Relay technology is pretty much straight electrical engineering and only borders on electronics, so it's not difficult to sketch out what is required. The bulk of the effort needed to get a system like this working would be in the construction, wiring and power supplies. If you were costing your time at a commercial rate, there would be no way it would be more economical to build this sort of thing from scratch against buying a commecial unit. However, if you regard it as a learning exercise, it's a relatively straightforward project, and, provided certain rules are followed, one that is likely to have a successful outcome.

Is this for a console or preamp that has single unbalanced TRS insert jacks or one that has separate balanced insert send and receive connectors (XLR or TRS jack) and panel switches for changing the mixer signal route to come from the insert receive?

Thanks for the reply Boswell. its for multiple balanced insert switching box, id like to be able to flip order of inserts one and two potentially?? and I am hoping to inter-grate into my new passive transfer console we are opening a second room at our studio thus the interest in building another console. as far as these projects are concerned its more about learning and having a deeper connection with my equipment

Do I understand from what you said that you would like to be able to chain effect units in an insert loop, but have the sequence of units in the loop (as well as by-pass) selected by relays? That's not a particularly difficult requirement technically, but getting it all housed and powered would take some care and thought.

I designed a unit some while back that performed a similar function (automated patch panel), with the relays controlled by MIDI strings. In this way, the initial patch settings could be held on a track as part of the recording but the setting could be changed dynamically by the mix engineer as required. This was quite a bit more complicated than the unit you described as it involved a microprocessor to interpret the MIDI stream and switch the appropriate relays, but it might give you an idea of the type of unit that is possible to construct.

Will Brouder: Welcome. I can help! I designed this type of thing for 30 years at the network level. There is much to say and this will probably take more than one post.

First – You need to be able to draw. In the old days, I used a chisel on the cave wall. Later I used a Leroy Pen. That’s all ancient history now. There are plenty of computer options. The gold standard is Autocad but that is expensive and just overkill for electronic drawing. Microsoft Visio is a less expensive alternative, but even that is overkill. I am now using Corel Painter X3 but that is only because my wife uses it for art. When you get right down to it, you can easily and cheaply make a simple drawing in Microsoft Excel!!! That’s right, give it a quick try before you spend any cash on something fancier.

Next – You need to decide on power. I always use 24 volt DC, positive ground. We call it ‘Plant Battery.’ That’s only because I am historically ingrained with it. You should NOT do that. Using positive ground makes it really difficult to drive relay coils with NPN transistors. Use negative ground either 24 or 12 volt DC.

Next – You need to decide on a relay. I can give you the benefit of years of experience. The absolute best thing to use is a Potter & Brumfield KHS 17 D 11. These things have gold bi-furcated contacts within a sealed, shielded container. The bad news is that they cost around $80 each these days. IMO unless you can score a deal on ebay, that price makes them unreasonable to use.

Don’t give up. There are reasonable work-arounds. Get anything with gold contacts and double-up on the poles. Use a 4 pole relay to switch two poles of audio. That will give you the advantage of the bi-furcation without the cost. Don’t worry about the shielding unless you are switching at mic level…..which isn’t a great idea anyway. A sealed container is nice, though as it will retard (not stop) oxidation of the contacts.

Next – how are you going to mount them? There is a Greenlee KHP punch. It goes for around $300 but you can often find them much cheaper on ebay. Unless you are making a very large matrix, probably a socket is a better approach. Something like this….

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…..next post, some design considerations.

Good Luck!

Here are some design thoughts……

First – Please seriously consider Boswell’s post recommending MIDI or computer control. Here are some fairly inexpensive things to look at that may inspire you…..

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Relay logic can get to be a dark art bordering on religion. In the 1950s, elevators, railways, telephones, & broadcast networks were controlled by thousands of relays. By the 1980s, even the strongest proponents of it had stopped and moved to microprocessors. I used Z80s in radio and my TV counterparts used TI 9900 processors. It can be tempting to throw some relay logic at a simple system but it is a dangerous road to go down…..particularly if you are on your own and don’t have a relay Jedi master to guide you. Yes, there are old, moldy textbooks….I have most of them….but even the most advanced of them don’t to the depths that we used to take relay logic.

