HELP with designing a headphones distributor please

[MicMaven]

why...the width of the PCB trace...would be a concern]

THANK YOU, gang, for the uptake on trace width. Duh ... pretty "oblivious" once you explain it ! Proving I am still a novice.

This thread illustrates how much consideration goes into building a simple, yet important, device just to play a single signal through a number of headphones at once. Very helpful. James

 

[MidnightArrakis]

THANK YOU, gang, for the uptake on trace width. Duh ... pretty "oblivious" once you explain it ! Proving I am still a novice.

This thread illustrates how much consideration goes into building a simple, yet important, device just to play a single signal through a number of headphones at once. Very helpful. James

[pretty "oblivious"] -- oblivious [ uh-bliv-ee-uhs ]

adjective
unmindful; unconscious; unaware (usually followed by of or to):
She was oblivious of his admiration.

forgetful; without remembrance or memory:
oblivious of my former failure.

Archaic. inducing forgetfulness.


[pretty "oblivious"] -- obvious [ ob-vee-uhs ]

adjective
easily seen, recognized, or understood; open to view or knowledge; evident:
an obvious advantage.

lacking in subtlety.

Obsolete. being or standing in the way.

>> Apparently, MicMaven was "oblivious" with knowing to use the word "obvious"!!! ( just kidding..... )

(SORRY!!! It's just that after studying "Journalism" and also being the Editor of my college yearbook, things like this just jump out at me and I am somewhat compelled to make a correction. No harm meant.....really)!!!

/

 

[MicMaven]

(SORRY!!! It's just that after studying "Journalism" and also being the Editor of my college yearbook, things like this just jump out at me and I am somewhat compelled to make a correction. No harm meant.....really)!!!

Ha! Mr. Editor, THE JOKE IS ON YOU! There is no need to apologize for embarrassing yourself!

That is what cultured literary folks call A PUN. You know, a play on words intended to be humorous, i.e., funny. Like when Foghorn Leghorn says, "That's a Joke Son, Don't you get it?" There is no need for correction, although I concede my family of English teachers taught it is unnecessary to identify a pun with quotations. And yet, I break that rule by putting puns in quotations TO MAKE IT OBVIOUS (not oblivious...) IT IS A PUN, as an aid to understanding and cue stuffy OFs who may not recognize the intended humor!

As a published philospher, professional legal writer, former Associate Editor of the Quarterly Proceedings of the Heraclitean Society, with a doctorate degree in something or other, myself, I really appreciate puns because they introduce a modicum of humor in anotherwise tedious technical discussion. Bill Keane, the cartoonist famous for the newspaper comic, The Family Circus, once said, "Puns are the number one form of humor ... but nothing is lower than one . . ."

So, lighten up - it is all in good fun, ya know !! James

View attachment thats a joke son fogleg21.mp3

 

[sonolink]

The traces here are handling at most the power needed for 20 headphones, maybe 4 watts or so. Not really like speaker cables, which can run 100+ watts at 4 ohms, i.e. amps.

Sorry, what I meant to say was that they are on the output side of an amp, like after an output transformer, so they are less prone to picking noise.

So in general I would suggest keeping traces as wide as practical between the RJ-45 connectors

All traces are right 0.5mm wide

Provided the lid fits well (???) you will be fully RF shielded, so I have no concern for "RF across the copper pour" (unless it comes in on the speaker lines).

The lid fits really well and the whole enclosure is aluminum (that's the main reasons I chose it)

Since I am currently plagued with a recent sheet metal cut on my thumb I can't over-emphasize the value in pulling the pour back from the edge of the board.

The pour is 0.3mm clear from the edge so it should be ok. The PCBs will be manufactured by JLCPCB

As for the transformer, winding resistance can be misleading, but is OK as a beginning indicator. To quickly test a transformer put 1V (or more, for an output transformer) of 1KHz audio across the higher resistance winding, and measure AC the voltage at the lower resistance winding. A budget dual-trace digital scope is handy for doing this as you can see if either waveform differs from a clean sinusoid, and get the RMS readings as well. The ratio of voltages is the turns ratio, and the impedance ratio is the turns ratio squared. Given that the transformer is rated 8 ohms on the output use the impedance ratio to determine the input impedance. And don't be concerned if something falls 10% - 15% different from what you expected, it will still work.

