Author Topic: RF filtering between phone audio output and plasma speaker  (Read 559 times)

Offline John123

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RF filtering between phone audio output and plasma speaker
« on: March 11, 2021, 05:59:45 PM »
Hello,

I've whipped up a plasma speaker using a flyback transformer and TL494 (sounds pretty good and loud for the lowish 30 watts power consumption), however when the high voltage is running the touchscreen on my phone goes bananas. The phone is connected to the circuit as an audio source using the 3.5mm audio jack.

What sort of filtering would I need to prevent the high voltage from interfering with the touchscreen? This is what I've currently got but it's not enough. The "clip on ferrite bead" is on the aux cable between the phone and circuit.


When I use a Bluetooth to aux adapter as a means of isolating the phone from the circuit the screen works fine. The high voltage is about 20kV open circuit and about 30 watts, switching frequency is 40khz. The circuit is 24v battery powered so all floating, the phone is also on its internal battery power.

Maybe an audio isolation transformer and more aggressive common mode/differential(?) choke? I recall this happening years ago too with an older circuit design except it was a laptop and its touchpad going haywire, back in those days phones didn't have touch screens however so it was never a problem using phones as an audio source.
 
I'm guessing the capacitive touch screen is being charged up by the high frequency high voltage with one end of the loop being completed using the audio cable.

Any advice would be appreciated!
« Last Edit: March 11, 2021, 11:20:05 PM by John123 »

Offline davekni

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Re: RF filtering between phone audio output and plasma speaker
« Reply #1 on: March 12, 2021, 06:24:34 AM »
Sounds familiar, in that I recall the audience for my SSTC demonstrations noticed that they couldn't use phone touch-screens.  They had to start filming before I ran the coil.

Of course, sending the audio over an optical cable would be ideal isolation.  For electrical, my guess is that you need a larger common-mode ferrite with more turns.  For 40kHz, a core intended for transformer use will be better than one intended for EMI.  A large ferrite core for GDT or an E-core with as many turns as fit.  If you want to go further, two cores, first one with many turns, then another with fewer turns to lower capacitive coupling between turns.
David Knierim

Offline John123

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Re: RF filtering between phone audio output and plasma speaker
« Reply #2 on: March 12, 2021, 07:30:36 PM »
Thanks Dave, I'll look into common mode ferrite filters then. How would it play out having both channels on one core? Would it induce audio into the adjacent channel?

Oddly the bluetooth receiver to 3.5mm aux adaptor works fine around the high voltage, it's just not very convenient at times.
« Last Edit: March 12, 2021, 07:41:04 PM by John123 »

Offline davekni

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Re: RF filtering between phone audio output and plasma speaker
« Reply #3 on: March 12, 2021, 08:39:37 PM »
Tesla coils don't generate much noise at bluetooth frequencies (GHz).  On the other hand, I think touch screens run in the kHz range where Tesla coils operate.  Touch screens are looking for tiny affects of a person's capacitance to the phone, so easily overwhelmed by Tesla coil fields.

Most audio cables are shielded - coax - but not good for RF frequencies.  If so, there is no issue with running both channels together for common-mode filtering.  If there are any large loop areas where the two channels are separate, then a bit of individual common-mode chokes could help.  That is if the loop is close to Tesla coil magnetic field where it would induce a voltage between one cable's shield and the other.

If using unshielded cable, then there will be some capacitive coupling between channels with the extra length needed to wind around common-mode choke cores.  Still not likely to be an issue as long as there aren't any large loop areas where the ground wire(s) is away from the signal wires.
David Knierim

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Re: RF filtering between phone audio output and plasma speaker
« Reply #4 on: March 15, 2021, 07:23:17 AM »
Thanks again Dave!

The audio cable isn't shielded I don't think, its just a basic 3.5mm to 3.5mm lead but I made sure to keep the audio paths very close together on the board.

Also I forgot to ask, should I count the 1k resistors as part of my high pass filter or just the 2.2k? Does the series resistor and capacitor even work as a low pass filter or does one end of the capacitor need ground?
« Last Edit: March 15, 2021, 07:31:39 AM by John123 »

Offline davekni

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Re: RF filtering between phone audio output and plasma speaker
« Reply #5 on: March 15, 2021, 06:04:41 PM »
Also I forgot to ask, should I count the 1k resistors as part of my high pass filter or just the 2.2k? Does the series resistor and capacitor even work as a low pass filter or does one end of the capacitor need ground?

