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Author Topic: CBS Audimax etc.  (Read 20050 times)
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K9ACT
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« on: October 05, 2007, 08:59:12 AM »

I picked up a CBS Audimax, Volumax and Dynamic EQ at the hamfest Saturday and need some help understanding it.

After replacing a transistor and zener in the power supply, it seems to be running but I don't know if it's working because I do not understand the terminology in the set up instructions.

It refers to DB, VU's and other broadcast lingo that leaves me hanging.  I know what the words mean but don't now how to apply them here.

It says the test input should be 4 db above the normal level on the master VU meter which is not much help.

If I apply a .1v 1kc sine wave to the input  and diddle with the input and output levels, I can hear it in earphones hooked to the output.

After warmup, the meter reads 0 gain and adjusting the input level, the needle goes to the right but nothing makes it go to left of center.

If I put a meter on the output, the level follows changes on the input so it does not seem to be doing what it should do.

I measured the level of the output of the mic preamp on my audio chain with 100% modulation and that is how I came up with the .1v as a normal level for the Audimax but beyond that , I am clueless.

Any help will be appreciated,

js









 

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Bacon, WA3WDR
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« Reply #1 on: October 05, 2007, 09:43:00 AM »

Typically, broadcast and recording gear operates at 0 VU = +4 dBm.  The 'dBm' is based on an assumed 600 ohm impedance, regardless of actual impedance, so 0 dBm is about 0.775V RMS, and +4 dBm is about 1.228V RMS.  In other words, it's just an AC voltmeter, marked in decibels, with an arbitrary "0" level.  A Volume Unit is 1 dB.

Home and semi-pro audio gear operates at about 0.25V RMS = 0 VU, which is -10 dBm on the 600 ohm based scale, or -14 VU on the 'pro' VU scale, regardless of the actual impedance, which is nominally 10K ohms in semi-pro gear.

For pro gear such as the Audimax, a signal of 0.1V RMS is about 22 dB low.  You need to try a signal around 1V RMS to 5V RMS or more.

I seem to recall that the Audimax could be very slow about raising its gain after loud audio caused a reduction in gain.  There was a 'hang' time, such that it would not change gain at all for a while if there was silence after a loud sound knocked the gain down.  Then after a delay period, it would begin to raise the gain again.  But if there was program audio louder than 20 dB below 0 level, the Audimax would skip the delay period and begin raise its gain right away, because it assumed that if there was program material at such a level, it should play it loud, but if the input level was lower, then it was just hum and noise, and it should let it stay down.

Are these for AM or FM?  The AM Volumax had a negative peak clipper, as I recall.  Maybe it is configurable in all units.  But someone was saying that the AM and FM versions of these units are significantly different.



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John K5PRO
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« Reply #2 on: October 05, 2007, 01:33:03 PM »

The FM version of the Amax and Vmax (broadcasters called them the Maxx brothers) has 75 uS preemphasis figured in. You'd have to disable all that circuitry to flatten the response for AM. The Amax has a very sophisticated (for its day) algorithm that holds level for a while, so as not to bring up background noise during pauses in audio. You have to get the levels right at the input for it to process properly. Both units are made for balanced audio throughout, so you really need zero to +4 dBm push pull audio, not a single ended -10 to -20 dB typical consumer audio level.

These units will need to be checked thoroughly, to make sure that the coupling capacitors haven't started being leaky, and causing level shifts among the amplifiers. I went through mine, and replaced most of them on the Amax, haven't finished the Vmax yet, or maybe it was the other way around.

The Volumax is a final limiter, while the Audimax is the compressor/gain rider. I think i got that right, its been a few years.

The old models used a tube (nuvistor) in the gain control element. The newer single high units replaced with a FET i believe.

As for the CBS dynamic presence equalizer, that is a neat idea, but most stations gave up on them. They are supposed to keep voice in the same levels as music, and have an adjustable EQ that automatically changes as the program content does. I have one, but have never checked it out yet. I believe they had 'artifacts' that would be noticed if not properly set up. Or maybe always.

