The old version of io connected the output of the synthesizer directly to the amplifier. [see: figure] This wasn’t too much of a problem since, though unbalanced, the signal was low-impedance, line-level.

There were, however, a couple of less desirable aspects to this. One was that the drivers (the woofer and the tweeter) would not be receiving signals tailored to their frequency response. Another was that there wasn’t any easy way to alter the signal level going to each driver.

io 0.0.1 beta++ will have a pre-amp that links the synthesizer output to the amplifier. Each channel will pass through a filter (optionally bypassed), a gain stage and a balanced line driver.

pre-amplifier outline

I haven’t decided whether the filters will be first-order, second-order or some combination of those (e.g. first-order for the bass driver, seconds-order for the treble). I’m planning to build this component into a separate board so that various filter response types and frequencies can be easily tested out.

The boost stage is something that I needed when dealing with some amplifiers in the past (I generally asked that the venue / space / promoter provide the amplifier). The actual gain amount will be set by an internal trim pot, but I’ll put a volume pot between this and the balanced line driver.

I initially planned to build the balanced line driver from a few op-amps and discrete components until I discovered the existence of ICs that implement these. (For example, the TI DRV134 and the Analog Devices SSM2142.) I plan to use one of these in the pre-amp.