Basic Mixer

[blort1961]

I need to mix 2 headphone audio outputs together to 1 headphone...but I need to lose as little fidelity as possible.
I need to mix audio that has hypnotic cues that are mixed into it with another source, and I am concerned with losing any fidelity that may effect these subliminal cues.
I dont need any level controls, as I can control the output of the 2 sources...so its just a simple, high-quality mix.

Yah Yah Yah...I know...hypnosis is stupid...so lets keep those comments down to a dull roar, and focus on the question at hand

 

[AnalogKid]

Are the two audio sources stereo (four total audio signals) or mono (two total audio signals)?

Two stereo output sources (four total audio signals) into one stereo destination: left1 + left2 = left-out, right1 + right2 = right-out ?

IF yes
AND IF you have extra volume available
THEN you can do this with four resistors, no active circuits of any kind.

What are the audio sources? iPhone, MP3 player, 8-track tape, etc.?
What is the headphone? $1.99 ear buds, $1000 Grado, 1970's Koss, etc.?

ak

 

[blort1961]

The sources are a both Apple MP3 audio players, and just a standard set of sony earbuds...sometimes a standard pair of Sony over the head headphones
All audio sources are stereo

 

[audioguru]

I agree that resistors can simply do the mixing with no loss of fidelity but each signal level will be cut in half (-6dB) that is not much loss.
Use 1k resistors in series from each MP3 player and join them at the left mix-out and the right mix-out. For a headphones output the mix outs must feed a headphones amplifier.

 

[blort1961]

-Audio, did you get my email?
-So just a standard LM386 Amp??
-Is each resistor in series with each l&R output or or just 1 resistor on each Right output? In other words 2 resistors for each feed, or 1 resistor on each feed?
The illustration is backwards , as it was meant to multiply 1 input to several outs, Im looking for 2 ins and 1 out, but Im looking for the other way around. It basically shows my question of 2 resistors for each leg of each feed, except using 1k as you suggested

 

[blort1961]

As well, if I wanted to keep the 2 channel stereo could I add a basic stereo amp such as a TDA 2822?

 

[audioguru]

The LM386 and TDA2822 produce some hiss and distortion therefore are not hfi. The little TDA2822M is obsolete and the TDA2822 is no longer made in a DIL through holes package.
There was a website forum devoted to hifi headphones amplifiers that use audio opamps not cheap little power amps. The site was called Headwize with many excellent headphones amplifier circuits but the website is closed.
Google has some good and some bad circuits. Here is a very good old one that can use more modern opamps today: https://tangentsoft.net/audio/cmoy/

 

[blort1961]

Audioguru...shows just ho little I know about amplifiers.
The schematic that you point to is really nice and easy. So if I required a stereo version, do I just use 2 circuits, common ground, with a dual audio taper pot?
Of course with the 1K resistors input that you suggested??

 

[audioguru]

The OPA2132PA dual opamp costs a few bucks but is worth it.

 

[gophert]

The OPA2132PA dual opamp costs a few bucks but is worth it.

AG,

If you look at that CMoy headphone amp, the design is asking that poor little op amp to directly drive a pair of headphones speaker (usually 32-64 ohm). Is that simply the best schematic you found with a quick google search, or, do you think it is a decent circuit?

 

[MikeMl]

The earbuds usually have an impedance of ~35Ω per ear, while the Sony headset is likely 8Ω per ear. It will be difficult to cover this with just a passive network of resistors...

 

[audioguru]

It is too bad that the old Headwise website is gone since it specialized in headphones and their amplifiers. The article I posted with the Cmoy headphones amplifier says, "It’s powerful enough to drive very inefficient headphones to thunderous volumes from even weak sources, and it sounds excellent". The old opamp used has a max output current of 40mA.

The passive set of resistors are at the input of this amplifier, not the output. The optional series output resistor allows the opamp to produce more voltage swing while driving the low impedance headphones.

 

[blort1961]

what do y'all think of this amp, uses a NE5532 P:
**broken link removed**

 

[gophert]

what do y'all think of this amp, uses a NE5532 P:
**broken link removed**

I might buy it just for the case. Very nice design. The only issue is that you need external power (a battery does not fit inside). The good news, you can use a 9V battery with a barrel connector attached, or, a 9 to 15V wall adapter.

 

[blort1961]

It is a nice case...they do have a external power jack, which is handy...Im not certain the power consumption, but I will use a wall-wart, and maybe throw in some more electros on the power jack, just in case

 

[audioguru]

The NE5532 is a dual opamp so only one is needed for a stereo headphones amplifier. But why is the Chinese kit using two of them? I wonder if they are real NE5532 ICs or cheap Chinese copies of them?