73
John
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K9ACT
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« Reply #3 on: October 05, 2007, 03:33:18 PM »

Good stuff.  I had to run out to get a 9V battery for my audio gen which now goes up to about 1.5V and everything is now making sense.  I can set a level and then reduce the input considerably before I hear any change in the output level and the gain goes up on the meter.  So, it seems to work.

This one has the Nuvistor.  That was a learning experience all it's own.  Heard the word  before but had no idea what they were.  This one has 19v on the plate.... pretty strange.

I believe it is the AM type.  Not sure how to tell.  The manual mentions a stereo option but this does not have one.

I hooked it to the Vmax and the meter went to the green there is leveled output so that seems to at least run.  I decided it was time to quit while ahead.

js
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John K5PRO
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« Reply #4 on: October 05, 2007, 06:55:34 PM »

Jack
This was posted in 2005, found in archives of a broadcast mailing list:

> For my own personal edification, I'm trying to create a list of all the
> models of Audimax and Volumax processors that were offered by CBS and
> Thompson-CSF.  Here's what I've come up with so far... any additions or
> corrections would be greatly appreciated.
>
> Audimax (original) - mono, ungated, tube-type
> Audimax II - mono, ungated(?), tube-type
> Audimax III - mono, gated, tube-type
> Audimax III, Model 444 - mono, gated, solid-state with Nuvistor tube
> Audimax III, Model 411 - two 444's paired together for stereo
> Audimax IIIS, Model 445 - stereo?
> Audimax 4440A - slimline mono, all-solid-state
> Audimax 4450A - slimline stereo, all-solid-state
>
> Volumax 400 - mono AM, wideband
> Volumax 410 - mono FM, wideband
> Volumax 411 - stereo FM, wideband
> Volumax 4000 - slimline mono AM, wideband
> Volumax 4010 - slimline mono FM, wideband(?)
> Volumax 4100 - slimline mono FM, dual-band with HF clipper
> Volumax 4101 - slimline mono FM, dual-band with HF limiter
> Volumax 4110 - slimline stereo FM, dual-band with HF clipper
> Volumax 4111 - slimline stereo FM, dual-band with HF limiter
>
> There was also the companion "Dynamic Presence Equalizer", with models 450
> (2RU) and 4500 (slimline), both mono.
>
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flintstone mop
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« Reply #5 on: October 05, 2007, 09:48:41 PM »

The Audimax was a nice piece of equipment in its day. It was neat to see the needle swing around during operation and the "hang time" was designed for playing classical music. If there was a sudden quiet spot in the music, the Audimax would not increase its gain giving you that difference in volume the composer created in the musical piece. But GAWD help you if there was a tick or click defect in the vinyl record. That would catch the "attention" of the audimax and UP with the gain and record noise and rumble from the turntable would be the main sound on the air. If the audimax was in the slow position and if you didn't push the volumax needles past the green (FM version) and you had a nice Gates stereo generator and exciter to the FM transmitter, you had a great sound on FM back in the early 70's. I mean this was 30db separation and 30-15000 hz audio. If the volumax needles were near the red all of your high end was trashed and an FM station could sound like a good AM station....back then. THEN came the ORBAN boxes in the late 70's.
This was during my FM days at Georgetown University, volunteer DJ/engineeer at WGTB 90.1
Fred boogie Cresce
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Fred KC4MOP
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« Reply #6 on: October 05, 2007, 10:18:12 PM »

I had a car radio (Motorola, if I recall) in my 1962 Mercury Comet that used tubes in the RF and IF, with 12V on the plates.  It worked OK Fine!  The audio output stage was a transistor.  I forget what tubes they were, they might have been regular 12BE6 - 12BA6 stuff.  I put a 20K ohm-per-volt voltmeter on the AGC, and peaked the RF and IF tuning.  It worked great!

