Sunday, August 2, 2009

Galaxy Audio Hot Spot PA5X140 and PA3

These two small powered monitors are in service at the keyboards at Hillcrest E-Free Church in Seward NE. Each have two inputs and are gain-staged to accept instrument- and mic-level signals. However, they're actually fed the line-level outputs from the keyboards, with the second input coming from the mixer in a music-plus-one monitoring arrangement.

Because the input signals are rather hot, all the gain pots are almost closed off completely, giving only a few degrees of rotation between too-loud and off. It was time to do something about it.

The PA5X140 used a C-law pot with an inverting opamp to get the large gain range needed to handle mic to line-level inputs. One end is the input, the wiper is connected to the inverting input, and the other end is connected to the opamp output. Plotted is the gain-versus-rotation of the stage using a linear pot.

The easy solution was merely to adjust the gain of the power amp from 38 dB down to 18 dB, effectively raising the as-used pot position from 5% to about 30% rotation. With only a single resistor change (next to the 1/4in TS jack), un-modding this unit will be trivial if needed in the future.

The beige-faced Hot Spot is, I believe, an ancient PA3. Even the schematic provided from Galaxy had little in common with the actual PCB. Volume control for the vintage model was the normal attenuator with make-up gain. The smoking gun, though, was the pot marking "104KA" -- a 100k linear pot. Volume controls "feel" best with an exponential R-vs-rotation slope to be linear in dB.

Quick mod #2 was adding a shunt resistor of 10k from wiper to ground to fake loggify. Rod Elliott of Elliott Sound Products calls this A Better Volume Control.

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