DIY Stereo (Headphone) Eurorack/Buchla output module - TL074 op amp

This is a quick and dirty guide to building a headphone output.

I have a odd  combination euro / buchla performance case which needs a bespoke headphone output.

 

The first question is what is a  safe headphone range?

There is no one answer.

As far as I can tell there isn't a standard for a headphone out.

And the type of headphone you use is important.

Most headphones have an impedance around 16-32 Ohms with some crazy "audiophile" headphones having higher impedance (e.g. 300 Ohms). 

DJ headphones generally range from 25 to 70 ohms.

 

 

 The ancient 0dBU commercial line out specification is that the voltage needs to be able to drive 1 milliwatt to a 600 ohm load. For a sine wave, this means a voltage of 0.77 volts RMS (2.2 volts peak-to-peak).

 

The "line level" out on something like a CD player, for example, typically has an output impedance of 500-1000 ohms, whereas a headphone out will usually have a much lower impedance to be able to drive headphones.

 

Eurorack audio signals are hot. ... generally 5V peak to peak. Eurorack treats audio & CV the same.

You shouldn't plug heahphones directly into any jacks.

Thus a Eurorack headphone output should reduce or attenuate the audio from 5V to about 1V p to p.

Buchla on the other hand treat audio & CV differently.

Audio is always at line level (1V p-p) so you can safely plug headphones into a audio output.

So a passive audio mixer is usually ok (though not great) for this .

 

Sometimes you may also wish to boost the audio slightly if the venue you are playing in

has lots of external background noise.

 

There are plenty of great headphone modules out there in euroland if you're not into DIY

 but if you have some time and enjoy learning electronics, they are easy to build.

 

This post uses op-amps for this.

Mainly the TL 074. Though you could try swapping the op-amps for something better

if you socket the IC first.

My mate Justin has suggested the OPA2134 & OPA 4134 as alternatives.

 

 

I decided to try to build one in a 1U format.

I chose the TL074 (quad op amp) mainly as they are cheap and I have a stack of them in my store.

Some pics of the module

Keep in mind this is still experimental / prototype stuff

These are the rough schematics to build a simple 1 channel  stereo mixer.

It's a basic unity gain inverting configuration. In the first half the left & right inputs enter the inverting inputs of the first op-amps. Since the output signal from this op-amp is inverted, the 2nd op-amp  re-inverts it. 

Thanks to Ken Stone for the original circuit.

Adapting it to reduce the final output will require some tweaking of resistor values.

 

The two resistors at the left (labelled 1 & 2) of the schemo are important.

They determine the level of amplification.

 

To reduce 5V to 1V 

try a 100K resistor for 1 

and 20 K resistor for 2.

 

I also tried simply changing the value of resistor 1 only.

If you fix resistor 2 at 100k, change resistor 1 to about 500k ohms.

This will reduce the output by 20%

Just experiment till you get what you want.

(I like an output a bit louder than 1v pp)