Author Topic: Schem for a phantom powered FET circuit for a ribbon mic  (Read 15028 times)

PRR

Re: Schem for a phantom powered FET circuit for a ribbon mic
« Reply #20 on: December 09, 2019, 01:47:26 PM »
> what do the BIAS points connect to? The in+ and in- will be +48 DC

There's 3 "bias" dots. Connect the dots.

It won't be 48V on top because P48 is 48V in series with 3.9k. If the amp does any real work it sucks current and sags the 3.9k. DC analysis should be trivial. The whole thing acts as about a 4k resistor. There's about 24V on the XLR pins.
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anotherjim

Re: Schem for a phantom powered FET circuit for a ribbon mic
« Reply #21 on: December 09, 2019, 04:46:47 PM »
I saw this...
https://www.bumblebeepro.com/shop/rm-7-active-diy-ribbon-mic-kit/
The preamp board looks like x4 BJT's doing the work. If you can beat the 75Euro that board costs, you can bee happy.
Croeso i Diystompboxes.

There is no aspect of human endeavour that cannot be improved with cheese.

tubegeek

Re: Schem for a phantom powered FET circuit for a ribbon mic
« Reply #22 on: December 10, 2019, 01:46:41 AM »
> what do the BIAS points connect to? The in+ and in- will be +48 DC

There's 3 "bias" dots. Connect the dots.

Just to each other, nothing else. OK

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It won't be 48V on top because P48 is 48V in series with 3.9k. If the amp does any real work it sucks current and sags the 3.9k. DC analysis should be trivial. The whole thing acts as about a 4k resistor. There's about 24V on the XLR pins.

So, if we are using this between a mixer and one of the 3 mics in the world that actually need to see +48 or they won''t work, we're out of luck. And if we are actually running a ribbon mic, we just want to make sure it's properly floating and not grounded at pin1. Yes?

Meanwhile I wish I had a nice ribbon mic....
"The first four times, we figured it was an isolated incident." - Angry Pete

"(Chassis is not a magic garbage dump.)" - PRR

PRR

Re: Schem for a phantom powered FET circuit for a ribbon mic
« Reply #23 on: December 10, 2019, 01:55:29 AM »
No normal ribbon needs Phantom. The few "Active" ribbon mikes should not need preamps.

Phantom is intended for ONE mike load. If a mike actually depends on that fact, you can't also tap-off power for a preamp.

So I don't get why you expect a pass-through on a single-serving interface?

If you have to smoke your cake and eat it too, get another in-line Phantom supply.
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tubegeek

Re: Schem for a phantom powered FET circuit for a ribbon mic
« Reply #24 on: December 10, 2019, 02:05:21 AM »
No normal ribbon needs Phantom.

This is all hypothetical, mind you, but I'm envisioning using the built in phantom power on a mixer to run this booster for a traditional/low-output/passive ribbon. Such a mic just needs to be protected from phantom, not powered by it, so I'm just checking in on that.
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The few "Active" ribbon mikes should not need preamps.
Indeed.
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Phantom is intended for ONE mike load. If a mike actually depends on that fact, you can't also tap-off power for a preamp.
So I don't get why you expect a pass-through on a single-serving interface?

I think you misunderstand me - I'm just making sure I fully understand what might happen, not hoping/asking for anything different.
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If you have to smoke your cake and eat it too, get another in-line Phantom supply.
Yeah, or use a condenser with adequate output or a preamp with adequate gain & thus no need for this gizmo. The hypotehetical combinations are unlikely, I understand.

Just doing a thought experiment, but as you know, my thought-lab is woefully ill-equipped.
"The first four times, we figured it was an isolated incident." - Angry Pete

"(Chassis is not a magic garbage dump.)" - PRR

merlinb

Re: Schem for a phantom powered FET circuit for a ribbon mic
« Reply #25 on: December 10, 2019, 05:39:03 AM »
I don't know how helpful this is now, but I feel obligated to shill Artur Fisher's 'Bumblebee' ribbon mic pre. I can't find a schem for it, but he has schems for his guitar DIs which are sure to be similar:

https://www.bumblebeepro.com/shop/bb-d1-jfet-active-direct-box-diy-kit/

https://www.bumblebeepro.com/blog/discrete-op-amp-jfet-active-di-prototype-first-test/

http://www.diyaudiocomponents.com/p1-inline-ribbon-mic-preamp

« Last Edit: December 10, 2019, 05:52:03 AM by merlinb »

PRR

Re: Schem for a phantom powered FET circuit for a ribbon mic
« Reply #26 on: December 10, 2019, 01:38:21 PM »
...Artur Fisher's 'Bumblebee' ribbon mic pre. I can't find a schem for it, but he has schems for his guitar DIs which are sure to be similar:

The back-end could be similar. The front-end must be very different; mike is not guitar. We want balanced, and hiss-optimized for 150-1k. Guitar is unbalanced and typically hiss-optimized for >10k.

Also mike booster makes about no sense unless it has a better noise figure than your console's preamps. There's typically ample total gain available. Good mike wires laid with care can run long distances without crap pickup. Since the $10 preamps of today do NF <3dB, this booster needs to be exemplary to justify itself.

But some folks want gain = more-more-more.

(There is a marginal case where you are trying to clip vintage/flavored preamps with a soft source.)
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