Coati Console Project -
Stage 1 : Initial Evaluations of components and modules

Preliminary Prototyping

At the time of writing, two preliminary prototypes have been assembled and tested individually and with each other. Initially the component modules were fixed to a fibreglass board about 20cm by 30cm square, using a hot glue gun but later, as modules were being swapped in and out for testing, the majority of the components were left loose to make it easier to change the configuration. Testing of the loosely assembled consoles proved the design concept to be viable, with signal routing all working as intended, and suitable audio levels being achievable throughout the system.

Internal wiring and connections

Connections between the modules was super easy thanks to the standard JST pin header pre fitted to many of the modules purchased for testing. It quickly became clear that it would save a great deal of time and effort if as much as possible of the internal wiring could be done using JST terminals and pre-assembled connector cables. These connections also reduce the amount of soldering and potential soldering errors, making assembling and repairing the console a much more accessible prospect as per the design spec.

In many cases it was fairly trivial to fit JST headers to modules that did not come with them, but for some components this was much harder due to inconsistent pcb holes or pin. Given the huge advantages the universal use of JST connectors offers, we would strongly recommend choosing modules and components on this basis, even if it means some additional upfront coss..

Signal routing

Our early attempts at wiring for the multiple signal paths required between the various elements of the consoles without recourse to JST headers throughout proved frustrating. There are basically four audio signal channels (three relay language buses plus the floor signal) to be distributed to five different internal elements (the two ‘receiving’ sections, the microphone section, the tx output conectors, and the console daisy chain connectors). Given mono pairs, that’s at least eighty connections to be made in the signal paths alone. Since most of our modules are stereo, and we’ve currently chosen to maintain use of twin channels where possible internally, that adds an additional 50% more connections, bringing the total up to over 120!

Fortunately, most of the modules already featured JST headers, and when we purchased a load more, we were able to design a simple and effective signal distribution hub based on a single vero strip PCB board. Obviously this doesn’t completely eliminate the need for soldering, but it does massively reduce it, and means that the vast majority of the remaining soldering is in assembling the signal hub board itself which is pretty easy to do and to test it has been done correctly. We may well consider offering pre-assembled signal hubs to further simplify other peoples construction of consoles in the future.

Power rails and supply voltages

There are at least three modules requiring connections to power, more likely four, and at least three more if not using passive switching. For simplicities sake, it seems sensible to take a common supply rail approach incorporated into the signal hub board. This could consist of multiple voltages to supply different voltage needs for different modules, however another lesson learned early on was that although it is possible to supply different modules with different voltages, such as 12v and 5v, it is probably unwise to opt for a design requiring multiple internal voltages when it is so easy to standardise on one during the choice of modules. We blew up several headphone preamps during module evaluation due to losing track of the fact they needed 5v max.

We opted to stick to 12v nominal supply voltage throughout. It is no doubt possible to put together a successful design based entirely on 5v modules, which may offer some advantages in terms of potential battery powered operation, but given time constraints and the fact that most of our evaluation modules ran on 12v, we decided not to pursue the 5v option further at this stage.

Amplifier Modules

While evaluating the various pre-amplifier modules we purchased for the purpose, we noticed that two of the amplifiers we had bought to use as headphone pre-amps seemed also to work okay as mic pre-amps. Given the design criteria calls for a modular construction that aids easy troubleshooting and repair, the idea that we could use identical amplifier modules throughout is an attractive proposition.

The best amp was also the largest and most expensive (https://www.circuits-diy.com/ne5532-dual-low-noise-op-amp-datasheet/). It was also the only one we purchased for evaluation that features a built in volume control. A built in control reduces the amount of internal wiring and additional components, therefore making assembly quicker and less complicated, but it would no doubt limit console control layout options significantly. When we attempt to assemble prototypes in cases, we will have a greater understanding of which is most important, the advantage of having less restrictions on control placement, or that of having less wires and extra connections to make.

The amp uses two 5532 chips, one for each channel, and I experimented using just on of these amps with two different mono signals and two independent volume controls and headphones. These amps have a gain of x5 gain but that can be increased by changing a couple of resisters. Another thing worth mentioning about this module (and another we will talk about later) is the use of socketed 5532 op-amp chips. The 5532 is also used in the Coati Spider so that means unified spares across the range of equipment.

