Introduction: Battery Powered Parking Lot Chorus

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I sing in a community chorus. At least I used to before the pandemic. For spreading disease, singing together indoors is one of the worst things you can do. We do Zoom meetings, but it’s not the same as singing together.

This Instructable is a recap of what a friend and I did to get the group back to singing and still stay safe. It is an idea copied from others and there’s little original here. In fact other folks have done a wonderful job of documenting their efforts and have made it easy for people like us to follow in their footsteps. But I don’t find an Instructable about this yet. So here goes…

The rudiments are:

  • A wireless mic for each singer.
  • The singers stay in their cars with FM radios tuned to a given frequency.
  • Singers can sing and simultaneously hear everyone else.The mic signal feeds into a receiver, and the receiver feeds into a mixer. The mixer feeds an FM transmitter, which sends at the given frequency.
  • Amazingly, there is seldom any noticeable latency or feedback.
  • The director is outside at central location with a headset mic and headphones to hear the combined chorus.
  • On our system the headset mic is wireless, but the headphones are plugged into the mixer directly.
  • The director’s keyboard is plugged into the mixer directly also.

Cars are parked in an arc so everyone can see the director. Unfortunately there aren’t a lot of parking lots large enough for cars to be parked this way AND have an electrical outlet. Therefore, battery power is needed (tenuous connection to the Battery contest). The intent of this Instructable is to provide a reference to chorus or choir who wants to try the same approach.

Supplies

Major Components

Links to Resources

http://thedenneys.org/pub/music/driveway-choir/ Very extensive, well documented trail of what this group did to set up their system.

https://www.facebook.com/groups/hotrodsandharmony Ongoing log with an increasing number of driveway and parking lot singing groups

https://www.facebook.com/groups/586692128695002 Ongoing log with an increasing number of driveway and parking lot singing groups

Step 1: ​Wireless Microphones and Receivers

Range

Wireless microphones typically come as a set (often 4) with a suitable receiver included. For a parking lot chorus, range is important. The better wireless sets promise to transmit over 400 feet. But this is line of sight, unobstructed distance. Windshields and steering wheels may reduce this distance. For our chorus we finally settled on GTD mic sets, which promised the largest range (over 400 feet).

Interference

The mics with a decent range are all UHF (versus VHF) and most transmit on the 500 megahertz band. There are also UHF mic sets which use 900 the megahertz band. On the lesser expensive mic sets, each mic is pre-set to a fixed frequency within its frequency band. There is no way to vary the frequency. So if by chance there is interference from external sources (UHF TV station for example), that mic may have problems. We tried three brands of wireless mic: GTD and Phenyx (500mhz range) and Tonor (900mhz range) .

Wireless Microphones

The Phenyx brand of mics set worked fine and seemed like a quality product. But I opted against them and in favor of GTD mostly because it’s way too easy to mute a mic by accidentally pressing the main button. And there is no active indicator that says you’re muted. The advantage of the Phenyx set is that there are gain controls for each mic on the front where you can use them. In the end, my thought was that we’d get the gains all set and then leave them alone. On the GTD mics, an LED changes to amber from green when you’re muted and one can “lock” each mic so it can’t be muted or turned off accidentally. All the mics we tried took 2 AA batteries each.

Wired Mics

Wired mics are much cheaper and can be plugged directly into the mixer. But each wired mic occupies a mixer input, and long wires can quickly become a mess. We decided against this.

Receivers

Receivers typically have a way to control the gain (volume) of each of their separate mics. In a chorus setting, adjusting individual gains is not very convenient or practical since we don't know which singer is on which mic. On the GTD brand of receiver, the individual mic gain controls are on the back and need a tiny screwdriver. Usually these gain controls are set to somewhere that works and are then left alone. The receivers have antennas (antennae) on the back. It makes sense to have these so they have an unobstructed “view” of the singers.

On the back of the receivers is an XLR outlet for each mic. These can be fed into the mixer, but for a chorus this would quickly occupy all the mixer inputs. On the back also is a ¼ inch outlet where all mics are mixed together. This is typically how the mixer is fed since there is only one wire per receiver.

Step 2: The Rack

If you are going to have more than 4 singers, you'll need multiple receivers. Stacking the receivers in a standard 19 inch audio rack is the most convenient and compact way to secure and transport them. You’ll need to build or buy such a rack. Make sure it's sturdy enough to support a stack of receivers (six for us) when picking it up from the top.