Thanks a lot for all the tips and advice Hubbub!!

 

[sonolink]

Quick question please: I'm using this tone generator and a small digital scope to check the transformers. The signal is on Ch1.
500mv is the grid size right?
So that sine wave is 1v peak to peak right?
What is the value at the left top corner? (V 20.00mv)





Sorry but I've only used this scope to calibrate a 1/2inch tape machine I've got....

So if I hook the Primary winding to the tone generator output and the secondary to the scope I get this:



Is that 125mv ish?
Any ideas? I'd love to understand that image

Thanks for all the help!
Cheers
Sono

 

[Hubbub]

You have the ability to show 2 traces, I would look at both the primary and the secondary simultaneous so any oscillator loading is accounted for. Each scope channel has its own scale factor, keep in mind that value is at the scope input - unless the scope allows for setting the probe division ratio (usually 1x or 10x) as well. Straight wire is 1x, but adds some capacitance loading, which is a don't care for these low impedances at audio frequencies.

Make CH1 the primary and CH2 the secondary. Adjust the oscillator level to fill the screen (8 divisions) with CH1at 500mV/div, so +2V top to -2V bottom for 8 divisions. 4Vp-p is the main thing to remember. With CH2 at 50mV/div you should see about 6.25 divisions peak-peak, ~6.25x.05=~0.31Vp-p (assuming 1300 to 8 Ohms). 4 to ~0.31 is your turns ratio = ~12.9, square that to get the impedance ratio 12.9*12.9=~166.5 to 1. With an 8 Ohm secondary the primary is 166.5 times that, or about 1332 Ohms.

 

[Hubbub]

In the first pic CH1 is 500mV/div, yes, centered on the grid so +2V top of screen and -2V at the bottom. The sine wave is +1V to -1V, or 2V p-p.

The top left corner is +2V at 3ms (6 divisions at 500us/div) before the trigger point. The trigger point shows as mid-screen by virtue of the little arrow at the top of the center vertical grid line.

The second trace with the secondary shows about 5 divisions p-p at 50mV/div, or 0.25V p-p.

2 to 0.25 gives an 8:1 turns ratio, 64:1 impedance ratio, or about 500 to 8 ohms. Results may be different, and more accurate, if you measure using two simultaneous traces.

 

[ccaudle]

500mv is the grid size right?

Assuming the display matches typical digital 'scope conventions, yes, indicating 500mV per large division.

So that sine wave is 1v peak to peak right?

The positive peak is four divisions above the negative peak, so 2V peak-to-peak.

What is the value at the left top corner?

It has no label, you will have to read your user manual to find what that value indicates.

Is that 125mv ish?

Postive peak at +125mV and negative peak at -125mV, or close to that.

As Hubub pointed out you are doing things the hard way, your display indicates that the scope has two channels, connect the primary and secondary sides at the same time to the two channels and you can see them overlaid. That will also allow you to account for any change in the voltage of the generator if you measure with a load attached to the secondary.

 

[Hubbub]

Thanks to ccaudle I now better understand your question about the "top left corner". I suspect the scope is displaying the CH1 average value there. (Apparently the scope has measurement capabilities, it might have RMS and p-p as well.) Even if the input is AC coupled - one typically uses that for audio - the scope can have some non-zero DC offset in the amplifiers and A-to-D converter, and the values displayed seem about right for such a DC offset at the vertical deflection factors being used. Notice that the displayed number is the same fraction of the scale factor in both scope images, which leads me to believe it is an (insignificant) error value inherent in the scope.

 

[sonolink]

Ok guys, sorry for being silent but I had quite a busy weekend gigging and recording.
I'll try to get those measurements using both channels of my scope.
So if I get this right, the signakl chain would be:
-1 kHz osc -> primary -> scope ch1
- secondary -> scope ch2
Is this correct?