Most of the smaller cheaper pre-amp modules were bare boards without JST connectors and no potentiometer for volume control. In order to even be able to evaluate in use, we had to piss about adding connectors or cables, supplying power, and adding volume control. This added lots of extra variables so the results were not really conclusive.

The TDA1308 stereo headphone preamplifier board module was incredibly cheap, under 50 cents each. The voltage range for these modules is 3V-6V and we blew up several applying 12v by mistake. Even when correctly powered, they were unimpressive, providing insufficient amplification for our needs. We found these modules needed much higher input levels on the floor signal than the NE5532 pre-amps. This might not preclude their use (and their price and size is certainly very attractive) but it raises the question of how might different consoles, built to the same design spec but using different amps, work together if their input level requirements are significantly different.

Microphone preamps

Initially, none of the dedicated microphone pre-amps that we managed to evaluate at this stage, seemed to provide sufficient amplification to get the desired signal level. We resorted to adding an additional pre-amplifier in series, the dual channel XH-A902 (a 5532 based amp with convenient JST connectors and sold as a general audio preamp). Using two amplifiers later proved to be unnecessary as the XH-A902 actually seemed to produce sufficient amplification by itself. Incidently, I tried it as a headphone pre-amp but it did not perform well in that application. It’s a 12-30v module and the specs say it has x5 amplification as standard. https://www.sinoning.com/product/operational-pre-amplifier-module-ne5532-amplifier-front-panel-digital-audio-module/

Among the most potentially interesting mic pr-amp was the SSM2167 based module which the datasheet described as improving vocal clarity in communications and public address systems with gain dynamically adjusted by a control loop to maintain a compression ratio set by a single resistor and varied from 1:1 to over 10:1, preventing overload and eliminate popping while a noise gate prevents amplification of background noise or hum (https://www.analog.com/media/en/technical-documentation/data-sheets/SSM2167.pdf). This is a 3v to 5v device so was a pain to include in our tests. https://www.aliexpress.com/item/1005001699213929.html

Supplied with its own 5v power supply we eventually got it working and it was buzz free, instead there was a high pitched whistle. This turned out to caused by the power supply, initially the 5v li-ion battery bank we had sourced specifically with the idea of trying battery powered consoles. On a hunch I tried a different 5v supply, a standard mains USB phone charger, and the noise went away!

We tried various other ways of supplying 5v but had limited options available having either inexplicably blown up. or found non functioning, all of the other power supply modules we had sourced for evaluation. We tried an automotive 12v USB adapter we had lying about but it also introduced a nasty whistle noise. Another one had no whistle but instead introduced some kind of digital noise. Basically this pre-amp demands a supply clean power supply. We built a 5v supply from an LM371 but somehow the voltage crept up over 6v and blew the module. With only one remaining we stopped further evaluations. Since the amp can be supplied with as little as 3v we can also try powering it direct from a li-ion battery with no DC to DC converter, as that would supply 3.5 to 4.1 volts.

Another small mic pre-amp we tried was based on an AD828 chip. This one didn’t have the fancy features of the SSM2167 but it did have a wider voltage range for we could run it on 12v. The amplification level proved disappointing however. It is possible to increase the gain by changing a resister on the board. We might give it a go out of curiosity but really don’t want to be recommending a module that needs such modifications.

https://www.aliexpress.com/item/1005004144327496.html

VU Meter

I could not figure out a way to provide an off-the-shelf audio ‘clipping’ warning LED as featured on the ALIS consoles. Instead I opted for a VU meter. The module we sourced was based on the KA2284 and rated at 3.5V-16V DC and cost under one euro. The voltage range makes them suitable for consoles built for 5v supply or 12v. The board is badly designed as the LEDs do not protrude above the level of the other components, making it hard to incorporate into a case while allowing the LEDs to remain visible. That’s annoying but not intolerably so, although sourcing an alternative module might make sense.

