The Make-Me-Mono Adaptor

Sometimes the best solution is the simplest
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WGLS has a speaker system that plays the station’s air signal through ceiling speakers all over the building. My first week working there, I noticed something odd when a Beatles song came on.

WHERE DID THE VOCALS GO?

As many know, early Beatles songs were known for their “ping-pong” stereo — they put the vocals in the right channel — and they were missing. Obviously, only the left channel was feeding the speakers. To me, this was unacceptable, and it needed to be fixed right away.

WGLS gets its air monitor off a tuner that also has RDS. That way, they can use the tuner for double duty — to listen to the station, and also to make sure the RDS is behaving correctly. From there, the signal went into an ancient LPB distribution amplifier, which fed the rest of the station, including the speaker amp, an old Lafayette 1-watt-per-channel transistor amplifier from 1966. It was the oldest piece of equipment in the station. One of my predecessors had paralleled its two channels together (it has output transformers), and apparently, the right channel had died about ten years ago. It was past time to make some changes in the monitor system.

Schematic diagram of the mono adaptor.

Schematic diagram of the mono adaptor.

I called my counterpart in the TV department at the college and asked him if he had any newer distribution amplifiers. It turned out that he had a pair of Sigma ADA 108 mono audio DAs that he had just retired and was willing to give me. All that was necessary to make them perfect for this purpose was to replace their input op-amps with better ones (the Sigma uses LM301s — an ancient op-amp) and replace their filter capacitors. A couple of hours later, I had two perfect mono audio DAs that were ready to go into service.

THE PARALLEL(OGRAM) PROBLEM

Upon pulling the LPB DA out of the rack enough so I could get to its terminal strips on the back, I saw a potential problem: The Lafayette amplifier had been paralleled across another output — unbalanced! That output was used to feed the logger for the station, the very one that had a low-level hum on it — forever!

I cut the wires to the Lafayette and – presto! No more hum on the logger. The new DAs went right in and resulted in an immediate improvement in the sound of the stations air monitor. But what to do about the speaker monitor problem?

When I looked at the schematic of the LPB DA, I saw that it was actually a pair of 3-watt-per-channel stereo power amp ICs connected in balanced bridge mode (BTL) with 120-ohm resistors connected from the amp outputs to the individual DA outputs. Why not jumper out the 120-ohm resistors on one output and use this to replace the Lafayette amplifier driving the speakers? Seemed like a good plan — but how would I get a true mono signal to drive this unit?

I opened up the LPB DA, replaced its filter capacitors, jumpered out the resistors on its last outputs, and soldered pennies to the heatsink pins on the power ICs (but that is another story). When I hooked it to an 8-ohm speaker, it sounded great. The Lafayette amplifier could now have a long overdue rest.

THE MAKE-ME-MONO ADAPTOR

I still had the problem of coming up with a mono feed for this “new” speaker amplifier. As I was thinking about this, a student walked past my office, handed me a small plastic box some costume jewelry had come in and asked me to put it in my trash can. This gave me the inspiration for the make-me-mono adaptor.

The Make-Me-Mono Adaptor is built in a plastic jewelry box.

The Make-Me-Mono Adaptor is built in a plastic jewelry box.

The basic adaptor is designed to plug in between the FM tuner output and the DA input. Four RCA jacks (input and output) are mounted to opposite sides of the plastic box. Their grounds are all connected together, and a wire connects the centers of each channel’s input and output together. This means that it is completely transparent to the DA that it feeds.

There is a small circuit board with one NE 5532 dual op-amp and one LM386 connected as a rail splitter (if you have read my earlier articles, you know that I use the LM386 to split single ended DC power into +/– DC all the time).

One half of the 5532 is configured as a summing amp that gets its inputs from the jacks connected to the input (tuner). The other half is an inverting amp that makes up the other half of the unit’s balanced output.

Since the summing amp actually inverts the audio, we use the output of the second op-amp as the + output, essentially inverting the other output back into phase (and also giving us 6 dB more output voltage).

With a –10 dBV input, the output of the summer is –4 dBV, which is plenty to drive most equipment, including the LPB DA.

The entire circuit is composed of just the ICs, a few 10K and 47-ohm resistors and two capacitors.

HOW DOES IT WORK AND SOUND?

In a single word, excellent! It plays nice and loud. Since it comes directly off the tuner, the audio quality is great. We also did two other things that anyone using an amp and speakers that are not part of a constant voltage speaker system should do: ALWAYS wire the speakers in SERIES (+ of one to – of the next one). If you need to lower the volume of a particular speaker, simply put a half-watt resistor across that speaker. We had to do that for the ceiling speaker just outside the station entry door.

The uses of this adaptor are only limited by your imagination.

Dana Puopolo is chief engineer at WGLS(FM), Rowan University in Glassboro, N.J. Read past articles on radioworld.com, keyword “puopolo.”

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