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Volumax Setup




 
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Author Topic: Volumax Setup  (Read 7089 times)
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K9ACT
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« on: February 20, 2008, 11:50:59 AM »

I am having trouble understanding the setup instructions for the CBS Volumax and am hoping someone out there has a copy or is familiar enough with it to help out.

It starts out with a 1kc sine wave in, with the input level adjusted to red/green line on the meter. 

Next step is to increase the input signal 10db which pushes the meter off scale to the left.

Aside from figuring out what 10db is, it's not clear if this means increasing the osc output or just increasing the input level by that amount or if it makes any difference.

When this level is set, we are told to adjust the transmitter output level for 30-40% modulation.

Question here is how does this relate to the speech amp gain control?  I just set this in mid position and use the VMax output to set this level.

Having done all this we are supposed to note a typical 30% mod envelope on the scope.  If it looks flat topped, reverse the output leads.

In my case it looks normal either way with no flat topping.  What doe this mean?

We then are told to reduce output level to zero and feed the 1 kc sig to reach the red/green line again.  This is the show stopper.... What happened to the +10 db setup and what was the point in doing this if we are back here again?

Next we are told to press the Mod Set switch and adjust the output level to produce the max desired modulation (whatever that means) and the scope pattern should should look like fig. 3.3.  This looks like gross baselining and flat topping at top and bottom.

We then release the switch feed in normal program, adjusting level for occasional peaks into the red region.

I don't understand how one adjusts for desired mod level if the envelope looks like that.  When the button is released, the envelope looks like something less that full modulation and when speaking into the mic, it never baselines.

So, it seems to be functioning but I am not sure if it is really set up properly.

Finally, there is a nasty hum as set up but I discovered that if I reverse the leads to the speech amp the hum goes away.  If I understand this thing correctly, that would reverse the phase of the Vmax set up and be worse than no Vmax.

However, this is not the case.  Other than eliminating the hum, it seems to make no difference which way output is phased.

Any help will be greatly appreciated,

Thanks,

js

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John K5PRO
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« Reply #1 on: February 20, 2008, 05:19:42 PM »

Jack
Is yours a true AM Vmax, that has asymmetric processing? Or an FM unit?

You should follow the instructions exactly. I have set them up before, and thats the way. Isn't there a time delay inside of the  modules, which is why you test it that way? I think it looks for program material before it begins operating, so as to not pump up noise floor. It was a fairly intelligent piece, for its day and for strickly analog processor.

You need balanced audio through your system to minimize noise and hum pickup around transmitting sites.

10 dB changes in audio, are done using a scope or voltmeter calibrated in dB. Usually a set of 600 ohm attenuators are used, or a sig gen with calibrated switches.
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flintstone mop
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« Reply #2 on: February 20, 2008, 05:48:38 PM »

Yea Jack,
make sure that is an AM unit. FM would not be nice for Ham AM radio. You CAN defeat the pre-emphasis, though. Is it really in wacked out condition? You will need a audio level meter calibrated for 600 ohms. Or you can use your DVM in the A.C. mode and calculate the voltage levels from the decibel readings. Most DVM's are calibrated for 600 ohm audio. Typically what we Ham ops do is re-cap the P.S. and it's ready to go. If there is something wrong in one of the black magic modules, you're in trouble!! I have never seen a need to calibrate this stuff. You might need a matching transformer before you notice any phase difference. Hopefully your TX can input balanced audio. If not, then you'll need a matching transformer.
And John, The feature you mentioned about no audio and the device just sits there waiting for audio sounds like the AUDIMAX, also made by CBS. It was great for FM broadcasting at an easy listening or classical music station. If there was sudden silence in the music the Audimax would quit following the audio (front panel meter)and wait until there was a little higher level in the program material and once that happened it would resume it's function. Not so nice if there was a click or pop in the vinyl record source.
I occasionally listen to old tapes from WGTB FM and it was an "alternative Radio station at a Catholic College in Washington DC", of all places. And it was superb stereo FM with just the simple Audimax/Volumax and a Gates FM-3 transmitter. The 'phone company provided flawless class A phone lines between the studio and TX, about 7 miles. 30-15K and a miniscule phase shift.
Sorry for all of the blabber
Fred
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Fred KC4MOP
K9ACT
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« Reply #3 on: February 20, 2008, 07:32:39 PM »

Yea Jack,
make sure that is an AM unit.