Having a VU meter proved to offer additional unintended features. When the power to the console is switched on, the VU meter lights up briefly, a good visual indicator that power is present without having to have a specific power indicator. Furthermore, the presence of the VU meter provided an extremely useful troubleshooting tool when diagnosing audio issues. By incorporating a probe lead, it was possible to trace an audio signal throughout the device in order to find problems in the signal path such a loose connectors or a failed amp. These amps can also be used to check voltage level, so in a battery operated console they could also provide a battery test function. It might also be possible to build functionality into the console for testing the capacity of the 9v batteries used to power the spiders, which might prove to be really handy.

Channel selection switching

Various methods of signal switching have been considered and explored. Mechanical switches are cheap but can introduce noise and degrade with use, while relays offer noise-free switching and greater reliability but are much more expensive. There are Integrated circuits that can provided switching at low prices and great reliability but are far complex to construct, and the addition of microcontroller-based switching provides advanced control options and a little more cost.

In our preliminary prototypes we tried two different active relay based switching modules and three different passive mechanical switches. All seemed to work adequately but none can be considered ideal. The mechanical switching no doubt provided the simplest option, and maybe also the cheapest.

The cheapest of the mechanical switches required the most soldering, nothing too challenging but every additional solder point adds to labour cost and things to go wrong. The more expensive mechanical option came as a module with JST headers and connecting cables. This made direct comparisons based on price difficult to assess but there is little doubt we’d recommend the plug and play option.

Also hard to assess with such limited short term evaluation is how long the mechanical switching option might continue to work flawlessly. It is possible that long term use would introduce crackles and other noise to the audio signals while switching. For these reasons we believe mechanical switching is most suited to the input selection elements of the console where some noise may be more tolerable, rather than the output switching side which could effect many more users downstream. Carrying spares would probably resolve issues about possible degradation of mechanical switching over time.

With one exception, the switching methods we had available to us for evaluation. all suffered from another potential drawback, they all switch sequentially through the options. For example, if your switch is currently on position 1 and you need it on position 4, you must pass through positions 2 and 3 to get there. In the context of the console, when switching from input channel 1 to channel 4 you would briefly hear the signal coming in on channels 2 and 3. This is not a problem, however, on the output selection side of things, as you switched from 1 to 4, you would briefly be outputting a signal on buses 2 and 3, potentially creating interference and disruption to others using those channels. For this reason, ideally the output selection system would not be sequential but instead allow you to jump from 1 to 4 directly etc.

We had one non-sequential switch to evaluate, mechanical interlocking push button switches. This was relatively cheap and seems robust, but as mentioned earlier, given time these mechanical units are likely to wear or become dirty, and may then start to introduce noise while switching. For these reasons, as mentioned before, this mechanical push button switch may be best suited to the input selection function rather than output selection, and therefore the benefits of non-sequential switching are of little relevance.

Ideally we’d use a non-sequential relay based switching system but could not locate a pre-exiting off-the-shelf module to do this at a competitive price. We may look at whether the sequential relay switching modules can easily be modified to non-sequential use but it is probably non trivial nor a good use of time. The relay boards we evaluated were of two types, either controlled by a four position rotary switch or a single push button switch. All provided LED channel indicators which was nice as not only did it make much more obvious at a glance which channel was selected, but also the bonus feature of providing power on indicators. And yes, the relay modules need power, adding to the number of power connections required in the console. They need at least 7v (up to 24v I think) so not ideal in any console design based around 5v modules. While the relay switching was noiseless, it was not better than the mechanical switching so it seems that almost doubling the total component cost of the console can not be justified, especially as these modules did not address the sequential switching issue.

There are a number of options for non sequential non-sequential switching which avoid the potential issues of mechanical switching and the expense of relays. That option is solid state switching controlled via either logic ICs or microcontrollers. As discussed elsewhere, microcontrollers offer a number of other possible advantages and advanced features, but logic ICs would be cheaper. Our preliminary prototypes have not attempted to make use of either of these systems. We will hopefully do these evaluations at a later stage.

