On 8/11/2010 7:02 PM, Jim Brown K9YC wrote:
> On Wed, 11 Aug 2010 18:42:08 -0500, Stuart Rohre wrote:
>
>> You may
>> need to transform the audio to a higher voltage to improve your reception
>
> Almost everything in this email is wrong.
>
> See my tutorial on headphones in http://audiosystemsgroup.com/RFI-Ham.pdf
>
> It's in an Appendix.
>
> Most modern radios are designed to drive almost anything from 8 ohms up,
> and are optimized for about 8 ohms to about 200 ohms. No matching required,
> no matter how hight or how low the Z. With modern phones, the higher the
> impedance, the less loudness you're likely to get from a modern output
> stage.
And so the voltage swing is limited to keep from overdriving the low
impedance heaphones. Sometimes a voltage step up or impedance match will
get more signal to higher impedance headphones and that is most
conveniently accomplished with a transformer, such as an 8 ohm to 600
ohm transformer. Such transformers ARE available from Mouser and
probably digi-key among others.
73, Jerry, K0CQ
>
> On the other hand, the very old crystal headphones are very high impedance,
> and also very sensitive -- 40 years ago I used them to do field service
> troubleshooting on audio systems, and I could even hear (barely) mic level
> with them! Fidelity was not great, comfort was even worse.
>
> 73, Jim Brown K9YC
>
>
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