Notes on connecting Speakers and external DSP
(courtesy Mark N8ME and John NØKHQ)Speaker Isolation Info
The external speaker jack on the Jupiter cannot tolerate a grounded speaker (this is documented in the manual) and thus cannot be used with some external DSP filter boxes. According to John (NØKHQ), he actually damaged his radio and had to send it back to TT (for repair). I was luckier, once I removed the ground the radio was fine.
I received a tip from Carl, N4PY, on isolating the speaker output so that a grounded speaker or filter can be used.
Take two Radio Shack Public Address (PA) transformers, catalog number 32-1031B, and connect the secondary outputs from one to the other. Then connect the primary of one of them to the Jupiter speaker output and the other primary to the speaker or filter. These transformers have multiple taps for both primary and secondary. As long as you are consistent and use the same taps on each, there should be no problem, but I use the taps that used the full extent of the windings (there is a schematic on the package). A more elegant solution would be a single 1:1 transformer, but I haven't been able to locate one that will handle the speaker output power.
The Pegasus does not have an external jack, but can be easily adapted to have one. Inside the Pegasus, the internal speaker is connected to the PC board with a two pin plug / jack typical to a lot of internal connetions in TT radios. I wired up a normally closed RCA jack to a cable with a mating plug and jack. Since I didn't want to modify the Pegasus case, I mounted the jack on the end of the leads in a plastic 35 mm film container. The jack is configured so that if nothing is plugged in, the internal speaker is connected, and if something is plugged in, the internal speaker is disconnected.
Note: the transformers mentioned above, are pretty large and heavy. I did not attempt to install them inside of the Pegasus. Instead, I mounted them into their own box and plug it into my speaker output jack on the Pegasus.
73, Mark N8ME
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| Images courtesy Kevin KA2NUE | Courtesy N8ME | |
Comment from John NØKHQ: (Feb 10, 2004)
From the ext. spkr. jack on the rear of the Jupiter run a 2 cond wire to the (c) common and (8) ohm taps on the "secondary" side of the xfmr. Run a 2 cond wire from the (c) common and (10) watt taps on the "Primary" side to your grounded speakers or, in my case a "Hear-It inline DSP module". There is no reason why this small mod wouldn't work with other DSP units.
Works great with no loss of audio dynamics. Best of all the smoke is still encapsulated in the Jupiter components where it belongs.
As I mentioned in an earlier email RS has discontinued this item, but some stores may still have some.
I still have (3) left. If anyone has a problem finding the xfmr have them send me an email.
So, with the IF gain adjustment and the addition of an outboard DSP unit the Jupiter received noise level is zip.....totally removed........its kind of spooky!
Update: February 17, 2004 (courtesy John NØKHQ)
The Radio Shack transformers have been discontinued. A replacement transformer from Electronics Plus Inc. (415) 457-0466 will work just fine. The part no. is LM25-10 cost is $7.95 + shipping they are a tad smaller than the RS model.
Update: February 24. 2004 (courtesy Kevin KA2NUE)
Hi Ken, Just got thru performing my version of the "speaker isolation mod" for the Jupiter. I say my version only because I could not locate in any of the audio transformers that Mark (N8ME) was using in his mod. It seems that Radio Shack is doing away with small parts line altogether. My mod basically is the same however I used the miniature audio output transformers (Radio Shack P/N 273-1380). The miniature transformers work well and no discernable difference in audio, even at high volume, no heat, smoke either. I connected the two secondary back-to-back (blue to blue and green to green) and each of the primaries for input and output. Hope this helps the troops. Kevin // KA2NUE )