Using an Optimod 8100A in a studio

I got an Orban Optimod 8100A in for repair. The power supply was the problem, the only tantalum cap on the board had opened up. That is strange since tants usually short, not this one. The symptom was that the + 15 volts would slowly decrease until it got to about 12 volts and everything on the front of the unit started flashing just before the Ua723 voltage regulator went POP.

That is not what this article is about. This article is about getting good flat audio out of the 8100A so you can use it in a studio for processing.

FM radio stations use what is called 75us, microsecond, pre-emphasis on the audio being shipped to the transmitter from the studio. They do this because of the natural tendency of the audio being sent being ‘dulled’ down. With pre-emphasis you boost the highs a lot, like 10db, and at the other end de-emphasize, or remove, the screechy highs to get back what you originally sent in the first place. They have been doing this forever.

This is what makes an FM processor unsuitable for being used in a studio where you want to do the absolute least coloring of audio that you can while processing it as far as compression and expansion and a whole host of other things. That involves de-emphasizing the audio.

The best way I have found is to disconnect the wires from the test jacks on the back, replace the RCA jacks with 1/4 inch mono jacks (you probably won’t even have to make bigger holes) and add a resistor and cap to the output wires. In the pictures above you can see the before and after. There are a couple 2 or 3db dips in the de-emphasized output around 400Hz but that can be taken care of with a good parametric equalizer or other device.

It is a pretty simple process. All you need is a 7.5K resistor, or good quality pot (I used 10 turn 20K pots) and a .01uF cap.

That is it. Now can go ahead and sweep the output to an audio spectrum analyzer and tune the resistor for the best and flattest output possible.

Oh yeah, the processor is not damaged in any way so if you get the chance to buy an FM radio station cheap you can still use it.

Let’s keep the old stuff alive and in service as long as we can.