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Old 16th January 2013, 04:57 AM   #11
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I know it's an old thread, but I see you DID update it in July '012. Why did you abandon the notion of using 48v phantom power? Wouldn't ohm's law prevail and give you suitable current at, say plus minus 18V? It seems to me starting out with 48volts/amps/current is much better than splitting 9volts or even lipo 12volts? Also, did you succeed in getting a suitable case, and would you be willing to post your schematics? I'm fitting a boundary mic in my latest prototype acoustic that I just finished. I was thinking of just using phantom power for everything. Here's a photo showing the condenser mic built into the hand-carved bridge.
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Old 16th January 2013, 10:37 AM   #12
teleman is offline teleman  Norway
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Firststly, congratulations on your nice unusual guitar! Unusual shape too! This may go very well in the AGP Forum!

I opted for BJT & JFET ics known to me at that time & the circuit consumes fairly high current around 50mA. So this ruled out single 9v battery.Then I tried 2 x 9v in +/- as a virtual psu, but this didn't sound as good as it did on +/-12v DC. The mic is then biased directly from the circuit board & since the mic was fitted on the guitar & very close to the onboard preamp, I found phantom powering & balanced output unneccessary.

If you're planning using p.power, then remember that the maximum current it can ouput & carry through a balanced guitar cable would be only 5mA! But this shouldn't be a problem if you're going to fit the battery inside the guitar. Why then use p.power? If your circuit requires only 5mA max on a single supply then p.power is viable if you're planning on external psu/phantom powering.

I know that there are opamps which can work on +/-18v...but then you'd be driving these very hard & hot,reducing their life cycle IMHO. This shouldn't be a problem for just +18v.

One thing discovered was that fitting my heavy preamp/case @350g actually reduced the timbre & loudness of the guitar considerably,at least when played acoustically! However, this didn't have much detrimental effect when elctrified. I've now made an ABS case,actually modified, & glued together from various stock cases...which is much lighter & doesn't affect the guitar sound as much as it did with the aluminum case. There were no EMI or RF issues either what so ever.

As for the circuits The charge circuit is exactly the same as the one shown earlier here by someone with just the 2 bias resistors removed for dual supply. The mic circuit is the much loved ,modded & discussed "Linkwitz" 3 wire modified. Summing/mixer/output is just a 2 input virtual earth opamp with low noise BJT like NE5532,LM4562 or similar for this. JFETs will not work well with long cables & tend to oscillate. These also have fairly high o/p impedance. JFETS are no good for output stages,I'm sure you know this already!

Last edited by teleman; 16th January 2013 at 10:50 AM.
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Old 18th January 2013, 04:22 AM   #13
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WOW! Thanks teleman for answering! 'Teleman' (i'm guessing because you either play a telecaster, or worked for Ma Bell as an audio engineer...or something). Thanks for the updates. I can't believe that only 5ma survives the cable run for a standard length XLR mic cable. If that 5ma potential is for 48volts, wouldn't that increase when the voltage is lowered (ohm's law)? Believe me, I'm no electrical engineer, that's why I depend on the input from some of you guys on this board. I think it is still an elegant solution for this particular design. I just want to use the electret (linkwitz modded of course) with no option for e.q., undersaddle piezo, or even a volume control. I've mixed sound on many different boards, and they all (even the cheap ones) have much better noise specs and sound shaping (parametric e.q., effect sends, etc.) than ANY onboard preamp. Especially since most onboard pre's are only 9volt. If you think I'm crazy for trying, please let me know, and keep in touch. I don't know if you even play acoustic as your main instrument, but you can see that I'm not running with the pack. I'm a firm believer in DIY, and stepping outside the box. If you can help me out, I would much appreciate it. I have looked into the LM4562, and decided on a balanced line driver instead - the LME49724. Check it out. I'm looking into a 'shoeps' style balanced front-end with the (phantom powered, mind you) 49724 outputting to a mixer console. My acoustic clientele would probably have an audio guy running sound for them, anyway.
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Old 18th January 2013, 05:01 PM   #14
teleman is offline teleman  Norway
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Thanks Marvinkite,
Good guess/assumption.Yes, I do own & play a Tele..albeit only a 1977 Ibanez clone. I Had a Washman Falcon briefly,but sold it quickly. It was no surprise that my user name was a giveaway. Lol. As for being a tech at Ma Bel...never been near that place! don't know even where the h--- it is. I'm actually in a different part of the globe! I wish I had done my share behind desk,as it was & still in front of it, on stage, mostly I guess!

