PORTABELLABZ

The pA726

The pA726 is a CA3046-based analog exponential converter with thermal oven compensation as featured in various analog oscillator circuits with great stability and accurate tracking.
Thanks to the connections layout matching the (in)famous ľA726 (UA726) pinout, it can be used as a direct drop-in replacement for this rare and expensive part.

The pA726 can work on +/-12V or +/-15V and replace the ľA726 in any other circuit, for a new build or for repair.
It can be mounted with component legs directly to the ľA726's place or to standoffs, chassis, case etc... thanks to the mounting holes and wired to the ľA726's place.

The pA726 is available in 2 formats to match the classic TO-100 pinout or the 208 PCBs DIP10 pinout.


FAQ

Does the pA726 sound as good as the original ľA726 ?
The exponential conversion is about pitch only and has nothing to do with the timbre. Unlike some assertions one can read on the interweb, having a ľA726 or another exponential converter doesn't affect the sound in any way, being NOS or new.

Does the pA726 installation need other modifications to the circuit ?
Some adjustment of the CV scaling resistor might be needed, same as if a ľA726 is replaced with another ľA726.

Does the pA726 need to warm up ?
The circuit needs some time to warm up and stabilize, evolving pitch during 5-10 minutes after power up is normal behavior.

Does the pA726 ensure stable pitch ?
When the circuit is warm no drift occurs.

Does the pA726 track as good as the original ľA726 ?
In a 208r rev2, I can get perfect tracking on more than 5 octaves with the pA726, which is a little higher than the ľA726.

Does the pA726 work with equivalent ICs ?
The pA726 works with TL071, TL081 or CA3146, CA3086, UL1111 etc... as well.



pA726 with the ľA726 TO-100 round pinout (44x26mm)

The pA726 needs a ground connection. The N/C pin 7 of the ľA726 emplacement can be used, there's a connection to this pin on the pA726 PCB but an extra connection between the former ľA726 pin 7's pad and ground must be added on the board which will receive the pA726, otherwise the circuit won't work as it should.
If for any reason this pin 7 can't be used as a ground connection, don't solder it and rather connect the pA726's 0V pad to ground.

On the 208 PCBs pin 2 is connected to ground and a bridge can be done between pins 2 and 7 to achieve the ground connection (pin 2 is not permanently wired to 0V in case the pA726 in used in a circuit with different layout).


pA726 -TO-100 shipped untracked

BOM, build notes and calibration procedure




pA726 for 208 with DIP10 pinout (64x29mm)

It was designed with the 208 in mind and can be mounted to the solder side of boards 6 and 7 with components legs.
An alternative mounting for the ones willing to make it easy removable is with 2 IC sockets (with round pins, if 10 pin ones can't be found, shorten 16 pin ones) : one on the pA726, one on the 208r board. A cable tie will fasten both together.

Thanks to this handy layout, the pA726's calibration in a 208 can be performed with all the boards in place : the test point, trimpot and jumper are directly accessible like the 208's other trimpots.

The pA726 PCB features two emplacements to connect the former ľA726 pins : the vertical one for board 6 (near the 78L10) and the horizontal one for board 7, both are connected in parallel and only one at a time can be used.
Square pads show the ľA726 pad (pin 10).
On this new revision, pin 2 is directly connected to 0V and the short with pin 7 is no longer needed.


pA726 for 208 PCB shipped untracked

BOM, build notes and calibration procedure




pA726 in an Elektor Formant VCO
Many thanks to David Brown for kindly sharing his nice pics





pA726 in a 208r






The PCBs come unpopulated and without parts. All modifications are at your own risk and I assume no liability for personal injury or damage to equipment or loss of use caused directly or indirectly by the use of any of the PCBs. Although the installation is quite simple it should only be performed by those experienced in electronics.