Smartphone to hifi amp : do I need anything?

[snk]

Hello,
I would like to be able to plug my smartphone on my venerable hifi amp (either on the AUX in or TAPE in).
I have tried using a simple minijack to RCA adapter cable (and it works), but I guess the sound might be improved if I could match the phone out impedance/level to what the amp is expecting at the AUX/tape input?
Is a simple cable enough, or should I try to build some kind of buffer/ impedance matcher?

[GibsonGM]

I wouldn't hesitate to try a buffer, and just use 'aux'.   Does it sound dull and noisy when you do get it to work?

[snk]

It doesn't sound noisy, but if i put the phone onto the amp, I hear some digital interference. That kind of noise goes away if i put the phone 60 or 80cm away from the amp.
I don't really know if it sounds dull or not : In fact, it sounds great with rock or library music, but when i'm playing bass heavy electronic music, the bass is a bit dull. But right now I don't have very good speakers connected to it, so i'm unsure if it is an impedance issue or just the speakers which can not handle the bass range well enough.

[GibsonGM]

Could be a few different things, sure.  It should be easy enough to see what it sounds like w/a buffer, and if you think it needs more gain, you could just reconfigure the opamp to do that.    Not sure how you'll get rid of the interference other than keeping the phone away a little (?)  I mean, you could put it in an enclosure, LOL...

[Rob Strand]

You don't know what circuit is driving the outputs.   What you might even find is the headphone output is D-class.   

You might need a dummy load to clean-up the outputs and set the correct damping for any output filters.

So maybe try (on each output):

Phone ----- 47ohm to 100ohm to ground ---- series 1k ----- 1n to 2n2 to ground --->   Line In

Experiment with the dummy load value upto say 470R, maybe down to 22 ohm.   Higher values will draw less current (from the phone battery - and probably not a big factor on a phone) but will probably be less successful.

Beyond that you could feed *all* the wires going to line-in through a large ferrite bead/tube,  wrap through a couple of times if you want to try something.

If you find you are setting the phone output level to a very low level then that means to the junk from the phone will be high relative to the signal going out.   So another thing to try is to replace the 1k series resistor with a 4.7k series resistor followed by a 1k to ground.  That will reduce the level to the amp and let you dial-up the phone output.

[snk]

Thank you both

Just to make it clear : I don't have any issue per se... I just want to know which are the good/best practices when it comes to plugging a smartphone into a vintage hifi amp (as obviously such devices didn't exist when the amp was designed).
The interference noise is not an issue, as I can just put the phone slightly away and the noise vanish.
Later this week I will plug bigger speakers to check the bass range (now I only have nearfield monitoring).
My phone is a quite cheap Wiko with nothing fancy, by the way:)

[Rob Strand]

I added this the same time as you posted:

If you find you are setting the phone output level to a very low level then that means to the junk from the phone will be high relative to the signal going out.   So another thing to try is to replace the 1k series resistor with a 4.7k series resistor followed by a 1k to ground.  That will reduce the level to the amp and let you dial-up the phone output.

[snk]

Thank you, Rob.
I am setting the phone output level nearly to the maximum

[Rob Strand]

I am setting the phone output level nearly to the maximum

Wow, I'm very surprised.   Normally headphone output is high-ish.

[snk]

Not on this one : it's the same when i plug it on the aux input of my car player, as well as on another hi-fi amp i had : I need to crank the phone level, and turn the amp volume a bit to reach unity gain with other sources.

[davent]

No idea about this but is there a line out available on a smart phone so you're not going through the headphone amp? With ipods (but not the shuffle) you could connect to line out if you wanted to play through another headphone amp or hifi system.

[Rob Strand]

No idea about this but is there a line out available on a smart phone so you're not going through the headphone amp? With ipods (but not the shuffle) you could connect to line out if you wanted to play through another headphone amp or hifi system.

My understanding is the speakers with the phone docks send the audio over the USB connector in digital form. The only analog output is from the  3.5mm headphone jack.     (There may be exceptions on some model phones but I expect these days it's all digital.)

I think you should try the dummy load and the output filter between the phone and the amp input.

[anotherjim]

The old amp has a high input impedance. 20k is now more usual for phono line inputs. Probably more sensitive to noise in this case. Robs suggestion of a simple load and filter should be good.

Low audio output from consumer devices expected to work 'phones is a new norm - they are limited to save your hearing.
Some audio codecs can sense if line or phones by load resistance, but to use it needs OS/driver support and that rarely happens.
My laptop defaults to limiting and I can only get around it by installing the full audio driver and selecting to disable the headphone limiter in its control app. This is probably not even an option for some smartphones.

[Rob Strand]

Low audio output from consumer devices expected to work 'phones is a new norm

Interesting info.   (I guess that's going to screw-up people who use their phones as a function generator.)

[anotherjim]

Depending on the phone, Apple used to make line-out easy with a docking station but I don't know if it's been upgraded out these days. They probably expect you to teleport the audio over Bluetooth to some other device that can only receive. The real problem is DRM, they don't want to make it easy to record or broadcast streamed content.

[snk]

Hello,
Thank you all for the tips.

I built the small circuit (using 47R resistors and 1nF cap, as well as small ferrite beads) :
- When I put the phone just on top of the amp, it doesn't change much the amount of interferences.
- But I also noticed that I am using a very crappy 3.5mm jack to RCA adapter : so I think that getting a nicer cable would help.
- I also noticed that when the phone is 40 or 50cm away from the amp, there is no noticeable noise.
- Also, the interferences are intermittent, it doesn't occur all the time (I guess it is just when the phone is checking if i have any message or stuff like that, when it is idle, or just playing music, there is no noise).
- I have ordered bigger ferrite beads, but it will take several weeks to arrive. In the meantime, I will have buy a better cable

All in all, I really can live with that, as i don't have buzz or hum, and removing the phone from the amp top make the noise go away.

Still, when I will have the bigger ferrite beads, I'll build the circuit again, because such thing can be handy from time to time, and I want to know if it works and improve the signal.

[Rob Strand]

I built the small circuit (using 47R resistors and 1nF cap, as well as small ferrite beads) :

Did you have the 1k resistors?   You need those for the filter to work.

You can increase the 1n cap for more filtering perhaps 2n2 max.

- I also noticed that when the phone is 40 or 50cm away from the amp, there is no noticeable noise.

I think that result is a good indicator for the problem.  Place the phone close to the amp with the audio playing (as normal).

Putting the filter at the AMP end of cable might have more success.  Perhaps not by much.

Then try pulling the audio cable out of the amp.   Is there still noise?  that would mean two things:
Noise is RF based and is getting straight into the amp circuits or getting onto the audio inputs.
Next insert shorting jacks into the amps audio jack.   Does the noise disappear, reduce, or stay the same.

Beyond that, a bad cable won't help.   Also the ferrite tube/ring/bead do need to be those larger types and the whole cable should thread through the bead, once or twice.    The ferrite should be located at the phone end.

[anotherjim]

Some amps just aren't protected from the radiation from mobile phones. It can get in via speaker wires.

[Rob Strand]

If you set the phone to Flight Mode it will remove anything to do with the stronger intentional RF but it won't do anything to help EMC issues  - which is more likely the cause if it's RF related.

[snk]

By the way, this is the schematic I drawn (and then built) after Rob's explanations : could you confirm it is correct?