[TenTec] 3.1 KHz IF Filter for 2nd IF in Omni 6

Gary J FollettDukes HiFi dukeshifi at comcast.net
Mon Jan 11 10:45:22 EST 2016 

I don’t disagree with the things you have said about the role of the DSP in restricting the Omni 6 SSB sound. It is a major limitation.

However, I had experienced significant audio frequency restriction in previous tests I had done with plain Omni 6 radios that had no DSP. Perhaps that was the result of some of the audio filtering that was present in those radios.

However, I believe I am probably the only person who has had access to this filter pair as they were very costly one-up devices.

The result was that, with the DSP and all on-board audio circuitry bypassed in this very late Omni 6+ that I have, the use of the 3.1 KHz filters in both IF’s made a very dramatic change in the audio quality, when compared against the stock 2.4 KHz filters. No amount of operating the PBT with the 2.4 KHz filters in place could produce the sound quality I hear with the 3.1 KHz filters.

In addition, Inrad does offer a 2.8 KHz filter for the Eagle and the Orion 2. Neither of these passes the CW signal through the IF roofing filter. Why would they offer that?

I  think there is a need to define the term “restricted response” in terms of a ham receiver. 2.4 KHz is plenty of bandwidth for any receiver to produce acceptable communications quality audio and, as you state correctly, the Omni 6 (any version) provides “acceptable communications quality audio” with the stock 2.4 KHz filters in place and the DSP bypassed or not present). But when I set any Omni 6 beside an Orion or an Icom Pro series radio, the sound from the Omni is fatiguing because it does not sound natural. I’m not looking for hyper-bass, just sound that is natural (to me), as close as possible to that which I would hear with the person in the room with me. With the 3.1 KHz filters in place, and the DSP bypassed, this Omni 6+ sounds VERY natural.

I have been a music audiophile for probably 40 years, and have built output transformer less vacuum tube power amplifiers from scratch in order to give me the sound I wanted with some significant bass (but not to excess). Therefore I am pretty experienced in knowing what to look for when I make changes to an audio product, which is essentially what we are talking about here.