It is AM.

>You CAN defeat the pre-emphasis, though.

I don't understand what this means or why I would want to do it

> Is it really in wacked out condition?

As far as I can tell, it's in perfect condition.  I just do not understand the setup instructions.

>You will need a audio level meter calibrated for 600 ohms. Or you can use your DVM in the A.C. mode and calculate the voltage levels from the decibel readings.

But the dvm reads in volts and I need to convert to DB, right?

What I do have is a VU meter on the output of the mixer but I am not sure how to interpret is as far as increasing the level 10 DB.  If I set the initial level to 0 VU, the scale only goes to +3.  The is a +8 range switch but this still doesn't get me there.

>You might need a matching transformer before you notice any phase difference. Hopefully your TX can input balanced audio. If not, then you'll need a matching transformer.

So what is a matching transformer and where do I put it?  The speech amp is just a solid state 20W bridged amp with an RCA connector on the input with the shield side grounded to the cabinet.

js


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WA1HZK
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« Reply #4 on: February 20, 2008, 08:19:39 PM »

Jack
Do you have the manuals?
I can loan you mine so you can copy them.
Audiomax & Volumax.
Keith
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K9ACT
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« Reply #5 on: February 20, 2008, 11:35:23 PM »

Jack
Do you have the manuals?
I can loan you mine so you can copy them.
Audiomax & Volumax.
Keith

Got all three including Dynamic Presence Eq. The problem is that I do not understand the procedure for the Vmax and what I do, produces results but I do not see any difference when I reverse the leads.

I did some putzing with a transformer between the Vmax and the speech amp and it cured the hum but I still see no difference by swapping leads.

It's not the right transformer but it isolates.  What do I need to match 600 ohms to the input of a TDA2003 amp?

Thanks for the offer,
js

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W1DAN
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« Reply #6 on: February 21, 2008, 10:13:44 AM »

Hi:

There are charts and java calculators to convert db to volts. 10DB is a wide voltage range though. I'd use the mixer's VU meter and just increase the tone 10db using that meter. It will be close enough.

Try this link about the history of the VU meter:

http://www.dorrough.com/dorrough/techdoc/multimedia/mmedia.htm

You are very close and you'll get the processor going well.

73,
Dan
W1DAN
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K9ACT
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« Reply #7 on: February 21, 2008, 10:32:17 AM »

Hi:

There are charts and java calculators to convert db to volts. 10DB is a wide voltage range though. I'd use the mixer's VU meter and just increase the tone 10db using that meter. It will be close enough.

Try this link about the history of the VU meter:

http://www.dorrough.com/dorrough/techdoc/multimedia/mmedia.htm

That link doesn't seem to work and I need to confirm that Vu's = Db's.  The Vu meter reads in Vu.

js


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W1DAN
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« Reply #8 on: February 21, 2008, 11:15:15 AM »

Hi:

Yes VU is essentially DB, but an average (not an issue for tone).

Try this explanation:

http://arts.ucsc.edu/ems/music/tech_background/TE-06/teces_06.html

http://howthingswork.virginia.edu/page1.php?QNum=1063

Dan

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John K5PRO
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« Reply #9 on: February 21, 2008, 05:01:50 PM »

One more thing, you won't notice asymmetric processing with a Volumax using tones. They are symmetric already. You have to use your voice, or a music recording to see it change. Then, when you change polarity of the audio, you will note that the output will drop, depending on the polarity of your peaks (+ or - going on a scope).

Highly recommended that you also look at a scope on the output terminals to make sure it isn't causing any clipping or changing your waveforms of tones or voices.

Your TDA2003 or whatever it is has a low output Z, capable of driving a speaker. It should have no problem driving 600 ohm input Z of Vmax. However, since one side of it may be grounded, you might have to transformer couple to get all the advantages (common mode noise rejection) of a balanced instrument such as a volumax. I believe the Vmax already has an input transformer, if so, then ignore this concern.

