# Hiring somebody to draw a stereo headphone amplifier circuit

#### allisonr

Joined Aug 16, 2016
27
Quality sound for solid state amplifiers means primarily headroom and low negative feedback.

Headroom is to minimize clipping. Low negative feedback is to avoid transient distortions.

If your input signal is so strong that it requires attenuation, going high gain is the opposite of what you want to do.

For most headphones, a headphone amps primary job is impedance transformation. (Voltage) gain is typically the least of your concerns.
Sounds would be distant and low, gain would be adjustable, that sound level limiter is there just to avoid hearing damage from microphones bumping/brushing when moving around.

#### dannyf

Joined Sep 13, 2015
2,197
Sounds would be distant and low,
What kind of headphone amplifier is this? It sounds more like a microphone amplifier.

#### allisonr

Joined Aug 16, 2016
27
What kind of headphone amplifier is this? It sounds more like a microphone amplifier.
There would be two microphones picking up sounds,which are then amplified and outputted over headphones

#### GopherT

Joined Nov 23, 2012
8,012
Quality sound for solid state amplifiers means primarily headroom and low negative feedback.

Headroom is to minimize clipping. Low negative feedback is to avoid transient distortions.

If your input signal is so strong that it requires attenuation, going high gain is the opposite of what you want to do.

For most headphones, a headphone amps primary job is impedance transformation. (Voltage) gain is typically the least of your concerns.
No, you misunderstand again, Danny. She wants his gain to better hear birds and bees as she hikes in the Swiss alps but she doesn't want her eardrums broken when her hiking partner talks to her. (Sorry, I assumed Gender based on name Allison, I didn't consider it could be a surname).

#### dannyf

Joined Sep 13, 2015
2,197
There would be two microphones picking up sounds
you need microphone amplifiers.

#### ronv

Joined Nov 12, 2008
3,770
Not exactly, since I'd like microphones to not be that directional but either omni or cardioid.
I don't have any preference over parts, microphones included, hence I'm entrusting the person picking up this offer.
About gain, as anything else in this task, as performant as the cost constraints allows. I wouldn't mind to hear nightingales singing behind the hill.
I know there have to be compromises between noise and gain, I'd be inclined to indulge on the gain side since being adjustable, I'd always switch to slightly less gain and higher audio quality, however as previously said, the only constraints are parts price.
I ask because part of what you worry about depends on what you are trying to do.
If you are trying to listen to little critters at some distance away you worry about the mic noise. If it's just general use you probably won't be able to tell the difference in any of the mentioned chips. But if that's the case you don't need real high gain.

#### GopherT

Joined Nov 23, 2012
8,012
There would be two microphones picking up sounds,which are then amplified and outputted over headphones
So, I was just playing at my work bench and set something up. The type of headphones makes a difference as does position of the microphone. If the headphones are the in-ear type without foam, the sound that escapes can be picked up by the micophone and generate huge feedback.

The wind noise is amazing - and there are not even leaves on the trees. I protected the microphone so wind across the mic is minimal - there is just more noise that you think across branches, evergreens, and tree branches.

I don't know if you want omni directional microphone (like a hemisphere or a 120-degree cone from the mic of directionality) or if you want a narrowly focused microphone. Finding the balance will be trial and error. I used a 120-degree omni-directional electret microphone that cost $0.25 each (more expensive mics are not amazingly better). I was getting some noise from the long wires on my breadboard but a pcb would make everything much quieter. I used an NE5532 op amp but it was definately overdriven by connecting to a 64 ohm load (left and right channels in series). Some switches to filter low frequencies would be good to block low frequency leaf/wind noise. You won't get the full effect of bear growls in the woods but it shouldn't hurt bird chirp noises too much. Thread Starter #### allisonr Joined Aug 16, 2016 27 No, you misunderstand again, Danny. She wants his gain to better hear birds and bees as she hikes in the Swiss alps but she doesn't want her eardrums broken when her hiking partner talks to her. (Sorry, I assumed Gender based on name Allison, I didn't consider it could be a surname). This is a bit stereotypical but exactly what I'm looking for. By the way yes, I'm female. I ask because part of what you worry about depends on what you are trying to do. If you are trying to listen to little critters at some distance away you worry about the mic noise. If it's just general use you probably won't be able to tell the difference in any of the mentioned chips. But if that's the case you don't need real high gain. There would be distance involved since very few species keep singing when approached at less than 40 meters. So, I was just playing at my work bench and set something up. The type of headphones makes a difference as does position of the microphone. If the headphones are the in-ear type without foam, the sound that escapes can be picked up by the micophone and generate huge feedback. The wind noise is amazing - and there are not even leaves on the trees. I protected the microphone so wind across the mic is minimal - there is just more noise that you think across branches, evergreens, and tree branches. I don't know if you want omni directional microphone (like a hemisphere or a 120-degree cone from the mic of directionality) or if you want a narrowly focused microphone. Finding the balance will be trial and error. I used a 120-degree omni-directional electret microphone that cost$0.25 each (more expensive mics are not amazingly better).

I was getting some noise from the long wires on my breadboard but a pcb would make everything much quieter. I used an NE5532 op amp but it was definately overdriven by connecting to a 64 ohm load (left and right channels in series).

Some switches to filter low frequencies would be good to block low frequency leaf/wind noise. You won't get the full effect of bear growls in the woods but it shouldn't hurt bird chirp noises too much.
I guess that using those in-ear "isolating" ones would generate the least feeback loop, so I'd use them.
As for microphones, a pair of 120° omni would be nice and close to human hearing pattern, with anything narrowly focused being less than ideal.
An on/off low frequency cutoff filter would be a nice addition too, good spotting on this, didn't think about it.

#### AlbertHall

Joined Jun 4, 2014
11,610
Incidentally the standard way of waterproofing a microphone is to put it in a condom and, apparently, this makes little or no difference to the sound.

#### ronv

Joined Nov 12, 2008
3,770
Here is a pass at a schematic:
Also the LtSpice file for those that would like to play.
The overall circuit has a gain of about 900 and is flat from 20 to 20Khz.
Noise for the entire circuit is about 10uv at 50% gain.
Distortion of about .24%. Less than half that without the clamp diodes.
!6 ohm headphone with sensitivity of 113 db.
Someone speaking normally at 10 meters should sound louder than if they were standing next to you.
A "bump" will be very loud but probably not fatal.
Mic is a low voltage electret.
Having said all that I think @GopherT is right the ambient noise will drive you nuts. That is why long distance listening devices are very directional and often have "mechanical" gain or "cancellation" before the microphone.

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