Simple Headphone Volume Control

DIY STUFF
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Hey everybody! I’ve been trying to figure out how to make a simple volume control circuit that takes a synth signal and outputs it to headphones. So far it’s been a lot more confusing than expected. I was hoping this inverting op-amp configuration would attenuate the input signal and that’d be it, but after looking at the output things seemed off. For one the output seems to be clipping for some reason.

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Any clues on what might be causing this?
Alternatively would a mixer accomplish the same thing or does that only work for speakers?? :eyes:

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4 ohms on the output of a TL072 ?
That’s way beyond it’s capabilities…

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Here’s the headphone output from the Befaco OutV3:
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They use an NE5532 which apparently can source more current than TL07x.

From the Erica Synths DIY Output:

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Uses a headphone driver chip, LM4808, which however is available only in SMD packages.

Barton Stereo Outs:

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This does use a TL0x. I have this board but haven’t built it yet so can’t comment on how it works.

YuSynth:

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TL072 and then BC327, BC337 transistors.

MusicFromOuterSpace:
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LM386 audio amplifier.

Seems there are as many approaches as there are designers…

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One problem you have is the 4 Ω load is too small. Headphones generally aren’t below 8 Ω and can be as high as several hundred Ω.

I simulated the Barton Stereo Outs output stage:

and it looks okay with no load, but with a 4Ω load it… fails:

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But with 47 Ω, which is what my headphones are, the distortion is gone:

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I still have some doubts about this circuit but I’ll breadboard it before soldering, if all else fails I can bypass it in favor of a real headphone amplifier.

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Thanks for all the suggestions on different designs. I’ll pick them apart and go with one of those.
I’m surprised that the 47 ohm load isn’t having similar distortion as the 4 ohm. That doesn’t seem like that big of a difference in resistance.

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It’s a factor of 12 or so larger.

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Just a little note on the Befaco OutV3. It looks like the design works but the NE5532 draws a good bit of current (about 800mA).

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Have you built the circuit? (the outputs have 75 ohm in series, so making one opamp use 200 mA takes some effort…)

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Where do you have that number from ?
Officially it’s : 24 mA on +12V, 26 mA on -12V.
So maybe up to 50mA on each rail is possible, but 800mA ?
That’s over 9W (or 18W if that’s 800mA on each rail), I doubt an 8pin DIP can dissipate that much.

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Whoops it looks like I made some mistakes when I made it the first time. After simulating it again it looks a lot more reasonable (peaking at about 70 mA per rail).

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The only thing that still sticks out to me is that there’s all that current drawn from the positive rail, but hardly any drawn from the negative rail.

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Thanks for the schematics! I’m thinking about building a headphone out based on the Befaco version. I have to admit, impedance has never been very clear for me (but building a modular helps a lot understanding electronics).

I see that there are 75 ohm resistors for the output. Do I have to change this value for 47-50 ohm headphones? Or is it all good as it is?

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Should be fine with the 75 ohm resistors. I tried it with 32 ohm headphones and didn’t have any issues.

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Went ahead and tried this one out. Seems to work good for my ears! For a thru hole solution this is probably what I’d use. I will say it gets very loud could probably blow my headphones if I kept turning it up. Maybe increase the 75ohm resistors a bit?

For my project I’d prefer to use smd this time however so I’m going to order some LM4808 or whatever it is for smd

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Yea I forgot to mention it’s definitely able to go louder than you’d want it. I used a dB meter app and it maxed out around 100 dB(C).

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Any clues as to why they’d use 75 ohm resistors here? It seems oddly specific.

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I’d guess they want to limit the output current for low impedance headphones, without limiting it too much for high impedance headphones. Or they cargo culted it from someone else’s design.

(headphone impedance tends be in the 8−600 ohm range, and 75 is just above the geometric mean of those two numbers, which is nice I guess – having the same impedance on both sides maximizes power transfer.)

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I build one of the Befaco Outputs for Kosmo. Using DT880 phones (250Ohm) the perceived volume is “accurate” at its maximum level, though I’d wish it would be “a bit” louder.
Edit: The circuit was quite simple to build, that’s why I picked it.

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I finally built this and it’s working perfectly. Thanks!

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For the volume control, it looks like most designs are using a dual gang pot. Are these linear or logarithmic?