Ground loop with guitar amp FX loop: any suggestions?

dokwok2

Goal: use my iPad 2 to process sound in the FX loop of my tube amp.

Signal chain: amp FX loop send --> 1/4" cable --> Apogee interface --> iPad 2 --> iPad 2 headphone out --> 1/8" stereo to 1/4" mono converter thingy --> 1/4" cable --> amp FX loop return

Problem: high-pitched whine that sounds like a ground loop

Any suggestions?

Durwin99

Assuming there isn't a problem with your cables, I would suspect the problem to be related to volume levels from the headphone out of the iPad back into the iPad - the amp would be expecting an instrument-level signal whereas the iPad headphone out would be delivering an amplified signal - I'm not sure what the differences might be but perhaps you're driving the return of the FX loop too high - have you tried with much lower input & output volumes to/from the ipad? I have never tried to do a setup like yours, but I do have a 1/8" stereo to 1/4" mono cable specially made up for this purpose, just haven't got around to plugging it all in yet. If you don't get another solution, I can try it myself in a couple of days, just travelling right now!

dokwok2

@Durwin99 said:
Assuming there isn't a problem with your cables, I would suspect the problem to be related to volume levels from the headphone out of the iPad back into the iPad - the amp would be expecting an instrument-level signal whereas the iPad headphone out would be delivering an amplified signal - I'm not sure what the differences might be but perhaps you're driving the return of the FX loop too high - have you tried with much lower input & output volumes to/from the ipad? I have never tried to do a setup like yours, but I do have a 1/8" stereo to 1/4" mono cable specially made up for this purpose, just haven't got around to plugging it all in yet. If you don't get another solution, I can try it myself in a couple of days, just travelling right now!

Thanks for these suggestions. I just tried everything again. Send out to the Tonestack on the iPad works, and I can hear the signal through the iPad's own speaker. But when I plug in my headphone-return-cable, I hear the high-pitched noise for half a second and then nothing. The light on my Apogee turns blue, too, which indicates that the amp has stopped sending its signal, I believe.

Ganthofer

@dokwok2 & @Durwin99 - most of those stereo to mono adapters aer really only ment to be used as Mono to Stereo adapters (e.g. sending a mono signal into a stereo input). Typically the Left and Right are just shorted together inside, feeding the mono signal to both. If it is an adapter like this and yoou use it the other way around (feeding a stereo signal to a mono input) the Left and Right channel will be fighting each other if the Stereo signal coming out is not truely mono, which could eventually damage the output drivers or shut them down if the have built-in protection. The first link below gives an explanation as well as circuits for a passive Stereo to Mono summing circuit:

http://www.epanorama.net/circuits/linesum.html

or

http://www.stufinnis.co.uk/stereotomono.html

Does the light on the Apogee turn blue if you don't plug the patch cable into the Amp Effects Loop In and play? This could be a good indication of whether its the shorting of the Stereo to mono or the Impedance mismatch that is causing your problem.

@dokwok2 - you say you can hear the signal through the iPad's own speaker, have you also tried with headphones? Also, as @Durwin99 mentions, the impedance will be wrong connecting the iPad headphone jack to the Amp effects loop In. It is possible that the iPad output is shutting down either because the stereo to mono is shorting the Left and Right together or it is sensing that the impedance is out of the acceptable range. The former requires finding or making an adapter ment for summing Stereo Left and Right into Mono (as mentioned above). The latter could be solved by using an adapter (Lo-z to Hi-z) or DI box of some kind. I use a a microphone that has a Lo-z balanced XLR connector with a Lo-z to Hi-z (transformer) adapter that comes out to a 1/4 Hi-z tha I plug into amps and effects pedals that are ment to be used with guitars (Hi-z). They are relatively cheap:

http://www.sweetwater.com/store/detail/LittleIMP

and in your case you would also need to make or find the unbalance (1/8 or 1/4 mono) to XLR adapter/cable:

D.I.Y.
http://shure.custhelp.com/app/answers/detail/a_id/650/~/how-do-i-wire-an-xlr-to-1/4-adapter-cable?

