Phantom adapter for electret modification??

Thread Starter

melodomail

Joined Jul 16, 2022
5
I got some of these tiny electret mic adapters and there was one faulty so I tryed to copy the layout schematic of this. As the components are all SMD. Maybe it will be helpful for someone..

The addiction I would like to this schematic is a HPF with a 3point switch (Flat-200hz-400hz) but I don't know how to modify this schematic as my electronics knowledge is limited. Is there anyone that can help me to modify this????
electret phantom board.jpgpcb.jpgPhantom PRE Schematic copy.png
According to SMAART analyzer results, there is a build in HPF around 50-63hz but it is low enough for the application I need to use it. electret phantom board.jpgpcb.jpgPhantom PRE Schematic copy.png
 

Alec_t

Joined Sep 17, 2013
13,077
I can't believe there are several resistors with a value of only milliOhms.
The board has several resistors marked '222', which indicates a value of 2.2kOhms. Note that the third digit indicates the number of zeroes following the first two numbers.
 

ag-123

Joined Apr 28, 2017
259
it partly depends on the quality of your electret mic, some "cheapo" electret mic are "horrible"
https://forum.allaboutcircuits.com/threads/lm358-electret-mic-amp.184812/post-1709481
https://forum.allaboutcircuits.com/...nded-electret-microphone.184747/#post-1705927

I did a circuit that uses LMV358
https://forum.allaboutcircuits.com/threads/lm358-electret-mic-amp.184812/post-1706624
It didn't use any caps, so in theory it goes down to DC.
The trouble is the electret mic is so insensitive, so I'd need to use large amplification.
And in the end, the pink noise from the mic is so large, any sound received by the electret is within that 1/F noise spectrum and you cannot separate that noise from sound, all sounds are part of that 1/f pink noise mixed.
Strangely, it is possible to hear background sounds despite the extreme pink noise pollution.
 

Thread Starter

melodomail

Joined Jul 16, 2022
5
I can't believe there are several resistors with a value of only milliOhms.
The board has several resistors marked '222', which indicates a value of 2.2kOhms. Note that the third digit indicates the number of zeroes following the first two numbers.
There are lines above these numbers so...?
 

MisterBill2

Joined Jan 23, 2018
12,746
Altering the low end of the response involves changing capacitor values, and that takes room that may not be available. So alter the response with the filter in the mixer board.
 

Thread Starter

melodomail

Joined Jul 16, 2022
5
Ok, maybe the values were my fault. So lets try some photo-edition of the schematic...
I hope someone can spot now where the low cut filter is, so I will try modify it...

*Excuse me for my GrEnglish!;)
Phantom PRE Schematic copy.jpg
 

MisterBill2

Joined Jan 23, 2018
12,746
If the circuits shown in post #1 are correct, then only C3 is in series with the audio, nd so reducing the value of capacitor C3 will raise the lower cutoff frequency. Actually, it will just affect the bottom end of the frequency response curve. It may be possible to cut the trace just past C3 and install another chip cap in series, but adding a switch with multiple choices seems like there is no room for it. And what sort of application requires cutting the low end of the frequency response?
 

Thread Starter

melodomail

Joined Jul 16, 2022
5
If the circuits shown in post #1 are correct, then only C3 is in series with the audio, nd so reducing the value of capacitor C3 will raise the lower cutoff frequency. Actually, it will just affect the bottom end of the frequency response curve. It may be possible to cut the trace just past C3 and install another chip cap in series, but adding a switch with multiple choices seems like there is no room for it. And what sort of application requires cutting the low end of the frequency response?

Thank you for your reply. I will try this..
It is for a friends DIY mic on a traditional instrument.
 

MisterBill2

Joined Jan 23, 2018
12,746
For many microphones a great deal of the characteristics come from the acoustical characteristics of the housing. So that may also affect the frequency response.
 

Thread Starter

melodomail

Joined Jul 16, 2022
5
Yes I also told him about the housing.
Finaly I found that the caps that affect the frequency of HPF are C1 and C2 (1st post schematic).

But is there a way to affect the slope of the filter??
Any Idea??
 

MisterBill2

Joined Jan 23, 2018
12,746
C1 and C2 appear to be shunting the microphone itself,and so they will affect the higher frequencies. C3 is in series with the audio flow and so it will affect the low frequency roll off. Reducing the capacitance of C3 should raise the low frequency roll-off, but it is not a sharp roll off because it is a one-pole filter. ( one resistance and one capacitance., To get a steeper roll-off curve you will need more sections or an active filter to replace C3.
 
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