FM Volumax

[W8ACR]

I came across a rack mount unit labeled "FM Volumax". Nineteen inches wide, maybe 1 1/2 inches thick. Is this usable as an accessory for an AM station, or should I feed it to the dumpster?

[W7TFO]

Ah, one of the Max brothers.  Audi was the other.

I have some lit of that somewheres round here, I'll dig it up next time I'm home...

73DG

[K5UJ]

Ron,

It's a piece of 60s or 70s FM broadcast processing gear.  I'd contact Barry Mishkind barrym at oldradio.com and as him if he can put it out as a free offer to his broadcast list subscribers.  Someone might want it for the shipping cost and I think that's preferable to scrapping it. 

Please consider that it is not made anymore.  It might be something someone wants -- the vintage audio processors are actually collected by some people -- to them scrapping it might be like scrapping a Johnson 500 to us.

Rob

p.s  the FM volumax lacks the diode clipper but you might still try it as a limiter for AM.   more information here:

http://www.rwonline.com/article/the-cbs-audimax-and-volumax/17232

[Opcom]

yeah don't trash/strip it. Trade or sell to someone if you don't want it.

[Pete, WA2CWA]

I came across a rack mount unit labeled "FM Volumax". Nineteen inches wide, maybe 1 1/2 inches thick. Is this usable as an accessory for an AM station, or should I feed it to the dumpster?

Which model number? There were several.

[W8ACR]

"Volumax FM 400" sounds right,  but I'm not sure, and it's not here in front of me now. I'll check next time I go out to the farm.

Ron

edit: I did a web search and the one I have looks like a Volumax 4100 or 4110. There is one currently on Ebay "buy it now" at $625.00!! Mebbe the dumpster will have to go hungry.

Ron

Edit #2 Hey Pete, great minds think alike.

[Pete, WA2CWA]

"Volumax FM 400" sounds right,  but I'm not sure, and it's not here in front of me now. I'll check next time I go out to the farm.

Ron

There's a FM Volumax 4100 up on ebay right now with a price of $625. 

[KB5MD]

It makes a nice shelf to sit non rack mountable equipment on.  I have an old radio station satellite receiver holding a monitor scope and watt meter.

[W8IXY]

The FM Volumax is a good final limiter, as long as the average input is relatively well controlled.   It was made to work with the Audimax audio level controller.

The AM version of the Volumax was "flat" in frequency response.  The FM version was a split band design that handled the flat base band and a separate control for the pre emphazed high frequencies to include the 75 microsecond pre-emphasis curve.  That roughly corresponds to a rising 6db per octave response starting a bit above 400 Hz and peaking about +18db at 15 kHz.  To make it compatible with the FM broadcast exciters at the time, the was an L/C/R de-emphasis network in the output audio path that had so much phase shift that even with the relatively good limiting in the FM volumax, you still could get up to 40% overshoots on high frequency peaks out of the exciter.

I had a lot of experience playing with those devices back in the 60's.  In the FM Volumax, you can actually adjust how much effect the high frequency pre emphasis has.  You can build a low pass filter to attenuate whatever audio above the desired cut off frequency.  If you keep a bit of the pre emphasis, you can mimic the effect of pre processing HF EQ.  However since the FM Volumax handles base band and higher audio frequencies separately, You may eliminate the need for any outboard high frequency EQ.

Back in the late 60's, I had an AM broadcast lineup, in order, of a mono Audimax 400, a mono FM 400 Volumax with de-emphasis disabled, followed with an AM Volumax 400.  It was the forerunner of what was eventually adopted as the NRSC EQ curve for AM, except we at the time didn't have to filter out stuff above 10 kHz.  The transmitter was flat to almost 15 kHz, and that AM signal sounded just like FM on a wideband receiver.

So, keep that FM Volumax.  Modify it slightly with an LPF on the output, disable the de emphasis, and tweak how much drive you give to the pre emphasis section, and you will have what will be one of the better sounding stations on the air.  (Provided you have clean audio up to the limiter!)

73
Ted  W8IXY

[W7TFO]

What Ted said...

73DG

[k4kyv]

That pre-emphasis curve ought to give plenty of "presence rise" to your AM audio.  Just remove the de-emphasis network at the output and use about a 5 kc/s cut-off brick-wall low pass audio filter between the speech amp and the limiter, to keep adjacent channel slopbucketeers (and AM ops) happy. Maintain good low frequency response, flat down well below 100~ so that the audio sounds balanced, not tinny.

But I'm not sure how the built-in high frequency boost affects the limiting action.  If they do the pre-emphasis prior to the peak limiter stage, it ought to work excellently for AM.  If they do it after the limiter, you might not have uniform peak limiting all way across the audio spectrum. For AM, you want the frequency response to be as flat as possible over the widest possible frequency range, past the limiter stage all the way to the modulated RF stage.

[Steve - K4HX]

Is there an echo in here. W8IXY already posted:

Modify it slightly with an LPF on the output, disable the de emphasis, and tweak how much drive you give to the pre emphasis section, and you will have what will be one of the better sounding stations on the air.  (Provided you have clean audio up to the limiter!)

That pre-emphasis curve ought to give plenty of "presence rise" to your AM audio.  Just remove the de-emphasis network at the output and use about a 5 kc/s cut-off brick-wall low pass audio filter between the speech amp and the limiter, to keep adjacent channel slopbucketeers (and AM ops) happy. Maintain good low frequency response, flat down well below 100~ so that the audio sounds balanced, not tinny.

But I'm not sure how the built-in high frequency boost affects the limiting action.  If they do the pre-emphasis prior to the peak limiter stage, it ought to work excellently for AM.  If they do it after the limiter, you might not have uniform peak limiting all way across the audio spectrum. For AM, you want the frequency response to be as flat as possible over the widest possible frequency range, past the limiter stage all the way to the modulated RF stage.

[Todd, KA1KAQ]

Is there an echo in here.

Ha! You've noticed this too? Check out the FT-243 crystal thread about crystal cut affecting oscillation. 

To the VoluMax, I'm with the others, Ron: it's worth saving, even if only for parts. As with real estate, 'they're not making anymore'. I bet you could score a few bucks from someone out there using similar models, probably a number of interchangeable parts. Though it sounds simple enough to modify for continued use.

[ssbothwell KJ6RSG]

i wouldn't take that $625 "buy it now" price too seriously. that particular volumax has been up on ebay for a long time and there are no completed sales for volumax limiters on ebay.

i bought a cbs audimax 4440 for $150 last year. i think that is a fair price for the 'max brothers.'

[W8ACR]

Went out to the farm today. The unit is an FM Volumax 4100. Maybe somebody has a manual??

Ron