Telephone Switchboard style RCA Selector

[SammyJ]

Hello Bottleheads,
I'm finding myself in need for a simple RCA switcher, to switch my many inputs and outputs.  At the moment, I've got:

IN:
BH Reduction
DAC

OUT:
SS Amp
Crack (mid-build)
Stereomour (Eventually)


I'm constantly messing around with home-built audio components, so expandability would be great here.  Rather than purchasing a cheap RCA switch, I figured it'd be easy enough to build one -- 4-way switch in, 4-way switch out.  Easy, right? 

Then, I thought, it'd be kind of cool to just make a matrix of inputs, and have a telephone switchboard style selector, using a Male-Male TRS cable.
(https://c1.staticflickr.com/1/96/275510039_ba71e38131.jpg)


So, if I were to create a series of paired RCA inputs (8 in total), and connect each pair to a singular TRS, would that work?  Any thoughts here?  I'm wondering if I'd be prone to introducing noise anywhere given that the TRS cable would be external to the box.  For the box itself, I'd likely build it in a metal project box, then build a wood box around it to match my BH gear.

Since this is entirely passive, do I need to worry any about resistances?  TRS plugs are shorted when not in use (as i understand), so are there any forseable issues, or is this as simple as I may think?

thanks!
S

[Grainger49]

A shorting plug/jack will likely smoke the DAC.  The reduction is capacitor coupled so the shorted output will not damage it.  So for the inputs you probably want to leave them open not shorted when not in use.

Most inputs, all I can think of, are happy, and very quiet, being shorted. 

[SammyJ]

A shorting plug/jack will likely smoke the DAC.  The reduction is capacitor coupled so the shorted output will not damage it.  So for the inputs you probably want to leave them open not shorted when not in use.

Most inputs, all I can think of, are happy, and very quiet, being shorted. 

Thanks Grainger, for the reply while on the mend.  So it would seem like 8 of these would be the ticket?

http://www.parts-express.com/neutrik-rean-nys230-1-4-stereo-jack-non-switched--092-124

Thanks!

[Grainger49]

Yup, the 1/4" plug will carry Right hot, Left hot and ground from each source.  Then for the amps it will carry it to the amps in the same way.

You get to mix and match. 

I'm assuming that if you are not feeding a source to the amps they will not be in use.  Otherwise you might inject noise to the amps with the open inputs and the attached wiring.

[SammyJ]

You get to mix and match. 

Exactly what I was going for, a la switchboard operator style.

I'm assuming that if you are not feeding a source to the amps they will not be in use.  Otherwise you might inject noise to the amps with the open inputs and the attached wiring.[/font][/size][/color]

Hmmm, yes, I do my best to turn off at least my tube based equipment when not in use.  Given that I'll likely have the space, I might consider adding switches to each RCA pair to prevent this.  Maybe a switched instead of unswitched TRS jack?  Though I guess the noise would likely be introduced beyond the switch and in the wiring to the RCAs...

[Grainger49]

Yes, the output jacks can be safely switched.  That takes the right/left hots to ground through the cable.  There might be a fraction of an Ohm to ground but that is safe.

[Paul Joppa]

Do some searchong on audio studio practice - this is a standard approach in that world.

[fullheadofnothing]

You're asking about a patch bay, you can probably find a used one for less than you'd spend on jacks.

It's also probably worth taking a moment to ask "why?" Switches are a useful method to avoid swapping cables. A patch bay is a useful method to have a re-configurable setup that is rarely repeated with maximum flexibility. Essentially, you are swapping patch cables to avoid swapping interconnect cables. Doing this adds unnecessary connectors, cables, and solder joints to your setup with the risks of adding noise pickup and a more complicated setup to debug should a faulty connection arise.

[SammyJ]

It's also probably worth taking a moment to ask "why?" Switches are a useful method to avoid swapping cables. A patch bay is a useful method to have a re-configurable setup that is rarely repeated with maximum flexibility. Essentially, you are swapping patch cables to avoid swapping interconnect cables.