BUY (plus a female female 1/8 or 1/4 adapter or something - depending on if or what you use for the Stereo to mono conversion)
http://www.sweetwater.com/store/detail/PXM110?product_id={product_id}&adpos=1o5&creative=54989267161&device=c&matchtype=&network=g&gclid=CJ_im6zr2dACFUQdgQodx5QCIA

dokwok2

@Ganthofer said:
@dokwok2 & @Durwin99 - most of those stereo to mono adapters aer really only ment to be used as Mono to Stereo adapters (e.g. sending a mono signal into a stereo input). Typically the Left and Right are just shorted together inside, feeding the mono signal to both. If it is an adapter like this and yoou use it the other way around (feeding a stereo signal to a mono input) the Left and Right channel will be fighting each other if the Stereo signal coming out is not truely mono, which could eventually damage the output drivers or shut them down if the have built-in protection.

I bought the adapter with this in mind, but your analysis seems correct.

This afternoon I tried a different interface and signal path:

Tube amp --> FX send --> Akai EIE instrument input 1 --> iPad 2 --> Tonestack app with no amp, just delay --> Akai EIE line output 1 --> FX return --> Tube amp

That worked, but the signal strength is extremely faint going in and out of the Akai unit. I don't understand this: the FX send is also supposed to be a line level out (10dBV Nominal). I tested the FX loop again, with an analogue delay pedal in place of the interface and iPad, and that worked as before.

Ganthofer

@dokwok2 - I can think of 2 possibilities;

• there are multiple place to adjust gain using the Akai EIE and iPad (ToneStack). It might need to be adjusted in multiple places to get everything balanced from input to output or it may need added buffering (see overview info below). Also, if you have AudioBus (I'm assuming you probably do ;-) or AUM and try using it to pass the audio from the EIE input to the EIE output with no other app involved. Adjusting the input gain to see if it is possible.
• there may still be an impedance mismatch load the signal down. If you have the manuals/specifications for the Amp and EIE, they should/might give the expected input/output impedance. are there any switches for the effects loop (sometimes they have a line/instrument or mixer/amp or ... To allow changing the impedance.

Some overview info on effects loop

http://proguitarshop.com/andyscorner/fx-loops-explained

dokwok2

@Ganthofer said:

• there may still be an impedance mismatch load the signal down. If you have the manuals/specifications for the Amp and EIE, they should/might give the expected input/output impedance. are there any switches for the effects loop (sometimes they have a line/instrument or mixer/amp or ... To allow changing the impedance.

Thanks, Ganthofer. I'll check into this. With more experimenting, I've discovered that the amp's FX send signal gets much stronger when I turn the amp volume up. I don't understand this -- I thought the volume control only affected the power amp, not the preamp feeding the FX loop -- but I can see the difference when I watch the meters in Audioshare or Tonestack.

Ganthofer

@dokwok2 - the link from my previous post talks a little about the amp. There is a difference between using the gain, the volume and the overdrive ( if your amp has one) and the effects loop output versus the amp power output.

dokwok2

@Ganthofer said:
@dokwok2 - the link from my previous post talks a little about the amp. There is a difference between using the gain, the volume and the overdrive ( if your amp has one) and the effects loop output versus the amp power output.

Thank you, Ganthofer. My amp is a Monoprice 15W tube amp (= Laney Cub 12r with spring instead of digital reverb). It has two channels (15W and 1W), with adjustments for gain, volume, presence, bass, mid, and treble. After reading the link you provided, I asked some gents on the Tele forum, where I learned about this amp in the first place. They answered as follows:

Many, if not most, amps have the effects loop in between the pre and power amps, so they work as you have discovered . It would take more complex circuitry than the 1940s and 50's designs still in use in amps today to ensure constant level at the effects loop out. Even if you chose to add that circuitry, your amp would have only one volume, since the fx out would drive the fx at a constant voltage .

All tube amps have the loop between the pre and post. Some, like Rivera have a special tube driven circuit for this, as well as knobs for the send and return levels. Amps like a Mono have to cut costs, so master effects all.

I'll probably fiddle with the Akai EIE some more; as you mentioned, it has several knobs that affect signal level.