balanced mic to phantompower to unbalanced input!?

perik

New member
hi!

Im going to build a small and simple preamp. the only thing im thinking of is how to go between the balanced XLR output on the microphone to the unbalanced input in the preamp.

First I was thinking of:

XLR
Pin 1 (Shield) ---> - (ground)
Pin 2 (Hot (+)) ---> + (signal)
Pin 3 (Cold (–)) ---> - (ground)

but then I was thinking that maybe Im in the future will connect Denecke ps2 or something similliar (or build one myself) phantompowersupply in between so that the setup will be like this:

condensermic ---> phantompower ---> unbalanced input in preamp

Ive heard that the mic will be damage if you would connect it as above (1 -> ground, 2 -> signal, 3 -> ground) into a phantompowered input. This make sense.

My question is:
Do I have to worry about this if I connect it like this: condensermic ---> phantompower ---> unbalanced input in preamp if I have got the cable with 1 -> ground, 2 -> signal, 3 -> ground beween the phantompower unit and the unbalanced input???

can I connect it like this instead on the unbalanced input on the preamp. with not connecting the shield into anything:
Pin 1 (Shield) ---> without any connection
Pin 2 (Hot (+)) ---> + (signal)
Pin 3 (Cold (–)) ---> - (ground)

or will I have problem with noise?

or do I not have to worry about this because the phantompower is before the unbalanced input?


thanks for your help!

Perik
 
Perik,

Why would not you add a transformer input to your pre and make it balanced? It would be the easiest solution, but still keep the things easy.

Otherwise, you could also use a separate Phantom PSU to power the mic, then decouple the output of the PSU with 2 47uf capacitors before pins 2 and 3 of the XLR. In this case you will be able to jump pin 3 of the XLR connector to the ground, and feeding your unbalanced pre. Although never tried this, I don't see the reason why it would not work.
 
thanks for your reply!

how do I build a transformer input then??

so your're saying I should do like this: (repeating just to make sure Ive understood)

Mic:......................external phantompower:...................unbal in preamp:
Pin 1 (Shield) ---> Pin 1 -----------------------------------------> (ground)
Pin 2 (Hot (+)) ---> Pin 2 (Hot (+)) -----47uf capacitors--------> + (signal)
Pin 3 (Cold (–)) ---> Pin 3 (Cold (–)) ----47uf capacitors--------> (ground)


this right?

is this the only solution or is it possible to do like this as well (the input in the preamp is already for miclevel)

Pin 1 (Shield) ---> without any connection
Pin 2 (Hot (+)) ---> + (signal)
Pin 3 (Cold (–)) ---> - (ground)


or like this if Im not are using the external phantompower:

Pin 1 (Shield) ---> - (ground)
Pin 2 (Hot (+)) ---> + (signal)
Pin 3 (Cold (–)) ---> - (ground)

are any of these wrong? in what case which one or is it possible to use both solutions?

thanks

P
 
anyone ...

Will this solution work:

condensermic --<XLR>--> phantompower --<modified XLR>--> unbalanced input in preamp


modified xlr:
Pin 1 (Shield) ---> - (ground)
Pin 2 (Hot (+)) ---> + (signal)
Pin 3 (Cold (–)) ---> - (ground)


with a "phantompowersupply unit" in between? Its only when the phantompower comes straight from the input where the "modified XLR cable" is connected i risk to destroy my mic?


thanks a lot for your help! very much apreciated!!!

/ p
 
Hit Google. There are about 100 DIY plans for preamps and they all have balanced inputs. You woul dbe better off looking at a tried and true design IMO then us guessing.
 
Why not just make a Preamp with a Ballanced Input/Output?? You can do this without a Transformer by useing a Ballanced Line driver or createing a Dual inverted output useing an IC.....

If you need help with this I have a little experience Building preamps and designing preamps and can show you ho to Create a Ballanced input/output without useing an audio transformer and how to implement Phantom power ect....

I am actually working on designing a Ballanced Low noise Preamp with a Pentode Tube stage and a whopping 75db+ of Gain, It has 3 Gain stages(Pre,Post and Tube) so you can dial in what type of sound you want and the Tube stage can be enabled and disabled with a switch, It also has Phantom power and a 20db Pad.....
I worked straight for over 6 hours last night designing this pre (the second incarnation of this design) and have gotten about half way through the Tube stage and still have to finnish designing the tube stage and the Fet Impedance buffer and the Dual Inverted Line driver for createing a Ballanced output and then comes the Prototypeing and Debugging stage which will take probably a month if I am lucky and if all is well this should be a Very Cool little project.....

