Api 312 Brent averill edition..

[MisterCMRR]

You're correct, the Jensen JT-115KE is properly damped with only the 150 kΩ resistor (and keeping the capacitive load on the signal side under about 100 pF (under 50 pF preferably). The late Deane Jensen was very proud of creating a design that didn't need the series RC damping (or "Zobel" as it's widely called). Most of its damping comes from a combination of winding resistances and cleverly distributed interwinding capacitances.

 

[JW]

Thanks very much MisterCMRR

(and keeping the capacitive load on the signal side under about 100 pF (under 50 pF preferably).

Are you referring to C6 and C7 in the original 312 schematic? (the 1000pF caps on the transformer's primary?) Because the only other cap on that side of the opamp would be the one in the zobel that wouldn't be used with the Jensen input transformer.

Anyway, do you have an output transformer preference in combination with the above mentioned Jensen inputs? I'm guessing not wired 1:2 because of the massive gain (?). . . I'm building a DIY 8 channel unit. These two channels with the Jensens I was thinking would be nice for ribbon mics.

 

[MisterCMRR]

Thanks very much MisterCMRR


Are you referring to C6 and C7 in the original 312 schematic? (the 1000pF caps on the transformer's primary?) Because the only other cap on that side of the opamp would be the one in the zobel that wouldn't be used with the Jensen input transformer.

Anyway, do you have an output transformer preference in combination with the above mentioned Jensen inputs? I'm guessing not wired 1:2 because of the massive gain (?). . . I'm building a DIY 8 channel unit. These two channels with the Jensens I was thinking would be nice for ribbon mics.

By "signal side" I mean the transformer secondary feeding the amplifier input. Normally there is only the load required to shape frequency response (resistor, or resistor + RC network). But sometimes users will connect the secondary to the amplifier with a shielded cable and then wonder why frequency response doesn't meet spec. Cable capacitance can have quite a negative effect because of the high impedance of the circuit. Other tips for getting the most from an input transformer include tightly twisting primary and secondary pairs (separately, of course) and being sure to connect the secondary low-side (ground) to the same physical point in the circuit as the ground end of the gain-setting resistor in the op-amp or the ground end of the cathode resistor for a tube.

Regarding output transformer selection, there is no magical "pairing" of transformers. The choice of output transformer will be driven by maximum output level (generally determined by supply rail voltages) and the output impedance of the stage driving the transformer, which should always be as low as possible (under 50 Ω is best) to reduce THD in the transformer. The core material type also has a strong influence on transformer THD. In the Jensen part numbers, an "L" means low (50% nickel) and "M" means high (80%) nickel. The 80% nickel parts, at any given output level, will have lower THD and slightly higher size and cost. Incidentally, all input transformers are made with 80% nickel core material.