Help with analog VU meters

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ModRob
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Help with analog VU meters

Post by ModRob » Fri Nov 01, 2013 8:39 pm

Moved past one hurdle, now onto I hope a bit easier ground...
Using an old mixer, and a consumer cd deck input to it. The -10 level will barely move the VU needles when plugged directly to the meters, so when I input to the console, the meter activity is near nothing. Routing is pretty simple now; through wiring, switches, Shallco channel attenuator pot, to a bus, then to the master volume pot. Then out to feed an outboard stereo amplifier. Also, the stereo amp has to be potted up quite a bit more then when listening to other sources (sound still clean though).
So, is there a cheap, down-and-dirty circuit/thingamabob/device I can install to ramp up the level to drive the meters better?

Thanks much for any ideas...

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Re: Help with analog VU meters

Post by CeaSaR » Sat Nov 02, 2013 12:57 pm

First things first, you don't have it plugged into a high level input, do you? It should be mated up to standard line level, ~300 mv. That should make things work as normal.

If that is already the case, are you using the correct output of the cd player (most likely yes as consumer grade usually only has 1 standard output) or is there a "software" setting to adjust the output level on the CD player?

Ruling these out, you could use a preamp between the CD player and the mixer. You definitely need to boost the levels. Go commercial or roll your own. There are tons to choose from out there on the net or even here on this board.

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Re: Help with analog VU meters

Post by ModRob » Sat Nov 02, 2013 11:48 pm

Hey...nice to "see" you again my friend, and thanks much for the help...

Not sure of the channel input levels, but on this console I believe everything is based on 600ohms. But, I've removed anything that altered my signal (a 600:150 ohm transformer that fed into the Shallco attenuator), and believe what routing I have in place now is only wires, couple of switches, and that attenuator. I thought I'd at least see the level at the meter that was there when wired straight to it, but no.
The CD unit has just the one normal output. I did try and hook up a phono preamp but got very low signal, along with terribly garbled signal (either distortion or what I'd describe as maybe "just barely there, fuzzy" sound). I believe it was the same result if I plugged it in before the console, or after console and just before the amp...

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Re: Help with analog VU meters

Post by Externet » Sun Nov 03, 2013 6:27 am

And why did you insert that Shallco atenuator in the audio path ? It is not needed or improperly set.
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Re: Help with analog VU meters

Post by ModRob » Sun Nov 03, 2013 9:23 pm

The Shallco is one of the most "important" parts of this effort. Let me explain:
This console is the focal point of my internet radio station-but with a twist. It's going to be "run" just like a station did in the seventies. Yes, I could have done it much easier with a modern board, but I am hard-headed wanting to stick with the "old school" feel of the consoles of that era. (You think that's silly? You should see the vintage turntables I made from scratch to replicate the old Russco units! Just for effect--after all the real deals, when and if you can find them, are terribly expensive).

I think I've stumbled onto a "solution" although probably not technically correct at all. But I'm more of a "try it-if it works, then so be it" kind of guy, and just too darn old and hard-headed to really start from scratch in learning the correct theory. Although I'd love to, as I'm fascinated at how electronics work, but at my age, my learning capacity has become more-and-more difficult. So I learn what I can as I go, and yes, that is thrilling for me when it works. But most all of my "successes" have been because of folks like you guys on here helping me along, and teaching me as well...

I'm sure the Shallco pot has some weird values to it and is probably not the best way to go for my project, but...again, it's "working" and I now have dang good audio all the way through. So at this point, I'm trying to add back some of the needed features that I disabled, due to my eliminating a lot of the old amplifier and related circuitry. Mainly the VU meters, and the monitor mute feature when the mic is engaged. The main "issue" I have created is the whole low level thing, as when I output from the console to feed the typical home stereo amplifier/receiver, I have to run the volume of the receiver up quite a bit from normal, so I know that the level coming off the console is low, which leads me to believe that I don't have enough level going in to be able to drive the meters. I think I mentioned before that when I ran straight from the CD unit to the meters, I did get a little movement. So my thought process tells me to get more level somehow. My solution thus far is I have built a little CMOY headphone amplifier, so I added it inline on the feed to the meters, and it worked! (I'm sure you're probably cringing at this moment, thinking "what in the you-know-what is he doing?" That AIN'T right! haha) Again, if something works, and I'm not burning the house down, then I'm calling it somewhat of a success...
A friend of mine a couple hours away has a -10 to +4 converter he's going to give me when I can get there to get it...maybe that's the correct way?

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Re: Help with analog VU meters

Post by CeaSaR » Mon Nov 04, 2013 2:51 am

I don't have time for a full reply, but I will get back to you after work. I have some more questions,
but if you could supply a make and model of the mixer, or a link to the specs/user manual and a good,
clear photo of the input connection panel (and connectors/adapters used), perhaps we could better
answer your dilema and comment about your work around (as to why it seems to work).

I think I know what you did, but don't want to finalize my assumption until more info comes in. :grin:

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Re: Help with analog VU meters

Post by ModRob » Thu Nov 07, 2013 11:34 pm

Console is Harris Stereo 80. I found a PDF manual on-line (not sure of link at this moment, but found it by googling the "Harris Stereo 80" phrase and it came right up).
Not sure if it would help you since I've done some pretty drastic changes to it. Again, mainly I have too low a level to fully drive the meters. I think what I'm trying to ask is, is there a less technical way to get more level to the meters?

