Possible signal interference with audio circuit

Thread Starter

Steven Woody Wood

Joined Mar 16, 2016
1
I've designed a 5th order Sallen Key High Pass Filter (HPF) (Passband>=1KHz) that feeds into a LM386 amplifier into an 8 ohm speaker. The input signal to the filter is a probe tip which picks up stray frequencies inside walls and other nearby metallic objects or antennas and sends that signal to the amplifier to generate a tone so the user can detect if something is present. The main board and filter board is powered by a 9 volt battery.

I tested the HPF board independent of the main circuit board and all works to spec. I then integrated the HPF into the main board and used a 1KHz tone generator connected between the tip and GND of the circuit and I can hear the tones perfectly without any distortion.

I then connected one end of the tone generator to a metal plate so it can radiate that same frequency so I can test if the probe works as intended to be used. This is where the probe circuit has failed to work and I can't hear anything at all.

Any input to this issue is appreciated as its the first time I've ever encountered this 10 years as an engineer.
 

Papabravo

Joined Feb 24, 2006
17,311
Why do you think the tone generator will do anything to the plate? Why do you think the high pass filter with a probe tip will pick up any vibrations at all? What kind of an engineer are you?
 

OBW0549

Joined Mar 2, 2015
3,566
Any input to this issue is appreciated as its the first time I've ever encountered this 10 years as an engineer.
At this point, my input would be that you need to post a schematic diagram showing exactly what you've built. Without that, any advice we could give you would be based purely on wild-assed guesses and would waste both your time and ours.
 

#12

Joined Nov 30, 2010
18,223
I think this one is not coming back. Probably already found out a metal plate isn't a speaker (at 1KHz) and a metal probe isn't a microphone.
 

Papabravo

Joined Feb 24, 2006
17,311
I think this one is not coming back. Probably already found out a metal plate isn't a speaker (at 1KHz) and a metal probe isn't a microphone.
I was actually hoping to learn some new physics here. Oh well...one can but hope for better outcomes.
 

ci139

Joined Jul 11, 2016
1,696
I've designed a 5th order Sallen Key High Pass Filter (HPF) (Passband>=1KHz)
this thing alone is worth to see - it's extremely difficult to find any example diagram of it in web
random test
stray frequency паразитная частота
stray frequency 雜散頻率
雜散頻率 疑似周波数
疑似周波数 Spurious taajuus
Spurious taajuus spurious frequency
spurious frequency паразитная частота
- if this thing occurs in building and construction - may have importance ???

also the search of [human body as a signal ground] returned a Figure 5
 
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OBW0549

Joined Mar 2, 2015
3,566
I think this one is not coming back. Probably already found out a metal plate isn't a speaker (at 1KHz) and a metal probe isn't a microphone.
Yeah. He hasn't logged onto AAC since Friday night, after several of us called for schematics so we could see what the heck he's doing.

My own wild-ass guess as to what's going on is that he is trying to construct some sort of "E-field sniffer" to probe for mysterious emanations from god-knows-what, and doesn't understand that he can't simply hang a wire probe onto the input of his filter circuit without going through some sort of high input impedance buffer amplifier, such as a bootstrapped FET voltage follower, and expect to pick up much of anything.

Who knows what he was expecting to find, other than mains hum coming from power wiring or the high-frequency crap that's spewed out by switching regulators and CFL lighting. Secret signals from space aliens?
 

#12

Joined Nov 30, 2010
18,223
Who knows what he was expecting to find,
Termites is a possibility. I have heard people say they could, "hear the termites munching during the night".
An amplified version of that sound might be a diagnostic tool and I think it would have a content at 1KHz.
 
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