DIY API 312 questions

still not finished my clones. :green:
have some more questions:
where to connect the ground wire for the balanced output ? On the centertap of the output transformer ?
and where to add an output gain control ?

Hi Andre,

Fore API 312 amp cards the 0 volt/ground from the PSU and all signal cable screens shall be connected to pin 3, but you can use pin 15 for the screens, because it is same as pin 3.

The level gain control "output gain contol" shall be connected between pin 9 and 10, and you can use a 22 kohm (25 kohm) reverse log potentiometer with the left and wiper terminal linked and connected to pin 10 and the right terminal connected to pin 9.

--Bo

Thanks Bo, but muy question actually was: where and how can I put an output level control, so I can crank the input stage to taste and have full control on the output.

Andre,

Ok, I avoid this question and instead talk about the normal gain potentiometer, because there are no good way to connect a "output level" control on the 312 card.

There are only one op-amp so you can not isolate a potentiometer between two op-amps, and you can not connect a potentiometer between the 2520 op-amp output and the 2503 output transformer primary because the transformer must have a very low-Z drive.

So the only way to do that is to do a low-Z pad after the transformer secondarys, you can use a stepped balanced constant 600 ohms pad with many stepps that is feed from the all three secondary windings configurated in series.
Or you can use a simple three stepped pad there you can switch between the three secondary windings and get 0, -6 and -9 dB attenuation.

--Bo