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Old 21st February 2012, 04:18 PM   #1
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Exclamation My Headphone amp series resistor on output

I built myself an headphone amp about a year ago. It uses one half of a jrc4556 opamp per side(L/R), for the output. Its a single supply design with 1000uf output coupling caps
I recently changed the design by taking out the two 100ohm resistors in series with the outputs. I use a 6m headphone cable extension with it.
My question is, is it ok to run it without those resistors? I have never had any problem with oscillations since taking them out.
I want to keep the output z low!!!
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Old 21st February 2012, 09:08 PM   #2
agdr is offline agdr  United States
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No output resistors on the JRC4556/NJM4556 are OK.

The O2 headphone amplifier uses NJM4556 chips as the output with 1 ohm output resistors. Even at that the 1R resistors were needed for balance since the amp uses both halves of the chip in parallel per channel (two chips total). With just one half of the chip per channel, like yours, the amp probably would have had no output resistor at all. That amplifier has been measured on good equipment, with capacitive cable loads, and the published results are very good.

One of the keys is the chip apparently has current limiting. The data sheet does not say it explicitly, but the data sheet graphs show it. So the output resistor isn't needed to protect against TRS plug shorting. The data sheet doesn't give a load capacitance graph, but with 70mA per half capability it seems to be fairly stable.

An interesting read on the benefits of low output impedance for head amps:

The "0-Ohm" Headphone Amplifier: The Sonic Advantages of Low-Impedance Headphone Amplifiers | Benchmark Interaction
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Old 22nd February 2012, 05:31 AM   #3
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Thanx! Thats exactly why i want low z. Increased damping factor!!
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