HELP with designing a headphones distributor please

[Brian Roth]

Apologies....but I have NO interest in allowing any remote controls to diddle anything in my hacienda! The wall mounted wall switches that control the Romex cables seem to do what I need. <g>

I recall a few years ago when working a studio project and they were fiddling with "new" internet enabled "light bulbs". A friend knew "whazzup" and began changing the ;lght level and color via the IP connection from across town.

Screw that...I can make my own adjustments locally!

Bri

 

[Voyager10]

Ok. So I guess I should return it asap.
Otherwise would a cap (say 10uF) on L+ and another on L- be a temporary fix until I get the amp replaced?

Cheers
Sono

Just one cap should do it, given that you're connecting the load between L+ and L-.

Regarding the crosstalk - it's probably worth checking with a DMM whether there is any measurable resistance between any of the A channel wires and any of the B wires on the connector. Might be some dirt/gunge forming a partial connection?

 

[sonolink]

Just one cap should do it, given that you're connecting the load between L+ and L-.

You mean that the cap should be between L+ and L- (parallel to the load), or between L+ and the load?

Regarding the crosstalk - it's probably worth checking with a DMM whether there is any measurable resistance between any of the A channel wires and any of the B wires on the connector. Might be some dirt/gunge forming a partial connection?

Thanks Voyager10
I'll do that and report back.

Cheers
Sono

 

[Voyager10]

The cap will need to be in series. Bad ASCII art ahead...

L+ ---|(---<Load>--- L-
      cap

The + end of the cap will need to be pointing towards whichever output has the more positive DC voltage.

 

[Hubbub]

FWIW if one channel has an output offset of 4V either the amplifier is not working correctly or the input is not AC coupled as it should be. If the former you can't depend on either output pin of that channel working properly unless confirmed with an oscilloscope that each side of the bridge has the expected gain. But if the input is not properly AC coupled you can test for that. TDA7297 pins 4 and 12 should be within a few millivolts of each other. If they are not close one should be near half the supply voltage, that's the good side. Look at the traces and coupling cap on the other side to see if there is some unexpected DC path to ground/circuit common.

 

[sonolink]

The cap will need to be in series. Bad ASCII art ahead...

L+ ---|(---<Load>--- L-
      cap

The + end of the cap will need to be pointing towards whichever output has the more positive DC voltage.

Thanks!

Regarding the crosstalk - it's probably worth checking with a DMM whether there is any measurable resistance between any of the A channel wires and any of the B wires on the connector. Might be some dirt/gunge forming a partial connection?

With the cat6 cable disconnected from the amp output I measured resistance between all A channel wires and B wires. They all read OL.




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I just took the amp out of the box, plugged the PSU in, and measured again across L+ and L-.
No voltage.....I'm seriously puzzled now...

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Ok so the 4Vdc issue is solved. I've replaced the standoff screws with nylon ones and the voltage is gone

 

[Hubbub]

I got tripped up with that same issue packaging a headphone amplifier some years back. Shorted one input supply to ground because the PCB designer didn't think much about mounting and put live copper around the mounting hole.

 

[sonolink]

I still got noise from the PSU though (it changes and sometimes disappears when I touch around) but I think it'll disappear when I put the B-circuit inside a box and connect the shield. Well I hope so.
Still busy drilling....


A probably dumb question just appeared in my mind:

- Shield is going to connect all the B-Boxes enclosures through the Cat6 cables to be finally connected to the PSU negative terminal (GND), creating a wonderful Faraday Cage

So, the A-Box enclosure should also be connected to that spot, right?

What about the 2 jack input sleeves? Since they will be connected to some ground in the mixer, should I connect Shield to the PSU negative terminal or to the jack input sleeves? Or both?

Cheers
Sono

 

[sonolink]

Bumpety bump.....

 

[ccaudle]

should I connect Shield to the PSU negative terminal or to the jack input sleeves? Or both?

You are mixing up different things again.
Shields are an external metallic enclosure.
Shields connect to other shields, so cable shields connect to conductive enclosure shields.
The power supply connection to the enclosure is so there is no uncontrolled potential between power supply reference node and the enclosure that would result in capacitive coupling of shield potential into the circuit. No external shield should connect directly to the power supply other than the one connection on the inside of the chassis.

 

[Voyager10]

Yes, the enclosure should be connected to some 'ground' rather than left floating. I'm assuming power supply negative and the input ground (jack sleeve) are connected together (via the amp PCB), so you get to choose where to make the connection to the enclosure. Grounding is a hugely contentious topic here on GDIY so expect lots of conflicting advice...

Personally I'd make the enclosure connection from the PCB, close to the IC ground pin, and ground the heatsink as well if it isn't already. IME chip amps will readily burst into oscillation if there is stray capacitance in the wrong places, so tieing all large bits of metal to chip ground is a good precaution.

 

[sonolink]

I'm assuming power supply negative and the input ground (jack sleeve) are connected together (via the amp PCB), so you get to choose where to make the connection to the enclosure.

Ok I'll check that.

Grounding is a hugely contentious topic here on GDIY so expect lots of conflicting advice...

I just noticed
Also, maybe my English doesn't help

Personally I'd make the enclosure connection from the PCB, close to the IC ground pin, and ground the heatsink as well if it isn't already. IME chip amps will readily burst into oscillation if there is stray capacitance in the wrong places, so tieing all large bits of metal to chip ground is a good precaution.

Thanks for that tip Voyager10!!
Cheers
Sono

 

[sonolink]

I'm glad to report that finally I've got a dead silent A-Box now and that the crosstalk is almost bearably noticeable. You REALLY have to crank it up all the way to hear it, so I'm really glad
Oh and by the way, this thing has got PLENTY of volume.

Connecting shield (enclosure), heatsink and RJ-45 shield to negative terminal of the PSU did the trick. Input jack sleeves were indeed connected already connected to PSU negative terminal, as you rightly assumed, Voyager10. So now any other B-Box that will be connected, will be shielded through the Cat6 cable shield and it's own enclosure.

As soon as I have some B-box finished I'll report back.

 

[sonolink]

Boxes B coming along nicely







Five done......15 to go!!





Again, thanks A LOT to all of you for your help, time, ideas and suggestions, especially to Hubbub and Voyager10.
I hope I can buy you guys some beers one day!!

Cheers
Sono