I now play ony acoustic & enjoy every minute of it! Enough said.

Ok, here is my take on mikes & preamps..so take it with a pinch of salt will ya?

Most condenser onboard pre amps are designed to accept 48-52v phantom power (dc) @4-5 mA directly through balanced mic outs from mixers. In addition, onboard pres are also generally powered by a 9v PP3 battery, providing an unbalanced output. As a rule ph.psu is designed to deliver only this small current & is stipulated as the industry standard protocol. So,to facilitate this, very low current ics are employed in the pre-amp designs. However, it won't be any problem if you only want to feed the mic with 48v @4mA! To quote you,"If that 5ma potential is for 48volts, wouldn't that increase when the voltage is lowered (ohm's law)? " yes, but then please remember that guitar cables can carry only very small ac signals of few mA & the cable may well melt it's sheathing/insulation & short circuit the whole thing in the process!. Another alternative would be is to use 12v supply aka "T" powering. Then again this too delivers only up to 12v @ fairly low currents. You can certainly power any condenser mic by p.power,but one has to aware of current constraints of driving high current pre-amps through signal cables.

As for using the mic pre amp without any eq or gain control, will not work IMHO as you're assuming.Let me explain. A high gain,un eqed mike/ pre with a gain of 20dB or so will behave quite differently when performing measuring tasks in free air to being inside an enclosed chamber,i.e.the guitar body! Since the mic may likely to have dips & bumps in it's freq.response, it will definitely howl & feedback without any proper gain control,eq or feedback limiting circuit! Mics are difficult beasts to tame, especially in confined, enclosed spaces. So driving high level unfiltered signals directly to a mixer board is certainly not a good idea! Ideally,you'd need a low gain input stage, some 6-12db eq control in the least to make it viable. Besides, Just imagine having to depend entirely on the sound engineer every time you get uncontrollable feedback or need to make simple adjustments such as volume & so on? Bad idea i.m.h.o!

Good on you mate that you too are thinking out of the box.Believe me, I too spent the best part of last 4 years experimenting & doing things out of the box with some mixed results. However,in my case, it was mainly due to my lack of solid technical knowledge, which allowed me to imagine solutions beyond any theoretical reasoning or possibilities! Yes, I did solve a few unusual problems in my ignorant bliss...yet could have saved my self a lot time & frustration, if only had I looked up & read these subjects beforehand! I probably would have wasted even more valuable time if it wasn't for some good help & advice from a few kind fellow members in this & a few other forums.

Do experiment by all means,but also use & take on board tried & tested methods & advice from our peers as well. Read as much as you can on every stage of the subject! Sorry if I sounded patronizing!....never meant to be though!

Keep on....

Last edited by teleman; 18th January 2013 at 05:14 PM.
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Old 19th January 2013, 12:35 AM   #15
teleman is offline teleman  Norway
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Marvinkite, this is in response to your earlier question you posted.

"Why did you abandon the notion of using 48v phantom power? Wouldn't ohm's law prevail and give you suitable current at, say plus minus 18V? It seems to me starting out with 48volts/amps/current is much better than splitting 9volts or even lipo 12volts?"

I think you've misunderstood phantom power supply am afraid! Phantom power is just +48v single rail dc supply with a max. current capacity of just 4-5mA. This voltage is first tapped from either the main or a dedicated psu of a mixer,mic preamp or a dedicated phantom psu unit!

This is voltage is then split by 2 matched 6.81 resistors & fed to +2 pin (non inverting) & -3 pin (inverting) of all or a selected few XLR mic inputs of a mixer or mic pre-amp! Hence this is also known as active balanced mic input or passive when ph.power is switched off.

Phantom supply is primarily to power condenser mics which require 48v supply voltage, but can also supply most condenser mics requiring 9- 48v,since low voltage mics have their own internal attenuator circuits to accept the required lower supply.
If an onboard pre amp is designed with low current ics to consume only 4-5mA in total,then p.power supply can be supplied to the unit through an XLR & the pre-amp output is taken through the same XLR +2/-3pins at the same time!

