 0
5 - 0.43 ?
6 - 9.09 !
7 - 9.09 !
8 - 9.61

Obviously a problem/bias issue with the second half of the opamp, all possible solder blobs and mis-wires have
been checked for using a mag-glass and resistor values also.... so what have I done wrong chaps ??
Thanks for any pointers,
Marty.

[slacker]

You need a decoupling cap between the diodes to ground and pin 5 otherwise the diodes will pull the input down to a diode drop above ground and the second half of the opamp won't be biased properly.
I think you could also solve this by connecting the diodes to vref instead of ground.

[Mark Hammer]

Right.  There are cases where you can "borrow" the bias voltage from a preceding stage, but here you need to wipe the slate clean with a DC blocking cap and rebias.  I have walked down that precarious path myself....too many times.

[MartyMart]

Of course .... slaps head, adds cap and the circuit springs to life ! ... I should know better 
Thanks Chaps, much appreciated as ever
MM.
BTW - it sounds like poo ... never build a "make it up as you go along" circuit and expect too much !!

EDIT : Improved with a few cap changes, not bad now ...

[Mark Hammer]

BTW - it sounds like poo ... never build a "make it up as you go along" circuit and expect too much !!

EDIT : Improved with a few cap changes, not bad now ...

There are two approaches to making a double clipper:

• Treat the first stage as a way to provide a steady high level and deliver a longer-sustaining distortion.  The Big Muff does this.
• Treat the first stage as a tone-shaping stage and a way to extract a different quality of clip from the second one. Remember, what you feed it determines what you get out of it.

I think you CAN make it up as you go along, but you need to start out with a clear sense of which of these two roads you want to walk down, and design accordingly.

[MartyMart]

Thanks Mark, good point - IIRC it was half a board from a 2x dual opamp idea that went wrong, so I've
adjusted it into something else - hard to go back to "unmarked" boards from a year ago and figure out
what you were trying to build !
Considering that I really did just "make it up" it's not too shabby sounding, I seem to have a lot of rather
large 220n/470n caps that I can't use in these circuits ( muddy ) and seem to prefer the 4n7 to 22n range
I/O and decoupling of dist "toys" !!
MM