My first try at a rack was built around a plastic bread crate. However, it wasn’t quite wide enough and it didn’t have room at the back for the antennas to stick out. The second try was a little bigger (clearance of 19 inches in width), had an open back for the antennas, and was hardwood and plywood construction. The mixer is secured to the top with wooden block which clamp it down.

Step 3: Mixer, Transmitter, and Car Radios

Mixer

A mixer is necessary because the signal coming from the receiver is not strong enough to be fed directly into the FM transmitter. A mixer also allows the output of multiple receivers to be combined, with adjustments as needed. For example, if the sopranos and altos are on one group of receivers, and the tenors and basses are on separate receivers, it would be simple to give more volume to the lower voices. Receivers feed the mixer via ¼ inch patch cables. An electronic keyboard or other instrument can be plugged directly into the mixer also.

Transmitter

Our initial attempt was with a very inexpensive ($10) FM transmitter, the kind marketed to people who don’t have an auxiliary jack or Bluetooth option in their car and want to play tunes from their smart phone. This turned out to be very weak and didn’t transmit nearly far enough. We then bought a FM transmitter that is adjustable from 1 watt to 7 watts of transmit power. 1 watt has proved sufficient so far. The frequency it sends can also be adjusted. We just selected a frequency where there didn’t seem to be any audible radio stations.

Initial tries with the 1 watt transmitter gave us a lot of humming and interference. This seemed related to the input signal wire being anywhere near the power wire. We probably could have isolated the wires in some way to solve the problem, but our solution was to power the transmitter directly from the 12v battery. And, we got rid of another wall wart. Another observation is that there is less noise when the antenna is placed on a car roof or even on a piece of sheet metal. We have an extension wire so the antenna can be several feet away from the transmitter body.

FM Car Radios
Almost every car on the road has an FM radio, so this seems an easy solution for hearing the mixed chorus. There is one caution however. Newer cars may have digital electronic FM receivers, and while this is great for just listening to the radio, these radios seem to have a noticeable delay or latency. This puts what one hears on the radio a fraction of a second behind the chorus. For some cars, the delay might be able to be defeated (Subaru Live mode). But for most, you need to give up on the car radio and use a simple portable FM radio. I had to do this in my fancy new car (grumble grumble).

Step 4: Battery Power

Getting Rid of Warts

Receivers require electrical power. They all come with AC adapters (herein referred to as wall warts, or warts) that supply 12v DC power. The wall warts require a power strip, but the strip is quickly occupied by the warts which are often too wide to allow a neighbor. I was also concerned that having 6 or more of these pulling power would be too much for our 500 watt inverter. In addition, it might run the battery down too fast (for a 2 to 3 hour rehearsal). As luck would have it, all brands of receiver we tried run on 12v DC. Why not power them all directly off our 12v battery and eliminate the wall warts.

12v Power Strip

We would need a power strip to provide multiple sources of 12v DC power. I couldn’t find one to buy so I built it myself. We would also need a male to male cables with 5.5mm plugs, one for each receiver. I bought a kit of 5.5mm plugs and sockets and built the power strip you see in the picture. I added a volt meter so we can keep an eye on how much battery we have left. The kit of plugs, plus a set of 5.5 2.1 male plugs, also gave me the parts needed to solder up cables needed to bring 12v power to the receivers.

Fine point: The GTD receivers accept 12v via a 5.5 outside diameter socket. But the inside diameter (+ side) is 2.1mm. Unfortunately I had built the power strip with 5.5 2.5mm sockets. So the cables I made have 5.5 2.1mm males on the receiver end, and 5.5 2.5mm males on the power strip end. Note that the Phenyx receivers have 5.5 2.5mm sockets.

I did a little test to see how much power was saved with this approach. Having 4 receivers powered by the inverter drew 2.61 amps from the battery. With the receivers turned off, but the wall warts still plugged in, 2.01 amps was used. That means the receivers themselves use only about .15 amps each (60/4). The inverter turned on but with nothing plugged in pulls about .5 amps. That means the receiver wall warts take about .377 amps each ((2.01-.5)/4). Good riddance. I didn't test the transmitter wall wart, but I expect it is about the same in overhead. So for a system of 6 receivers and a transmitter we may be saving about 2.64 Amps (7x.377).