73,

Gary
W0DVN



> On Jan 11, 2016, at 7:09 AM, Barry N1EU <barry.n1eu at gmail.com> wrote:
> 
> I just wanted to post a fuller follow-on comment, now that I'm no longer
> dependent on a cell phone to post.
> 
> People are free to believe whatever they want to believe, but in actuality,
> there is ONE and only one reason for the restricted ssb rx audio passband
> in the Omni 6, assuming you don't have narrow xtal filters installed and
> assuming you have the BFO oscillators properly aligned.  Actually a single
> picture tells the whole story:
> http://omni6.wdfiles.com/local--files/rxaudio/o6spectra_text.jpg  - with
> the dsp processor out of the signal path (top trace), the response is only
> determined by the filter bandwidth.
> 
> All the Omni 6 rx audio goes through the dsp processor.  The dsp processor
> has several principal functions, and you can't simply bypass it without
> there being issues.  But one obvious dsp effect is rolling off the rx audio
> spectrum at the bottom and at the top, by design.  Ten-Tec designed the
> Omni 6 from the beginning to use 2400hz filters and optimized the dsp
> processing for those filters.  It only becomes an issue if you want fuller
> fidelity (especially on the low end).
> 
> The Inrad 2800hz 9MHz filter was introduced NOT to improve ssb fidelity,
> but to produce a better sounding cw transmit signal.  You can search the
> reflector archives and find all the information from ~15 years ago.
> 
> Over the years I've done A LOT of experimentation on improving the ssb
> receive response, but it's not a simple matter.  I succeeded in the end,
> but I also came to realize that the Omni 6 is essentially/inherently a
> superb cw radio (perhaps the best ever) and the Orion is the much more
> suitable radio if you want better ssb receive fidelity.
> 
> I also found that the Omni 6 carrier null level is marginal with the Inrad
> 2800Hz 9MHz filter installed, even after re-aligning as Inrad recommends.
> I've done this on several Omni 6's and the carrier null is shallow and I've
> always thought the amount of carrier still being transmitted was enough to
> make me a bit uncomfortable using the rig in ssb with that filter installed.
> 
> 73, Barry N1EU
> 
> 
> 
> 
> On Sun, Jan 10, 2016 at 10:03 AM, Barry N1EU <barry.n1eu at gmail.com> wrote:
> 
>> 
>> On Jan 10, 2016 1:18 AM, "Gary J FollettDukes HiFi" <dukeshifi at comcast.net>
>> wro
>>> 
>>> 
>>> This supports the notion that the restricted audio arises in the IF
>> combination of filters and their respective center frequencies and overlap.
>> 
>> Sorry but this is absolutely not true
>> 
>> 73, Barry N1EU
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>>> 
>>> The only other p=lace where such restricted frequency response could
>> take place is in the very first ceramic filter that is used prior to any of
>> the 9 MHz roofing filters. I have never tried bypassing this just to see
>> what impact it has on the audio response. I will wait for that until I see
>> hows the filter arrangement in this particular Omni 6+ sounds.
>>> 
>>> 
>>> I can’t wait to hear how this Omni sounds tomorrow with the high buck
>> 3.1 KHz filters in both IF’s. I have high hopes...
>>> 
>>> Gary
>>> 
>>> 
>>> 
>>> 
>>>> On Jan 9, 2016, at 10:58 PM, elespe at lisco.com wrote:
>>>> 
>>>> That is why I asked.
>>>> I have the 2.8khz INRAD filter and the radio sounds fantastic and gets
>>>> great audio reports but keeping the carrier supression better than 34
>> db
>>>> on both sidebands is impossible without moving the bfo which, without
>>>> doing the math on the entire radio frequency scheme, seems to have a
>>>> trickle down effect on screwing something else up along the way.  If
>> the
>>>> bfo is wrong doesn't that reflect in the readout as well?
>>>> I love the INRAD "wide" filters but am going to change the 1st if down
>> to
>>>> a 2.4khz model to keep everything else in check.
>>>> Along the line of sounds great I somehow stumbled across a speaker that
>>>> fits the original hardware and hole and actually makes the OMNI VI
>> sound
>>>> better than any external speaker system I have tried on it. Just dumb
>> luck
>>>> and Parts Express---who would know.
>>>> Paul K0UYA
>>>> 
>>>>> As I recall, when one purchased the Inroad “wide† SSB filter for
>> the
>>>>> first IF, one needed to adjust the BFO frequencies in order to get
>> proper
>>>>> positioning of the carrier on the filter slope. I do not (yet) know
>> the
>>>>> adjustment range for the BFO’s bu I am guessing that the BFO
>> adjustments
>>>>> in this one must have pushed that frequency another 300 Hz, or the BFO
>>>>> crystal(s) were changed out.
>>>>> 
>>>>> Anyone who had spent $1000 to have custom filters made up would
>> certainly
