Console audio levels

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kcbooboo
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Console audio levels

Post by kcbooboo » Sun Sep 06, 2015 2:08 pm

I'm working on some Stereo Line Input (SLI) modules for an Auditronics 200-series console. These boards have two line inputs with level pots for each channel of each input. All inputs are high impedance (not terminated with 600 ohms). These boards feed the Audition and Program busses in the console. Separate Stereo Line Output (SLO) modules feed the Program and Audition outputs and these have left and right level pots, so there are effectively two gain pots per channel. I could feed in 1V, adjust the input modules for almost anything between 0.1V and 10V, then adjust the output modules for the desired line level.

The linear fader on the front of the SLI module is marked from +15 to -50, which I presume is the module gain in dB.

The SLI has active (op-amp, not xfmr) balanced inputs, however I'm feeding them with an unbalanced source, so I've grounded all the negative input pins and am feeding my signal into all the positive input pins. This might have an effect on the overall signal level that the board sees. I only mention this because I can't quite get 15dB gain through the board the way I've connected it and I think there is some number of dB lost in this configuration, possibly as much as 6dB.

Several questions:
1. If there is a difference in signal level between a balanced and unbalanced input, how many dB different would they be?
2. What should the internal Program and Audition buss audio levels be?
3. If I set the input fader to 0, I presume that should result in a 0dB gain through the board, but that might depend on questions 1&2 above.

Thanks for any insight and guidance.

Bob M.

TPT
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Re: Console audio levels

Post by TPT » Sun Sep 06, 2015 4:18 pm

It really depends on what source you are normally going to use with that channel--that's why there are the level adjust pots. Typically, the assumed input level is +4 from a balanced device, an unbalanced device usually is built with a -10 db level into 10 K-ohm. I have a couple of Auditronics 2500 series consoles--different console, but probably not that different.

So-- my first two pots are satellite feeds, I try to set typical levels so that the meters peak "0" when the fader is aligned with the -|- mark. Since I can't change the satellite receiver output levels I would use the little set pots on the channel modules. Then I have two CD players on the next two channels. The CD player outputs go through a -10 to +4 unbalanced to balanced amp (which is also the best way to connect unbalanced sources in order to avid ground loops, hum & other nastiness). A test CD gives me a reference tone--I adjust the gains on the matching amplifier in order to set the CD playback levels on those channels. Again, to the mid-range dash mark reference.

Now, let's assume I had to connect another unbalanced source, but without a matching amplifier. These devices are typically -10, rather than typical +4 line level. Not the best solution, but I would then adjust the set pots to get enough gain through the channel so the VU's peaked near zero. Had such a problem recently when we were covering a 10K race, using two way radios in the lead truck. Receiver we got had about -20 level, since the fixed level output was designed to feed a laptop computer. I had anticipated this, so used a little Art direct box--but if I hadn't had that device I would have set the console channel input to maximum hoped the processor AGC would bring it up.

kcbooboo
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Re: Console audio levels

Post by kcbooboo » Sun Sep 06, 2015 5:30 pm

Part of the problem is that I only have the SLI modules on the bench. While I can adjust the input level pots, I don't know what level the module is supposed to put out. If the VU meters are on the Program and Audition busses, I don't have those and I don't know what level they've been calibrated to.

Perhaps there are just too many variables.

Bob M.

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kkiddkkidd
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Re: Console audio levels

Post by kkiddkkidd » Mon Sep 07, 2015 6:54 am

It has been years since I worked on a 200 (1996?) but I recall that the VU meters are independently adjustable from the independently adjustable output busses which are independently adjustable from the module inputs. I found one with the input levels extremely offset to compensate for the misadjusted VU meters which might have been misadjusted to compensate for misadjusted output levels.
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Re: Console audio levels

Post by TPT » Mon Sep 07, 2015 7:00 am

I don't think you will be able to tell what levels these modules are supposed to put out without a mainframe--for one thing, I would guess the mixing buss is unbalanced, of some indeterminate impedance. So the module output impedance is also a guess.