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« Reply #7 on: October 06, 2007, 07:15:43 PM »

I have this gear. I bought copies of the manuals. Let me know if you need them scanned.
My gear was in tough shape & took a while to get running and set up correctly. It works F.B.O.M. driving all of my transmitters.
Keith
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K9ACT
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« Reply #8 on: October 07, 2007, 12:37:13 AM »

I have this gear. I bought copies of the manuals. Let me know if you need them scanned.


That must have been before some kind sole uploaded them to BAMA.  I got them both before I started poking around on the stuff.

But thanks for the kind offer.

I think I have them both working but I just blew out the LM380 on my preamp so I don't have enough poop to try it out but this brings up a question:

My 2.5W audio pre-amp has a 4 Ohm output.  Can I drive the Amax with this or do I have to somehow get it to 600 ohms?

But back to manuals, I can't seem to find anything on the Dynamic Presence EQ.  You don't happen to have one of those, do you?  And where in the chain does this go?

js






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Bacon, WA3WDR
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« Reply #9 on: October 09, 2007, 10:14:08 AM »

A 2.5W amp expecting 4 ohm output will produce about 3.16V RMS which will drive the Audimax with about +12 dBm (ref 600 ohms).

If you convert it to 16 ohms you will have about 6.3V RMS, or +18 dBm (ref 600 ohms).

If you convert to 600 ohms you will have about 39V RMS, or +34 dBm (ref 600 ohms).

Where does the Audimax start to compress?  I would want at least 20 dB and preferably 30 dB of headroom above that.

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« Reply #10 on: October 09, 2007, 11:24:15 AM »


But back to manuals, I can't seem to find anything on the Dynamic Presence EQ.  You don't happen to have one of those, do you?  And where in the chain does this go?

js

I have the 4500 manual available.
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John K5PRO
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« Reply #11 on: October 09, 2007, 02:58:58 PM »

Using an LM380 to drive an Audimax? You will sacrifice some of the quality of the Amax using a 380 like that. 380 is really an output stage for radios, for speaker drive. It suffers from crossover distortion. If you had two op amps, you could make a nice pseudo-balanced line driver that would do it FBOM.
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« Reply #12 on: October 09, 2007, 05:22:52 PM »

Yup
I drive mine with a symetrix 528. That gives 600 ohms of balanced output at around "0" level. The gain controls on the CBS stuff can compensate for a lot of the gain settings. There are also instructions in the book about building attunuators.
Keith
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K9ACT
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« Reply #13 on: October 10, 2007, 12:20:24 AM »


Mine is a 450 if that makes any difference.  Keith says he has the manuals but I don't know if that includes the DE.

If either of you can get one to me, I will be happy to pay what ever it's worth.

But first of all, I really don't understand what this thing is or if I even want to use it.

I am currently using a graphic EQ with the last three channels all the way up and it turns my audio from mush to crisp and clean.  What will this thing do and would I still use the Graphic EQ?

js




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K9ACT
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« Reply #14 on: October 10, 2007, 12:29:06 AM »

Using an LM380 to drive an Audimax? You will sacrifice some of the quality of the Amax using a 380 like that. 380 is really an output stage for radios, for speaker drive.

It's all I had to play with when I got it running and learned that my audio generator didn't have enough poop to exercise the Amax.   I ran the audio  into this little amp and when I got it up to about 1.5 V, the Amax started acting like it should, viz., reducing the input drove the gain meter up and no change in level could be heard in an earphone.

I'm sort of a babe in the woods on this one but I was thinking of just running it into and output transformer backwards if I need  more volts.

js


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John K5PRO
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« Reply #15 on: October 10, 2007, 12:46:19 AM »

Yes, you would be well served to feed the Maxx brothers with a balanced audio feed, otherwise you can have RFI and other noise problems (light dimmers, etc), due to common mode pickup on your audio conductors. Either use a simple op amp circuit to convert single ended to balanced, or drive it with the balanced output of EQ unit or whatever mike preamp you are using, if you have dynamic or condenser mike. You can also use a 600:600 transformer, if your EQ has enough drive. Sounds like it doesn't. You need a line level driver, from the output of whatever EQ or mike preamp you are using. There are numerous chips made specifically for this, by Analog Devices, for example. Or use a couple of op amps. You need a power supply to do this, however, preferably a bipolar supply unless you use capacitors for audio coupling and run single-ended amplification.