External connectors

There was some concern about the 3.5mm stereo jack headphone sockets we purchased for evaluation. When using some headphones, it appears that the chassis mount sockets required the plug to be pulled out a few millimetres to ensure sound could be heard in both ears. It may relate to some the fact that some of the headphones we were using had been designed for mobile phones and had four bands on the plug instead of the three used on a standard stereo jack, or it may have been poor quality sockets with inaccurate tolerances. This requires further testing with more typical headphones and maybe sourcing alternative sockets. We’ve now located sockets that come mounted on PCBs with JST connectors and we will purchase some to test and most likely recommend these or similar in order to reduces soldering needs.

Another connector issue noted related to some of the RCA socket units being evaluated, the bare units not fitted to a pcb and supplied with JST headers. These bare units had solder pins that did not comply to the spacing of our vero strip boards and being flat rather than round, they would not go into the holes even when the spacing was adjusted. We managed to squeeze the pins into a shape that could be forced into our PCBs but it was fiddly and not something we can expect others to do. On this basis we would definitely recommend the pre-assembled modules with JSTs rather than the bare connectors.

The RJ45s came pre-fitted to a pcb with an eight pin header. Other formats were available but probably none more convenient. Since we are putting four bus channels down eight wires, we are constrained to four unbalanced mono pairs, and since almost all our other modules and connectors involve two channel signals and 3 pin JST connectors, we have to switch from 3 to 2 pins for each bus, thus resorting to a different cabling solution than used elsewhere in the console. Eight wire ribbon cables maybe the solution, especially if we sourced them pre-formed with the appropriate JST connectors fitted. What is certain is that a solution to this will massively reduce soldering complications and is well worth pursuing further.

Update, later tests suggest that the cross talk issue is related to the way we have utilised these RJ45 boards, with cross-talk present even when no cat5 cable is plugged in, but stopping when the RJ45 boards are disconnected from the signal hub. This requires more investigation.

It’s worth pointing out here that we assumed the twisted pairs in the cables would correlate sequentially to the order of the pins on the breakout board but actually this is not the case at all. This can not be the source of our current cross-talk issue, but it would very likely cause an issue if not addressed. The correct pin pairs should be as follows. Pins 1 & 2, Pins 3 & 6, Pins 4 & 5, Pins 7 & 8. This means we can’t easily use an off-the-shelf ribbon cable with pre-fitted connectors to link the RJ45s to the signal hub.

The combo XLR-1/4” jack microphone sockets proved to be fabulous, certainly a worthwhile choice compared to the other options. However it came without any pinout information and was a bugger to figure out. Good instructions for others will be essential as I wasted many hours getting it right and I think some of my earlier failures in evaluating mic pre-amps was down to having not wired the mic correctly.

Physical observations

It was noted that the completely assembled preliminary prototypes required a much larger footprint for the various modules and wiring than previously imagined. Certainly they could have been assembled with a more compact layout. Had our connecting wires been selected by required length rather than the standard sizes we had available, there may have been less clutter. None-the-less, it does seem unlikely the consoles could be made quite as small as first predicted, and even if it could be done, it’s probably not a good idea as it would make assembly and any subsequent repairs, much more difficult. Finding a compromise regarding the size would be well worth while as reducing the weight and volume is part of the design criteria and will mean reduced transportation costs for events.

We will get a more realistic assessment when we begin to make enclosed versions of the prototypes.

Console tests

Beyond evaluation of the individual components and modules, our initial tests of the console prototypes proved very promising. The design worked as expected, with all the signal switching elements performing well and audio levels being maintained across the system and also when shared between consoles. However, some care was required to adjust floor input level and microphone signal levels to be adequately equal at the headphones, although this is to be expected. At one stage the floor level, when received on the second console, appeared a little lower than on the first but this somehow resolved itself so may have been a connection or soldering glitch.

Noise

There was, at various stages of testing, quite a lot of noise in the form of buzzing. Importantly, this did not occur when monitoring floor signals, only when listening to the mic. It was inconsistent, seemingly worsened by handling the microphone and improved or worsened when touching various cables, connectors or components. Sometimes there was very significant levels of noise, but at other times it was negligible. Given the haphazard assembly of components laid out on the table with soldering iron, power supplies, cables and microphones all criss-crossing each other, it’s hardly surprising we experienced buzzing noises at times. At the next stage of prototyping, with components more neatly assembled in a case, we figured we’d have a clearer idea of whether this interference was genuinely a problem or not.

