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Spark calibration of microphones
Spark calibration of microphones
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Old 24th July 2021, 05:04 AM   #301
1audio is online now 1audio  United States
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Spark calibration of microphones
Some more input from my research on this spark generator. The internal supply charges the cap to approx 500V before discharge. Anything smaller than about .2 uF and it doesn't charge all the way and doesn't work.

With this arrangement and 500K Ohms (5 100K in series) I get a peak voltage before breakdown of about 6 KV. It takes about 60 uS to charge and nanoseconds to discharge the stored charge. Its now definitely a single discharge.

Getting a good current reading is quite hard. The current into the 500K is about 500 mA. I suspect the current at the discharge is higher. I'm using about 4' of RG59 to transport the voltage to the spark gap. Out of the generator there are two HV wires which is not good for EMI. I'm going to replace this with a single coax all the way to the spark gap. I hope that reduced the radiated noise.
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Old 24th July 2021, 05:35 AM   #302
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Sounds like a plan. My thinking has been to have a balanced line for HV so the wave cancels out at the microphone, but that may just be fanciful thinking, or may only apply to bare capsules with minimal shielding or where an internal node is exposed through the front.
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Old 24th July 2021, 11:45 PM   #303
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Spark calibration of microphones
So, with a coax from the coil to the gap. Put the spark gap in a metal can and I still get EMI on the scope once I connect probes. Att a 100Meg resistor in series at the coil end and it still sparks and still EMI. Less at least. This is a really good EMI source.

Distance helps, however the active circuitry in the power supply gets a big DC shift with the spark and starts to return but is still offset when the acoustic pulse hits. I will switch to a different power supply and see if that helps. My hopes for this are dimming fast.
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Old 25th July 2021, 12:09 AM   #304
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Spark calibration of microphones
This shows the EMI problem. Upper trace- microphone. Lower trace trigger from spark generator

Holds true with two power supplies/buffers. When the spark happens there is a big DC shift that takes a long time to recover. The acoustic pulses are near the center.

I may need to abandon this and revert to using a ribbon tweeter or electrostatic actuator when possible. I'll keep thinking about this in any case.
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File Type: png Spark EMI.PNG (314.4 KB, 22 views)
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Old 25th July 2021, 12:23 AM   #305
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I made it work by using ring cores and ferrite beads everywhere. Rings cores on the HV wires help, as well as individual beads for the + and - output wires, both near the gap and at the coil.

My battery powered sparkers do not need to be plugged in, so the area they can radiate in can be confined to a small area using a relay to isolate a remote trigger.

Any wires leaving the spark generator needed to be capacitively or inductively isolated.
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Old 25th July 2021, 01:09 AM   #306
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Spark calibration of microphones
After some more fiddling here is what I get recording. The emi pulse is the first transient. The DC offset starts there. The two acoustic pulses are due to a reflection, the first is the more valid one. I found that despite the full isolation of the channels in the Tek connecting the sync makes for a big dc issue on the other channel. Disconnecting it seems to remove the DC, at least on the scope. Still seems there on the recording setup. Enough for today.
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Old 25th July 2021, 01:12 AM   #307
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Spark calibration of microphones
I think this spark generator just makes too much EMI. I have ferrites etc. all over. The coax was to make the radiating window smaller. I have isolated from the AC line. The scope and the mike are both battery powered. I even get stuff on the scope with the input shorted.

I have been dealing with this stuff testing surge strips for years. I think its the current peak that makes the most trouble. I have a 100 Meg resistor in series inside the connection to the HV terminal in the coil and it still has too much energy. However this is a 12V coil getting hit with 400 V. Maybe I need a different coil 20:1 instead of 2000:1? The discharge is using the stored charge in the coax (8' of RG59) as part of its energy, Maybe all of it with the 100 Meg series resistor, unless that's arcing inside. I'll check it. If it is the terminal resistance will have changed.
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Old 25th July 2021, 02:05 AM   #308
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The coil looks like an automotive coil, but now I wonder. Surely 400V into a 12V coil would fry the secondary insulation in no time. Maybe it is custom wound by a factory that makes the standard kind.

I suspect it is arcing through the resistor, and I wouldn't assume that it will be noticeable in the resistance value.

The corner frequency of 100Mohm and 130pF is 12Hz, there is no way the 100Mohm resistor could charge the coax fast enough to reach sparking voltage.
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Old 25th July 2021, 07:29 PM   #309
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Spark calibration of microphones
I went back at this today. Made some changes.
1) 100K at the coil going into the coax.
2) switching around the output of the preamp poser supply.

Those two changes got the EMI down a lot, to the point I can make OK measurements. The 100 Meg resistor is now 88 Meg, definitely arcing. The 100K seems to handle the stress. Its a 2W size.

Now that I can make an OK recording its time to figure out the acoustics for a decent measurement.
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