As for the DPE, I have one too, and was told by a broadcaster that it wouldn't add much to a MAXX brothers setup already. It was a box that CBS developed to apply sliding EQ to stations with high density voice along with music, pre-Optimod. It emphasizes the presence band, in the audio frequencies that voice uses. However, it is also capable of working with music, althoug hams don't care about that.
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Steve - WB3HUZ
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« Reply #16 on: October 10, 2007, 09:01:55 AM »

Quote
What will this thing do and would I still use the Graphic EQ

The EQ affects the frequency response, the Audimax the level, amplitude or dynamics. Both can be used together.

I'm a little concerned that you would need to set last three sliders of your graphic EQ to maximum boost. There must be some serious high frequency loss elsewhere in your system.
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WA3VJB
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« Reply #17 on: October 10, 2007, 09:45:08 AM »

I think the 4500 is the little skinny 1U rack unit with edge meter, rather than the taller rackspace, older sister with the conventional meter.

You might want to read Mike's treatise on processing. It's got some good context.

http://amfone.net/ECSound/processing101-1.htm

I think I had an Audimax for a while, if it's the one with the center-reading meter indicating gain or reduction. I believe the NuVistor was soft in mine, because it would limit but not really boost an input as needed.

Later went on to more exotic stuff, and I have no idea what I ever did with the Audimax. There wasn't any book, I remember that.

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K9ACT
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« Reply #18 on: October 10, 2007, 10:01:56 AM »

Quote
What will this thing do and would I still use the Graphic EQ

The EQ affects the frequency response, the Audimax the level, amplitude or dynamics. Both can be used together.


Roger but the question is about the Graphic EQ and the Dynamic EQ together.


>I'm a little concerned that you would need to set last three sliders of your graphic EQ to maximum boost. There must be some serious high frequency loss elsewhere in your system.

I get the feeling sometimes that it is in my vocal chords.

My born-again AM evolution started with a Kenwood/Amp, Ranger/Amp and finally my homebrew and I managed to hook up with HLR on all three and was totally deflated with his never changing assessment..... "mushy, no lows, can barely understand you".

It all went away with the EQ and the only common element was my vocal chords.

js


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KA1ZGC
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« Reply #19 on: October 10, 2007, 11:25:39 AM »

Quote
What will this thing do and would I still use the Graphic EQ

The EQ affects the frequency response, the Audimax the level, amplitude or dynamics. Both can be used together.


Roger but the question is about the Graphic EQ and the Dynamic EQ together.


For now, just use the graphic EQ.

The graphic EQ allows you to recover any energy that was insufficient in your incoming audio (in your case, the highs) and cut any energy you have in surplus prior to entering the dynamic EQ.

You may not need the dynamic EQ at all. It's role in life is to adjust and readjust the equalization based on changes in the incoming audio's properties (going from voice to music, and vice-versa).

Given that you won't be going from voice to music and back, you should probably go without the dynamic EQ until you get the graphic EQ set where it should be. Otherwise, the changes you make to the graphic EQ will get swamped by the dynamic EQ putting things back where it thinks they belong (you'll be shovelling it against the tide).

I'm a little concerned that you would need to set last three sliders of your graphic EQ to maximum boost. There must be some serious high frequency loss elsewhere in your system.

I get the feeling sometimes that it is in my vocal chords.

My born-again AM evolution started with a Kenwood/Amp, Ranger/Amp and finally my homebrew and I managed to hook up with HLR on all three and was totally deflated with his never changing assessment..... "mushy, no lows, can barely understand you".

It all went away with the EQ and the only common element was my vocal chords.

Nope.

Unless people stop you during a face-to-face conversation and ask you to repeat yourself because your voice is mushy, with no lows, and they can barely understand you; your vocal cords are not the issue here.

