audio attenuator headphone to line

My recollection is that "line level" is related to the level that would drive a telephone earpiece, and be generated by a telephone's carbon microphone. In other words, line level is not greatly different from headphone level.

Just as anyone would adjust a volume control to set their personal listening level, you would need to adjust the amplifier output for the desired line level. The only network needed might be for impedance matching to the line. If your headset amplifier has a very low output impedance, a series resistor equal to the line impedance may be all that is needed.

What kind of "line" are you working with? Some are balanced, but the circuit you refer to is for an un-balanced line. For a balanced line you may need an audio transformer.

Fred

the line input is a 1/4" TRS socket on an m-audio microtrack II.

Strangely enough I haven't been able to find any proper specs on the microtrack but I know it commonly takes microphones and such. Headphone output is too 'heavy' for a decent recording into this input so I need to attenuate it. I can't use any boxes or such because the headphone device and the microtrack will be used on the move, often in the open, on foot, running on batteries. Thus I have to integrate any attenuation into the cable. Thanks for any further advice.

The resistor values you suggested give an attenuation of 10 : 1 which is probably rather too much. OTOH, I'm surprised that you find a headphone output too 'heavy' to feed a standard line level input. The output from the headphone socket of an iPod, for instance, feeds a standard line level input on an amplifier, without issue. I do it every day with a variety of different amplifiers, as I use my iPod as a source of test music for amplifiers after repair. Even if the output from the headphone socket does seem a little OTT to drive the recorder's line input, you can reduce it by turning down the volume control on the source device.

If, however, you still want to externally attenuate the headphone signal, prior to driving it into the recorder, then a ratio of 2 : 1 might be more appropriate, so just use a couple of equal value resistors . You're never going to match the impedances properly, as the output of the source device's headphone socket, is likely to be somewhere between a few ohms, and 32 ohms. The line input on the recorder will likely be around 47k ohms. Just use two

1k resistors. If that doesn't give enough attenuation, double up on the 'top' resistor to say 2k2.

Only thing that might make all of that wrong, is if the input that you are trying to use on the recorder, is not a 'line level' input. I say that because you talk about the device "commonly taking microphones". A microphone input is probably anything up to 100 times more sensitive than a line level input. If this is the case, then you might need an attenuator of

50 : 1 or more. In which case, try 47k for the top resistor, and 1k for the bottom one.

Arfa

On 7/10/2010 9:06 AM tg spake thus:

According to that propaganda sheet, the Micro Track II will accept input from "studio monitors (and/or headphones)". So can't you just plug it directly into the headphone output?

(Not sure what they mean by "and/or" here. I've always held that this usage ("and/or") is needlessly confusing and should never be used, as "or" as it's commonly understood is not an exclusive or, so it means one, the other, or both.)

--
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TG-

At the bottom of Key Features, a note reads: "For information on using dynamic mics with the MicroTrack II recorder, please visit the MicroTrack II product page on m-audio.com." This type of microphone apparently uses one of your 1/4" TRS inputs.

I went to the web site and found the recorder's User Guide. Under Recording, it mentions that you should "2. Turn down the input gain of MicroTrack using the [LEVELS] buttons on the front panel." I think this may be enough to solve your problem of the headphone output being too 'heavy'.

Fred

Replace that circuit temporarily with a 10K pot. Adjust the level to what you want. Measure both sides of the pot relative to the slider and make up a fixed attenuator to the same percentage values - the actual total load across the headphone output isn't critical. A total of 1-50K should be ok.

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thanks for all the replies here. I tried a few resistor combinations (from arfa's response) and what worked best in the end was a 22k resistor on the positive and a 47k resistor linking the positive and the negative. I made a cable with this setup and was able to get nice clean recordings from the headphone socket at normal headphone volume.

I'm glad what I said helped you to get a result that you were pleased with, but I'm a little confused. Are you sure that the resistor values that you've stated are correct, and the bottom one wasn't 4.7k rather than 47k ? As stated, the attenuation provided would be very small - in example numbers,

1v in, around 0.7v out. With the bottom resistor at 4.7k, the attenuation would be something over 4 : 1, which seems more realistic ?

Arfa

nope, just checked and the bottom resistor is 47K. in your previous post you did say 'a ratio of 2 : 1 might be more appropriate,' and you were right, this - as it turned out - is a 2:1 ratio.

Not quite. As stated that your series resistor is the lower value of 22k, and the shunt one is the higher value of 47k, this is a 1 : 2 ratio ...

Still, if it does the job that you were looking for, not worth getting bogged down in maths and definitions :-)

Arfa