Hello guys. This is my first post, but I am a stalking the forum for quite some time. So, I have a Denon PMA-757, and I am having some problems with using my phone (LG V20) as the source. No matter where I connect it (DAD, PHONO, VIDEO/AUX) with a simple RCA to aux cable, I get severe distortion and almost no volume. Same happens with the speaker outputs. However, my dad initially used a Luxman T111L Tuner as the source, and it works like a charm, both with headphones and speakers. What could be the cause for the phone combo not working?
P.S. I hope I am posting this at the correct section of the forum
P.S. I hope I am posting this at the correct section of the forum
Welcome to diyAudio 
Your phone should connect to one of the 'high' or line level inputs such as Video/Aux or CD. Not phono which is for low level pickup cartridges.
Two possible reasons for the distortion:
1/ You are simply applying to much signal and overloading the input stages.
2/ Its possible the phone output is using a switching Class D output stage that relies on having a low impedance load (typically headphones) to reduce the noise and hash that can sometimes be produced.
The first thing to do is to prove the amp is OK on a chosen input using something like your tuner or a CD player. Then using the same input connect the phone and try altering the volume of the phone to see if that helps or not.
If that's no good then I would suggest you try adding a low value resistor across each output (L and R) from the phone. That easy to do with a suitable lead. Try something really low like 33 ohms.
Your phone should connect to one of the 'high' or line level inputs such as Video/Aux or CD. Not phono which is for low level pickup cartridges.
Two possible reasons for the distortion:
1/ You are simply applying to much signal and overloading the input stages.
2/ Its possible the phone output is using a switching Class D output stage that relies on having a low impedance load (typically headphones) to reduce the noise and hash that can sometimes be produced.
The first thing to do is to prove the amp is OK on a chosen input using something like your tuner or a CD player. Then using the same input connect the phone and try altering the volume of the phone to see if that helps or not.
If that's no good then I would suggest you try adding a low value resistor across each output (L and R) from the phone. That easy to do with a suitable lead. Try something really low like 33 ohms.
First of all, thanks for moving the thread to the correct section. I can confirm that the amplifier works properly with the Luxman tuner (tested few minutes ago) on the dad input. Connecting the phone to the same input results to the pre-mentioned problem. Moving the phone away from the amplifier didn't work. I have tried with an HTC M7, and had the same results. The V20 has the so-called quad dac circuit, which has an amp too. Switching it off did nothing too. So I guess one of the following days, when I find some time, I will make a modified cable, with the 33ohm resistors, and see what happens.
Ok, so, I built the cable with 2 33ohm resistors. This has improved things. Connecting the phone to Tuner input, I can finally listen to music. There is a problem though. The amplification the amp can provide is considerably less. I have to put the phone to 50 out of 75 volume steps, and have the amp turned up to about half the way. Also, now there is a lot of hissing, the kind of one you get when you connect sensitive earphones to an amp, and turn all the volume up, with no music playing (normally the cheaper ones). I can notice the hissing even with music playing. (Is this the noise flour?) Connecting the Luxman tuner though to the tuner input, and I get none of that, just pure sound, no hissing or anything, and a lot of volume headroom.
So it sounds like the resistor theory has proved what the issue is...
It may be that you can't get this 100% perfect because of the nature of the phone's output stage.
Things you can try though would be to make sure you turn the volume up on the phone, turn it up as loud as possible up to the point distortion sets in. That will maximise the available headroom and minimise noise vs signal level.
You could also try experimenting with the resistor values, both higher and lower. 33 ohms was a good compromise that approximates to a typical headphone impedance.
It may be that you can't get this 100% perfect because of the nature of the phone's output stage.
Things you can try though would be to make sure you turn the volume up on the phone, turn it up as loud as possible up to the point distortion sets in. That will maximise the available headroom and minimise noise vs signal level.
You could also try experimenting with the resistor values, both higher and lower. 33 ohms was a good compromise that approximates to a typical headphone impedance.
Hmm, ok, I will give it a try. I found some 18 ohm resistors, and I must have some at around 100, so I'll give those a try too. Thanks for the help
You can use resistors in any combination to bring the level of the phone down to that of a traditional source. That includes using a resistor across the output as we are doing now, and also using a resistor to load the stage optimally and then placing a divider across that resistor. So 3 resistors in total per channel.
Beyond that and its purely down to the phone and what comes out of the socket.
Beyond that and its purely down to the phone and what comes out of the socket.
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