Author Topic: How to Add a Slow Rise RC Circuit to Green Pre  (Read 800 times)

Phrazemaster

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How to Add a Slow Rise RC Circuit to Green Pre
« on: December 23, 2010, 02:54:05 AM »
Hi, I've been researching for hours here and still just don't quite understand how I could go about implementing a simple slow-rise RC circuit into a Green Pre. I've read everything I could find on RC circuits, and posts on slow rise phantom here, etc.

I know it's just a resistor and capacitor in series, but I have some questions on how to do this, and the effect it might have on the microphones. I am including a schemo of the green pre v14 board, and my very crude understanding of how to extend it.

1) Simplest Question. The Green Pre circuit board has two outputs for Phantom Pwr at the end of the board, for powering the mic.

a) Can I simply add a resistor and capacitor in series with one leg of these outputs and take it to a SPST switch? (The board is supposed to just have a SPST switch right at that place).
b) Does it matter which leg of the output I put the RC on? Since the outputs on the Green pre are not labelled, I have just used "connector 1" and connector 2" in the diagram in the following post.
c) Can I just choose the values of R and C to get the time ramp I like? Example: T=RC. Take a 5 second ramp time and use R=5K, C = 1000uF.
d) If this will work fine, will I be inadvertently adding junk to the phantom power supply (distortion/noise) - I'm still not sure if phantom power, supplied to the mic, affects the audio directly (sorry, I'm really trying to understand this all but I'm slow at it).
e) Do I need to use an electrolytic or film cap, or does it matter?
f) Is this just a totally stupid idea? I want to use my green pre with some great mics, but I'm a little concerned about sudden phantom power hitting them and damaging them. I thought using a slow rise phantom would protect them, and also eliminate some pop when they are turned on.
« Last Edit: December 23, 2010, 02:58:25 AM by Phrazemaster »
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Phrazemaster

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Re: How to Add a Slow Rise RC Circuit to Green Pre
« Reply #1 on: December 23, 2010, 02:55:41 AM »
Since I haven't mastered multiple attachments in a post yet, here's my crude rendition of taking the phantom power off the end of the green pre and adding an RC circuit.

Thank-you in advance for all replies. Understand, I'm not an idiot, but I play one on tv.

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Harpo

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Re: How to Add a Slow Rise RC Circuit to Green Pre
« Reply #2 on: December 23, 2010, 12:57:35 PM »
1) Simplest Question. The Green Pre circuit board has two outputs for Phantom Pwr at the end of the board, for powering the mic.
The green pre circuit has a single electronic (not cross-coupled) balanced output. If there is phantom voltage on these output pins, you or better said connected gear will have a severe problem. A condenser mic is supplied by phantom voltage applied to the green pre input thru the 6k8 resistors. Same value 6k8 is much more important than absolute value of these resistors.
If you're talking of the switch for engaging phantom power, that's not two outputs, but the switch pole connected to +48V and one throw position for passing this voltage to the mic thru a 100R and prementioned 6k8s.

Quote
a) Can I simply add a resistor and capacitor in series with one leg of these outputs and take it to a SPST switch? (The board is supposed to just have a SPST switch right at that place).
You can do lots of things with a resistor and cap in series, but not passing a DC voltage. This will get blocked by the cap.

Quote
b) Does it matter which leg of the output I put the RC on? Since the outputs on the Green pre are not labelled, I have just used "connector 1" and connector 2" in the diagram in the following post.
If it would work (it doesn't), it would matter. The switch pole is permanently connected to +48V. You want to ramp up this voltage every time you engage the phantom switch, not only once when powering on the green.

Quote
c) Can I just choose the values of R and C to get the time ramp I like? Example: T=RC. Take a 5 second ramp time and use R=5K, C = 1000uF.
If it would work (it doesn't), the 5k resistor would be in series to the paralelled 6k8 resistors. If the cap was blown short and would be passing the +48VDC, this additional 5k would limit the current for a connected condenser mic to 5.7mA instead of 14mA.

Quote
d) If this will work fine, will I be inadvertently adding junk to the phantom power supply (distortion/noise) - I'm still not sure if phantom power, supplied to the mic, affects the audio directly (sorry, I'm really trying to understand this all but I'm slow at it).
This arrangement will only pass junk to the mic. The cap will only let pass the remaining ripple voltage, hardly enough to power a mic.

Quote
) Do I need to use an electrolytic or film cap, or does it matter?
would be electrolytic, rated for at least this 48V. You could substitute your single 1000uF cap by 1000 pcs of 1nF film caps in parallel to build an expensive and space consuming antenna instead.

Quote
f) Is this just a totally stupid idea? I want to use my green pre with some great mics, but I'm a little concerned about sudden phantom power hitting them and damaging them. I thought using a slow rise phantom would protect them, and also eliminate some pop when they are turned on.
Some condenser mics come with a built in switching supply. These might misbehave with a ramped up phantom voltage, so this ramping might not be a perfect idea.
Ramping this +48V could be done with a transistor, a R/C connected to the base and a protection diode for the transistor if the switch gets disengaged but a following smoothing cap (47uF) is still charged.

Phrazemaster

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Re: How to Add a Slow Rise RC Circuit to Green Pre
« Reply #3 on: December 23, 2010, 01:39:53 PM »
Thank-you Harpo for your time. Clearly, I need to just leave the design as-is. Not that there is a thing wrong with it!

Your eyes have saved me a lot of pain. Merry Christmas!

Mike
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