Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: punkykatt on December 17, 2010, 03:55:45 pm
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Hey guys, Just built a 2 channel low Z mic switcher to switch 2 mics in and out of a vocal hamonizer. When one mic is in the other is true bypass and vise versa. The switcher works flawlessly with standard type Shure SM-58`s, "BUT" they want to use their phantom powered mic`s, which when switched make a loud audio pop. Im sure the pop is caused by the phantoms 48vdc power running on the hot and cold Pins 2 and 3 of the XLR connectors. The switching is done with 4 DPDT relay switches. Anyone have an idea how to get rid of the pop? Any help is much appreciated. Thanks punky
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Been shuffling through my notes and came across this: " To stop switch pop, install a resister with high resistance, 3.3M or higher across switch contacks". Do you think that will work?. Space to install resisters is very limited. I will probably have to remove the relay pcb and tack solder them on the underside. I would hate to go through all that work to find out they still pop. Any help is much appreciated. Thanks in advance. Punky
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yes, one solurion is a 1M or more resistor. I think Butteryliscious posted a circuit for relay noise suppression using diodes. A capacitor might also work.
Possible causes - the relay coil itelf is inducing a pop into the signal; SW-contact noise. Experiment: maybe jerry-rig a simple SPST sw (no relay) -- outside the box -- for a phantom mic. Does it pop when switched ON/OFF? If so then fix that; then apply the solution to the relay inside the box.
Last ditch solution: a time delay relay so the pop is gone by the time the mic kicks in. But the delay ould be annoying for stage use.
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Thanks jjasilli, I dont believe the pop is coming from the relay coils because using the non phantom power mic`s with the phantom on the PA head turned off there is only a faint click when switching which is acceptable. The loud pop is when the 48vdc phantom power is turned on in the PA head and the phantom mic`s being used. The 48vdc phantom power is on the same conductors as the signal, XLR pins 2 and 3, pin 1 is used as negitive ground. I think its the making and breaking the dc voltage contact thats causing the pop? I could be wrong tho. I will have to get that PA head and a phantom mic in the shop to do the outside the box experiment you suggested. Good idea.
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http://www.symetrixaudio.com/kb/SX202_ug.pdf
It seems you are not alone. Checkout p 9 of the above manaul for a commercially made dual mic preamp. It requires muting the speakers or headphones when switching phantom power on/off tr aviod a nasty pop.
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I tried the jerry rig outside the box without the relay experiment. I used a DPST switch to switch both legs at the same time. Tried 1M to 3.3M resistors and also .1 caps across the switch contacts. Still loud pop. I looks like they are going to have to use the dynamic mikes with this switcher. Thanks again for the help. Punky
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Put Phantom supplies BETWEEN mikes and mix/switch unit.
You can't (simply) switch both legs at the SAME instant. When red lead breaks and blue lead is still contacted, there's a 20V-40V POP, and again when blue breaks. So you can't switch here.
> install a resister with high resistance
That's for the common case of SMALL stray leakage such as a coupling cap left open tna then connected. The resistor keeps the cap leakage bled to zero.
That won't work when you MUST have BIG voltage to make the mike work.
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A thought on the subject, based off my very limited knowledge of Mics, Balanced XLR connections, and phantom power.
Given:
XLR 3-pin. ONE pin is considered a ground, and the other two are 180° out of phase for noise canceling benefits.
The usual cheat for an unbalanced signal is to short one (usually - ) to ground.
WHAT IF: Diodes were used to keep the mics powered, BUT in the switching process the diode is then shorted to allow AC to pass. The unsellected Mic retains power via the diode, yet AC is blocked. Removes power-up noise, but I'm not sure if that would actually get rid of the popping noise from switching mics.
All this is based on the premise the Phantom power AND the audio signal are opposite polarity.
Educate me if this thought, and premises are flawed.
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> premise the Phantom power AND the audio signal are opposite polarity.
No. The audio is balanced. The Phantom power is not.
A key problem is: BOTH mikes must stay powered UP. They are sure to thump as power goes off and on. The thump may be several seconds.
And they can't share the same mike jack. Each mike needs +48V through 3.4K. Two mikes on one 3.4K source is not sure to work.
So two Phantom supplies.
After that, blocking caps, ~~10K drain resistors, and a DPDT switch into a NON-Phantom input.
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> premise the Phantom power AND the audio signal are opposite polarity.
No. The audio is balanced. The Phantom power is not.
I was not intending to imply that the phantom power was balanced, just sharing one signal line. But I do thank you for your knowledge on this.
And they can't share the same mike jack. Each mike needs +48V through 3.4K. Two mikes on one 3.4K source is not sure to work.
So two Phantom supplies.
Now THAT I didn't know. And, that is the major key right there: power loading which rightly does kill my thought dead.
Thanks for the Education PRR.
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> not intending to imply that the phantom power was balanced
It's hard to see without a chalkboard. Words tangle.
For instance: "just sharing one signal line".... Phantom is on both signal lines. I bet you know this, but "one signal line" may be confusing to someone with little knowledge of Phantom.
Punky's problem seems simple, and I have started to type several cheap-tricks, and seen a flaw in every one. The cheapest/fastest answer seems to be a micro-mixer with at least two Phantom mike inputs. I have seen such near $89. But that outputs line-level and I gather Punky wants to go into a Mike input. And the $89 micromixers tend to occupy a lot of space (even though there's not much inside). So even that's not as neat as we could like.
Musician's Friend shows:
Nady SMPS-1X Phantom Power Supply $19.99
Nady SMPS-2X Dual Phantom Power Supply $34.99 (Expected: 12-28-2010)
ART Phantom II Pro Phantom Power Supply $49.00 (Expected: 12-28-2010)
ART Phantom I Studio Mic Power Supply $50.00
Two Phantom supplies for $35 is interesting. A simple DPDT switch wired into 1-1/2 mike cables should select the desired mike cleanly; ~~2K pull-down resistors may be needed if Nady could not afford to include pull-down at that price.
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Twas the afternoon of Christmas & I've nothing to do
so I sketched up a little schematic for you
This circuit might work that is what I think
But someone should check it cuz I've had too much to drink
R3 through R6 should be 1 percent
Trust me on this one it's money well spent
C6 through C9 should be very small
So there is no high frequency attenuation at all
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> This circuit might work that is what I think
> But someone should check it cuz I've had too much to drink
> C6 through C9 should be very small
> So there is no high frequency attenuation at all
Santa got some good egg-nog this year?
C6 to C9 should be LARGE for no LOW frequency loss.
Large will be electrolytic so observe polarity.
There's also a subsonic random noise/rumble issue but probably moot for stage-work.
Because caps leak and large caps leak lots, we need bleeds to keep the switch at zero DC voltage. These should be close tolerance. They do not have to be AS precise as the Phantom resistors. But hey, 1%ers are cheaper buy the dozen, so why not.
Pin 1 on the output should usually be tied to all other pin 1s and the case.
This is of course "two phantom supplies" (and a switch). And I'm not sure you can buy all those Phantom parts and jacks and case for what Nady asks for the SMPS-2X. But if metal-chopping and debugging 48V supplies is a recreation instead of a pain, then Joy To The World.
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Santa got some good egg-nog this year?
no jagermeister... we killed most of a bottle - he more than i :angel
fortunately it did not impair his cooking skillz... baked tater w/ bacon (i fried the bacon), sour cream, homemade biscuits, and we split a large NY strip - it was grilled to perfection!
buuurp!!! :angel thanks buttery...