There are about three different styles of headsets, generally speaking.

A PC- or phone-style headset will use an electret mic and have one or more low-impedance earphones. Typically, a bias voltage is expected and provided by the device for the electret microphone, and an impedance or load on the MIC line of about 18 kohms. Earphones may be 8 ohms to 64 ohms, typically 16 - 32 ohms. Higher impedances are common in newer equipment at either or both ends of the earphone circuit.

A General Aviation headset will typically use an electret microphone and one or more high-impedance earphones. This electret microphone will also expect a bias voltage to be provided by the radio, but the microphone impedance will be lower, approximately 50 ohms. The radio microphone input impedance may be 50 ohms to 300 ohms. The earphones will be 150 to 600 ohms, often 300 ohm earphones are used and the two earphones wired in parallel to halve the impedance to 150 ohms.

A Military Aviation headset will typically use a low-impedance dynamic microphone and one or more low-impedance earphones. This type of microphone does NOT require a DC bias voltage, nor should one be provided for the microphone, nor is a DC bias voltage expected. These microphones typically have low impedances; five ohms is not unheard of and rather common. Impedances may be up to 50 ohms or so. The earphones are much more conventional, 4 ohms to 16 ohms, wired appropriately to arrive at about 8 ohms.

In summary:

PC style headset:

Mic Load: ~18 Kohms

Mic Impedance: ~10 to 40+ Kohms

Earphones, stereo: 8 to 32 ohms each

Tip: Mic

Ring: Mic Bias Voltage

Sleeve: Mic return

GA headset:

Mic Load: 50 to 600 ohms

Mic Impedance: 50 ohms

Earphones, mono: 150 - 600 ohms, typically wired to 150 - 300 ohms

Military headset:

Mic Load: ~5 - 50 ohms

Mic Impedance: ~ 5 - 8 ohms

Earphones, mono: 4 - 32 ohms, typically wired to 8 ohms.

The military headset can be wired directly to a 3.5 mm plug and connected to a computer. The microphone side will require a transformer to convert the impedance, and a blocking capacitor to keep DC from going through the transformer coil and magnetizing it.

The General Aviation headset can be interface with by using common audio interface methods associated with telephony. Most telephone interface equipment expects a nominal impedance of 600 ohms. Older audio equipment may use this impedance as well for "line" connections. Most "line" inputs today are 10 Kohm or higher.

DC blocking capacitors are a must (0.068 uF, 0.1 uF and/or 1uF) as well as RF bypass capacitors. Motorola has used 1500uF capacitors on the output of bridged audio amplifiers. Values such as 10uF may cause filter effects to the audio. Remember that audio distortion is comprised of even and/or odd harmonics of the fundamental. Therefore, filtering above 5KHz will not cause problems for your audio in either direction. The RC formula for resonance (Xc) will give the resulting curve.

There are a lot of transformers to choose from on the surplus market and from electronics houses. A transformer that goes from 1 Kohms to 8 ohms is the same as a transformer that goes from 625 ohms to 5 ohms. Similarly, some 600-ohm transformers, repeat coils, and older microphone / audio transformers can be wired to go from 50, 150, 250, 300, 500, 600, or 1200 ohms.

The important thing to keep in mind is the turns ratio. A transformer with a 1 Kohm primary and an 8 ohm secondary is also equivalent to a 625 ohm primary and a 5 ohm secondary (125:1). Likewise, a center-tapped 600-ohm transformer primary can be connected as two 300-ohm windings in series, however the phasing of those windings determines how they can be used. Two independent windings that make up 600 ohms can be placed in parallel with each other, giving 150 ohms. Don't worry about power handling capability since most headphones and ears can't take more than about a half-watt of power without taking damage. Two of these transformers, coupled one after the other from low impedance to high impedance to low impedance to high-impedance should transform from 5 ohms to 312.5 ohms to about 39062.5 ohms, which should be enough to interface with any modern gear.

There are small audio transformers that can be obtained from the parts houses. I recommend checking with Mouser, Jameco, All Electronics, Surplus Sales of Nebraska, Fair Radio Sales, Electronics Goldmine, Electronic Surplus, Mendelson's, and any other outlets you may be able to locate for surplus equipment.

Don't go audiophile to the nth degree on this one. Audio is rather forgiving, and transformers and capacitors are the only way out. A low-impedance dynamic microphone behaves just like the element in a ribbon microphone (which is often in the milliohms). A transformer will work best here. These headsets were only designed to do 300 - 4000 Hz for voice. Some were designed for loud environments, and function well there. They were not designed for HiFi audio. Don't worry about audio response above 5KHz.

Crystal mics and carbon mics are another topic altogether. If there is interest, I will try to cover them here.

If you need more information or don't understand something, feel free to email me at the address given in the registration information for this domain.