Ooops...just thought about this, and it might be a factor...
I now have my main audio feed that feeds into a left input board, and a right input board. Off of that feed I tapped cables out to an outboard mixer (geez..a little redundant huh?). That little mixer will feed out two stereo feeds back to the console. One will be for the Program; one will be for the monitor circuit (control room monitor volume). The meters are currently still stock wired from the meter back to the left/right input boards, but NOT connected back up to anything. My intentions have been tapping off the Program feed to the meter inputs. Maybe this is nowhere near correct, but since I was able to get low readings by connecting the meters straight off the cd output RCA connectors, then I figured it would work, if I could just get a high enough level to further push the meters...
One day I plan to make things a little more proper, but for now I'd so like to get my project off and running...

Again, thanks so much for your ideas.

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Re: Help with analog VU meters

Post by Externet » Fri Nov 08, 2013 6:08 am

ModRob wrote:...a little CMOY headphone amplifier, so I added it inline on the feed to the meters, and it worked! (I'm sure you're probably cringing at this moment, thinking "what in the you-know-what is he doing?" That AIN'T right! haha)
There is nothing wrong on using a headphone amplifier to boost a 'line' level signal. It can yield an even better lower output impedance.

Just keep in mind that the level shown on the VU meters must match the modulation limit. In other words, how much the meters deflect is irrelevant; they have to be 'true' to the next stage. Red zone should start distorting.
Note: common and suggested practice is to peak at -7 dBu for digital audio.

From my former boss; it may give you a good headache; but it is all true ----> http://www.orban.com/pdf/Maintaining_Au ... y_2011.pdf

Miguel
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Re: Help with analog VU meters

Post by Robert Reed » Fri Nov 08, 2013 5:09 pm

Liked your link (Orban). Although I didn't read the whole paper in detail, there were quite a few chapters that were very interesting and informative. Mr. Orban was quite the man!

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Re: Help with analog VU meters

Post by ModRob » Sat Nov 09, 2013 12:55 am

Thanks Miguel...I feel a little better now...whew! But I tell you folks, the deeper I go, the further I seem to get away from anything that I thought I used to have a grasp on...for example;
This is baffling me to no end--I have my main audio feed that exits the input/output L/R boards, and tied into speaker terminal strips. So this is my control room monitor setup, and pots up and down with the master vol. pot (another side issue is that since I did this "customization" the stereo vol. pot only works on about the first half or less of its range). I want to "sub" off this feed with another that is not affected by the monitoring master vol. pot. Everywere I've tried to tap into the feed, it ends up controlled as well by the master monitor volume. I keep going around in circles...why is this eluding me?

----/------------------------------------program feed not affected by console master mon. pot
/---/-----------------------------------
/ /
/ / ------------monitor feed for control room
/------------------

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Re: Help with analog VU meters

Post by Externet » Sat Nov 09, 2013 6:35 am

ModRob wrote:...the stereo vol. pot only works on about the first half or less of its range...
Is there any limiter switch or AVC (automatic volume control) set to on ?
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Re: Help with analog VU meters

Post by ModRob » Sat Nov 09, 2013 4:44 pm

No, nothing like that in place. Basically all I have is audio feeding in, through one master/multi-routing switch for the particular channel, then thru a Prog/Aud selector switch, then through the master vol. pot on out. I want to think I'm missing the basics on wiring a pot. Since the pot is a variable resistor installed within the circuit, wouldn't that same resistor affect the whole circuit? Or is it as simple as if I tap into the feed BEFORE the point of the pot, the new feed wouldn't be affected? Am I making sense here, or am I just complicating it?

again....-----------------audio feed--------\----------------to control room monitor setup, controlled by console master
(.................................................) \---------------feed for Program out, to not be affected by console master

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Re: Help with analog VU meters

Post by Externet » Sat Nov 09, 2013 5:51 pm

An improperly hooked potentiometer may give you that sort of troubles.
There is 2 ways to connect a variable resistor (potentiometer):
In series - hook signal in to one extreme and signal out to centre wiper. The other extreme left unconnected.
Divider - hook signal in to one extreme; common ground to other extreme and centre wiper will be signal out.

Are your console inputs balanced and the CD player unbalanced ?
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Re: Help with analog VU meters

Post by ModRob » Sun Nov 10, 2013 12:06 am

Ah ha! You might be on to something there...I'm thinking it's still wired in the original format (divider, which I don't quite understand) but since I did some extensive "remods" then I believe I need to change it to "series"...

Inputs balanced? I don't think so...CD player comes out RCA outs (-10) with standard 2 condr. cable and runs into the console to solder to two posts (red, black) on the Left Input Board, and the same to the Right Input Board.

Be leaving work in a couple hours; can't wait to look into that area. Thanks for raising my "excitement level" again...haha...

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Re: Help with analog VU meters

Post by JerryR » Sun Nov 10, 2013 5:55 am

ModRob wrote:Ah ha! You might be on to something there...I'm thinking it's still wired in the original format (divider, which I don't quite understand) but since I did some extensive "remods" then I believe I need to change it to "series"...

Inputs balanced? I don't think so...CD player comes out RCA outs (-10) with standard 2 condr. cable and runs into the console to solder to two posts (red, black) on the Left Input Board, and the same to the Right Input Board.

Be leaving work in a couple hours; can't wait to look into that area. Thanks for raising my "excitement level" again...haha...

According to the Harris Stereo 80 manual, it seems all inputs *are balanced/600ohm* your CD player output is UN-balanced(RCA ).
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