1. We would like to remind our members that this is a privately owned, run and supported forum. You are here at the invitation and discretion of the owners. As such, rules and standards of conduct will be applied that help keep this forum functioning as the owners desire. These include, but are not limited to, removing content and even access to the forum.

    Please give yourself a refresher on the forum rules you agreed to follow when you signed up.
    Dismiss Notice

Hiss on Input2?

Discussion in 'Axe-Fx III Discussion' started by kartman, Jun 22, 2018.

  1. kartman

    kartman
    Expand Collapse
    Inspired

    Joined:
    Jan 1, 2015
    Messages:
    212
    Likes Received:
    134
    Location:
    Ottawa, Canada
    I have what I hope is a simple issue with Input2. I'm running FW1.10 and only have an instrument cable plugged into the rear instrument input. IN1 block is on the grid and all is good.

    I put IN2 on the grid and I'm getting a constant hiss when there is nothing connected to either of the Input2 L or R connections. I've tried shifting L ONLY, STEREO, SUM L+R and there is nothing showing on the input meters page. On the Input2 block, the gain meter is basically 0.0db with the gate threshold at -80db.

    Bypass the block and the hiss goes away... IN1, IN3, and IN4 are all silent....

    I'm sure this is some sort of user error but I'm confused. I've used IN2 for a second instrument since getting the AF3 but I've never noticed this hiss before today. If I plug an instrument into IN2, the hiss stops too.

    Thoughts?
     
  2. chris

    chris
    Expand Collapse
    Legend!

    Joined:
    Jul 31, 2008
    Messages:
    25,715
    Likes Received:
    16,812
    by design. don't add input 2 if you aren't using it.
     
  3. onj

    onj
    Expand Collapse
    Regular

    Joined:
    Jun 23, 2014
    Messages:
    58
    Likes Received:
    10
    whats the design that makes it hiss? Thanks.
     
  4. REDD

    REDD
    Expand Collapse
    Forum Addict

    Joined:
    Mar 1, 2012
    Messages:
    1,725
    Likes Received:
    1,496
    Location:
    Missouri USA
    What if you are using it?
     
  5. chris

    chris
    Expand Collapse
    Legend!

    Joined:
    Jul 31, 2008
    Messages:
    25,715
    Likes Received:
    16,812
    It doesn’t hiss when something is plugged in. So not to worry.
     
    REDD likes this.
  6. kartman

    kartman
    Expand Collapse
    Inspired

    Joined:
    Jan 1, 2015
    Messages:
    212
    Likes Received:
    134
    Location:
    Ottawa, Canada
    OK. 'By Design' seems to imply a purpose. Can we get a hint? I'm curious now! :)
     
  7. Rex

    Rex
    Expand Collapse
    Legend!

    Joined:
    Jul 22, 2010
    Messages:
    19,953
    Likes Received:
    9,708
    The Input 2 jack is non-shorting. I used to know why that choice was made, but the reason escapes me at the moment.
     
  8. yek

    yek
    Expand Collapse
    Moderator
    Moderator

    Joined:
    Sep 30, 2009
    Messages:
    20,904
    Likes Received:
    24,705
    Location:
    The Netherlands
    IIRC it's because the Combi connectors on Input 2 support high-impedance sources such as guitars and basses.
     
    ccroyalsenders likes this.
  9. Rex

    Rex
    Expand Collapse
    Legend!

    Joined:
    Jul 22, 2010
    Messages:
    19,953
    Likes Received:
    9,708
    That’s it. TRS jacks can be normalized to short to ground when there’s no plug inserted, and that cuts down the main source oh hiss. But Input2 uses TRS/XLR combo jacks. If you did that kind of normalization on a combo jack, you’d get the XLR pins shorted together.

    Easy fix: don’t use the Input2 block when there’s nothing connected to it.
     
  10. #10 FractalAudio, Jun 23, 2018
    Last edited: Jun 23, 2018
    FractalAudio

    FractalAudio
    Expand Collapse
    Administrator
    Fractal Audio Systems Moderator

    Joined:
    Jul 29, 2008
    Messages:
    12,938
    Likes Received:
    85,462
    We wanted Input 2 to be able to support both line level sources and instruments. A guitar needs a very high impedance input impedance (1 Mohm). Line level sources typically see input impedances around 10K but work just fine at higher impedances.

    The self-noise of a resistor is proportional to the resistance. Therefore a 1M resistor will have 100 times the noise power (20 dB!). However the input resistance is shunted by the source resistance so it effectively doesn't contribute to the noise figure.

    A combi-jack does not have a shorting contact on the 1/4" tip contact like a regular 1/4" jack. The whole reason a regular 1/4" jack has a shorting contact is to short the input to ground when nothing is plugged in. This shorts the noise of the input resistance to ground. Without that shorting contact and nothing plugged in you get the noise of that input resistance and since it's 1M it's significant. Plug something in and the noise will go away. Or simply don't use Input 2 with nothing plugged in.
     
  11. jjozwia

    jjozwia
    Expand Collapse
    Forum Addict

    Joined:
    Jan 18, 2011
    Messages:
    1,002
    Likes Received:
    475
    Location:
    MFC-101 Editor, eJozSoftware.com
    This post makes me feel much better. Maybe missed seeing something about this in manual and have a SR with this as one item in inquiry. Figured it was hardware issue having experienced similar sounding hiss at one point in a II. But that was with things plugged in or not on front input. Thanks for clarification
     
    kartman likes this.
  12. kartman

    kartman
    Expand Collapse
    Inspired

    Joined:
    Jan 1, 2015
    Messages:
    212
    Likes Received:
    134
    Location:
    Ottawa, Canada
    Thanks for the explanation.
     

Share This Page