A hum in your system could never be "generic" and is most likely local to your specific setup. A product (from Yamaha) would not 'feature' such a thing. Without hearing the hum a better description is needed. There is 60 cycle electrical hum (common ground hum) easily recognizable by its characteristic pitch (between the second lowest Bb and B). Some ground issue with USB connections can manifest themselves as a high pitched zinging noise.There is a 'hum' that can be produced when audio is routed incorrectly and is more of a buzz - this is produced when audio routed out is routed back into your system and a loop is created. This can be a full bodied insistent/aggressive buzzing that builds up and then just sustains.

I can not play MIDI via the CP4 and hear iPad audio through the CP4 Aux in jack at the seem time without this awful hum.

Also it is not clear what your application is exactly (what you are trying to do) with both audio into the iPad and running a music record/playback program. Not sure, are you describing everything connected or just some of the items connected... As ground hum is usually a result of a community of connections altogether.

I'm wondering how you have this assigned, (perhaps it is some kind of copy protection built in to prevent you recording commercially released music to an app)... I don't know exactly what or how you are routing this signal to pull this off. Please provide details. Are you using the iOS InterApp audio routing, or are you just assuming the iPad can multitask on its own?

Thank you Bad Mister. Let me be more specific and detailed. All I am trying to do is have the CP4 play a sound on the iPad via MIDI from the CP4 into the iPad (using any synth app but primarily Garage Band or Cubasis- it does not matter) and have the sound created in the iPad played out from the iPad into the Aux In jack on the CP4 and audio from the CP4 (which then includes both the CP4 instrument and iPad audio inserted through the Aux In jack) played out into my mixer as usual.

This Hum/Buzz issue only occurs when both of the following connections between the iPad and CP4 are made at the same time. There is no buzz/hum if only one of these connections is made and there is no and has never been a buzz/hum ever under any other circumstances with my set up (my set up is simple, CP4, MG10XU, Yamaha powered studio monitors):

1. USB cable from CP4 to iPad camera connection kit inserted into iPad. This connection is to transmit MIDI between CP4 and iPad.
2. 1/8" to 1/8" audio cable from iPad headphone jack into Aux In jack on CP4. This is the transmit audio from the iPad into the Aux In jack. I am not transferring or recording audio from the CP4 into the iPad.

When both of these cables are connected as described above at the same time and the CP4 is on, as soon as I turn on the iPad the buzz/hum starts instantly and continuous whether or not any specific iPad application is selected and when no application is selected (if instead of connecting the audio cable into the Aux In jack on the CP4 I connect a 1/8" audio cable to my mixer instead of the CP4 there is no buzz/hum).

If I simply want to play music from the iPad through the CP4 without any MIDI and jam along, there is no problem. But when I also connect the MIDI USB cable then there is this problem.

It seems to me that some kind of loop is created when both of these cables connect between the iPad and the CP4 at the same time. I have tried another 1/8” cable and another USB cable with the same results.

I would really appreciate it if you or someone at Yamaha or another forum member could make the same connections to see if they get the same results as I do. If they do not, then I will accept that there is some electrical issue unique to me. If they do, then there needs to be a fix.

Thank you again.

With reference to this above discussion, it seems to me that my earlier statement is correct that there is an inherent flaw which produces a loud buzz/hum when connecting both the camera connection kit/USB for MIDI from the CP4 to iPad and the iPad headphone jack to Aux In on the CP4 at the same time. As I mentioned, this occurs by just making these concurrent connections and turning on the iPad without any app selected. It never occurs in any other configuration of connections between iPad/mixer/CP4/powered studio monitors- I have tried them all.

Now I have found a way to greatly reduce the buzz/hum and I was wondering why my work-around is effective. If I engage the "Pad 26db" buttons on the mixer channels for both CP4 R/L lines coming into the mixer and then increase the volume of the mixer to compensate for the reduced volume the buzz/hum is 90% gone.

Just curious what the "Pad 26db" buttons really do and why this helps.

As always, thanks in advance.

The role of a Pad in audio is to protect the next stage (device) in the chain from receiving too much signal (gain). A 26dB Pad will reduce the incoming signal by 26dB, thus matching the input to the strength of the incoming signal.

The input stage must be able to receive a wide array of incoming levels, for example, weak signal from microphones to strong signal from mixers and keyboards. If you ever look on the back of your studio monitors you might find a Volume scale that looks like it works in reverse. You might turn the knob up to 12 o'clock where it reads +4dB and you continue to turn up until you reaches -10dB... Well this makes sense when you begin to look at it in terms of "gain staging". A signal coming in with +4 level will need you to turn the amplifier less that when a weak signal is plugged in.

You are matching input sensitivity.

Engaging the -26dB Pad into your signal path better prepares your receiving device for the "hot" signal you are attempting to plug in.

Signal-to-Noise is always represented as a ratio... Where if "noise" is given a value of 1 you want your "signal" way above that. Any and every circuit makes noise, you want the useable signal so far above the noise floor that it is negligible. Had you played signal into that input without gain staging the signal down 26dB you would have overloaded and distorted that device. You have matched the receiving device's input to the strength of the sound source.

The average person thinks the big control on an amplifier is Output (because, yes it does ultimately affect the output) but that big knob is actually an input sensitivity setting. "Pads" are devices that "knock down" the signal in one fell swoop (instead of on a knob) so it can safely (without distorting) be handled by the next stage of the audio chain.

Whenever you see a switch that selects between Mic and Line, you are basically inserting a "Pad". Microphones are extremely weak and need the most from the amplifier. Line level signals are powerful and require less work from the amplifier. Make sense? When you select "Line" as your input you are inserting a PAD.

Thank you Bad Mister for your great and understandable explanation and taking the time to reply. I got it. In all these years I never before understood why there was a Pad reduction or why +4 was "less" than -10. The beauty of your explanation is that you have taken a very difficult concept to understand and explained in a simple way, like a "Gain For Dummies" (that's a compliment in case you haven't seen these series of books). Thanks again!

Follow Up: Bought the UD-BT01 bluetooth/wireless MIDI interface. Works great with iPad audio via headphone jack to CP4 Aux In port. No more electronic looping or buzz/hum. Problem solved.

Sweet!