Cheers
Sono

 

[Hubbub]

Yes, correct

 

[sonolink]

How about like so?



Cheers
Sono

 

[Hubbub]

Is the upper trace CH1 at 5V/div or memory A at 200mV/div? I suspect the latter because of the ratio (450:10) and the 90 degree phase shift. Turn off the memory and display both in real time, expect near zero or 180 degree phasing. Don't be afraid to drive the primary harder so the secondary signal is less noisy. And make sure both probes are set at 1x scale factor.

 

[sonolink]

I've reset to factory settings the scope and autocalibrated it, just in case...

This is what I get now. Makes sense?

 

[Hubbub]

Better. But hard to detail the amplitudes as you have the traces confined to just over 1/4 screen. Run with CH1 at 2V/div and CH2 at 500mV/div to get things larger, adjust the positions so everything is on-screen. It helps if you set the bottom peaks on a grid line, now only the top peaks need to be looked at for amplitude. Right now I see about 13Vp-p on CH1 and about 2.4Vp-p on CH2, a voltage ratio of about 5.4 for about a 30:1 impedance ratio. 240:8 is not hardly 1300:8 like I was expecting, but may still be usable for this application. When loaded the transformer winding resistance comes in to play, that will increase the effective voltage ratio. I would suggest you hack together one pot, transformer and earbud set, drive it from some old hi-fi receiver and see what happens. [Also, the HDS1022 should be able to give you Vp-p numbers of the signals presented, may be better than looking at things by eye. I could not find an HDS1022M manual online quickly.]

 

[sonolink]

Man, I had no idea I could do and check SO MANY things with my scope. I guessed it's not THAT shitty after all

Check this out:

 

[Hubbub]

Input is 7.2*2=14.4Vp-p, output is 5.2*0.5=2.6Vp-p, voltage ratio 5.54, impedance ratio about 30.7:1 or 245:8. Pretty close to the 240:8 calculated from less detailed curves before. So I'm thinking the transformer you are testing is not 1300:8 as was advertised IIRC. With a load that is two 32 Ohm earbuds in parallel you would see a 16 Ohm load, the transformer input looks like 490 Ohms impedance. Might work just fine with a 5K linear curve pot to give some semblance of a logarithmic adjustment. If the pot is already a log curve then the pot value should be probably be closer to 1K ohms. I still suggest hack one unit up just to try. Hope that helps.

 

[Hubbub]

Just for grins, sweep the oscillator up and down in frequency to see how the transformer behaves. 300 to 4000 Hz is critical, anything below 100Hz and above 8kHz or so is probably unimportant for speech intelligibility. Chances are the transformer is a poor performer below 100Hz, but may be good out to 30-40kHz.

 

[sonolink]

Just for grins, sweep the oscillator up and down in frequency to see how the transformer behaves. 300 to 4000 Hz is critical, anything below 100Hz and above 8kHz or so is probably unimportant for speech intelligibility. Chances are the transformer is a poor performer below 100Hz, but may be good out to 30-40kHz.

Ok, so I found the way to show Vp-p. Here's the 1kHz again..You were spot on!




Here's 300Hz:



And this is 4kHz:



I bought B5k pots so I should be ok. As soon as they arrive I'll whip up a test box and report back.

HUGE thanks for the scope experience. I've ALWAYS used it to calibrate azimuth on the 1/2inch tape machine I use for giving tracks or stems some tape compression and mastering.
It's really an awesome instrument

Cheers
Sono

 

[sonolink]

Hi guys,
All the components finally arrived yesterday so I whipped up a quick breadboard this morning.
This thing sound amazing, WAY over my expectations, PLENTY of volume and enough quality to listen to music so speech will be just fine.

Again, I'd like to warmly thank ALL OF YOU for your ideas, time, suggestions and help. Thanks to you I could help and provide my friend with a cheap and reliable system for little cost. I wish I could buy you all a nice cold beer!!
Thanks again.
Here's a pic of the breadboard. I'll post pics of the real thing asap.

Cheers
Sono