We wondered if using a different power supply might have an effect on the buzzing issue but ultimately this did not appear to be a factor. That said, it is very likely that the frequency of the buzzing was that of the inverter used to generate the AC power we used while conducting tests (the test location is off grid). We hoped that the inclusion of noise suppression components such as chokes, capacitors and ferrite beads, could significant reduce future interference issues.

Update - later re-evaluation of microphone pre-amps saw the buzzing noise eliminated so it appears the problem was literally being amplified by using an unsuitable pre-amp module for the microphone. This is great news and suggests near noiseless operation is achievable without resorting to additional noise elimination strategies beyond appropriate selection of the pre-amp modules used.

Cross-talk

Another form of interference observed during this early testing stage was ‘cross-talk’, with the floor signal being faintly audible on the bus channels when the consoles were linked with the cat5 daisy chain cable. This is not totally unexpected, although it was hoped that the twisted pair construction of the cat5 cables chosen for the interlink cables would have eliminated the issue. It’s worth noting that the same phenomena is often observable with the existing Coati consoles. As with the buzzing noises, it may be that the problem goes away when the prototypes as properly assembled and wires not criss-crossing in an unorganised birds nest. However, even if the issue persists, the level of the cross-talk interference observed is very low and is unlikely to cause any problem or disturbance to the users of the system. We can also play with shielded cable options.

Stage 2 : Evaluation of building prototypes into cases

Two prototypes have now been built into an enclosures. The cases for both have been cut from an aluminium plastic sandwich board and glued together with a hot glue gun. This material will theoretically provide shielding. Modules have simply been glued in place for the time being.

The first of these second stage prototypes uses all passive mechanical switching. It also uses only one headphone pre-amp module, along with a number of custom made cable splitters and combiners that allow the one amp to service to separate mono audio channels and send them to two independent headphones. I also found that I needed only one pre-amp for the microphone, using the single 5532 amplifier module.

This configuration probably represents the simplest and cheapest design. The component cost of the specific combination of components used when this prototype was first assembled and tested is estimated as follows:

x2 4 way rotary switches (double pole) €3.00

x1 4 way interlock push button switches €4.00

x1 ¼” jack chassis socket €0.50
x2 RJ45 boards with header pins €3.20

x8 RCA board with JST €2.50

x2 DC chassis barrel socket €0.50

x1 power switch €0.50

x3 volume ports (1 with JST, 1 bare) €3.00

x1 3 way toggle switch for floor to bus €0.80

x1 VU meter €1.00

x1 single 5532 op-amp used as pre-amp for microphone €2.00

x1 dual 5532 op-amp used as pre-amps for both headphones €4.50

x2 3.5mm chassis stereo jack sockets €1.00

x1 combo XLR/Jack socket €1.50

x24~ 3 pin JST pcb connectors €2.00

x22 cables with JST one at least one end €6.50

x16 10cm female to female wires to connect to header pins €1.00

x16 header pins €0.10

x5 knob tops €0.50

~50cm of two core red and black power cord €1.00

about half a vero strip board €1.00

Total component cost = ~€40 (not including case)

(prices based on aliexpress inc shipping, rounding up, benefits from some pro rota bulk lots)

Physical Issues

At first I attempted to assemble the components in a case measuring 20 x 15 x 4 cm but this proved too cramped, mostly due to inadequate depth. I had at first started to glue the modules on the bottom of the case, and the switches etc. on the top panel, then close the two parts. However I found this was impossible with my initial layout and even had that not been the case, getting the wires to behave when closing the top would have been very challenging. This process taught me that the best approach is to build upside down, with all the modules and switches etc being attached to the underside of the top face of the console, then the bottom of the case is attached when complete. This makes assembly less complicated and would make disassembly for repair much more convenient too.

Since I could not physically accommodate my assembly into the 20 x 15 x 4 enclosure, I switched to a wedge shaped design 9cm tall at it’s highest. In this configuration there was plenty of room inside. This is important and a case of this size footprint could be 3D printed.

