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Impedence mismatch problem in audio project

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gringoben

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I'm working on a project which involves some old conference interpreter consoles from around 2008. They were designed to work with seperate distributor boxes into which you plug a floor signal from the venue PA system and it passes that to all the consoles. In return, the consoles pass their interpreted audio back to the distributor boxes as six seperate audio signals so that they can be output to transmitters. The problem is that there are only a few working distributor boxes left, and their use is quite limiting in terms of the configurations possible. For this reason I have been modifying the consoles to have their own outputs, and make it possible to input the floor signal without the distributor boxes.

On the whole, everything is working great, but the floor input, the most import element, is not working. With the majority of consoles, no floor signal can be heard, even though it is definitly there, however it does work on some of the consoles, although not necessarily all of the time. All the other channels work fine, but the consoles clearly do something a little diferent with the floor signal.

My conclusion is that there is some kind of impedence mismatch. If I play with the levels from the floor mixer (all around line level) I can sometimes hear something on some of the consoles, although it goes from very faint or non existent, to totally distorted, with no middle ground. It's odd because on a small number of the consoles, it just works. I do know that over time, some modifications were made to the consoles, including changing some resisters on the mic pre-amp to boost gain, but I don't see that this impacts on the floor signal, and besides, the consoles that work seem to include boosted and non boosted - I can not establish a pattern.

So, my question is this, how do I modify the input from floor in order to better match whatever the consoles expect? I am thinking either a mod to every single converted console so that the signal is 'corrected' immeadiately after the floor input jack, or a box or special cable that sits between the floor mixer and the console the floor signal is brought into.

I've tried a 600ohm 1:1 transformer with center taps configured to provide what I imagine is 2:1, and it kind of worked, briefly, on one console. Not exactly a definitive solution. I also tried what I think is an L-pad using a variable resister but is was a 22k ohm pot and I suspect that it well beyond the range needed. I know next to nothing about impedense matching. I also don't know how to measure the input and output impedences of the existing kit, and to make it even harder, I don't currently have access to a working distributor box as they are in short supply and needed for events.

Update: As well as the transformer and half hearted attempt at an L-pad, I also tried putting the floor through a 5532 based amp before putting it into the consoles. This was interesting because it worked, kind of, for a bit, although with lots of hum. I assumed the hum was power supply related and tested that by disconnectiong the power to the amp. The noise did not go away but interestingly the audio remained , although quieter. It appears the insertion of the amp provided the required impedence match, to a degree, regardless of it being an active circuit. More confusion mostly.

Any advice?


p.s. I have schematics of both the consoles and distributors.
 
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Your symptoms do not match the linear behavior of impedance ratios but rather the nonlinear behavior of feedback with crosstalk and spurious oscillations above gains >1
 
Here are the schematics. I don't know enough to figure out the input and output impedences from these.


The 'mixer' schematic is for the 'distributor' and seems incomplete but has values for the 'floor' part of the board.

The 'main' schematic is for the console.
 

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Can you post the schematics.

My money would be on corroded switch contacts, but there really isn't enough information in the original post to say for sure.
I can't imagine this being the case. Each console has two 'listener' sides to it, and so two different toggle switches which switch between floor or the relay channels. I've found no console in which one side works and the other does not. Therefore I think it extremelt unlikely that corroded switching could explain the issue.
 
Your symptoms do not match the linear behavior of impedance ratios but rather the nonlinear behavior of feedback with crosstalk and spurious oscillations above gains >1
hmmm.... please tell me more. I have been extremely confussed by the unpredicatable nature of the issue. I was testing over 50 consoles a few days ago and took the opportunity to test the working ones alongside the modified consoles I've been working on. I found less than 1 in 6 would accept the incoming floor signal, and some of those did did work would then randomly stop accepting the signal. I had hoped I could simply mark the ones that worked, or spot some kind of pattern relating to previous modifications, but nothing emerged. I am pretty certain it's not level related, and impedence mismatch seemed to tick the most boxes.
 
What type of connectors are used for the "Floor input" and what type of output connector on the floor mixer, and what wiring (cable type and pin connections) have you used between them?


(I suspect a signal config problem, such as both signal lines of a balanced output being connected to a single-ended input).
 
What type of connectors are used for the "Floor input" and what type of output connector on the floor mixer, and what wiring (cable type and pin connections) have you used between them?
(I suspect a signal config problem, such as both signal lines of a balanced output being connected to a single-ended input).

The distributor has TRS jacks. It is not clear whether they are balanced or not. In use I have only ever seen them used as unbalanced but the fact that they are TRS not TS gives me pause for thought. That said, the cables I've seen them used with use TS plugs, so go figure.

Now this is where it gets a bit odd - the connection between the distributor and the consoles is done using 9 pin sub D types connectors (as in RS232) and unshielded data cables. It works but if there is EFI or RFI, you can be sure it gets in. There are seven audio channels in the cable, unbalanced I assume, six relay buses and one floor, and the extra wire carries some kind of serial communication between the consoles so they know which send channels are occupied.

The consoles I have converted now have six TS jacks for unbalanced outputs, one XLR/TRS combo socket for floor in (unbalanced for now), and two RJ45s that carry three relay channels, plus floor, all balanced, using transformers. These, along with Cat6 cables, are used to replace the serial cable daisy chaining between consoles with something less prone to interference.

Along with these converted consoles, I've also made a simple breakout cable using the 9 pin D sub connector, that allows the six channels plus floor to be accessed directly from the 9 pin connectors on unconverted consoles. The floor issue is present on these and the converted consoles.
 
The distributor has TRS jacks.
TRS jacks are quite common for inputs, with the jacks wired balanced.
A TRS plug gives balanced line plus screen input, while a TS plug grounds one side of the input and connects the signal to the other input, so it also works fine.

If outputs are transformer coupled or have series resistors for protection, the same scheme can be used.


Re. the problem, I'd try probing from the audio source onward, to find when the path stops?

eg. Using a floating battery powered amp, or an isolating transformer with series input resistors plus volume pot at the output in to an amp, so you can test between any two points for either single ended or differential signals, without risk of shorting anything.
 
I've been studying the schematic for the distributor trying to figure out whether it's using balanced or unbalanced and I'm more confused then ever. Just looking at the PCB traces for the TRS jacks for the output and the one for the floor input, I've concluded the opposite of what I had originally though most likely.

It appears that on the outputs the sleeve is wired to ground on the PCB while the tip and ring are independently connected to the positive IN on the two channels of the op amp. I guess this means the outputs are balanced, despite them almost always being connected to unbalanced equipement (radio transmitters).

Meanwhile the floor (which makes most sense to be balanced as it comes from the venue PA system, usually capable of balanced audio, and often requiring very long cable runs), actually appears to be unbalanced as the schematic show the sleeve and ring to both be connected to ground while only the tip goes to the op-amp input and the output goes to pin 8 on the 9 pin connector which I know to carry the floor signal.

I found additional circuit diagram that shows two partial circuits, one I think shows an output on the distributor, and the other shows the input. I think from this diagram I should be able to assertain the output impedence of the floor output, although it doesnt show the component values, I think they are on the schemetic of the pcb.
 

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Your negative image is a nightmare. Schematics should have positive dark lines on a white background.
None of the opamp +inputs are biased properly with a DC voltage.
Also the inputs are missing coupling capacitors.
 

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