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 A726 (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 A726 in any other circuit, for a new build or for repair.
It can be mounted with component legs directly to the A726's place or to standoffs, chassis, case etc... thanks to the mounting holes and wired to the A726's place.

The pA726 needs a ground connection. The N/C pin 7 of the A726 emplacement can be used, there's a connection to this pin on the pA726 PCB but an extra connection between the former A726 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).

The pA726 is available in 2 formats to match the TO-100 pinout or the Buchla 208 PCBs (original or clone).


Does the pA726 sound as good as the original A726 ?
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 A726 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 A726 is replaced with another A726.

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 A726 ?
In a 208r rev2, I can get perfect tracking on more than 5 octaves with the pA726, which is a little higher than the A726.

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

pA726 with the A726 TO-100 round pinout (44x26mm)

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pA726 - TO-100 PCB
BOM, build notes and calibration procedure

pA726 for Buchla 208 with DIP10 pinout (64x34mm)

It was designed with the Buchla 208r rev2 clone in mind and can be mounted to the solder side of boards 6 and 7 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. Alongside to being solid and stable this mounting makes the board easy to remove if service is needed. You want to fasten it with a cable tie.

Thanks to his 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 A726 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.

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pA726 - 208 PCB
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 Buchla 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.