First test of first unit by itself

The prototype was initially tested independently from the second unit. Everything worked as expected. The sound quality when listening to an input on floor was perfect, as was listening to floor sent to the various buses. Switching between input channels did not introduce any notable pops or other noises and switching through output buses with the push button interlock switch also seems clean and without issue. Additionally there was no cross-talk noted at this time.

Picking up the microphone introduced a nasty hum that sounds like a 50hz buzz from the inverter supplied AC power. This noise was reduced significantly when I touched the front of the console. This suggests capacitive coupling is occurring. There are a number of methods I can try to prevent this from occurring, such as adding a capacitor in parallel with the input into the pre-amp, and earthing all of the modules to the case.

In later experiments I tried a different mic-preamp module and the noise completely vanished. It turned out that the pre-amp I had been using was actually the cause of the problem, or at least exacerbating it to notable levels. It seems that these noises can be avoided simply by choosing a more suitable pre-amp. I will need to source more for for evaluation.

Second unit

The second unit is of similar size to the first, a wedge measuring ~ 20 x 15 x 7cm. Again, all modules are attached on the underside of the top panel, through which are also fitted most of the switches etc. The rear, front and sides are attached to the upside-down top panel, and the various external connectors fitted.

This unit incorporates all active switching with three off the shelf four channel stereo relay boards fitted. This makes the unit considerably more cramped than the first. I deliberately installed what I consider to be the least suitable output switching module, one which cycles sequentially through the output buses using a single button.

Unlike the first unit, initially I have utilised two separate dual 5532 op-amps for the two headphones sections. This will allow for comparative evaluation of the potential to use just one, but fitting two in this test unit requires the modules to be stacked on top of each other. If figured that making it really cramped and complicated inside, with signal cables all in close proximity to each other and other parts of the unit, might help to establish whether having such a cramp design will introduce noise issues not experienced in less cramped and complicated designs.

In terms of component cost, this unit is considerably more expensive than the first.

x2 rotary controlled relay board board €28
x1 push button controlled relay board €14

x1 ¼” jack chassis socket €0.50
x2 RJ45 boards with header pins €3.20

x8 RCA board with JST (included with relay boards)

x2 DC chassis barrel socket €0.50

x1 power switch €0.50

x3 volume ports (1 with JST, 1 bare) €3.00

x1 3 way toggle switch for floor to bus €0.80

x1 VU meter €1.00

x1 single 5532 op-amp used as pre-amp for mic €2.00

x2 dual 5532 op-amp used as pre-amps for both headphones €9.00

x2 3.5mm chassis stereo jack sockets €1.00

x1 combo XLR/Jack socket €1.50

x24~ 3 pin JST pcb connectors €2.00

x26 cables with JST one at least one end €3

x3 knob tops €0.30Next

~50cm of two core red and black power cord €1.00

about half a vero strip board €1.00

Total component cost = ~€72 (not including case)

(prices based on aliexpress inc shipping, rounding up, benefits from some pro rota bulk lots)

The higher cost is mostly down to the relays, which cost more than the entire component cost of the first unit. There’s also the extra pre-amp. However, despite being significantly more complex than the cheaper unit, there was actually less soldering involved since the main switching elements all featured JST connectors. They also came with cables and RCA sockets, offsetting some of the price difference. That is not to say that I’d recommend the relay modules on this basis, it’s just good to be aware that the cheapest combination of components may not be the simplest or quickest to assemble.

Second unit individual tests

The basic functions all worked as expected with the exception of the three position switch that allows the user to switch off the mic and instead pass the floor signal to their selected output channel. I used a three way switch so that the middle position provides a microphone off function but I must have connected it incorrectly as the middle position still carried the mic signal (it works fine on the first unit). Whatever error I have made here will need rectifying as it may be causing knock-on issues.

The LEDs that indicate channel selection provided a convenient way to observe power was present (only the VU meter provides this information on the first unit). The sequential channel output was super annoying to use. The headphone output seemed a little more powerful than the first unit, but not significantly so. Unsurprisingly perhaps, this unit did seem to be noisier than the first, and the hum appeared to be reduced much less by simply touching the case.