Design tip #1 – Connect a diode backward across the coil. Anything will do, even a 1N4001. When a relay releases, the coil generates a backward spike that can produce a click in nearby audio. The diode shunts this. It also protects driver transistors from frying. One possible downside is that it does make the relay release slower…..that may or may not be an issue.

Design tip #2 – Static drain resistors. If you are switching devices that have transformer isolated inputs and outputs with no ground reference, large static charges can build up that are shunted when the relay operates. This produces a click in the audio. The simple cure is to drain the charge off with resistors to ground……the value is not critical but at least 100k ohms. 1 meg is fine. This may not be a problem in non-transformer equipment.

Design tip #3 – Sooner or later you will be tempted to do some audio switching in an actual switch. This is dangerous but can be done. Just follow the same precautions you would with a relay. My colleagues in New York completely stopped doing this at one point and paralleled every switch with a relay. Be careful!

Design tip #4 – Be careful about back-terminating a device if it is not being use in a path. Some devices will not operate properly unless they see a proper load. There is no current equipment that has this problem but some vintage devices (Older Ampex tape machines come to mind) need to always see a load or their metering and internal monitoring won’t work properly.

Again, good luck! I am very happy to see your DIY spirit!

All very good suggestions! and thanks for such a warm reception :) basically want to build some thing similar to the dangerous liaison (not as fully featured) or the spl switching unit. i want to build something very basic but would like to add the functionality of being able to change the order of insert loops for instance eq patched to loop 1 comp patched to loop 2, id like to be able to flip the order of so the comp and eqs orientation can be changed. even if its just on inserts 1 and 2. i basically am planning on building a relay controlled insert on/off box. i hadn't put much thought into the psu just yet but those are fantastic suggestions i had not considered.

as far as componentry i dont think i really need to make it midi controlled although that is a very very good idea being able to flip order and insert on/off via automation is definitely something worth considering, i want to kind of make a basic box for now and see where it takes me, i love building things for myself and could see myself getting into that concept, but im taking baby steps as it were lol! i was planning on using greyhill relays, they are used in one of my passive eqs and preform absolutely fantastic! thoughts on those? mberry: all those design tips are great! can you elaborate on the power supply and midi functionality? im going to check out that link now!

i had never considered a coil across the relay..... that makes allot of sense!! i have used static drains on all my passive equipment just as precaution so i would be using those as well. i guess what im most curious about is switching the order or sequence of the insert loops.. and sorry not greyhill relays... they dont make relays. its the potter ones you recommend.. (" I used a chisel on the cave wall. Later I used a Leroy Pen." made me chuckle :)

I have looked at the Dangerous Liason and the SPL MasterBay. I was not familiar with either. I've got to say that I am very impressed with both. I really like the Master Bay concept of inserting devices in the sequence that you select. The Liason compromises on this and does give you the 'flip' option & I guess that is reflected in the lower price. I will think about this. I have no personal use for it but the design challenge is very interesting.....especially how to pull it off at a reasonable price. I note that the Master Bay uses 164 relays in spite of the fact that it uses a CPU to do the logic!!!!!

One of the things that you might consider when looking at the Highly Liquid cards is their ability to do more than just give a GPI in response to a MIDI note. They can store patterns that are much more complex.

Again, good luck - be patient---I'll get back to you eventually.

thanks mberry, i find it all very fascinating! i have a liaison and its fantastic. the spl is also a good unit. i tried both but like the dangerous more build quality is much better. i look forward to hearing from you when you have time of course!

Will - your opening post talked of "saving some money" by your building a routing switch to use on insert loops. Is that need still active, or has the subsequent discussion involving Dangerous and SPL boxes made it a thing of the past?

If it's still current, you haven't said how much you would be prepared to spend on a ready-made device, and hence give us a way to judge whether building your own it likely to give substantial savings.

its a two way argument im having with my self, i would spend 2500 on parts just to build it my self, i use a liaison which i like, i would like to make one for myself to understand what its doing on a deeper level, and be able to troubleshoot my system better, more of a personal growth project and less of a saving money project. ideally id like to do it for under 2 grand and save a little not buying a second liaison as i dont really require the preset memory etc the parallel control is super nice and id integrate something like that. more or less just knowing how my kit is put together is worth the added expense expense as i get more out of it

and i would prefer a passive signal path.. i am no chris muth lol