Stereo Mic Preamp (with Phantom Power - diyAudio
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Old 12th September 2012, 12:29 PM   #1
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Default Stereo Mic Preamp (with Phantom Power

Hi Guys

I'm building a bit of a weird floor box for on stage. Its really a stage box that will fit around the mic stand, will have a DB-25 pin connector for a multicore to connect to. It will have DIs for my other instruments, xlr for vocals and a xlr monitor out . (I'm actually building one of these for each band member to speed up setup time! But they dont require this accordion system)

Part of this box needs to be a stereo preamp system for my accordion. Basically, my accordion will have two mics in it (treble side & bass side) and I want the sound man to be able to choose the balance between them.

So the box will have 5 pin XLR in (Two balanced signals from Acc) and have 2 balanced jack out. The output needs to be at line level as I want it to go into a stereo channel on the desk, but I still want it to be balanced out . The two mics in the accordion can be phantom powered, so I'd like to do that

I found this phantom power circuit. And I found this preamp circuit. That last project also mentions the INA103 chip that could pretty much replace that circuit.

So my question is how do I make the output balanced again (but at line level rather than mic?

I am very tempted just to buy two Behringer MIC200s and use those instead...

Cheers,
James

Last edited by jwheeler; 12th September 2012 at 03:28 PM.
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Old 12th September 2012, 01:11 PM   #2
AuroraB is offline AuroraB  Norway
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What mic's are you using inside the accordion?

Unbalanced to balanced - use tranformers! Gives you isolation as well.

The easiest way will surely be to use two mic input channels on a mixer. Then you have both tone controls and pan to set the image you want.
Or - use a mini mixer with phantom feed. I have Soundcraft Spirit Notepad, which should be perfect for the job ( not selling mine, though.. :-) )
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Old 12th September 2012, 01:15 PM   #3
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Bill Wall's DIRECT APPROACH - Microphones

I will be using this design of microphone when I build it
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Old 12th September 2012, 04:17 PM   #4
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It won't let me edit???
Am I able to do this (circuit on the right) with a transformer before running through the multicore.

Other circuits I've found like that say that it goes right before the input on the mixer, rather before the long length of cabling???

The other thing I found, which might just be the ticket is this (figure 3A). Which I guess just inverts the signal twice? Shall I just go for that one?
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Old 12th September 2012, 05:21 PM   #5
AuroraB is offline AuroraB  Norway
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The transformer will only adapt the single ended output you suggest to a balanced input.

If you intend to use the Panasonic capsules, you will need some form of preamp with power supply, at least to feed the built in JFET impedance converter. A 20-30 dB single ended preamp will give you better noise margin
Then you can follow with this....
ESP - Direct Injection Box for Recording and PA Systems

However, a mini mixer, like the one I suggested, will give you much better control. A stereo input on the mixer will only give you a bass/treble balance, not pan in a stereo image. For this you need to use two separate mic or line inputs. A couple of active boxes could also be used, but you stillneed one f the other circuits you suggested to feed the capsule.
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Old 12th September 2012, 05:43 PM   #6
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I am aware that the stereo channels will not be converted to mono before output, which would be ideal as we don't do stereo live mixing. However our mixer presents us with a work around. It has an Output B that can be set to work in mono, and be post main fader. So setting it's level to 0dB will result in a set of identical mono outs for Left & Right FOH.

I am actually planning to build a small mixer circuit in, but I decided it wasn't worth mentioning in the post as it is then getting complicated (using jack insertion detection to decide whether to run in mono or stereo).

My plan at the moment is to have:
Accordion Mics powered by -> Phantom Power into -> a Preamp (or Preamp chip) -> then balanced by circuit (See Figuire 3A) or transformer (Passive DI).

However before the balancing, I will mix the two channels together using one of the many mixer circuits on that ESP website. But only if only one stereo jack is inserted
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Old 13th September 2012, 03:27 PM   #7
Struth is offline Struth  Canada
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Hi Guys

Sound men cannot be counted upon to provide the balance of sounds YOU think is appropriate. They are mostly untrained and regardless of their background tend to just overpower the room with booming bass. SPLs at live shows are waaaaaay too loud for human perception to be accurate.

Your local system should basically mix the two mic sounds as you want them to be, then output a single signal for the accordion. If you have a guitar or other things you play alternately these can go through your local mixer as well.

Most sound men do not want to dedicate more than one channel of the house mixer to each instrument. They do not like miking cabinets and would rather DI everything. This is to their advantage not the musician's.

Can you tell I am dissatisfied with the state of modern live sound? The last good live sound I heard was in 1985. The band was King Crimson and they are very talented and therefore want the audience to hear what they can play. Overall SPL was loud but comfortable - not over 90db. In most small clubs drums do not need to be miked. The PA would handle voice and other quiet instruments. It should be possible for people to talk to each other at their table without shouting, as that causes more booze to be sold which is what the band is there for - to sell booze.

Have fun
Kevin O'Connor
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Old 13th September 2012, 03:48 PM   #8
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Quote:
Originally Posted by Struth View Post
Sound men cannot be counted upon to provide the balance of sounds YOU think is appropriate. They are mostly untrained and regardless of their background tend to just overpower the room with booming bass. SPLs at live shows are waaaaaay too loud for human perception to be accurate.
We have a good sound man who knows (and cares about) what we like. And he is very keen on making the sound be as good as he can make it.

Quote:
Originally Posted by Struth View Post
Your local system should basically mix the two mic sounds as you want them to be, then output a single signal for the accordion. If you have a guitar or other things you play alternately these can go through your local mixer as well.
You're probably right. Ok, I will make it only able to mono .

Quote:
Originally Posted by Struth View Post
Most sound men do not want to dedicate more than one channel of the house mixer to each instrument. They do not like miking cabinets and would rather DI everything. This is to their advantage not the musician's.
We don't use cabinets, our instruments are all acoustic. And the best we can do is use the built in pickups. We have a few instruments that are balanced out (my Taylor 614ce & our cellist has a fretless acoustic bass that has xlr out...). We only DI what we have to...

Quote:
Originally Posted by Struth View Post
Can you tell I am dissatisfied with the state of modern live sound? The last good live sound I heard was in 1985. The band was King Crimson and they are very talented and therefore want the audience to hear what they can play. Overall SPL was loud but comfortable - not over 90db. In most small clubs drums do not need to be miked. The PA would handle voice and other quiet instruments. It should be possible for people to talk to each other at their table without shouting, as that causes more booze to be sold which is what the band is there for - to sell booze.
In pubs we rarely mic the cajon as it is unnecessary . The bass carries well. The Djembé is too high pitched and quiet to not be miked.

As you can probably tell, we are not a normal band... We are trying to get the best sound we can without spending too much. I've been planning to mix the piano to mono too
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Old 13th September 2012, 04:19 PM   #9
Struth is offline Struth  Canada
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Hi Guys

Your situation is better than most.

Young guys that get on stage think they can crank it and often do. They end up with very bad sound. Unintelligible. No one can tell what they are playing or even if they are any good.

One thing to make sure of is that the mic cables to the accordion are very supple. There will be a lot of flexing, so some strain relief and/or a clamp on the outside of the instrument might be helpful.

Have fun
Kevin O'Connor
londonpower.com
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Old 13th September 2012, 04:23 PM   #10
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I have ordered the best connectors I know (they usually clamp the cable incredibly well!)

Neutrik FTW . The cable I've ordered should be good too. If its not flexible enough or doesn't seem of high enough quality to last, then I will avoid it

We are young, we are energetic, but we are adamant that sound is the most important part of what we do!
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