Right -- It'd makes perfect sense just to have a switchable input and switchable output.  The major advantage I see here is being able to have the flexibility in deciding how many of each.  It's just like swapping interconnects, but it's just physically easier to swap cables.  I'd like to do A/B comparisons, and given that I'm constantly tinkering, having the flexibility to add more inputs/outputs as needed is nice.

Doing this adds unnecessary connectors, cables, and solder joints to your setup with the risks of adding noise pickup and a more complicated setup to debug should a faulty connection arise.

Fair point, and this was a concern originally.  It's not the shortest path, but seems like a fun aside.

[Grainger49]

The easiest thing I see is to have dedicated outputs and have a 4 way two deck input selector switch.

[Paul Joppa]

Worth remembering, phone jacks were invented specifically for frequent, reliable inserting and removal - it's not an accident that it's easier than swapping RCAs. The reliable ones are "long-frame" - see picture page 7 in https://mikeriversaudio.files.wordpress.com/2011/08/patchbays.pdf - they are of course more costly!

Studio standard is one balanced line per jack, so you need two for stereo unless you get creative and non-standard and use the headphone wiring ...

Incidentally, RCAs were invented to connect a long-wire antenna to a radio, back in the days when radios were not at all portable. The are not designed for repeated swapping. For coax swapping, BNC connectors are the reliable ticket, widely used in laboratory gear of all sorts.

[Tubejack]

8 years ago, I wanted a multisource input/output selector box, capable of many in to many out, and didn't want to use mechanical switches. All my electronics except DAC's are tube DIY, including Bottlehead, and many own design. I focus on Analog and Tape (3 RTR, 1 cassette, 2 TT's, 1 Tuner, 1 CD deck, 1 ADC). Although not a 'purist' approach, having an interface selector box for multiple sources is at time convenient, and cuts down on the cable swapping, etc. Also for DIY, future projects can have only a single input, saving cost and complexity.

Inspiration was the Manley Labs SKIPJACK http://www.manley.com/msj.php, but it was limited to a bidirectional 2IN/3OUT, and cost $900. My unit uses relay boards, completely separate, essentially 2 x 2IN/6OUT or 6IN/2Out .... you get the idea. I can even use the 2 sections together, by "patching" them together.  Each section also has a switchable processor loop if needed.

I used a Par Metal case, and a clean 5V wallwart for power. Direct connection to all I/O RCA's with silver Teflon wire. All RCA's chassis isolated, and each pair has it's own separate ground connection to the I/O board. There is no signal ground to the chassis, the 5V is grounded to chassis, and a hardwire chassis ground to the total system ground.  (I do use a power conditioner that all equipment is behind.) Use of this device has not induced and hum or signal degradation into my system that I can hear or measure, using good quality interconnects.

[SammyJ]

This is incredible.  Skipjack was originally my inspiration as well, but at $900, it was just too spendy.  I love your silicon based approach.  I was actually considering rigging something up using a Raspberry Pi attached to a a touch screen.  Either super lo-fi or super hi-fi.  Decided with the former, but may eventually move to the latter.

Regardless, I love your approach.

[mcandmar]

The reliable ones are "long-frame" - see picture page 7 aboratory gear of all sorts.

What a lovely design, such a simple yet elegant solution.  Makes me wonder why we took a step back for headphone use as the TRS socket is obviously an inferior design with the way it shorts the contacts on insertion.

If i was making my own patch board i would be tempted to use XLR sockets, possibly even four pins and keep the grounds separate. They would have no problems with repeated connects/disconnects, only drawback is the cost of the sockets/plugs.

[Big Al 954]

That really is a nice looking switch box. I've been contemplating building something almost identical for a while as a passive source selector using relay switching. The only additional thing I would add for my build would be a Penny +Giles RF-15 10K log Rotary Fader. Not cheap ( around $400 for a stereo pot) but accurate and a really beautiful feel to them because of the type of bearings used.