HELP with designing a headphones distributor please

[sonolink]

C)

 

[Speedskater]

For headphone cables, it can be CAT 3,4,5,6,7 or 8.
Chose a cable with as heavy, stranded conductors as you can find.

 

[Hubbub]

For headphone cables, it can be CAT 3,4,5,6,7 or 8.
Chose a cable with as heavy, stranded conductors as you can find.

And make sure your chosen connectors will work with the conductors in the cable. All Cat-x cables are not the same!

Since this is a voice grade installation the optimum fidelity is not needed, and channel balance does not apply, so don't be concerned to go downgrade a little. That, and the cable handles a headphone signal, but not at the headphone impedance. The source is very lo-Z, the cables relatively low Z, and even 20 expected loads will be higher Z than even one 8-Ohm studio headphone. With maybe one full watt of output power needed, remember that POE can easily do 25W over Cat-5.

 

[sonolink]

One question: the headphones negative terminals cannot be connected to GND/Shield right? I'm asking because some jack panel mount connectors will connect the sleeve to chassis.

Cheers
Sono

 

[Hubbub]

It is OK to connect the headphone negative terminal to the cable shield / ground. Just don't connect anything on the headphone side of a transformer(s) to any of the 8 cable circuits. You are using (per post #114) a bridge-tied amplifier, TDA7297 based, and that amplifier wants a balanced, floating load. All four amplifier output lines must not connect to ground.

FWIW what you selected for an amplifier module should follow figure 3 of https://www.st.com/resource/en/datasheet/tda7297.pdf

 

[sonolink]

It is OK to connect the headphone negative terminal to the cable shield / ground. Just don't connect anything on the headphone side of a transformer(s) to any of the 8 cable circuits. You are using (per post #114) a bridge-tied amplifier, TDA7297 based, and that amplifier wants a balanced, floating load. All four amplifier output lines must not connect to ground.

I see what you mean now. Connecting anything on the headphone side of the transformer would be overriding it and defeating it's isolation purpose, right?

One more little question please.....

Trying to find a little bit more powerful amp than the 15w+15w TDA7297 based bridge-tied amplifier mentioned earlier, I found this TDA 7377 Class AB 30W+30W.

https://es.aliexpress.com/item/1005....0.0.796a7a9d63qmw2&mp=1&gatewayAdapt=glo2esp
Would this one radiate interference or would it be ok noise wise?
How about running it at 9v? Or would a higher voltage be preferable power wise?
Or do you think I'm overthinking it and the TDA 7297 will be enough?
I've already began ordering some parts and I'll be posting a sketch of my B Box idea in a bit.
Thanks a lot for all your advice and help
Cheers
Sono

 

[Hubbub]

For a given load the only way to get higher power into it is to drive it with a higher voltage. Power = voltage squared divided by impedance. The TDA7377 gets its higher power rating by tolerating a lower impedance load, which is not readily changed unless you find different transformers. Since you want more volume (more power), that means more voltage is what is needed. Run the amplifier at its maximum rated operating voltage, 18V, but the load is relatively light so it won't take all that much current at 18V. An 18V 2A supply is probably overkill. Note that the smaller TDA7297 as well as the TDA7377 runs at 18V maximum, so the more expensive amp doesn't help. (https://www.digikey.com/en/htmldatasheets/production/5227849/0/0/1/tda7377).

Like the TDA7297 the TDA7377 is class AB, it should not be radiating noise to other devices like a switching (class D) amplifier would. (Sorry for needing editing to get this right.)

 

[sonolink]

For a given load the only way to get higher power into it is to drive it with a higher voltage. Power = voltage squared divided by impedance. The TDA7377 gets its higher power rating by tolerating a lower impedance load, which is not readily changed unless you find different transformers. Since you want more volume (more power), that means more voltage is what is needed. Run the amplifier at its maximum rated operating voltage, 18V, but the load is relatively light so it won't take all that much current at 18V. An 18V 2A supply is probably overkill. Note that the smaller TDA7297 as well as the TDA7377 runs at 18V maximum, so the more expensive amp doesn't help. (https://www.digikey.com/en/htmldatasheets/production/5227849/0/0/1/tda7377).

Like the TDA7297 the TDA7377 is class AB, it should not be radiating noise to other devices like a switching (class D) amplifier would. (Sorry for needing editing to get this right.)

Thanks a lot for taking the time for that complete reply Hubbub

I'll stick to the TDA7297 then and invest in a good and quiet PSU.

I found this Mean Well which is supposed to be a good brand:
https://es.aliexpress.com/item/1005....0.0.3f1f7a9dbIuWsL&mp=1&gatewayAdapt=glo2esp
Thanks again!
Cheers
Sono

 

[Hubbub]

The Meanwell GST25A18-P1J is a 3% voltage tolerance 25W rated part, should work fine for feeding 18V to a TDA7297 amplifier that is pumping out no more than 10 watts, typically under 5. https://www.meanwellusa.com/upload/pdf/GST25A/GST25A-spec.pdf

That particular product line (GST25A) is handled by a major distributor here in the states, if it wasn't at least decent they wouldn't carry it. And yes, it's a brand I would use myself.

The shipping from that vendor is more than the supply, you might want to look for another source. How does RS (ex Radio Spares) compare? DigiKey Spain? Both carry the 18V supply.

 

[sonolink]

Excellent!! I will definitely check another supplier to reduce shipping. Just wanted to make sure it was ok to use that one.
Thanks!!

