Electronics > Projects, Designs, and Technical Stuff
Active audio mixer design & Op-Amp considerations
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supershirobon:

--- Quote from: Hero999 on November 28, 2018, 11:05:20 am ---
I would have thought the headphone drivers would have a lower impedance than 120R: where did you get that figure from?


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IEC set a recommended headphone amplifier output impedance of 120 a while ago. Apparently these days most amplifiers have an output impedance less than 50 or so, but for the worst case I figured 120 is the highest, but probably still used somewhere, impedance.
supershirobon:

--- Quote from: Bassman59 on November 28, 2018, 04:21:48 pm ---
--- Quote from: supershirobon on November 27, 2018, 07:46:37 pm ---If it is a problem, what are recommendations for an op-amp that works well for audio purposes with a single supply at 5V and can get pretty close to the rails (at least 0.5V or so from the rails)? Preferably one from Analog Devices/Linear, but not necessary.

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I know you asked for an ADI/Linear part, but I used the TI OPA2365 in a bus-powered USB audio design and it worked well. TI has a simulation models that works with LTSpice, if that's your concern.

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That's great actually, thanks! I'll definitely look into it.
supershirobon:

--- Quote from: David Hess on November 28, 2018, 02:47:10 pm ---The increased isolation from using an active mixer is usually irrelevant.  What might matter is the lower input impedance of a passive mixer but this is not a problem with low impedance outputs intended to drive headphones or a speaker.  A pair of resistors for each channel between 120 and 470 ohms will work fine.

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Thanks for the reply. Also based off other people's responses I'll probably go with a passive design instead then if it doesn't make much of a difference.
David Hess:

--- Quote from: supershirobon on November 28, 2018, 08:04:01 pm ---IEC set a recommended headphone amplifier output impedance of 120 a while ago. Apparently these days most amplifiers have an output impedance less than 50 or so, but for the worst case I figured 120 is the highest, but probably still used somewhere, impedance.
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Modern headphone amplifier designs tend to be from zero to 10 ohms maximum.  The only reason an output series resistance is included is to prevent oscillation with difficult loads like cable which can have 10s of picofarads per foot of capacitance.
Audioguru:

--- Quote from: supershirobon on November 28, 2018, 01:19:31 am ---Wikipedia says the Max Vpp for consumer level Line Level audio is 0.894 volts.
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No, they say 0.894V p-p is the nominal consumer line level. The nominal level is a timed average with loud peaks frequently exceeding the nominal level. The peak levels are determined by the crest factor that they do not say. I think a crest factor of 12dB (4 times) is common but it could be more. 4 x 0.894V= 3.58V p-p which is very close to the 5V power supply you are using if the opamp is not rail-to-rail. Clipping sounds baaaad! (But sounds good for acid rock).   
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