Noisy effects returns

[LemonTree]

Sorry about the cross post, think I had this in the wrong forum.....


is it normal to get a lot of hum and hiss on your aux returns? I read somewhere people use gates to control effects return noise when not in use but should they be that noisy in the first place?

They're clean with nothing plugged in but whenever I plug up my lexicon mpx100 or alesis microverb III I get a lot of noise. I've tried to balance it with the input/output knobs on the effect devices but to get an audiable return I need to crank the outputs.

The desk is an Allen & Heath saber 24/16/16/2

6 aux sends
6 aux masters
4 aux returns



I'm using send 5 for the lexicon and returning it (left/right) to aux 1&2 because they have dedicated eq

I'm using aux 6 to feed the Alesis and returning that left & right to aux 3 & 4

1 & 2 are pre/post switchable 3 to 6 are fixed post... I'm not sure I'm quite grasping the concept of this at all.

Can anyone help?

Alec

 

[jtienhaara]

is it normal to get a lot of hum and hiss on your aux returns?

Nope.

I read somewhere people use gates to control effects return noise when not in use but should they be that noisy in the first place?

A gate or expander should never be necessary for a signal processor.


There are a few possible problems that would cause noise. Generally speaking it's all to do with the cables. However in this case there may also be nominal level issues, or balanced / unbalanced issues, and possibly noise sources in your environment which could be re-positioned.


1) Cables: Cables pick up noise from all kinds of sources. Radio signals, CRT monitors, etc. It is always a good idea to a) make sure your cables are short and b) use quality cables.

Resistance to current increases exponentially with the length of a cable, and also with the thickness of the cable. 22 AWG (American Wire Guage) is roughly the "standard" for mic and guitar cables. 24 AWG (which is thinner) is fine for short lengths, too. 26 is getting too thin and should be avoided except for the shortest of cables.

There is also the question of shielding for the cables. It's worth paying extra for higher % shielding. And if you can find solid wire (instead of strands) for the cable then that improves conductivity too (though it's hard to find solid wire pre-made cables).


2) Nominal level: there are two main standards in the audio world today: +4 dbu nominal level ("pro" gear) and -10 dbv nominal level ("consumer" gear). If you send a -10 dbv signal to a +4 dbv input, you'll need to really crank the signal to get a semi-useable and rather noisy signal.

You'll have to check the specs of your devices to find out a) whether the effects are, in fact, sending -10 dbv nominal levels; b) whether the mixer is expecting +4 dbu nominal levels; and c) whether you can match them. Often mixers + Lexicon gear have switches to change between -10/+4.


3) Balanced / unbalanced: An unbalanced cable has one "hot" signal and a "zero" (ground / shield) channel that is used as a reference point. Unfortunately the ground channel in particular is very susceptible to noise, and if you have multiple paths for current to flow via ground channels (called a ground loop) then you have effectively made a radio antenna out of your studio gear.

That's why balanced cables were invented. A balanced cable has a "hot" channel and a "cold" channel. They are both referenced to the ground channel, but with opposite voltages. For example, when a signal hits +1.0 volts on the "hot" channel, it hits -1.0 volts on the "cold" channel. The signal amplitude is (hot - cold). Therefore noise on either of the channels is cancelled out, and ground hum is ignored.

If you plug a balanced output into an unbalanced input then, if it doesn't short something out, you will get exactly 1/2 the input level that you would expect, or approximately a -6 db drop.


4) Noise sources: make sure you don't run cables close to monitors, power supplies, power cords, ... There are lots of noise sources in the typical studio and sometimes just moving things around can eliminate noise.


Now there are solutions to the level-matching and balanced cable woes. Most audio dealers have fairly cheap boxes to solve the aforementioned mis-matches. However you'll have to spend some time looking at specs in your manuals to figure out where the issues lie.

Hope this helps. Good luck Alec!

Cheers,

Johann

 

[LemonTree]

thanks a lot Johan