The result is that with our 55ah battery starting at 12.9 volts (over 90% capacity), running the entire system for 3 hours brings the voltage down to 11.8 (30% capacity). It works, but taking it down to 30% is not ideal. Perhaps at break time we should turn off the keyboard.

Step 5: Setup and Safety

Before the advertised start time, the tech people should arrive and set up at the director’s location. This involves unloading from cars, connecting and starting the inverter, and powering up the mixer and transmitter. Our mixer accepts a thumb drive with MP3 files, so we play oldies on our frequency so as people arrive, they will be able to tune to our frequency. Our FM transmitter is now plugged in to 12v DC, and the antenna is placed on the car roof.

The director’s headphone and keyboard should be made ready. At this time also the receivers can be powered up. I suspect the keyboard uses a lot of power (certainly over 100 watts) so it shouldn't be turned on until needed.

The collection of mics should be sanitized and organized with a new disposable cover on each. Each mic should have its batteries replaced as needed. My suggestion is that the mics be organized by receiver, some sets for the sopranos and altos, and some sets for tenors and basses. I labeled each mic with the receiver number and the channel (1A, 6C, etc).

As singers arrive in their cars, ideally someone should direct them to park in an arc around the director. We put out orange cones to define the arc and encourage efficient parking. If 10 cars are anticipated, the arc can be 20 feet in radius. If 20 cars, the arc could be about 40 feet in radius. If windows are kept closed, there’s no reason the cars can’t be parked close together. As each car arrives, someone will select an appropriate mic (S&A or T&B), turn it on and “lock” it (to prevent turn off or muting), make sure the battery power shows full (3 bars), wipe it down with alcohol-wipe and take it out to the singer. At this time also FM radio reception can be verified.

The singers must understand that they must return the mics after the rehearsal/performance. Some of our singers have said they’d like to take the mics home with them. This should not happen since not all singers attend every rehearsal. They must understand that without the mic, the receiver is useless. And without the receiver, the mic is useless.

Step 6: ​Antenna Consolidation

With six receiver sets in a rack, each one with 4 antennas, the back of the rack is a porcupine mess of 24 antennas. We decided to copy some capable folks' lead Antenna Consolidation Document and combine the antennas, 24 down to 2. This allows the antennas to be moved away from the receiver rack to a better location for “seeing” the singer’s mics. It also allows the receiver rack to be oriented for ease of use rather than trying to point the rack so antennas face the singers. It also allows the receiver rack to be kept sheltered while the remote antennas can be exposed to the elements. The before and after pic shows we traded a mess of antennas for a mess of cables.

Antenna consolidation works like this: GTD receivers (and many other brands) use what they call a diversity scheme, where the 4 antennas on the back of each receiver are really only two bands, A and B. The A antennas are separated from B antenna by a few inches, so that if the radio frequency wave happens to hit the A antennas at a trough (low energy), the B antennas will see the wave at a peak. To consolidate then, all A antennas will be combined into one, and similarly for all B antennas. We end up with only two antennas. We use four 6 to 1 splitters (brings 24 down to 4) and two 2 to 1 splitters (brings us to 2).

All this splitting of course dilutes the signal, so it needs to be amplified. In-line amplifiers are installed at the remote antenna end. But to power them we need in-line power injectors. Injectors are at the receiver end and need to be given 12v. This “injected” power is put onto the coax cable, where it is used by the amplifiers at the remote end. The power injectors only accept coax connectors, so we had to make special cables with coax on one end and connectors on the other end to plug into the 12v power strip.

We tested the scheme first using only one receiver. We did find that on the power injector it does matter how the In/Out signal wires are connected. The side leading back to the receivers does not want the power.

LPI-2200 Power injectors were from https://www.blondertongue.com/lpi-2200.html

LA-2150A RF amplifiers were from https://www.solidsignal.com/search?keywords=LA-2150A

Step 7: Conclusion

The system works great and everyone is so happy to be able to sing together again. The pictures don't show the smiling faces in the cars, but they all very pleased.

I expect that having only 24 mics will soon be a problem. Out chorus is normally 60 to 70 people. We’ll have to think about how the system could be expanded. A larger mixer certainly, and another bank for GTD mic/receiver sets. We’d have a second rack and some longer ¼ inch patch cables to get from receivers to the mixer. And an additional 12v power strip and a bigger battery of course.

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