Patchbays – routing your signal
Note: This is a preliminary article and will cover the construction of a basic Arduino controlled patch bay/audio signal router. Users should be aware that manufacturing a patchbay as a DIY project can and will will be very expensive. The Samson S-Patch Plus retails for between R2000 and R2500.00 in South Africa. A parts B.o.M places this project at about R5 000.00. However, careful consideration of B.o.M (input types) and digital control will be infinitely cheaper than an electronic audio signal router off the shelf.
The Samson S-Patch Plus is remarkable in that it tidies up your flow, gets rid of unsightly cables, is quick and affordable. If you are like me you prefer to purchase pro series audio because it’s modular and often cheaper than commercial consumer integrated audio. So you can throw away the parts you don’t like and only add that which you deem necessary and very importantly, have some control from the PC or laptop.
This comes at a price of course, spaghetti! And if you are like the mechanic whose car never runs properly this spills down to never being able to find that missing plug or socket. To be honest, pro audio can be a nightmare.
Patching with your Mic Pre-amp
I am currently using a Scarlett 18i20 to patch, the routing is more than adequate for my purposes and it can all be done from the mix control. Was it designed for this purpose? The reality is, from an output perspective yes, from an input perspective, no! We need to be looking at electronic routing. We need to be looking at XLR sockets. We need to be looking at blocking Phantom power from sensitive ribbon and dynamic microphones. When routing line signals using an expensive sensitive microphone pre-amplifier is just a pure waste.
There some workarounds, The S-Patch being one. But it doesn’t have XLR and XLR patchbays are horrendously expensive. We can use XLR to TRS converters. Each time we add, convert and reconvert we add to the noise level. If price is a problem, which in most cases it is for home use then maybe it’s time to be looking at a home brewed solution, one which I will be attempting when I get the time.
Arduino and digitally switching your patchbay
As Arduino has gained in popularity there’s simply just hundreds of circuits out there setting one’s creative juices alight. Arduino controlled 4066 bilateral switches is a very strong contender. The max pass through current is 9~10mA which should be adequate to drive most line level inputs. The supply quiescent current is a fraction of a uA which is also ideal for battery power. Bandwidth of 40MHz makes them ideal for video switchers as well. Beware of static and unearthed equipment connections, these devices have extremely high input impedances. Multiplexing through logic 1s and 0s being mechanically switched to control lines is a solution in itself if the user only has a limited amount of pre-defined routes to make.
Users should be aware that the 4066 chip has an on resistance of about 100 Ohms – working line voltages and “realistic” input impedances / loads having more than a couple of switching paths in series can have an effect on performance.
Using relays for switching is often a better solution in that they are robust and one can hear mechanical switching (insecurities?) but the contacts need to be of high quality. Many audio enthusiasts prefer to use latch type relays otherwise reed relays, both comparatively cheap.
Hinton Industries manufacture a SwitchMix 8×8 summing router which should give us DIYers some food for thought.
The rough and tough schematic above is just one permutation of what can done using bilateral switching. Buffers are included to counteract losses but in reality may not be necessary.
In our follow up article we will make up the basics of an Arduino controlled patching bay using the 4066. For most audio work this chip should work fine – the Max series has a low Ron but is expensive.
Ed’s Note: In our wonderful world of electronics the subject of audio signal routing often comes up. Wafer switching soldered cables between different components is a simple and often rewarding exercise. Audio routing should be a simple and affordable exercise.
- Nexperia 74LVC2G66.pdf
- Texas Instruments CD4066
- Further reading:
- Ledgernote’s take on patching – excellent article on this confusing subject.
- Rod Elliott – MOSFET solid state relays. A must read article for those interested in muting and signal throughput.
- Maxim Integrated – Selecting the right CMOS analogue switch