[Help!] Weird, distortion on the left side only? Any help appreciated...

This has got me stumped! If anyone can help with this, I will gladly marry your sister - or promise to stay away from her, whichever you prefer...

I monitor DAW headphone mixes in stereo via an older Rane HC6 headphone amp.  Problem is: Whenever I monitor a bass guitar DI through headphones, I hear mild distortion through the left earcup of my headphones during louder passages.  This happens even when the bass guitar is panned hard right and NOTHING is being played through the left earcup!

Why?  According to the meters on my converters (and the meters in both my DAW and the soundcard's mixer app), I am NOT overloading the input signal.

I've tried:

- A different DI/preamp channel
- A different converter input channel
- A different converter output channel
- Different converters (I have three different Tango 24 AD/DA converters - problem is the same with any of them)
- A different bass guitar
- A different guitar cable
- Different, brand new and newly soldered cables connecting the headphone amp to my converter outs
- A different headphone amp (I've attached another Rane HC6 - same model, different unit, same problem)
- A different set of Sony 7506 headphones
- A different set of headphones from another manufacturer (Sennheiser)


Here's the signal chain:

  OUT: DAW > RME HDSP 9652 lightpipe > Tango 24 AD/DA converter balanced L/R out (via insert cable) > Rane HC6 hp amp > Sony 7506s

  IN: Bass guitar > DI input on Neve 9098 (or Trident iX one) > Tango 24 AD/DA converter balanced input > RME HDSP 9652 > DAW

Help!  What could be causing this?

Aj


P.S.  A suspicious clue, if it helps - the Rane HC6 headphone amp has a little LED signal present indicator, which lights up green whenever any input signal above -20dBu is present.  The left earcup distortion thing occurs whenever this light comes on!  Almost as if there is some sort of electronic interference caused simply by the light coming on.

While it is difficult to give a final conclusion on this without having the unit at hand LED metering can definitely cause distortion. There are some "class A" biasing schemes (i.e. the flowing current remains constant with LED on or off) which avoid this. Hard to tell if this is easily implemented in your unit; removing the LED would probably help too.

Samuel

Thanks for the reply Samuel...

Indeed, my intuition tells me it's the LED.  But that would be a design flaw, right? 

Logically, something just doesn't add up.  The Rane HC6 is a venerable old design,  Thousands of people still use it.  In all the threads online, I've never heard of anyone report this problem.  And yet...  even when I swap out for a different Rane HC6, the problem persists. 

Any ideas on the easiest way to disconnect the LEDs - in a reversible way, without harming anything?  Here's the unit's schematic:

http://www.rane.com/pdf/old/hc6sch.pdf

cut it off with a pair of pliers.
try and leave some length on the pins soldered to the pcb like that, you can solder another led later if you want.
sounds like something is shorting the signal to the led ??
open the unit, inspect visually, clean it, try again
best


edit: its weird that the problem persists with a different unit.
also, i would look here:

OUT: DAW > RME HDSP 9652 lightpipe > Tango 24 AD/DA converter balanced L/R out (via insert cable) > Rane HC6 hp amp > Sony 7506s

balanced via insert cable sounds strange, but then i don't know the Tango converter...
good luck !

Isophase, regarding the insert cable issue, the connection from converters to headphone amp is less than "ideal", but should be "good enough."  Or, at least that's the assumption I've been working on!

To be specific, the Tango 24 converters have individual balanced mono outputs.  The Rane HC6 headphone amp has unbalanced stereo inputs.  So, according to Rane, they recommend simply using a standard insert cable - two TS jacks (from the converters) wired to one TRS jack (into the hp amp).  It would be great if the Rane had stereo balanced inputs, but it doesn't.  (I don't know of any headphone amps that have stereo balanced inputs for each individual channel.)

Is there another way to do it?  Am I missing something?  Would like to try anything else before snipping internal LEDs.

It's weird that the problem persists with a different unit.

Click to expand...

Why--this is a design flaw and hence repeatable.

I've never heard of anyone report this problem.

Click to expand...

It's probably only audible under certain conditions. Wideband signals (i.e. not just a low-frequency bass guitar) likely mask the distortion. Also the use of less sensitive headphones will shift the signal-to-distortion ratio upwards.

One thing you could try is to feed one channel with a 20 kHz sine and the other with regular music. By increasing the level of the 20 kHz signal (which hopefully is high enough in frequency to be inaudible) you can make the LED go on continuously which should remove the distortion effect.

If this works I'd cut one side of R111 and R211; make sure there's enough wire left on both the resistor and PCB side so you can easily solder things back. If the opamp is mounted in a socket you can also bend pin 9 such that it doesn't make contact any more.

Samuel

Just wanted to follow-up for anyone following this thread (or with a similar problem)...

I replaced the Rane HC6 with an Art HeadAmp Pro 6, and the problem is gone.  As I had already tried another "working" HC6, I'm going to have to agree with Samuel that this falls under the category of design flaw (with the HC6).  Which still surprises me (there's nothing but praise online for the HC6), but the evidence is pretty damning.

The other problem I noticed with *both* of the Rane HC6 units is their noise floor was pretty high.  Not so high that I suspected a problem in the interconnect, but just enough hiss - even when nothing was connected - that I abandoned the idea of snipping out the LEDs, and just went with a newer unit from a different manufacturer.

The Art Pro model seems to work a treat - quiet, loud (is that a paradox?), and well-built.  It even has bass/treble controls on each of the six individual channels.  And no problems with the LEDs, despite having a multi-segment level meter on every output.

Problem solved.  Now we'll see how the Art holds up over time...

Aj