Did you mean "high pass" in the second sentence above?  The series capacitor works as high pass if the error-amplifier input impedance is finite.  The total resistance to calculate R/C time constant for high-pass 3dB cut-off frequency is 1k (from low-pass input filter) + 2.2k + error_amplifier_input_impedance.

Sounds like you are using a single cable with both left and right channels included.  That should be fine wound around ferrite core(s) for common-mode filtering.  No reason for separate filtering.
David Knierim

Offline John123

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Re: RF filtering between phone audio output and plasma speaker
« Reply #6 on: March 15, 2021, 10:08:43 PM »
Hi Dave, yes sorry my mistake I mean't high pass, probably helps not to post right before bed  ;D

I've drawn some annotations to make it easier to understand, the green resistors forming part of the input low pass filter are also part of the high pass filter too? The same resistor can be part of multiple filters?

Also won't the node circled in brown where the two channels meet have an effect on the corner frequency (I believe its called a passive summing network).

Thanks and sorry for the confusion.
« Last Edit: March 15, 2021, 10:14:57 PM by John123 »

Offline davekni

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Re: RF filtering between phone audio output and plasma speaker
« Reply #7 on: March 15, 2021, 11:09:18 PM »
Many such cables are shielded, although certainly the very cheap ones aren't.  Not overly important here since wires are all close together.

The two 1k resistors in each channel's LPF add to 2k per channel.  When combined in parallel at your brown node, that becomes 1k, thus the 1k + 2.2k in the HPF equation.  Impedance of the TL494 "-" input depends on the rest of the circuitry around TL494.  If there is feedback from the error amplifier to that same negative input pin, then the effective input resistance is about zero.  Thus total HPF resistance is 3.2k.

At the lower 3dB frequency of the HPF, the LPF capacitors are relatively-high impedance.  So just the resistance matters (all four 1k resistors combining to 1k net).
David Knierim

Offline John123

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Re: RF filtering between phone audio output and plasma speaker
« Reply #8 on: March 15, 2021, 11:44:17 PM »
Thanks Dave!

From the feedback pin of the TL494 to the inverting input I've got a 22k resistor for a bit of gain in parallel with 47pF for additional HF noise filtering. If this 47pF is made too large it starts cutting into the top end of the audio.

Here's the Jan Martis schematic I've based it on, its really much of the same except my output stage is single ended with a mosfet driver+snubber and the DTC pin set so the max duty cycle can never exceed 50% (idle state is 25%). I'm going to attempt adding peak primary current limiting at some point down the line.


Offline davekni

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Re: RF filtering between phone audio output and plasma speaker
« Reply #9 on: March 15, 2021, 11:57:45 PM »
Looks as expected, roughly zero ohms at the inverting TL494 error amplifier input.

At 50% duty max, you are using 1.5Vpp range of error amplifier output.  With gain of 22k/3.2k, that is 0.22Vpp audio input before clipping.  If you have any noise issues, you could crank up the phone volume and reduce gain (increase 2.2k resistor value).  The phone can likely output close to 1.5Vpp, allowing gain down to 1 (21k resistor instead of 2.2k, for 22k total).  Of course, if it is working fine from a audible noise/interference perspective, then no reason to change.
David Knierim

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Re: RF filtering between phone audio output and plasma speaker
« Reply #10 on: March 16, 2021, 12:34:01 AM »
The EU imposed limits on how much drive voltage headphone jacks can output on devices sold here, and there's still a warning if its cranked too high. The default gain on the original schematic was fine for old phones but newer ones need that extra gain unfortunately.

It doesn't sound too bad to be honest (like a portable speaker), if anything the flyback LOPT itself seems to dictate the overall tone of the sound. The best one so far was from a small TV with a core gap made slightly bigger with a layer of electrical tape (this was done for another project).

One of the worst ones was with a flyback LOPT from a much larger TV which gave the sound a slight crackle (probably due to internal capacitances). Monitor flybacks aren't very well suited as the internal capacitor just wants to join the circuit with bright loud snaps at the pins, plus I haven't put peak current limiting in yet so charging capacitors is very risky for the fet.

I've read old AC LOPT's are supposed to sound best for audio modulation but the only AC LOPTs I've got are DC ones with their internal diodes shorted from half-bridge resonant drive;D
« Last Edit: March 16, 2021, 01:02:21 AM by John123 »

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Re: RF filtering between phone audio output and plasma speaker
« Reply #10 on: March 16, 2021, 12:34:01 AM »

 


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