If you have a mainframe in working condition, just pop the modules in one at a time, and feed them with +4 from a test oscillator. Set them up so levels are balanced, & they zero at the -|- mark on the fader. If they don't set up that would indicate a fault, mostly likely an IC gone south. Caution: if these modules are like the modules in the Auditronics 2500, never swap them around with the power on. Blows chips.

Seeing the post above--that gets to be real fun. You really need an outside reference, e.g., a distortion analyzer, to calibrate the VU meters and match channel levels. You might be able to clip lead into the VU meter circuit before the set pots but after the meter switch & set them either from a +4 source or at least match them to a common level.

kcbooboo
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Re: Console audio levels

Post by kcbooboo » Mon Sep 07, 2015 10:30 am

Kevin said it rather succinctly. That's the problem. Everything is adjustable and no buss levels are specified.

I did find one place in the manual where it said a +4dBv input should produce a 0dBv output on the "Direct Output" pin with the fader at "0". I eventually settled on 1.0Vrms input producing 0.5Vrms output on the program and audition busses with the fader at "0". Those busses are single-ended and are fed through 10k resistors. The output modules connect to the Program and Audition busses. The VU meters monitor the output of the console; there's nothing on the internal busses at all.

I did try feeding the module inputs with a transformer and saw absolutely no difference in level vs the unbalanced feed, so I'll stick with the unbalanced configuration as it matches my HP 339A distortion analyzer / oscillator. Everything else on the board is unbalanced.

I now have my own "standard" and will adjust the individual boards to these levels when I get one on my bench. I don't have access to the console these came out of; I'm just being handed modules with the request to "fix" them. I made a simple test jig to plug them into and so far that's worked out just fine.

Thanks to all the responders.

Bob M.

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Re: Console audio levels

Post by BigRed » Tue Sep 15, 2015 9:40 am

Is this the 200 SLI module that uses an Allison Research/Valley People VCA module in each channel? If so that makes level setting, left/right level matching, a bit more important, just a word of caution . . . And there's a couple of adjustments for those VCA's as well that you shouldn't mess with unless you've got the appropriate test gear and a decent single malt to get you through the set-up.

Anyhow, it's been a while since I've worked on one of those but I've still got some documentation including a marked-up drawing for that module. Someone's notes on that drawing (not mine) shows that for a +4-dBu balanced input you should have 0-dBu at the insertion points ahead of the VCA's and the same 0-dBu level at the input to the “Mode” switch (the decoupled side of IC5a&b) and at the module's “Direct Out” output. That also means that for a unity gain console the summing/output amplifiers will have about 4-dB of gain. Some console manuals actually have a gain diagram that shows levels throughout the console, some even show that for the individual modules; don't remember if Auditronics did that, but I can check the 200 manual in a few weeks. (Let me know if you need a copy of that or anything else for the console; pretty sure I can get my hands on the full manual.)

You mention the slide fader and its markings. That should be set to the “0” mark to set the module's levels correctly. When the fader is set to that “0” mark applying a +4-dBu balanced signal to the module input should give that 0-dBu level at those two points on the module and a +4-dBu (or dBm) balanced output from the console. That also gives the board operator 15-dB “in hand” to allow for up to a 15-dB boost in level on weak materials. I REALLY encourage keeping the gain structure of all the modules the same. Don't “customize” input levels from module to module to compensate for -10-dBu unbalanced consumer grade equipment as that can affect the noise floor and clip levels on the individual modules and make it impossible to keep a "universal" spare module on the shelf. (MCM Electronics has their 555-15310 active “matchbox” for $65 so using something like that to match a “consumer-grade” source to the “professional” console won't break the bank or “damage” the audio any more than “dicking” with the internal settings of the console.)

