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Old 21st October 2010, 05:57 PM   #1
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Default output drive from PC headphone jack??

I am building an LM3886 amp to drive speakers, with my MAC/iTunes as the source. I will start off using the analog L/R headphone outputs to drive the input of the amp.

My question is ... what kind of load can the headphone jacks drive?
I am having a hard time finding such info in the MAC specs.

Eventually I will add a DAC. For now, I plan to use a volume control POT in the AMPs input stage. My plan was to use a stereo, 10K POT. Thus, the MAC would have to be able to drive the 10K load impedance. Is this an issue?


Rob
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Old 21st October 2010, 06:06 PM   #2
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I found this online .... still doesn't tell me the output drive current capacity ...

"Headphone sound output port

The iMac has a 3.5 mm sound output port for stereo sound output. The headphone sound output port is suitable for connecting a pair of headphones or amplified external speakers. When a plug is inserted into the headphone sound output port, the internal speaker and the Apple Speaker minijack are disconnected.

The sound output through the headphone sound output port has the following electrical characteristics:

output level 2.0 V peak-to-peak (0.7 V RMS), open circuit
source impedance 44 Ω
signal-to-noise (SNR) 90 dB unweighted (typical)
total harmonic distortion (THD) 0.03 percent or less "
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Old 23rd October 2010, 05:51 AM   #3
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Considering there are not many headphones around approaching 10k, think you will be fine using the headphone jack.

I would use a simple preamp with volume control myself. In fact, that is how I have my setup working right now. PC analog out to a passive pre/switchbox.

My Pot is 100k. It will be replaced with fixed resistor values soon enough.

I am curious if you have a line out or a way to set the headphone jack to line out mode. This is possible both ways on my machine. The card has a standard line out and I can additionally set the headphone jack to line out with the software.

If so, that would be more desirable. The headphone output is amplified so you could end up with little to no pot travel.
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Old 23rd October 2010, 02:52 PM   #4
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I may just keep the amp as a power amp, with no volume control. Then I can find a DAC with volume control.
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