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Author Topic: Fair price for audio processing equipment  (Read 4800 times)
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W8ACR
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« on: August 14, 2018, 11:12:58 PM »

My Broadcast transmitter was delivered today. The radio station owner offered to sell me some audio processing equipment as well. The four units are pictured below. Assuming they are in working order, what should I be willing to pay to get these units?

The last picture is the transmitter itself. 4-400's X 4-400's.


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K6IC
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« Reply #1 on: August 14, 2018, 11:34:47 PM »

Hi Ron,

Will pass,  for now,   on the processors ...

BUT,   IMO,    that CCA AM 1000D  is a very fine box.

Good Iron,   and,   first cabin isolation of the Plate Tank,  from all else.   Should be easy to get coverage from 160,  to 40 M.   20 should be fairly easy,   but  20 is a busy band,  and may not be too well-received for AM transmissions.

FWIW,   Have fun,  ...   all IMO,    Vic
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KL7OF
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« Reply #2 on: August 15, 2018, 09:05:11 AM »

Ron.  Those Boxes will all probably need to be recapped...especially the CBS Volmax..I have had several of these go thru my shack....They worked OK..I paid $25 a box 10 years ago...sold them for the same price...fun to play with.....Steve
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KD6VXI
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« Reply #3 on: August 15, 2018, 09:59:22 AM »

Well I'd offer 30 a piece..!

--Shane
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KA3EKH
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« Reply #4 on: August 15, 2018, 10:15:02 AM »

Had removed the same CRL audio stack, all three go together from an AM Broadcast station years ago and sold for $75 along with the NRSC junk that went behind them to limit bandwidth to 10 KHz. No big fan of CRL for AM or FM operations.
The CBS Volumax is a good peak limiter and was originally placed with a companion Audimax gain system and when they work correctly are good processors but keep in mind that they are at least forty or maybe fifty years old so they may have issues. They sell for stupid money on Ebay but I have not sold one in years and at that time thing I got around $100 for the box.
Good luck with the CCA, had a 5 kW AM that ran a CCA along with C Quam AM Stereo back in the  nineties that I took care of and hated that transmitter being that one was a strange mix of solid state and vacuum tube technology so some parts of the transmitter were modern and others were primitive. The audio driver used RCA integrated audio power amplifier IC that were something like SK-3740 or 41 or something like that that would randomly fail at the most inopportune times.
Never worked with the 1 kW so hope that’s a better box
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W8ACR
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« Reply #5 on: August 15, 2018, 11:24:27 AM »

OK, thanks for the replies. This helps. I plan to use an RE-27 mic. Is there a more modern single box of any sort that would suffice for decent audio?

Thanks again,
Ron
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W1DAN
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« Reply #6 on: August 15, 2018, 11:55:23 AM »

Folks:

I'd run the CRL set. Don't have to crank tool hard to get decent audio.

If you are not interested in the CRL set, I would be.

Dan
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« Reply #7 on: August 15, 2018, 02:46:52 PM »

All that broadcast stuff ran at 0 Db balanced so you need a microphone pre amp anyway. Get yourself an old production console and a couple cart machines and you’re on the way to building a late seventies AM Broadcast station!
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W8ACR
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« Reply #8 on: August 15, 2018, 10:21:14 PM »

OK, I made a deal to buy the audio processing equipment. All the original manuals are included. Will keep you all informed how things progress.

Thanks again for all the help. Ron W8ACR
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« Reply #9 on: August 16, 2018, 08:44:48 AM »

Ron:

All you need now is a mic amp. Ask the person at the radio station if they have a spare mic amp or processor.

Many things would work, from a homebrew MPF102-based preamp to a Symetrix 528 mic processor. I use a Symetrix 421M for the mic amp only, and do not use any other processing in the unit, just cuz I got the 421M cheap.

Let me know if you have questions in adjusting your audio.

Have fun!

73,
Dan
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« Reply #10 on: August 16, 2018, 09:51:12 AM »

I used a Shure M-267 for driving a CBS Volumax, they are plentiful and cheap. Have seen a lot at Hamfest for $20 The thing to watch out for is that on a real AM processor you may find that positive peaks can be hitting 125% modulation but negative peaks are limited so not to exceed 90% or so negative modulation so the polarity of the audio going between the processor and the transmitter is important. That will be a balanced line +,- and ground.



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W8ACR
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« Reply #11 on: August 16, 2018, 01:39:51 PM »

A couple more questions.

1. Doesn't the volumax and the CRL PMC unit do the same thing?

2. I have a Heil Classic mic, which is a dynamic mic with good frequency response. It's a good mic, probably not on par with the RE-27, but a pretty good mic. Rather than spend $500 on a new RE-27, I might try to use the Heil. It's output is listed at -72dBm. Most preamps seem to have around 60dBm gain max. Will that be enough for this mic? I believe the RE-27 is listed at about -53dBm output.

I've been doing a lot of reading on the subject of audio processing. It's starting to make some sense to me Wink Thanks again

Ron W8ACR
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« Reply #12 on: August 17, 2018, 12:32:15 PM »

Hi Ron:

Yes both systems do essentially the same thing. The CBS unit is simple, while the CRL system is complex-and thus can be more louder. If you want essentially a transparent sound, the CBS limiter would do. For a dense sound the CRL system would do that. The goal of audio processing is to prevent over-modulation and increase the average loudness of your voice so your audio signal would overcome the noise on the band. Many ways to achieve this.

Your Heil mic should work fine into any reasonable mic preamp. Yes a Shure M267 would work, but it is somewhat susceptible to RF. In the end, it does not matter much what you use to get your mike to "line level", or someplace around +4dBm average to feed either audio system.

Dan
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W8ACR
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« Reply #13 on: August 17, 2018, 02:37:52 PM »

Dan,

Thanks for the reply. I guess I should have stated my questions better. If I use the CRL system, should I also tack on the volumax at the end of the audio chain? That would seem redundant to me if the one CRL unit is doing the same thing.

Next, I guess my real question was: What level of audio signal do I need to adequately drive the CRL system. Obviously, -72dBm wont do it. Is there a ballpark figure that is generally needed as far as preamp output? -30dBm? -20? -10?  Sorry I'm so dense on this stuff. For some reason, RF circuitry is intuitive for me, but audio has always baffled me.

By the way, the preamp I bought is the "Presonus Studio Channel Tube Microphone Preamp".

Thanks, Ron
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« Reply #14 on: August 17, 2018, 04:24:58 PM »

Hi Ron:

The CRL set and the CBS limiter should be used separately and independently. Install the CRL set for a few weeks, twiddle the knobs and see what you get (would be good to make audio recordings for comparison), then one day swap it out for just the CBS limiter.

OK on your Presonus. that'll work fine. Adjust the preamp output to make the AGC400 input LED meters move about mid-range (that should be around +4dBu average). The AGC400 output would then feed the SEP400. The SEP output would then feed the PMC450 input. Each CRL unit should have yellow LEDs. Feed audio so that these LEDs flash on to your voice. You can set all front panel knobs to nominal for a start.

With the right-hand side output trimmer of the PMC-450 limiter turned down, fire up your transmitter into a dummy load. Talk and turn the PMC output trimpot up until you see modulation, someplace between 75% and 90%. Drop carrier and then flip the PMC's output leads that are feeding your transmitter. Transmit and see if your positive peaks are higher or lower. You want the audio phase that makes the RF waveform go higher.

Then turn the output trimpot up further until you almost cut off your carrier (pearling). This sets your negative modulation to near 100% You are then good to go.

Down the road you can play with transmitter LF and tilt EQ for that extra inch of modulation. It's in the limiter book.

Dan
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