[TheLaw]

Hello,
 
Working on a little "hearing aid" project.
 
The mic will be on the same board as everything else so it's not a matter of long cable runs and picking up noise on the way etc.
 
I have a question about how the different stages should be laid out.
 
Currently I have been thinking: Electret Mic->LC/RC filter -> Mic Preamp -> 3-Band equalizer -> Headphone amp -> Headphones
 
 
Is this the correct order? Does the filter go somewhere else? I want to bandpass 100-3000Hz.
 
Do I need a preamp? Can I run the signal of the electret directly into the equalizer? Is that a bad idea for some reason?
 
Preamp, equalizer, and head amp will all be op-amp based.
 
 
Thanks!

 

[wakibaki]

Why not take a look at ESP - Rod Elliott's electronics site. Here's a link to a page dealing with electret mics. You don't really need a preamp, an opamp buffer will do, this will drive any mixer/tone control.
 
http://sound.westhost.com/project93.htm
 
Look around the rest of the site, this is not the only page dealing with microphones, and there is a great deal of other specific and general knowledge about electronics.
 
Steer clear of inductors in filters, you don't need them for what you want and RC filters are much easier to deal with. The filter wants to go after the buffer. An active filter will be preferable, google Sallen - Key bandpass.
 
w

 

[TheLaw]

Alright thanks.
 
[size=12.222222328186035px]Electret mic -> Buffer -> BP Filter -> EQ -> Headphone amp -> Headphones[/size]
 
[size=12.222222328186035px]How that sound?[/size]
 
[size=12.222222328186035px]What kind of buffer is that? Current/voltage?[/size]

 

[wakibaki]

That looks fine.
 
Buffer means unity-gain buffer, which outputs the same (roughly) voltage as input, but has vastly improved current output and lower output impedance. There are dedicated buffers such as the BUF634 (integrated chip buffer) which has only a non-inverting input, discrete buffers, such as the diamond buffer (a famous topology) and any opamp can be configured as a unity-gain buffer by tying the output to the inverting input. You could use half of an NE5532 (dual opamp) and use the other half for an active filter. The NE5532 is a good audio opamp with reasonable current drive, very cheap, especially considering how good it is.
 
w

 

[TheLaw]

Right thanks! I realized that I had just said something dumb. I know what a buffer is. Dur!
 
However, Just a question, why does the signal need to be buffered? I understand what these stages do, but I'm not quite fully understanding the theory behind all of them. The microphone is a high impedance source. So the buffer has a high (infinite) input impedance. (Is this called impedance matching?) And then the output of that buffer is low impedance. Why do we need a low impedance source for use with a BP filter or a EQ? I guess that's my question. 
 
I mean the headphone amplifier is what is driving the headphones. What's the point of buffering?
 
Thanks so much!

 

[wakibaki]

True impedance matching is only really necessary at radio frequencies, and means that driving and receiving impedances are identical. This is because it is more important to conserve power at RF and power is reflected at an impedance discontinuity. The reflected power can destroy the device it is produced by, due to overheating. Matched impedances insure efficient (100%) power transfer which is important to maintain signal-to-noise ratio when the signal power is very small, such as the signal from an antenna. Most RF devices operate in a 50 ohm environment, although 75 ohms is used in TV.
 
In audio, power is more easily come by, signals are rarely very small and reflections are less of a problem. Power is not concentrated in a comparatively narrow band of frequencies. It's conventional in audio for the driving device (source) to have an impedance of about 1/10 of the receiving device (sink). This means that most of the output voltage appears across the sink and simplifies thinking about gain. It is not an absolute requirement. 1:1 can be acceptable and so can 1:100. 10:1 is likely to upset things.
 
In the absence of specific knowledge of the input impedances of devices in a chain the general rule is that sources should have a comparatively low output impedance, and it is known that most electret mics (which have a built-in single-fet amplifier) of necessity have an output impedance of about 1500 ohms, which in the general scheme of things, is a bit on the high side, so to maintain the low->high, low->high chain, a buffer (with an input impedance in the 10's or 100's of k) is interposed. 600 ohms could probably be considered a median impedance. Filters (an EQ is just a kind of filter) are also not immune to having their response affected by the impedances they see on either side. It's best to assume that they will work best when incorporated into an audio chain following the convention.
 
w

 

[TheLaw]

Thank you so much. Makes sense.