A PC- or phone-style headset will use an electret mic and have one or more low-impedance earphones. Typically, a bias voltage is expected and provided by the device for the electret microphone, and an impedance or load on the MIC line of about 18 kohms. Earphones may be 8 ohms to 64 ohms, typically 16 - 32 ohms. Higher impedances are common in newer equipment at either or both ends of the earphone circuit.

A General Aviation headset will typically use an electret microphone and one or more high-impedance earphones. This electret microphone will also expect a bias voltage to be provided by the radio, but the microphone impedance will be lower, approximately 50 ohms. The radio microphone input impedance may be 50 ohms to 300 ohms. The earphones will be 150 to 600 ohms, often 300 ohm earphones are used and the two earphones wired in parallel to halve the impedance to 150 ohms.

A Military Aviation headset will typically use a low-impedance dynamic microphone and one or more low-impedance earphones. This type of microphone does NOT require a DC bias voltage, nor should one be provided for the microphone, nor is a DC bias voltage expected. These microphones typically have low impedances; five ohms is not unheard of and rather common. Impedances may be up to 50 ohms or so. The earphones are much more conventional, 4 ohms to 16 ohms, wired appropriately to arrive at about 8 ohms.

In summary:

PC style headset:

Mic Load: ~18 Kohms

Mic Impedance: ~10 to 40+ Kohms

Earphones, stereo: 8 to 32 ohms each

Tip: Mic

Ring: Mic Bias Voltage

Sleeve: Mic return

GA headset:

Mic Load: 50 to 600 ohms

Mic Impedance: 50 ohms

Earphones, mono: 150 - 600 ohms, typically wired to 150 - 300 ohms

Military headset:

Mic Load: ~5 - 50 ohms

Mic Impedance: ~ 5 - 8 ohms

Earphones, mono: 4 - 32 ohms, typically wired to 8 ohms.

The military headset can be wired directly to a 3.5 mm plug and connected to a computer. The microphone side will require a transformer to convert the impedance, and a blocking capacitor to keep DC from going through the transformer coil and magnetizing it.

The General Aviation headset can be interface with by using common audio interface methods associated with telephony. Most telephone interface equipment expects a nominal impedance of 600 ohms. Older audio equipment may use this impedance as well for "line" connections. Most "line" inputs today are 10 Kohm or higher.

DC blocking capacitors are a must (0.068 uF, 0.1 uF and/or 1uF) as well as RF bypass capacitors. Motorola has used 1500uF capacitors on the output of bridged audio amplifiers. Values such as 10uF may cause filter effects to the audio. Remember that audio distortion is comprised of even and/or odd harmonics of the fundamental. Therefore, filtering above 5KHz will not cause problems for your audio in either direction. The RC formula for resonance (Xc) will give the resulting curve.

There are a lot of transformers to choose from on the surplus market and from electronics houses. A transformer that goes from 1 Kohms to 8 ohms is the same as a transformer that goes from 625 ohms to 5 ohms. Similarly, some 600-ohm transformers, repeat coils, and older microphone / audio transformers can be wired to go from 50, 150, 250, 300, 500, 600, or 1200 ohms.

The important thing to keep in mind is the turns ratio. A transformer with a 1 Kohm primary and an 8 ohm secondary is also equivalent to a 625 ohm primary and a 5 ohm secondary (125:1). Likewise, a center-tapped 600-ohm transformer primary can be connected as two 300-ohm windings in series, however the phasing of those windings determines how they can be used. Two independent windings that make up 600 ohms can be placed in parallel with each other, giving 150 ohms. Don't worry about power handling capability since most headphones and ears can't take more than about a half-watt of power without taking damage. Two of these transformers, coupled one after the other from low impedance to high impedance to low impedance to high-impedance should transform from 5 ohms to 312.5 ohms to about 39062.5 ohms, which should be enough to interface with any modern gear.

There are small audio transformers that can be obtained from the parts houses. I recommend checking with Mouser, Jameco, All Electronics, Surplus Sales of Nebraska, Fair Radio Sales, Electronics Goldmine, Electronic Surplus, Mendelson's, and any other outlets you may be able to locate for surplus equipment.

Don't go audiophile to the nth degree on this one. Audio is rather forgiving, and transformers and capacitors are the only way out. A low-impedance dynamic microphone behaves just like the element in a ribbon microphone (which is often in the milliohms). A transformer will work best here. These headsets were only designed to do 300 - 4000 Hz for voice. Some were designed for loud environments, and function well there. They were not designed for HiFi audio. Don't worry about audio response above 5KHz.

Crystal mics and carbon mics are another topic altogether. If there is interest, I will try to cover them here.

If you need more information or don't understand something, feel free to email me at the address given in the registration information for this domain.