Now I'm going to embarrass you Don! But you started this high fidelity post!
About 10-15 years back or maybe a little more, W4KKO and I built a passive audio filter section and stuck it in the line of one of his HB rigs. It was very flat from 10-2800, rolled off quickly to about 3500 and then went down like a rock. Took us about a week to get the darn thing right. We had it set up to be switched in & out with the flick of a single switch.
You just happened to be around as one of the "test subjects" on the first day of use and you could not report any difference with it in or out. We ask you several times about the audio with his voice and mine, filter in and filter out, no difference reported from you or several others. We really laid a bogus line on you & others about adjusting this & that!
Now get that mixer back out and let's do the old "this is microphone A, this is microphone B, this is microphone C and this is all 3 mixed together" thing again.
I probably had the 6 kHz AM filter in the 75A-4. With that, which I used 80% of the time in the old "AM Window" on 3870-90, due to congestion, it would be about impossible to tell any difference between audio out to 3000~ vs out to 5000~ and beyond. Most AM receivers on the ham bands are probably set up for similar bandwidths under congested condx. I have a 8 kHz filter which I use more often now down below 3750 where the congestion is less severe. It does make a big difference, even more so than comparing the 8 kHz filter to the 16k (originally used in an R-390A).
I have two low pass filters in my audio, with a switch to select the one of choice. One is a brick-wall filter that cuts off very sharply at 3400~. You can connect a signal generator, and at 3300~ there is negligible attenuation. At 3400~ it is 20 dB down or so. At 3500~ the modulation is undetectable on the scope. The other one begins to cut off somewhere about 5000~, and the attenuation increases more gradually with frequency, until the attenuation beyond 7500~ is sufficient to make the modulation undetectable on the scope.
Most of the time, I can switch between filters and the other operator cannot tell the difference. But if he is using a wider bandwidth at the receiver, most of the reports are that there is a very noticeable difference.
I also use a substantial amount of presence rise, beginning about 800~, and rising to about a 9 dB boost somewhere above about 2000~, and it is flat beyond that out to the cutoff point of the filter. In addition, the D-104 has its own built-in acoustical presence rise. The combined "presence rise" seems to compensate for the lack of highs above 3400~, so with that filter I often get reports of "broadcast quality" audio. Without the boost, reports are that the audio sounds "muddy."
For more information on the response curve I use, see some of the old articles in The AM Press/Exchange by George, W2WLR. My response curve is very close to what he recommends in his "enhanced AM" articles. I think he was onto something in those articles, but then got sidetracked and went off in a tangent in other directions.
I still use the combination microphone setup, which I developed in about 1976 while operating out of Cambridge, MA. It consists of a homebrew 2-channel audio mixer, based on the circuit in the old RCA Receiving Tube Handbook. Two microphones are mixed together in phase, a D-104 and an Electrovoice model 670 dynamic, which has practically no high frequency response ever since I removed the little high/low impedance transformer that was inside the case, erroneously thinking it had crapped out. But actually the lack of highs in the dynamic mic makes it a better choice for mixing with the D-104, since I don't get the cancellations at various frequencies in the upper frequency range that would otherwise create a "comb filter" effect on the audio response.
At times, under heavy QRM condx, the D-104 does better alone, but usually I get better reports with the combination. Everybody says the dynamic is too bassy, even with the presence rise switched in. The presence rise can be switched in/out separately in each microphone channel. I have a Sony dynamic mic that sounds very good over the air with the treble boost switched in, but I rarely use it.
I have noticed a subtle form of distortion, kind of a rattling sound, with the sharp filter, evidently due to ringing as the higher audio frequencies crash against the "brick wall" of the filter. Others have reported hearing it too, but only after I called their attention to it.
Cave City is a long haul for me, too, for such a small indoor hamfest. If I could hitch a lift from someone in this area, I'd go with them, but I don't feel like making the drive myself. It is about 100 miles from here and I don't feel up to the 3-4 hours of combined state highway and interstate driving for the round-trip.