Later it was established that it was a poor choice of amplifier for the microphone which was causing the buzzing noise. Unfortunately the only other mic-preamps we had available were 5v not 12v and this introduced new issues as we now needed 5v voltage power supplies. We soon found that those we had purchased for evaluation were either dead on arrive or blown up during prototyping experiments. Those still working introduced various noises, probable because they were switch mode rather than linear. Further progress on this will require additional research and sourcing more power supplies and mic pre-amp modules.

Somehow I neglected to consciously check for cross-talk when testing this console on it’s own, perhaps because I was in too much of a hurry to progress to testing both units together. This is an unfortunate oversight as I’ve since discovered that it’s probable that the source of the cross-talk issues are internal and probably related to the RJ45 module rather than caused by the cat5 interlink cable.

Testing both prototypes together

Noise grew much worse with the two units chained together, and touching the case of one or both units did not provide such significant reduction. The noise only relates to whichever bus channels are being used with the microphone, the floor channel or relayed floor channels are entirely without noise. I had not tried to incorporate any noise reduction strategies such as adding low pass filters etc. I have a very limited range of components available to try different capacitor values at this time. I will try playing with earthing the modules to the case. I’ll also try to power the units with 12DC from battery to establish for certain whether the noise is an AC hum.

Cross-talk
The tests revelled notable cross-talk that manifested as being able to hear the floor signal on all channels at much reduced levels. It was not annoying loud but definitely there.

It’s worth noting that by turning the level down on the device provided my simulated floor PA, the cross-talk almost disappeared entirely and the headphone amps had enough headroom to compensate for the lower level floor signal. It’s entirely possible that the cross-talk is actually a non issue in this context. However, it may be that when many more consoles are chained together, the floor level would need to be raised higher, perhaps exacerbating the issue. I can’t explore this further with only a few consoles.

Interestingly it didn’t actually appear that the cross-talk was dependent on the console interlink cables at all. Instead it appeared that the interference was occurring earlier, and would manifest even in a single unconnected console. I don’t current have access to an oscilloscope to use in attempt to figure out at which point the cross-talk is being generated. However, I built a third partial prototype in order to retest other pre-amp modules against a known configuration and discovered that I was experiencing cross-talk even with a greatly simplified system with only the bare minimum of cables and channels connected. I then removed the cables to the RJ45 i/o ports and the cross-talk instantly disappeared completely.

Bus conflict test

The situation in which two consoles attempt to use the same output bus was tested. The results were reassuring – basically the mic outputs get combined if similar levels and you hear both. However when floor was being relayed, it appeared to win over a microphone, regardless of volume. There was a moment when it felt like the floor relay had somehow locked the channel as even when I muted the source I still could not hear the microphone until I released the floor relay switch on the other console. But performing these tests by myself was difficult and I may have imagined it so I recruited a volunteer and tried again. Inexplicably, there was definitely some weird ‘locking’ going on. I really cant explain it. Magic, but not usefully so.

Sequential output selection test

I also tried to check what listeners might experience when the second unit, the one with the sequential output selector, ‘passed through’ the channel they were listened to. Surprisingly there was little discernable interruption at all, unless the floor was being relayed, in which case, yeah, there was a brief but intolerable interjection, the very reason why sequential switching on the output selection should be avoided.

Next steps

The priority is definitely to work on the mic noise issue. Unfortunately I may not have the range of components to work on this until my return to Europe, and certainly will not have access to an oscilloscope until I do. There are a few things I can try in the meantime and we will see if they progress things or at leat point towards a solution.

I will probably return to pre-amp module evaluation to confirm my gut feeling about sticking to the 5532 based amps. I’d like to retest the mic pre-amp modules, especially the one with the built in compressor, to see if there is any improvement on the noise issue. If they prove usable then I might be able to build a third boxed prototype as I could free up two amp modules currently being used as mic-pre amps. A third prototype would enable more representative testing, and also enable me to start to develop the layout of components within the case, as well as the lessons learned about the positioning of the controls and connections.