 

[sonolink]

Here's what I have come up with for Box B. The space between the RJ45 D-Type connectors and the PCB mount RJ45s is to allow for the cat6 short cables that connect the PCB to the D-Type connectors.




















All comments and suggestions are very welcome as usual.
Thanks
Sono

 

[Chrisfromthepast]

Why not use a single RJ 45 connector that accommodates ethercon or rj45 and solders straight to the pcb?
Like post 128?
Neutrik NE8FBH range of products
https://www.mouser.com/ProductDetail/Neutrik/NE8FBH-S?qs=Lrve3rxDCt%2B6nu4OiHmhKA==

 

[sonolink]

Why not use a single RJ 45 connector that accommodates ethercon or rj45 and solders straight to the pcb?
Like post 128?
Neutrik NE8FBH range of products
https://www.mouser.com/ProductDetail/Neutrik/NE8FBH-S?qs=Lrve3rxDCt%2B6nu4OiHmhKA==

Thanks for your input Chris. As MA "kinda" puts it, this is a one off build for a friend and I can't really make it too expensive for him.
The link you provided implies a cost of more than 350€ + shipping only for those connectors......need I say more?

Thanks anyway for chiming in and trying to help

 

[Chrisfromthepast]

The link you provided implies a cost of more than 350€ + shipping only for those connectors......need I say more?

Im sorry, I assumed this was for a A box and a B box. Whats the build quantity? There may be suitable European sources. $11 ish in singles.

 

[sonolink]

Im sorry, I assumed this was for a A box and a B box. Whats the build quantity? There may be suitable European sources. $11 ish in singles.

There's a total of 21 Boxes. One "A Box" that provides the amp, and 20 "B Boxes" that provide the minijack output channel switch and volume control for 2 users each box.
In total there would be 41 RJ45 connections (40 in pairs for the B Boxes and one on the A Box as the amp output.

 

[Hubbub]

Might be good to think tolerances and hole precision. Note that making a PCB attach / mate to one surface (the bottom) is easy, two surfaces (bottom and front) not too difficult, three surfaces (bottom, front, rear) doable but not perfect, and four surfaces is next to impossible. Yes, you depict a four surface solution. I would never do or allow such.

And how are you securing the externally accessible RJ-45s? Typically they PCB mount. No sense using two PCBs when only one is needed, provided the PCB attaches / mates to bottom / front / REAR only. (That approach allows using a rugged extruded metal case, such as https://www.aliexpress.us/item/3256805843987308.html, which is probably too big.)

 

[sonolink]

My idea is to secure the PCB through the potentiometers' and switches' screws. That's how I usually do it when I build guitar pedals. I'll put a couple of standoffs at the top of the PCB (where the RJ45 PCB mounted connectors are) screwed to the bottom of the enclosure. (I just noticed that they don't show on the pic). The only surface that really needs "mating" is the front panel with the pots/switches. The minijacks on the side just need to be in front of the hole in the enclosure, if you know what I mean.

And how are you securing the externally accessible RJ-45s? Typically they PCB mount.

I'm afraid there's a misunderstanding here. The externally accessible RJ-45s are Type-D connectors:



They are screwed onto the enclosure wall, a flat 15cm cable is plugged in it and the other end of the cable goes to the RJ-45 connector soldered on the PCB:


The "inside" flat cables aren't shielded, so the shield connection will be achieved connecting the GND plane of the PCB to the switches' shield, to the enclosure, to the externally accessible RJ-45 D-Type connector's shield, to external cable shield, to the PSU GND. The section between the amp output and the transformers will NOT be connected to that.

Is that ok?
Thanks for that enclosure link!!

Cheers
Sono

 

[Hubbub]

Now that I understand the packaging better I think you are good enough to be successful. Just make sure the holes to access the recessed headset jacks are large enough to accept the body of the mating connector. I've had issues with cellphone cases that wouldn't let me plug in an extension cable or adapter cable.

If you have to bend the board to get the opposite side jacks in place you have a serious assembly issue. The PCB needs to fit in place without being stressed or solder joints may fail.

 

[sonolink]

Now that I understand the packaging better I think you are good enough to be successful. Just make sure the holes to access the recessed headset jacks are large enough to accept the body of the mating connector. I've had issues with cellphone cases that wouldn't let me plug in an extension cable or adapter cable.

Very good point actually. I hadn't thought about that and the same thing happened to me with a cell phone protecting cover

If you have to bend the board to get the opposite side jacks in place you have a serious assembly issue. The PCB needs to fit in place without being stressed or solder joints may fail.

Damn, that's a very good point too!!
Thanks a lot Hubbub, I'll look into sorting that out

Cheers
Sono

 

[sonolink]

Now that I understand the packaging better I think you are good enough to be successful. Just make sure the holes to access the recessed headset jacks are large enough to accept the body of the mating connector. I've had issues with cellphone cases that wouldn't let me plug in an extension cable or adapter cable.

If you have to bend the board to get the opposite side jacks in place you have a serious assembly issue. The PCB needs to fit in place without being stressed or solder joints may fail.

I finally went with off board minijacks to make things easier. Thanks a lot for pointing that out and for your really helpful comments Hubbub!

I also realized I had to take into account the space the "inside the box" cable connectors take. So I redesigned the layout:








I also found a supplier for the Mean Well PSU with proper shipping rates!

Thanks again for your help!
Cheers
Sono