Possible advice for AM AUDIO?

[jefflaurence]

Okay..here's the deal. 1KW AM with studio directly accross a street from xmtr and tower. Station is run as a standalone with a PC running an automated modified adult standards format. Audio coming from the computer is great..clean, and dynamic.

At the XMTR is an Optimod AM..XMTR is an MW1. Both units test well..BUT the audio is excessive in the middle mids, and the low end is distorted..as is the upper end..(sounds like a massive rolloff after 3K)

There is also a weird "gating" effect i.e the cold endings of songs, and reverb seems to clamp quiet..Overall not a real good sounding station..but the modulation is where it should be...allbeit clipped.

I am good pals with the owner and he told me that the audio is actually delivered to the XMTR (accross the street) over a long run of un-shielded ROMAX cable! I am telling him that this could be a big issue with the cable picking up RF from the tower. A kind of internal feedback that would cause the distortion and weird fidelity? I think so! But I defer to you..any thoughts?

Also there is an APHEX Aural Exciter added after the Optimod in order to try and gain some high end? ALSO this must be a problem.

For a loud and pleasing AM music station..you guys MUST have a set of settings for the Optimod AM that you like. Would you mind sharing? I am advising him to run a low-power microwave link from the studio to the transmitter..then to dump the Aural Exciter. At that point he would be running the Optimod solo..so he could sure use some help.

 

[Nostalgia]

I have read about many new UNLICENSED short haul microwave links available today. I think across the street qualifies for short haul, and who would want to pay for coordination and licensing for a real STL for that distance. TESSCO advertises these alot for their "last mile" links but I bet some would work for analog audio. Anyone out there using any of these things?

Agreed that the underground wire is working as a big antenna, or worse, part of the towers ground system! Sucking up all that RF and depositing it at the input terminals of the Optimod, and back into the sound card on the computer. I bet putting a scope on that audio line would tell the tale.

Has anyone tried dragging the computer across the street some night and hooking it right up to the Optimod (and ditching the Aphex)?

 

[Tom Wells]

If (rectified) audio and/or RF of an AM are getting back into the audio feed wiring, you can either get feedback and oscillations, or the
strangely damped audio you're hearing. On my part 15, if the grounding isn't right on equipment, it gets back into the reverb unit, and
the result is WAY more reverb than desired.
One question is whether that feed is balanced or unbalanced.
If it's unbalanced, you might get an improvement just by setting it up as 600 ohm balanced line.
Assuming that means transformers, there would be some loss of definition, which probably wouldn't hurt
most adult standards.
I thought STLs were a little more transparent than they are; with WSM 650 in Nashville now
broadcasting from the transmitter location, it is definitely clearer and sharper and I can hear the difference on the air signal.

 

[rfburns]

The audio problems could be coming from a number of things. Replacing the Romex with shielded audio cable or an STL system of some sort would be the first thing. Move or get rid of the Aphex unit - the Optimod should connect directly to the transmitter input. What model / age Optimod? It could be a problem with the Optimod itself. If you can, borrow a known good AM processor and verify the Optimod doesn't have issues. If you can't, hook the program audio directly into the transmitter - run the transmitter at low power and make sure the program audio is at a low enough level to prevent overmodulation.

RFB

 

[OKCRadioGuy]

Some of the older AM Optimods sounded terrible if the unit had been mistreated with too much heat too many times. I had one like that. It sounded truely lousy. You might see about trying another loaner processor to see if you can make it sound better. If you can, you know where the problem is. You might be able to send it over to Bill Sacks to have it fixed, or possibly just get an AM Omnia One as a replacement.

 

[bilco]

That run of Romax scares me almost as much as the Aphex. Just for kicks, try feeding the transmitter with No processing. That will eliminate the Aphex and Optimod as part of your problem. Some very heavy duty toroids near the transmitter end and a good center tapped grounded transformer at each end of the Romax might help as well. That short a run, there are dozens of cheap RF solutions from older STL transmitters, to WiFi transmitters or even a part 15 FM transmitter for that short a run. I knew there had to be a good use for that Ramsey stuff!!!!
As to Optimod settings you can hardly go wrong with factory presets if you deliver clean audio to the boxes. The "gating" affect sounds like the Optimod may have problems but it could be to much RF.

 

[Randal Marshall]

What is the characteristic impedance of the Romex? Long cable runs need to be terminated with their characteristic impedance, otherwise it will sound exactly as you have described it.

Replace the Romex with balanced shielded audio cable and use TELCO transformers 110 ohm/600 ohm on both ends (110 ohm side towards the cable). Put .5 amp fast blow fuses on the equipment side (600 ohm) at both ends (4 total) for lightning protection. Ground the shield at the transmitter end. I've used this method at distances greater than 1000 feet and it works well.

Then mess with the Optimod - Aphex unit

 

[Chuck]

CAT-5 cable works amazingly well for audio, as long as it is balanced. With it, a shield really isn't necessary, although you can buy a shielded version as well. A single run gives you four balanced pairs. It is cheap and easy to buy. Any home center can sell you some.