Well if you turn down the "RF Gain" on a Corsair, virtually all the hiss
disappears leading me to point blame at the IF chain.
BTW: You Pegasus owners can use the headphone jack for an 8 ohm speaker if
you are handy with a soldering iron. Each lead on the jack has a 100 ohm
resistor and electrolytic capacitor in series with it. Just bypass them all.
You won't believe how good it sounds!
Steve Ellington
N4LQ@iglou.com
----- Original Message -----
From: "Duane - N9DG" <n9dg@yahoo.com>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Wednesday, May 11, 2005 9:58 PM
Subject: Re: [TenTec] hiss hiding
> --- Steve N4LQ <n4lq@iglou.com> wrote:
>> The Triton through the Omni series were all quiet
>> receivers. Narrow xtal
>> filters were added to the Triton to make it an Omni. On the
>> Omni C, a small
>> transistor amplifier kicks in when using the 500hz cw
>> filter. This adds just
>> a touch extra noise. Otherwise, the Triton and Omni are
>> neck and neck for
>> noise. Being single conversion rigs, they had few mixer
>> stages thus less opportunity for noise.
>
> The 580 Delta is almost in this same camp though a dual
> conversion design. I found its main limitation (other than
> poor AGC) is that the audio chain is rather noisy. Ten Tec
> radios of this vintage I believe all used the ubiquitous
> LM380/383 series ICs for their audio amplifiers. Theses
> aren't very good audio amps if you look their specs sometime.
> I'd bet that the RX audio of all TT radios of this vintage
> could be improved upon with an audio PA update.
>
>> Then came the Corsair. Passband tuning was added. To
>> achieve this feat meant
>> another mixer and another IF stage. The extra IF at 6.3mhz
>> eliminated the
>> 17m spur but added noise and lots of it in the form of hiss
>> or white noise.
>> In the original Corsair you just had to live with the
>> noise.
>> Then came the Corsair II. Still had the extra noise but
>> they added a tone
>> control. Turning this control to minimum treble cut down on
>> the hiss.....some.
>
> Like the models that precede it I've also concluded that it
> is the audio chain that accounts for 80% of this hiss in a
> Corsair series radio. The Corsair has a number of op amps
> that make up the active audio filtering, I suspect that the
> cumulative noise and distortion of those parts is not
> insignificant. They too use a somewhat noisy LM38x series
> audio output IC. My experiments last fall using the Corsair
> II as a 9MHz IF feeding a 9MHz detector (I/Q baseband audio)
> feeding DSP IF processing chain (PC sound card with SDRadio
> software) seemed to confirm this. I did not however try
> tapping the 9MHz after the PBT tuning but I suspect that
> there would be minimal differences than when I tapped the
> 9MHz signal immediately after the first 8-pole 9Mhz filter.
>
>> They came the Omni VI. Still hissy but now we have DSP to
>> help hide the
>> hiss. Works pretty slick as far as hiss hiding goes.
>
> After some playing with a recently acquired Omni VI (price
> was too right, couldn't pass it up) I found a new variation
> of audio chain limitations. From what I can tell the A to D
> is only 14 bits, I've never been convinced that 16 bits is
> enough for high fidelity audio, so needless to say I have
> reservations about 14. The Omni VI does however use a more
> modern audio PA IC; it is noticeably cleaner than the
> LM38x's. It is a TDA8611A, if you look up its specs it is
> considerably better than the LM38x. The Pegasus (Jupiter and
> Orion too I presume) use the TDA7056B. I always found the
> Pegasus to have excellent audio. The Peg's (Jupiter too) main
> shortcomings are the high-ish 1st LO phase noise and high
> frequency first IF, - therefore suffers from the typical wide
> roofing filter dynamic range limitation. Can't say I've
> noticed digital noise artifacts for the way I use the Pegasus
> though.
>
>> I played with the Flex-Radio SDR for a few weeks. I got the
>> same buzz. It
>> was as if my ears were directly connected to the ether!
>
> I also attribute much of the SDR-1000's *stellar* audio
> fidelity to the fact that you use an inherently clean audio
> chain by virtue of it being a modern PC sound card. Almost
> all such sound cards perform well for high fidelity music
> reproduction purposes so it is no accident that a radio based
> on them would have very good audio too.
>
>> Crazy thing was
>> useless as a transmitter and flaky as a june bug though.
>> It's SINGLE CONVERSION!
>
> Its software is changing weekly, at least the experimental
> code branch is. It is not the same radio
> performance/behavior-wise that it was as little as two months
> ago. Fun stuff.
>
> If you dig deep enough and get your head wrapped around the
> inner workings of the SDR-1000's QSD you will realize that it
> isn't really a mixer at all, it is actually a "sampler" that
> provides an audio baseband version of a small slice of RF
> spectrum. So in reality its analog RF section is a "Direct
> Conversion" radio in the truest sense of the word.
>
>> Some day I hope others follow this
>> lead and perfect it.
>
> It's just a matter of time before almost all radios will
> include many of the design concepts and approaches that the
> SDR-1000 is now pioneering.
>
>> Bottom line here is; too many mixers spoil the radio.
>
> Yepp.
>
> Duane
> N9DG
>
>
>
>
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