>>>>> have taken proper measures to ensure that transmitted SSB was properly
>>>>> generated.
>>>>> 
>>>>> I should know the answer tomorrow.
>>>>> 
>>>>> I have not looked yet, but it is POSSIBLE that the radio may have been
>>>>> rewired to route the TX 9 mHz IF signal through the position 2 filter.
>>>>> That is a lot easier to do the one might think, due to the use of
>> diode
>>>>> switching to select the desired filter, so it is possible.
>>>>> 
>>>>> The first IF filter that was in the position 2 is marked “48074â€
>> and
>>>>> the second IF filter in position 2 is marked 48058.
>>>>> 
>>>>> Can anyone tell me if these are the numbers for the “stock† SSB
>>>>> filters?
>>>>> 
>>>>> I can measure them fairly easily for bandpass but it would save some
>> time
>>>>> if anyone simply knew the answer to that.
>>>>> 
>>>>> I will, however, put a frequency counter on the BFO tomorrow and
>> measure
>>>>> the frequencies on LSB and USB to determine what was done to make this
>>>>> work. As you know, without adjustment of the BFO down the filter edge,
>>>>> both carrier suppression and SSB suppression would suffer quite a lot.
>>>>> 
>>>>> Gary
>>>>>> On Jan 9, 2016, at 9:54 PM, Paul Kraemer <elespe at lisco.com <mailto:
>> elespe at lisco.com>> wrote:
>>>>>> 
>>>>>> How is that possible?
>>>>>> The lsb and usb bfo frequencies are only 3khz apart
>>>>>> Paul K0UYA
>>>>>> 
>>>>>> -----Original Message----- From: Gary J FollettDukes HiFi
>>>>>> Sent: Saturday, January 09, 2016 9:05 PM
>>>>>> To: Discussion of Ten-Tec Equipment
>>>>>> Subject: Re: [TenTec] 3.1 KHz IF Filter for 2nd IF in Omni 6
>>>>>> 
>>>>>> I bit the bullet and took the entire bottom cover off.
>>>>>> 
>>>>>> I found that there is also a custom 3.1 KHz filter in the number 1
>>>>>> position in the first IF (9 MHz)! This is also an 8 pole filter with
>> the
>>>>>> IF cans on board to tailor response. It is equally well made as the
>>>>>> second IF filter is well made.
>>>>>> 
>>>>>> I’ll report tomorrow on how the radio sounds with quiet band
>>>>>> conditions and also get a transmitted SSB audio report. I may have
>> found
>>>>>> the Omni 6 of my dreams!
>>>>>> 
>>>>>> I have always hated the SSB sound of the omni 6 (all variations).
>>>>>> 
>>>>>> I contacted the fellow from whom I got this and he informed me that
>> the
>>>>>> original owner of this radio had these filters custom made for a
>> cost of
>>>>>> over $1000. I sill do not know what company made them but they are
>> top
>>>>>> quality all the way.
>>>>>> 
>>>>>> Apparently that fellow hated the sound of the Omni 6 as much as I
>> doubt
>>>>>> had the resources to take corrective action.
>>>>>> 
>>>>>> Hopefully these filters will offer some relief as the overall design
>> of
>>>>>> the Omni 6 is one of my favorites over any other radio design.
>>>>>> 
>>>>>> It’s also on of the prettiest radios going.
>>>>>> 
>>>>>> Gary
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>>> On Jan 9, 2016, at 8:17 PM, Mark S. Holden <
>> mark at holden-insurance.com>
>>>>>>> wrote:
>>>>>>> 
>>>>>>> On 1/9/2016 8:32 PM, Gary J FollettDukes HiFi wrote:
>>>>>>>> I have a 3.1 KHz bandwidth filter in one of my Omni 6+ radios. It
>> fits
>>>>>>>> perfectly into the TenTec socket and was obviously made for the en
>>>>>>>> Tec. However, it looks unlike any filter I havee ever seen in ANY
>> Ten
>>>>>>>> Tec product.
>>>>>>>> 
>>>>>>>> It is an 8 pole filter but it also had four small IF transformers
>> on
>>>>>>>> it, presumably to tailor the passband shape.
>>>>>>>> 
>>>>>>>> Has anyone heard of this filter? Does any know who made it? It is
>> too
>>>>>>>> perfect to have been a home brew job. It is built on thicker board
>>>>>>>> stock than that used for most Ten Tec filters.
>>>>>>>> 
>>>>>>>> I would assume that its intent is to improve received audio in
>> SSB. I
>>>>>>>> am going to try it in a few minutes to see what the radio sounds
>> like
>>>>>>>> compared with the others.
>>>>>>>> 
>>>>>>>> Gary
>>>>>>>> _______________________________________________
>>>>>>>> TenTec mailing list
>>>>>>>> TenTec at contesting.com
>>>>>>>> http://lists.contesting.com/mailman/listinfo/tentec
>>>>>>>> 
>>>>>>>> 
>>>>>>> I'd expect that to be a fun filter for rag chewing.
>>>>>>> 
>>>>>>> My Harris RF-550 rx has a 3.24khz crystal filter that's incredible
>> for
>>>>>>> SWL.  I also installed some crystal filters intended for the Racal
>>>>>>> 6790gm in my AOR 7030+
>>>>>>> _______________________________________________
>>>>>>> TenTec mailing list
>>>>>>> TenTec at contesting.com
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>>>>>> 
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>>>> 
>>>> 
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