Last edited by teleman; 19th January 2013 at 12:56 AM.
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Old 19th January 2013, 07:08 PM   #16
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Talking I'M SUCH AN IDIOT

First, I didn't even connect the dots with the British Union Jack right there! Uhgggg! Sorry Mate! Ma Bell relates to the famous Bell telephone corporation here in the U.S. that fell out of grace with government anti-monopoly regulators and had to be broken up into several spin-off companies. 'tele'-is short for telephone, and Bell Labs is a proper breeding ground for many audio engineers, which I thought you might be one of. I'm glad, however, to know you are a musician after all. I DID check out T power that utilizes lower resistance levels on balanced audio rails to allow up to 12vdc instead of pinching off the phantom 48 volts with 6.8k resistors. I can also bias the dc potential for the electret's FET source voltage with a 10volt zener, or series resistance drop (with a little more noise) and still drive an op amp with phantom. I agree that having some control over the dynamics of the mic would be wise, but this mic is NOT inside an enclosure. It is a boundary mic that utilizes the half-space of the guitar top as the pickup area. The guitar has an anti-feedback grille to help break the sound waves coming out ot the body, helping prevent feedback. Even the most sophisticated onboard mic pre's don't have limiters or threshold adjustments for gain stages, which modern mixers with effects all have. In short, I'm not thoroughly convinced that onboard pre's are better. I AM convinced, however, that microphones are the best transducer for acoustic guitars over undersaddle and under top (even dual placement) piezo elements or magnetic sound-hole type pickups. These produce unnatural sounds. Sometimes, less is more-if done correctly, which is what I'm after.
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Old 24th January 2013, 04:02 PM   #17
teleman is offline teleman  Norway
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Union Jack! I must be in the wrong country again! Lol!! It's norwegian actually!

Ok.So you want to power this boundary mic, right? Does it run on 9,12 or 48v dc? The whole idea of feeding 48v p.power is also to have balanced in/out buffer to the mic with low impedance, better noise & increased o/p level. Add a 100k? resistor in series for 12 volts. "T" powered (24v) circuit is a little cumbersome & uses only resistors

I think zeners are noisier than resistors. I totally agree with you on your reluctance to use UST,MPUs etc. I've been there & tried all that. However, you'd need to power the mic & the FET, hence require some sort of power source & a nice low noise balanced mic buffer circuit. All this can be built quite easily on a 1 sq.inch perfo board & fitted inside the guitar or on the mic itself!

Last edited by teleman; 24th January 2013 at 04:04 PM.
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Old 25th January 2013, 04:18 AM   #18
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Default OMG IDIDITAGAIN!!!

Sorreeee! I guess I'm that arrogant American who doesn't know his european flags. Soooo sorry. Norway is AWESOME! I have actually studied about the fjords and certain aspects of your Country (like it bordering old communist Russia). Way cool! Ummm, I guess I can send you a schematic that I have been working on, but I also want you to check out the LME 49724 balanced line driver. I would like your opinion if you think phantom power can drive it. Let's stay in touch, and I'll send you the two versions I have been breadboarding. Let me know if you want to see other pictures of my acoustic prototype, yeah?
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Old 25th January 2013, 05:56 AM   #19
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Default Phantom powered preamp

Hi Teleman,

it is no problem to find a low power op amp for your needs. There are some low power op amps on the market. I use the LT1014 in my Smart DI ( The Smart DI ) and still had some power left to feed a microcontroller and an lcd display. However pls. note that its always a balance act between low noise and power consumption. The low power op amps usually have a bit higher noise floor compared to the "high power" op amps. For my opinion the LT1014 is a good compromise between that and with the noise floor almost similar to a 084. The rest is just a bit power saving design. So, in your case you still can use a microcontroller and an lcd display for a build in tuner...

Have fun!

All the best!
Axel
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Old 27th January 2013, 04:45 PM   #20
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Hey tonaxel, welcome to the thread. This has to do with this acoustic guitar prototype that I made, with a boundary mic built-in to the bridge. (See pics) I'm trying to make a siutable circuit for an XLR output that runs on phantom power, so no batteries are required. I want this circuit to be minimalistic - no tone control, maybe just gain. I already have a version that uses a built-in unit with four-band E.Q., undersaddle piezo, condenser mic and a digital tuner (made in China, of course) as an option. I want this to have a true balanced output and a linkwitz mod source-follower type circuit that has higher SPL, lower noise, etc. Any ideas?
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