Phase

[SNG]

Building a dual channel SSL 9K pre-amp and confused where to place the phase.
Some units have it before the pre-amp and others, like the supergreen, after the pre-amp at the output connectors.
What is the best location to place the phase?

 

[audiox]

Output would be my choice. Switching noise it is not amplified and there is no phantom power causing trouble.

 

[JohnRoberts]

If the output is balanced that would be slightly preferable to switching "polarity" at the input, but only slightly.

JR

 

[MHanson]

If the output is balanced that would be slightly preferable to switching "polarity" at the input, but only slightly.

I'd love to understand the reasoning there.  My logic head has always thought it should be at the input, but if there's a real reason I'd love to know why. 

Thanks,
Michael

 

[abbey road d enfer]

This is not phase, this is POLARITY.
There is no absolute best position for the polarity switch, on a functional POV.
If there are balanced in and out, placing it at the output makes the wiring less prone to picking up interference, RF, hum...
As audiox mentioned, placing it at the input has the added inconvenience of producing more noticeable clicks, in particular when phantom power is applied.

 

[emrr]

think about the effects of ten years age (wear, corrosion, etc) on your switches, and then ask yourself whether you'd like to do things at a high or a low signal level. 

 

[SNG]

Thanks for the quick help, gonna place it between the Balanced Line Driver and the XLR outputs...

 

[gridcurrent]

The time tested order of input controls on mic preamps is:
1.  phantom
2.  polarity (yes, many of us still refer to it as phase)
3.  pad
Placing the phantom first ensure no DC clicks.
No problem with polarity switch the output,
but the above method works with balanced and unbalanced outputs.

 

[abbey road d enfer]

Nothing really justifies the universality of this organisation.
Well, phantom must obviously put first, but the pad could be put right after and the polarity last, that would not change a thing.
On mixers, originally, the polarity switch was in the input section because there was no output transformer in the channel, but with the increasing low cost of IC's, a separate polarity reverse stage was added, because it is easier to switch line-level signals than mic-level.

 

[gridcurrent]

"Well, phantom must obviously put first, but the pad could be put right after and the polarity last, that would not change a thing."
Placement does matter.  There are often 2 ways to accomplish a task, acceptable and better.  Switching at higher level reduces the chance of contact issues.

 

[SNG]

Okay think this will be the layout of my dual SSL9K pre-amp...

1. XLR Input
2. Phantom
3. The switch between MIC XLR and DI
4. PAD -20 dB
5. HIGH PASS Filter (maybe also a LOW PASS Filter)
  Still looking around for the best solution.
6. SSL 9K PRE-AMP
7. Dual Line Out Balancer
8. Polarity (Phase)
9. XLR Output

 

[abbey road d enfer]

Although it seems tempting to position the HiPass and LoPass filters before the active circuitry, it is not a very good solution.
First order (6dB/octave) HiPass filters are easy to implement, but do not really offer the expected performance. First order (6dB/octave) LoPass filters are too much dependant on the actual source impedance of the mic. Second and higher order filters impose the use of inductors, which are costly and may pick up hum. In addition, they expect to see a controlled source impedance, which is not compatible with the variety of microphone output stages, from 50 Ohms to 600 Ohms, active or transformer isolated; their complex input and output impedances may introduce severe frequency response aberrations and noise build-up.
IMO, filtering is best done actively at line level.

 

[JohnRoberts]

+1

Perform the HPF and LPF later at line level. There are even more reasons than already given.

JR