Introduction |
I am always looking for a way to improve the audio in my shack. Initially, I built what I called a Key/Audio Switch Box. I had several different speakers. I had a SB-630 (for the SB-101), a HS-1661 (for the HR-1680), and several other speakers. With that box I was able to try several different speakers, with a simple switch.
But none of them really made the audio from my radios sound any better. But then I came into the possession of a couple of small Public Address (PA) amplifiers and a set of speakers from a old surround sound system. One was a Raymer Model 790-6A PA amplifier with 6 Watts RMS output. The other one was a McGohan MS-103 PA amplifier with 10 Watts RMS output. I had two of the MS-103s. One was complete, but the other was pretty much a parts unit. The speakers were a from a RCA CD/DVD Home Theater System that had six speakers (2 front, 2 rear, 1 center, and 1 sub woofer). Of particular interest was the "center speaker". Center speakers are intended to enhance the center stage sound, which is usually where the vocalist(s) is(are) located.
I tried all of the speakers and found that the "center speaker" gave remarkable clarity to SSB signals. It seemed to be tuned to just cover the normal voice range, without enhancing the noise. Overall, a very pleasant sounding speaker.
I chose to use the McGohan MS-103 PA amplifier. The output power was a little higher and it had better input selection. On the right is a simplified diagram of the MS-103 two auxiliary inputs. The Line-Input (top left) is balanced and feeds directly into a 600Ω:15KΩ audio transformer. The transformer then drives the Aux 1 Input. The Microphone Input is intended for a 3-wire XLR style connection which includes a 200Ω Balanced Line and an separate ground. Microphones for this input are usually high end microphones with low output levels. The Microphone input connector feeds directly to a MC-2 type input transformer which matches the low impedance of the microphone (~200Ω) to the Microphone Amplifier input (~50,000Ω). The Microphone Amplifier then brings up the microphone level to effectively Line-Level (0.3 to 2.0 V).
All of the inputs are mixed together and fed to the Pre-Amp and the Power Amp. While I have not shown any detail for those sections, the Pre-Amp contains overall Bass and Treble adjustments. The Power Amp is capable of driving a standard 8Ω speaker. Or, it can drive 25V or 70V constant voltage speaker configurations. For more details on the McGohan MS-103 PA amplifier, go to my web page on the McGohan 10 Watt Amplifier Model MS 103 - Public Address (PA) Amplifier where I provide a detailed explanation and detailed diagrams of the amplifier.
The block diagram may look a little odd. The mixing variable resistors look to be in the wrong direction and there appears to be nothing to prevent the input from being shorted to ground. You need to realize that this is a high level block diagram and doesn't contain details of the actual circuit. If you look at the schematic you will see that the mixing variable resistors are shown correctly and there are components that prevent the inputs from being shorted to ground.
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