You should use a balanced sine wave generator to set levels. A 1-kHz signal should do the trick but check at 20-kHz to be sure that there's not a response issue (or use a swept function generator if you have one, slow sweep speed if you have to watch a voltmeter, fast sweep if you're using an analog scope works great too). Use one of those match boxes to get you there if your test source isn't balanced. If you don't have a test source eBay can be your friend. For measuring levels an AC voltmeter calibrated in dB (commonly part of a distortion analyzer) is terrific but any decent true RMS AC voltmeter should get you there with a little math. (My personal favorite is my elderly but trusty Fluke 8060A with it's built-in dB scaling function. It perfectly tracks the Tek AA501 across the audio band, is a lot more portable and doesn't have a ground loop problem.) When setting levels through the system use the external voltmeter and then set the cal's on the console's VU meters after verifying the output levels. As a “cross-check” make sure that all the gain “trim” controls are set at about the same points, comparing left to right at the same points in the audio path. If there's a big difference for any, left to right, you have a problem. And I think that the Auditronics 200-series had output transformers so they will have to be terminated to prevent ringing and overshoot issues. But make that measurement by first disconnecting the console's outputs from the plant and using precision terminations only. Then check levels again after re-connection to the plant. A difference of more than a fraction of a dB indicates a termination issue downstream. Another useful test is to watch the phase between left and right channels at the output while sweeping the input from DC to daylight. I've seen bad TL series chips cause a 45-degree phase shift (in ITC cart machines, showed while setting stereo head azimuth). Won't affect levels (except in the MONO SUM output) but will screw up the audio for sure.

And the several questions:

1. If there is a difference in signal level between a balanced and unbalanced input, how many dB different would they be?

+4-dBu is +4-dBu, (0 dBu being a voltage referenced to 0-dBm-600-ohms, which represents the voltage drop across a 600-ohm termination dissipating one milliwatt of power, but it's still a voltage reference, multiply that log of the ratio by 20 instead of 10, "voltage" vs. "power" systems in the analog audio world is a "whole'nuther" discussion . . and now my head hurts . . .), be it from a balanced or an unbalanced source or to a balanced or unbalanced “termination”. But it's complicated (yeah, I know; after that dBu vs. dBm(u) how could it be worse). A lot of different physical factors come into play. Is the source a transformer (possibly center-tapped to ground) or directly from an amplifier? Is the termination a transformer, true differential (instrumentation input) amplifier or the “cheat” that only uses the + and – inputs of a single IC, or something else? (A certain “popular” brand of 8 x 2 switcher that I checked out recently that uses that "cheat" has an input impedance that varies, substantially, with the input gain “trim” control; another reason not to “dick” with plant levels, not good!)

2. What should the internal Program and Audition buss audio levels be?

It appears from my notes to be 0-dB.

3. If I set the input fader to 0, I presume that should result in a 0dB gain through the board, but that might depend on questions 1&2 above.

Yes, that is the desired outcome. Unity gain through the console leaving 15-dB “in hand”.

kcbooboo
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Re: Console audio levels

Post by kcbooboo » Tue Sep 15, 2015 1:10 pm

Thanks for the reply.

Yes, these are the 200-SLI modules with the Allison VCA plugins that are surprisingly still available from multiple sources. I didn't even bother touching the four pots associated with them.

I had two manuals and scanned the necessary pages for the modules I was working on. I don't recall seeing a stage gain diagram in either one though.

I'm using an HP 339A distortion analyzer for my source and meter. It has true RMS metering and also does distortion measurements. It's completely unbalanced. That's fine for the module's outputs which are single-ended. I just connected all of the negative input terminals together and grounded them, and connected all of the positive input terminals together as my input.

I did split the "A" input terminals and fed them through a transformer. I found that they had the exact same sensitivity whether balanced or unbalanced, as measured at the input of the module. I have worked on other equipment that gave me different results depending on whether I used a balanced or unbalanced input source; I guess Auditronics has a good design.

Knowing that they should have +4dBu input to produce 0dBu at the "direct" output with the fader at "0" is useful but about four hours too late! I just finished checking out three more 200-SLI modules; all of those were set for about 0dBm input to produce 0dBm output with the fader all the way up, so I just left them there. They must be being fed by a rather healthy signal source.

I don't have access to the consoles themselves, just the boards that were handed to me with the words "please check these out" so that's what I'm doing. Besides balancing the four input level pots, I also test the frequency response at 10 Hz and 30 kHz, test the distortion at 1 kHz, check for output on the Audition and Program outputs, make sure there's output at the Cue terminal, and measure the no-input-signal output level (noise). It's up to the person who puts them back into the console to possibly readjust the levels to suit.

Bob M.

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