If you want to take a Look at the schematic let me know as I should be Finnished with it tonight or tomorow and if you need any help with the Preamp you are working on or need help implementing an Idea I will be Happy to help out if I can.....

Cheers
 
thanks a lot for your help.

ive tried the preamp now with this solution:

Pin 1 (Shield) ---> - (ground)
Pin 2 (Hot (+)) ---> + (signal)
Pin 3 (Cold (–)) ---> - (ground)

unfortunately it was not exactly dead quiet and not as powerfull as I hoped it to be. I was going to use it to take mic level into line level on my Iriver H120 to be able to make field recording.

just a question: can it that if I put up the voltage (at the moment from 9 v) to for example 18 v that the preamp will amplify more or has this nothing to do with the voltage?

do u know any other design or way for me of building as close as possible dead quiet preamp??? have u got any circuit u think would do the job? my two criterias are that it shall be small and portable (something like Denecke AD20 in size) and give me a stronger signal without adding noise. this was supposed to amplify 40 dB. not sure it gives so much amplification. dont think it does but otherwise I need something like 50 dB.

thanks for your reply

/ p
 
Hi, I think it was you who e-mailed me from the "Mic builders" yahoo group asking if I had any schematics for a Simple battery operated preamp...

Well I e-mailed you one that is very simple (2 Transistors) that should provide about 40db+ of clean gain and Run off of a 12v supply....

Check it out.....
 
no it wasnt me. can you email me as well? ive pm:d u mine email. have u tested it? no noise?

12 volt supply. hmm what kind of battery will provide that? well thats later problem. can it be that it doesnt amplify as it should if it gets to less volts. for exemple that it only amplifies 30 dB if it gets 9 volts or has it nothing to do with that?

thanks
 
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Hi, actually the Original curcuit was intended for a +/-30v supply but it was Modified by the designer for Battery opperation....You would have a lower noise floor and less gain if you ran it on 9v, but you would have much more headroom and less noise if you ran it useing 2 9v batteries in series to give +18v but you would have to adjust a couple resistor values.....

Since it is a Unballanced design there will be some noise introdeced through the Cables because of the Lack of CMR but if you keep your cables short you should be able to minimize this.....

Here is one that uses a 9v battery and also supplies Phantom power for Electret Mics and has a seperate Input for Dynamics and electrets:

http://www.ee.washington.edu/circuit_archive/circuits/micamp.html


Here"s the 12v version which probably has better performance and less noise but is a Bit more complicated, Scroll down half way down to see the 12v version:

http://sound.westhost.com/project13.htm


I also have lots of other Mic preamp schematics but not many Battery powered ones.....


Cheers
 
ok. i think i will try to build the second one. have u tried it?

in the description it´s said that
This is a design for a low noise microphone preamplifier, which is ideally suited to low impedance (600 Ohm nominal) microphones.

Im going to use it together with among others Shure SM58 with
Nominal Impedance: 150 ohms

and AKG c1000s wit Impedance: 200 ohms

how does this affect the preamp, that they are below 600 ohm?


how much expectations do u think I can have. can I expect it to be as quit as something u buy or is that to much (u believe so)

my main criteria is that it shall be quiet

thanks
 
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With a Lower Impedance you can expect better performance, The preamp should work well with Mics from 150 Ohms up to about 2k ohms without much loss in performance....

No I haven"t built any of these Battery powered ones as I haven"t the need for a portable unit but I am useing a simular type curcuit as a Front end of the mic preamp I am designing (it is closer to the 30v version accept it uses a +/-15v supply)...

The pre amp should have acceptable performance with a relitively low noise floor but it won"t have the Dynamic range of Higher end preamps but what can you expect for $5 in parts and something that can be fit into a Match box (excludeing the Battery).....

You could build something of Considerably better performance but it will also be much more complex and more expensive and probably need more Power.....


Cheers
 
have u got any more advanced circuit with better performance that´s still is compact?

one more question regarding battery:

can I use a transformer which I connecting to the wall which produces DC [U)]---[/U]) and in that way put up the power or can this harm the circuit. (cant be any difference can it?)
 
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Hi, no I don"t have any "battery powered" pre"s that have better performance, a More Complex Curcuit will draw More power which will make it so you won"t be able to use Batteries or the Batteries will die very Quickly....

You can use an Ac/DC adapter but you will have to Has some filtering to the Power Rails as there is a Lot of Ripple in the DC Comeing from an adapter so you need to add some Caps to smooth out the ripple, Just put a couple say 470uf Capacitors off of the Positive power rails and tie them to ground and bypass them with a 0.1uF poly Cap and this will smooth out the Power supply ripple.....


Cheers
 
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