Question about phase switch in mic pre´s

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API

API

Well-known member
Joined
Nov 25, 2005
Messages
869
Location
Sweden
Hi.

Why is it that the phase switch in most mic pre´s I have seen is on the input and not the output?
Is there any sonic difference to have it just after the input transformer than to put it before the output transformer?
Or even maybe after the output transformer?

The reason I am asking is because I am modding a Golden Age PRE73 (with great results as well!) with some new transformers and wiring and the phase switch is between the output transformer and the output XLR which I thought was a bit odd.
If there is any difference to have it just after the Input I want to rewire it that way.

If someone could elaborate I would be greatful.

API
 
There will not be a sonic difference between the phase switch on the input or the output, no matter what preamp.

1. Phase switch only on input means your switch won't have any effect on DI recorded material. Of course this only applies to preamps with DI input (which is after the input transformer)

2. Phase switch on output means your switch will affect all audio passing through the preamp.

hence, switch on output is just about always a better option.
 
Ok, that makes sense.
I did not think it made any sonic difference but I wanted to check.

On the PRE73 the switch sits after the Output transformer where the tap for the VU meter is.
Will try to split the signal by the transformer and take one cable to the VU and one to the output XLR.

API
 
It can make a sonic difference if the switch is on the input, because the signal is small.  If the switch contacts become contaminated then a small signal will have difficulty in "punching" through.  That is why many people use hermatically sealed ralays for this job in this position, because they can not be contaminated.  If the switch is on the output the signal is generally more beefy & less susceptible to this problem !
 
Rob Flinn said:
If the switch contacts become contaminated then a small signal will have difficulty in "punching" through.  That is why many people use hermatically sealed ralays for this job in this position, because they can not be contaminated. 
Click to expand...

Didn't know this. At least it's just a faulty equipment scenario, and won't affect much anything if one uses quality parts in the first place.
 
Plus, any extra wiring length is better done at the higher signal level, if possible and reasonable. 
 
One point of view is the switching noise. If placed at the input the switching noise is amplified. Switching the phantom power also results a huge pop.
 
bugfight said:
that link is wrong too, if you wanna nitpick.
a phase difference measured in degrees has no meaning on a complex signal...
Click to expand...

Don't wanna nitpick myself, just wanted to be regarded as such  ;)

These might be better ones (but the idea will be clear now, so back to topic):

http://www.sweetwater.com/insync/techtip.php?find=06/27/2003
is saying

First we should make the distinction between phase and polarity. The switch that you are referring to should be called a polarity switch. Though it is commonly called a phase switch (this is an industry wide problem, not something you got wrong), really all it does is reverse the polarity of the signal. 
Click to expand...

http://www.sweetwater.com/insync/word.php?find=polarity
Polarity In electronics, two points that have opposite electric potentials (one is positive, the other negative). This is not the same as being 180 degrees out of phase (although the results can be similar). Phase implies a relationship with time, polarity does not. What most engineers, consoles and preamps refer to as a "phase" switch is actually a switch reversing signal polarity.

Polarity is important when interfacing equipment, particularly speakers - you don't want one cone moving in while the other moves out. Some designers feel that maintaining "absolute polarity" (no polarity reversal in a signal chain) throughout a signal path is important.

While tests don't indicate that the ear can hear which polarity is correct, they do show that it may be possible to detect a difference between normal and inverted polarity signals. (Try it for yourself in a critical listening environment: Play a signal though a single speaker, then reverse the speaker wires and play the same signal again - remember to switch the wires back when you are finished!)
Click to expand...
 
API said:
Hi.

Why is it that the phase switch in most mic pre´s I have seen is on the input and not the output?
Is there any sonic difference to have it just after the input transformer than to put it before the output transformer?
Or even maybe after the output transformer?

The reason I am asking is because I am modding a Golden Age PRE73 (with great results as well!) with some new transformers and wiring and the phase switch is between the output transformer and the output XLR which I thought was a bit odd.
If there is any difference to have it just after the Input I want to rewire it that way.

If someone could elaborate I would be greatful.

API
Click to expand...
You can reverse polarity anywhere in the signal path, the end result will always be the same. But if you switch the mic signal, you are dealing with low level, so you have to have supearior quality contacts, and if it is a transless input, it's a potential source of serious clicks, especially with phantom power on. Switching after the input transformer is not a very viable option, because you're dealing with higher level, but also higher impedance. Note that some mic xformers have noticeably different perforformance when the secondary connections are swapped, because of the parasitic capacitance. Switching at line level has all the advantages of high level and low impedance, at the cost of a few added components. Almost all the mixers I know have a dedicated phase reversal stage, generally a basic opamp configured as an inverter. It has the advantage of using only one section of the switch , the other being available for a led indicator.
 
I had a quick look at the original 1073 and the switch is actually after the output transformer!
And it does make sense to switch the line level signal than the incoming mic signal.

Thanks for all the responses!

Regards,
API
 
Sorry to bump this old thread but I was wondering if it makes any difference whether you flip the polarity on the primary or the secondary of the output resistor transformer?

[edit] Sorry I meant transformer and not resistor :)
 
warpie said:
Sorry to bump this old thread but I was wondering if it makes any difference whether you flip the polarity on the primary or the secondary of the output resistor?
Click to expand...

Some variables in that question.  If you're talking about a step-down type with a large ratio in a single ended circuit, you may find treble response differences if flipping the primary.  Secondary can be more reliably recommended. 
 
Hi emrr,

Just a typical 600:600 output transformer.  Why would the top end response change?
 
This is just a guess but perhaps it is because the origin of that arrangement is from console channels with inserts and sends and other such things that might care about phase.
 
It's quite interesting though that many schematics and app. notes recommend polarity reverse at the input. 
 

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