Two things I haven't seen mentioned are important, here: what are you using for a microphone, and how is the rest of the graphic EQ set?

--Thom
Killer Agony One Zipper Got Caught
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K9ACT
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« Reply #20 on: October 10, 2007, 12:42:55 PM »

The current mic is a Reslo dynamic but I have a D104/Kobitone, a hand held dynamic, and a Electret lapel mic.  The Reslo has been voted best sound as of now.

However, while you were typing your message I was out there putzing with mics and it may be time to reconsider.  The EQ has a bar graph display on it and I noticed that I can nudge the 10kc bar with the D104 but not the Reslo with the EQ out and levels adjusted to match.

All the other channels are more or less flat at the present.  This is a very difficult thing for me to deal with without a/b'ing people to death. 

js
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flintstone mop
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« Reply #21 on: October 10, 2007, 05:57:24 PM »

I guess there's going to be a disadvantage of using home audio with pro stuff. The pro equipment needs that +4 or 8 dBm and the LM380 will add more grief to your project as it is unbalanced and will introduce more noise into the system. The MAX brothers are noisy already using late 60's transistor technology. But we're not too concerned here as this is Ham use, and the typical listener, or QSO will be over a noisy radio path any way. Some one local to you (10mi from your station) would hear circuit noise using a high quality wide band receiver.
Really good advice given to get the duo working properly. Make sure PRE-emphasis is disabled!!!
We'll be listening

Fred
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Fred KC4MOP
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« Reply #22 on: October 11, 2007, 10:18:33 AM »

Hiya Jack,

A few things struck me during our QSO last night. I don't know how well you were hearing me, as the static was tearing you a new one.

At any rate, the audio doesn't sound mushy, it just sounds, well... flat. This makes perfect sense, since you have the EQ set to pretty well flat below the top three sliders.

Getting a reading at 10kHz is making progress, but won't be very helpful on AM. Assuming your mod iron will even pass 10kHz to start with, most of us won't hear it. Most (not all, but most) receivers will pull in from 3 to 8 kHz either side of center, so 10kHz is out of the passband.

Not to mention that 10kHz audio usually winds up in a neighboring QSO, especially when we get all packed in at 5kHz spacing.

All you need to do now is find the right EQ curve and it sounds to me like you'll be in good shape. I would recommend cutting everything below 200Hz and above 6 or maybe 8 kHz. Set 200 Hz and 6-or-8 kHz for some good amount of boost. Then, between those two points, set the sliders so you have a smiley-curve whose bottom hits center somewhere in the 1-2kHz neighborhood.

That should give you something to start with, anyway, and some more on-air experimenting will be necessary to determine what kind of EQ curve you really want on that setup. Those numbers are out of thin air, but should give you enough presence boost to make you easier to copy and enough lows to keep you from sounding too shrill.

The important thing is that you are being heard, and the Audimax sounds like it's doing its job the way it's supposed to. Just some EQ tweaks, and you should be in good shape.

Good luck with it, we'll keep an ear out for you.

--Thom
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« Reply #23 on: October 11, 2007, 03:45:55 PM »

Jack,
 The audio equipment sounded good in KC last night. How did it go over with the midday net?                         
                                                                        73
                                                                        Mike
                                                                        KA0ARA
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K9ACT
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« Reply #24 on: October 11, 2007, 07:52:09 PM »

Jack,
 The audio equipment sounded good in KC last night. How did it go over with the midday net?                         
                                                                        73
                                                                        Mike
                                                                        KA0ARA

Everyone liked it but no one said it sounds like W9AD, darn!  And he didn't check in so I haven't gotten an opinon from him yet.

We did some A/B'ing with the EQ levels and everyone liked the settings that I had been using.

I tried the levels that Tom suggested and everyone missed the lows.

It seems to be working but I need some help optomizing it as the lingo in the manual is meaningless to me (sort of).  Even guessing at what the standard VU level might be, I can still only set it up for the 1kc sine wave and all bets are off with a mic.

js
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