The AM Forum

Well, still going thru the boxes and found my National Select-O-Ject SOJ-3 audio notch filter. It was one of the very first audio filters. It’s sort of like a primitive DSP filter, but for analog audio instead of digital. It was designed to plug into the back of most of the common National receiver models. It actually works fairly well, but has anyone else had any experience with one of these units? Is it a great little Select-O-Ject audio filter or did National waste their time and create big Select-O-Reject flop?

Are you hating on National Radio? Them's fighting words in Massachusetts, son!!!

Looks like a q-multiplier...

I don't understand the question? You said it works fairly well, so I'd guess you're seeing some benefit from  using the device? I think the original design may have come from an old GE Hamnote publication.

I use an Autek analog audio filter on my JRC rx.

Pete

This and a Q-mult are both active filters. One is a bandpass and one is a notch.

The only Select-O-Ject I have is the one built into my NC-125; for CW I like it better than the Q multiplier built into some of the other lower cost receivers.  It does a good job as an audio peak filter on CW and a decent job on hetrodynes on phone.  The outboard unit is pretty scarce so glad you found yours again!

I have several Autek QF-1's and QF-1A's hooked to various receivers and they do a good job.  My Drake R-4C with the narrow Sherwood CW filter and an outboard QF-1A is still my favorite for pulling weak CW sigs out of the noise on 160 and 80.  The DSP and full set of Inrad filters in my FT-1000MP is not so impressive compared to what the Drake/Sherwood/Autek setup can accomplish.

Looking at the skiz, it looks like you can run it in the Boost mode (bandpass) or Reject mode (notch). Looks like a neat addition to the shack.

The QST article, in the November 1949, page 11, was called Selectoject. A very detailed scoop on the homebrew design and working of this piece of equipment. Thr article is available in PDF format on the QST member's only section of their web site.

I have a homebrew one picked up from an estate.  I have never tried hooking it up to a receiver, mainly because no documentation came with it.

The select-o-ject works off the audio stages in the receiver.  The Q-multiplier, which also can work in the boost or reject mode, works at the i.f. stage.

The Q-multiplier should give a cleaner select or reject performance because the selectivity occurs earlier in the receiver circuit, with fewer active stages between the antenna and the selectivity circuitry.

Ideally one should use some selectivity before the first i.f. stage, and then mop up any filter blow-by with additional a.f. selectivity following the detector.

For cw I use a 300~ crystal filter in one of my 75A-4's, and a 500~ mechanical filter in the other.  I have a solid state variable pitch and variable selectivity audio filter installed between the output of the detector and the outboard AF amplifier.  I think it was made by Bencher.

Ever since I retired my 1935 vintage HRO in the early 80's.  Picked up my first one with slopbucket filter only, for $200.  Picked up my 2nd one with all 3 stock filters for $100!  It had a damaged (but usable) vernier drive/tuning knob assembly that I had to destroy in order to remove  from the receiver to allow me access behind the front panel, but I found a n.o.s. Collins replacement kit, virgin still in the unopened original Collins box and packaging along with assembly instructions, for $35.

I have made some reversible but significant mods in both receivers to enhance AM performance, but if I ever find something that is really better than what I have now, I'll probably replace it.  Or if I ever get a round tuit, I might homebrew my own.

Ive had my 75A4 since 1964 Don, bought it while at National. Its not exactly stock tho, made all the changes back then when the A4 was just another old radio.

The SOJ wasnt a big seller mainly due to the price. I have one but havent plugged it in for decades.

Carl
KM1H

I was researching a Select-o-ject SOJ-3 to sell on ebay and your posting came up. Very informative, being a trucker and using the C.B. radio for 30 years I could at least follow the basics.   What would be a fair price for a nearly new in box I could expect from an Auction?

Hey circus freak, welcome to AMFone.

You could search the completed listings on eBay, but a mint in box one I would expect should go for $100-200.  Perhaps more or less, depending on the unpredictable market at the time.

Here's one that's there now:
http://cgi.ebay.com/Select-O-Ject-Model-SOJ-3-Ham-Radio-Notch-Filter-/280705754220?pt=LH_DefaultDomain_0&hash=item415b5de46c

I sold one at Dayton two years ago for $65.

I like it on CW and it's fun to play with to make you jokers sound like Martians on AM.

Thanks all.  It's desired on Ebay, 4/1 lookers-to-watchers is a good ratio

    The NC-109 also has an octal socket on the back chassis panal for the Select-O-Ject.

Actually that's just auxiliary jack for connecting the crystal calibrator. There are no connections there (in its stock form) for a SOJ. The NC-109 doesn't need one since it has a fairly good crystal filter.