As for db to VU to volts, use the chart provided, or find a simple VTVM or audio voltmeter surplus, they are quite available, and show both volts and dB. 0.775 volts into 600 ohms I believe, is 0 dBm. Somebody check me on that. You can calculate any other voltage with db =20 x log (V1/V2).

I would avoid sticking the DPE in there, unless you have everything perfectly set up. The Audimax is also tricky to set up, but a good addition.
73
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K9ACT
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« Reply #10 on: February 21, 2008, 07:41:43 PM »

One more thing, you won't notice asymmetric processing with a Volumax using tones. They are symmetric already. You have to use your voice, or a music recording to see it change. Then, when you change polarity of the audio, you will note that the output will drop, depending on the polarity of your peaks (+ or - going on a scope).

That contradicts the setup procedure in the manual. A 1kc sine wave is used at the input and the scope views show asymmetry when reversing the leads.  Getting it right is part of the procedure.

>Highly recommended that you also look at a scope on the output terminals to make sure it isn't causing any clipping or changing your waveforms of tones or voices.

That's a given.

>Your TDA2003 or whatever it is has a low output Z, capable of driving a speaker. It should have no problem driving 600 ohm input Z of Vmax.

It's the other way around.  The Vmax drives the TDA2003 which is the speech amp.

>I would avoid sticking the DPE in there, unless you have everything perfectly set up. The Audimax is also tricky to set up, but a good addition.

They are quite straight forward to set and the instructions are clear.  I know when the Amax is doing the job but the DPE is sort of like an insurance policy.  Only called upon when needed.

Bottom line is, the manual on the Vmax says we should be able to change limiting from pos to neg by reversing the output leads and I see no such reversal.  It shows it both with a wave envelope and the trapezoid.

When set up according to the procedure, I never get base line crossings but I also never see 100% positive mod either unless I press the Mod Set button and then it over modulates in both directions.

js

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John K5PRO
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« Reply #11 on: February 21, 2008, 10:40:24 PM »

A tone should not have asymmetry unless you have DC bias on it to shift the zero point of the waveform. Hence, the Volumax shouldn't be changing that, unless it is doing some drastic clipping. The only way I could get higher positive peaks with mine was to feed it real program material, not tones, and then I could reverse the leads and see the peaks reverse polarity on a modulation monitor (Harris MW90).

When you said it shows it both on wave envelope and trapezoid, are you referring to looking at the modulated RF from your transmitter? You should also see the affect on the audio waveform on the output of the Vmax. Could the transmitter be unable to modulate more positive, so it was causing the lack of higher + than - peaks, no matter which way you connected the Volumax output to your speech amp?

I thought the DPE was supposed to be used to enhance talk on mixed format stations, it would dynamically shift EQ as needed to keep voices on par with highly processed music. I have one too, but I am not sure it is of much use on amateur bands.

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Steve - WB3HUZ
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« Reply #12 on: February 21, 2008, 10:44:53 PM »

Hook a scope up to the output of the Volumax. You should see more asymmetry. Your transmitter may not have enough modulator capability to reproduce all the asymmetry.


A tone should not have asymmetry unless you have DC bias on it to shift the zero point of the waveform. Hence, the Volumax shouldn't be changing that, unless it is doing some drastic clipping. The only way I could get higher positive peaks with mine was to feed it real program material, not tones, and then I could reverse the leads and see the peaks reverse polarity on a modulation monitor (Harris MW90).

When you said it shows it both on wave envelope and trapezoid, are you referring to looking at the modulated RF from your transmitter? You should also see the affect on the audio waveform on the output of the Vmax. Could the transmitter be unable to modulate more positive, so it was causing the lack of higher + than - peaks, no matter which way you connected the Volumax output to your speech amp?

I thought the DPE was supposed to be used to enhance talk on mixed format stations, it would dynamically shift EQ as needed to keep voices on par with highly processed music. I have one too, but I am not sure it is of much use on amateur bands.


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K9ACT
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« Reply #13 on: February 21, 2008, 11:36:52 PM »

Jack for $75-$150 you can get one of the Behringer boxes with mic preamp, amp, EQ, compressor, limiter, all in one box, ready to use and no headaches and full warranty.

