• 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.

Hiss on Input2?

kartman

Inspired
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?
 

Rex

Legend!
The Input 2 jack is non-shorting. I used to know why that choice was made, but the reason escapes me at the moment.
 

Rex

Legend!
IIRC it's because the Combi connectors on Input 2 support high-impedance sources such as guitars and basses.
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.
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
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.
 
Last edited:

jjozwia

Power User
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

Inspired
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.
Thanks for the explanation.
 
Top Bottom