[1000mp] Recording from headphone jack

[Bud Hippisley, K2KIR]

At 08:55 AM 2003-03-04 , N4ZR wrote:

>My only problem is that when I'm using headphones at a comfortable level, the audio level is a little low going into the sound card....Are there considerations of impedance matching and/or harm to the audio output amplifier that I need to consider in doing this?  Say, for example, that I wish to reduce the audio voltage to the headphones by a factor of 2 -- would I be safe just using 100 ohms in each leg of the voltage divider?  Or should I use some other value?

  For years, starting back when I had tube gear that often had a lot of fixed amplitude AC hum coming out the headphones jack, I used an accessory box that padded down and either switched between or combined ("multed") the headphone outputs from my two receivers.  I used a 100-ohm power resistor in series with each audio "hot" lead to the new headphone output jack on the accessory box, where I had a 10-ohm shunt resistor to ground.  I chose 10 ohms for the shunt to make the equivalent source impedance look like about 8 ohms to the headphones, which were Koss low-Z stereo phones. 

My design approach was to decide how many dB down I wanted to knock the hum, then calculate the series and shunt resistor values to give me: a.) that number of dB; and b.) a reasonable source impedance for the specific headphones I was using.  You could do the same thing by deciding how many dB lower than your SB16 input you want your headphone levels to be.  

Damage to the audio output stages in today's equipment is not likely to be a problem because transistor audio output stages are usually emitter-followers that don't care if they see an open circuit.  (Capacitance might be another matter, however.)

As an example, a stock TS-940 has a 100-ohm resistor in series with each (L & R) hot lead at its stereo headphone jack.  Kenwood obviously is not really worried whether you supply any resistance to ground beyond there or not.  Note that if you use standard low-Z stereo headphones, those 100-ohm resistors mean they've already built in part of the "hum filter" pad that I had to build externally for my old tube gear.

Bud, K2KIR