I wish I knew that when I paid $300 for this stuff.

This I will go shopping.

js
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K9ACT
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« Reply #14 on: February 21, 2008, 11:52:27 PM »

A tone should not have asymmetry unless you have DC bias on it to shift the zero point of the waveform. Hence, the Volumax shouldn't be changing that, unless it is doing some drastic clipping.


Sounds reasonable but I am just parroting what is in the manual.  If you happen to have one, I would like to hear what you have to say about the procedure for set up.

>The only way I could get higher positive peaks with mine was to feed it real program material, not tones, and then I could reverse the leads and see the peaks reverse polarity on a modulation monitor (Harris MW90).

But wouldn't that just be recognizing the asymmetry of the voice which is a different issue entirely?

>When you said it shows it both on wave envelope and trapezoid, are you referring to looking at the modulated RF from your transmitter? You should also see the affect on the audio waveform on the output of the Vmax.

Again, that is the procedure in the set up. However, I don't see how you could see the limiting without the modulated RF.  Not sure what to expect on the output.

 >Could the transmitter be unable to modulate more positive, so it was causing the lack of higher + than - peaks, no matter which way you connected the Volumax output to your speech amp?

That is certainly a possibility now that you mention it but it seems to me it is the same on my 811 rig which has separate supplies and plenty of audio.

>I thought the DPE was supposed to be used to enhance talk on mixed format stations, it would dynamically shift EQ as needed to keep voices on par with highly processed music. I have one too, but I am not sure it is of much use on amateur bands.

You are probably right but having paid for it, I wanted to use it.  The DPE has  voice or music modes which are separate positions of the mode switch.  I think in the music position it somehow is able to ignore music and do it's thing on voice only.

With the same person talking on a ham rig, it is probably pointless and no better than a Graphic EQ.

I have a lot to learn yet.

Thanks,

js
 


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W1DAN
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« Reply #15 on: February 22, 2008, 10:15:14 AM »

Hi JS:

Do not buy any new stuff yet. This is a good unit.

You should be able to get the Volumax working to your satisfaction.

Without reading the manual and having a Volumax in front of me, I'd pass tone through the box and set the G/R meter for 10db gain reduction.

Then put the scope on the output of the Volumax and using your microphone talk through your chain (no need to transmit at this time). Make sure you have some gain reduction while you are talking. On the output, one polarity of your scope trace should show a very slight clipping or flat-topping.

That should be modulating negatively when you transmit. If not swap the two wires leaving the Volumax output. This will swap the audio phase 180 degrees. One way will make the clipping polarity the negative going modulated waveform.

In the end the limiting and slight clipping of the Volumax will increase your modulation level.

Your vintage audio gear will always be worth more than the Behringer stuff.

Let us know what you find.

73,
Dan
W1DAN
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WA3VJB
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« Reply #16 on: February 22, 2008, 08:14:53 PM »

Over the years I have used five different broadcast-quality audio processors, each quite different from the other.

In each case, the best method of adjustment and initial setup was to record my voice on a tape deck, and by playing it back, use that as the audio source for scope observations, audible effects of compression, and basic hum/phase ascertainment.

As Steve points out, a lot of this can happen without the transmitter, especially the part about learning the interaction of various adjustments and their effects.

I've heard good things about the Behringer gear, and it's very hard to argue with the price point when you're buying new, warrantied stuff.  Worth an experiment within the satisfaction guarantee timeframe.

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« Reply #17 on: February 24, 2008, 12:03:53 PM »

I've got a Volumax 400 and a Marti something or other.. I've been counseled though to use an all-pass network before the processors.

The object is to phase shift the components of the random speech wave so that there is a much more equal amount of energy in the positive and negative polarities, then do the clipping.

I have not had time to make any experiments or measurements but the articles and e-mails seem to make alot of sense, as do the waveforms shown.

A good article is here -maybe I saw reference to this from Jim Tonne on this on this BBS? pls. excuse my bad memory, it's aggravating..
http://www.tonnesoftware.com/appnotes/allpass/allpass.html
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