I've seen pro gear with as little as 36V for phantom, and most modern mics seem to have no problem with lower voltages. Vintage mics may be more of a problem.
As for your traffo, I'd connect the screen to the chassis ground, but it does depend a little on your implementation. Most switches these days seem to be a minimum of 5A, so that shouldn't be an issue. Fuses - depends a little on the surge current of your traffo, but I'd start out with a 200mA slow blow, and see how you get on.
As for your traffo, I'd connect the screen to the chassis ground, but it does depend a little on your implementation. Most switches these days seem to be a minimum of 5A, so that shouldn't be an issue. Fuses - depends a little on the surge current of your traffo, but I'd start out with a 200mA slow blow, and see how you get on.
I have some preamps I built from some Mic preamp cards for a PA system and they only put out 24v DC for phantom and I haven"t had a single mic that didn"t work with them (and they sound great)..... the Phantom on these preamps have 3.4k resistors instead of the 6.8k ones on most phantom supplies .....
Thanks Al an minion for the replies. I'll take your suggestions.
Minion, I was remembering your earlier comments about using 24v phantom when I wrote that. I don't think I'll worry about the slight voltage difference for now. If it seems like an issue, it's only one resistor to change later.
Now I need to finish then preamp layout and boards. I'll be using INA217 chips. I'm still debating on having a balanced output or not but that is a topic for a new thread I'll start tomorrow.
Minion, I was remembering your earlier comments about using 24v phantom when I wrote that. I don't think I'll worry about the slight voltage difference for now. If it seems like an issue, it's only one resistor to change later.
Now I need to finish then preamp layout and boards. I'll be using INA217 chips. I'm still debating on having a balanced output or not but that is a topic for a new thread I'll start tomorrow.
You can pretty easilly add ballanced outputs ..... I used a Dual opamp but there are also ballanced line driver chips like the DRV134 and of cource you can use transformers .....
Thanks Minion. I got several free DRV134 from TI, so inwouldnuse those🙂. I just wonder if it is worth the work. I seriously doubt I would use the preamp remotely compared to the interface. I doubt my cable length would exceed 2ft. I also wonder about the driver increasing noise as it does have a 6dB gain.
I'll likely start a new thread tomorrow when
I'm not using my phone.
I'll likely start a new thread tomorrow when
I'm not using my phone.
Thanks again everyone!
My preamp discussion thread was just posted here: http://www.diyaudio.com/forums/anal...ic-preamp-critique-questions.html#post2588071
My preamp discussion thread was just posted here: http://www.diyaudio.com/forums/anal...ic-preamp-critique-questions.html#post2588071
The 6db increase of gain is a byproduct of it being ballanced ...... No matter which method you use to ballance the signal it will allways be 6db louder than if it was unballanced ...... it shouldn"t affect noise performance ....
Cheers
What is a reasonable power rating for the phantom voltage setting resistor. The resistor from ADJ to ground on the 317 in the phantom supply. This line carries about 47 volts. I'm currently using a 3300ohm 1/4 watt resistor. I find that the resistor gets warm, not hot. I can keep my finger on it, but this is with no load. Both the resistor and the IC get warm, I have an isolated copper heatsink on the LM317. I have 100ohm on the current setting resistor (adj to out pins).
Logically P=IE and E=IR tells me I= 12.5mA so P = ~0.59 watts. To be honest, I'm not really sure what factors I need to consider when sizing that. If each channel (6 channels) draws 10mA, and I say 100mA for all the phantom, that is 4.8watts of possible dissipation. If I apply a load resistor on the output to draw 100mA, the regulator and its resistors start to get hot fast, as does the load resistor.
Am I thinking about this wrong?
Logically P=IE and E=IR tells me I= 12.5mA so P = ~0.59 watts. To be honest, I'm not really sure what factors I need to consider when sizing that. If each channel (6 channels) draws 10mA, and I say 100mA for all the phantom, that is 4.8watts of possible dissipation. If I apply a load resistor on the output to draw 100mA, the regulator and its resistors start to get hot fast, as does the load resistor.
Am I thinking about this wrong?
Thanks Sy!
Why? Does the higher wattage rating keep the resistor cool protect it I'm the event of a high current or voltage situation? Is there some rule of thumb for how much to oversize something like this?
Why? Does the higher wattage rating keep the resistor cool protect it I'm the event of a high current or voltage situation? Is there some rule of thumb for how much to oversize something like this?
using a bigger wattage resistor certainly keeps things cool 🙂 take a look at the datasheet for the resistor they usually have a temperature graph which shows the surface temp of the resistor for a given wattage. I was very surprised when I saw just how hot they can run even at only part of the rated wattage. for an example have a look at this datasheet http://www.vishay.com/docs/31021/cpf.pdf it has a graph for the temp of 1W 2W and 3W variants. at 1W the 1W resistors are running at ~100 deg C above ambient, but the 3W ones are running at ~50 deg C above ambient. lower temperature should equate to better reliability.
Tony.
Tony.
Keeping it cool reduces noise and drift in the set voltage. It can help long term reliability as well.
Thank you both.
As size increases, what are the best types of resistors? Metal film then wire wound and cement??
As size increases, what are the best types of resistors? Metal film then wire wound and cement??
I'd avoid wirewound unless they're non-inductive. If you're a hound for expensive parts, bulk foil would work well. Personally, I use the "flameproof" resistors off the rack from Fry's and the DC I get is just as pure. 😀
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