Headphone amplifier

I found an old LM386 in my junk drawer I wanted to build a eurorack headphone amplifier with it. I used a TL072 buffer as preamp.
According to the data sheet (http://www.ti.com/lit/ds/symlink/lm386.pdf) it supports 5-18V supply voltage. I used the 12V from my modular but the output power is massive. I had to attenuate the signal at input with at least 100k:10k divider at the input for a 5Vp-p signal.
What are the typical eurorack levels?
Should I to for even higher resistors/potentiometer?
How do you usually handle this?
Should I use a linear voltage regulator (like 10V) or are the supply noise levels generally acceptable?

Typical eurorack levels are 10Vpp.

The LM386 has a power supply rejection ratio of 50dB, it’s not stellar but it should keep the PSU noise away. i wouldn’t bother adding an extra regulator unless some part of the circuit has a PSRR of 0dB :slight_smile:

Showing my ignorance: do you even need a power amp stage to produce headphone output? I can see why you’d want to buffer it but do you need voltage gain?

Power amp != voltage gain.
Power amp means it can supply a lot of power - which means the right combination of high current and voltage, depending on the headphone impedance.
IIRC, the voltage levels for headphones are usually in the range of 1Vpp, far below the eurorack levels.

I’ve build some simple headphone amps for monitoring purposes by using two simple opamp buffers in parallel per channel. Slap a potentiometer at the input as a volume control and you’re good to go. I can’t hear any degradation in sound quality and the power levels are far higher than I would need.

Take a look at this if you want more power than two opamps in parallel can provide.

You can’t connect headphones directly to a module output.

On most modules, the output signal comes from an op-amp, with a 1k series resistor which acts as a protection in case of mispatching.

If your headphone impedance is 25 ohm (conservative figure), you’ll get 25 / (25 + 1000) = 0.02x your output voltage on the headphones. Ouch! This doesn’t happen when the signal goes to a regular module, since the standard input impedance of a module is 100k.

And even if the module doesn’t have the protection resistor on its output, there’s no guarantee the op-amp it uses will be able to deliver so much current through a low impedance.

So the point of this Eurorack headphone amps is not so much voltage gain, but having something that can deliver enough current to drive a low impedance destination.

Thanks for that (despite this not being my thread!)

In respect of the OP, doesn’t the TL072 buffer stage that is already there do the job?

As I said, you want to be able to drive a load with some power. A single opamp channel won’t be enough for reasons of current capability and output impedance, you need at least two in parallel.
I would agree that for an LM386 no input buffer would be necessary.

Sinne the power amp have a feedback loop I want to have a buffer stage first. Otherwise the output impedance of the thing at the input will affect the amp. It’s much more reliable with a well defined high impedance input.

ah, okay! I stand corrected