Noob mic phantom power, cable, balun question

J

jviss

New member
New to audio engineering, but I've been around electronics for a while.

Condenser mic requiring phantom power, 20 to 30' balanced mic cable, into phantom power supply (no mixer yet) and then need an unbalanced 1/4" phone plug for input to a -10dB tape amp (Teac 3440).

I've looked at the available inexpensive phantom power supplies, but don't see one that has an unbalanced output.

Is there an inexpensive phantom power supply with a built-in balun and unbalanced output (RCA or 1/4")?

How is this normally done?

Thanks,

jv
 
If you use an XLR to 1/4" TS plug, it will unbalance the signal. That's no big deal. However, -10dBV is a designation for a *line* input (consumer level), not a *mic* level signal. As far as I understand it, for proper impedence matching, you would need a preamp with a -10dBV output, or a preamp w/ +4dBu output, and a line level shifter, such as:

http://www.8thstreet.com/product.asp?ProductCode=3104&Category=Recording_Accessories

Unless I'm missing something, first you need a line level signal.-Richie
 
Richard Monroe said:
If you use an XLR to 1/4" TS plug, it will unbalance the signal. That's no big deal. However, -10dBV is a designation for a *line* input (consumer level), not a *mic* level signal. As far as I understand it, for proper impedence matching, you would need a preamp with a -10dBV output, or a preamp w/ +4dBu output, and a line level shifter, such as:

http://www.8thstreet.com/product.asp?ProductCode=3104&Category=Recording_Accessories

Unless I'm missing something, first you need a line level signal.-Richie
Click to expand...

Thanks. I may have misunderstood what I need to do here! The -10dB refers to the RCA jack "Line" inputs (on the back), but hte 3440 does indeed have mic inputs on the front, which are 1/4" phone jacks, with a switchable attenuator. Which input should I use, or does it matter?
 
Yes, it matters. The mic inputs on your venerable antique are rated at 0.6-10K ohms, in other words, for high impedence mics, AKA high-z mics, which have not been an industry standard for a very long time. Unless your condenser mic is as old as the deck, it is most likely a low impedence mic. Fortunately, during the transition from high-z to low-Z PA's, lots of folks faced the same problem, so relatively inexpensive transformers were produced to solve the problem, and they are still made. So-here's the basic solution: First get a standard XLR to XLR phantom power supply. Use a standard XLR to XLR mic cable from the mic to the power supply. Use another standard XLR to XLR cable coming out of the power supply. Plug that cable into this, or any number of units like it:

http://www.8thstreet.com/product.asp?ProductCode=9767&Category=Recording_Accessories

This will transform your lo-Z balanced signal into a high-z unbalanced signal. Plug that into the mic input on the front panel, and you are good to go.

As suggested above, the alternative solution would be to plug the mic into a small mixer, which needs to produce phantom power, and use the RCA "tape out" from the mixer to the RCA line ins on the back of the TEAC. Most line outs today are +4dBv, but the tape outs are generally -10dBU. I prefer the first solution because- Not only is it fundamentally cheaper and simpler, but the TEAC already has mic preamps in it, which are probably better than the preamps in most cheap mixers. Make sure you don't have any ground problems, because a ground loop with an in line transformer into an unbalanced mic input will produce the most Godawful hum you have ever heard. Best of luck-Richie
 
Thanks for the excellent reply! More, ...

...so, I confess I'm embarrassed, since I should know this stuff. Taking the 3440 as an example, the specification is "0.25 mV (-72 dB)/600 Ohms or more." What does that mean?

I looked at a modern mic spec, for the AKG Perception 220 (which I have my eyes on); the impedance is specified as "<200 Ohms," and the load impedance as >=1000 Ohms."

My understanding is that the practice is not to attempt to match impedances, as one would for maximum power transfer, but rather to bridge impedances, to get maximum voltage at the interface. So, I assumed the low output impedance of the mic into the high impedance of the mic input would be fine. Not true?

So, looking at the spec for the adapter you reference:
• Input Impedance: 250 ohms (nominal);
• Output Impedance: 50,000 ohms (nominal);

I can't help but come to conclusion that this adapter's input impedance is too low for the mic (will load it too much), and for the adapter output to mic input, if the impedances are close, you will have a significant voltage dividing effect, and therefore a diminished signal voltage at the mic input.

What I thought I wanted, in the same package as the adapter you reference, is a balun (balanced to unbalanced transformer) that reflects a high impedance from the mic input on the deck to the mic cable, so maybe a 1:1 ratio or so.

But, what do I know! Open to comments.

Thanks.
 
You should be fine with the mic the way it is , if your 3440 has an input impedance of 600 ohms and your standard moderm mic has something like 150 ohms output impedance then the match up isn"t really all that bad ......

A low impedance signal into a high impedance input isn"t a really a problem accept that High impedance mic preamps are inherantly more noisy but there isn"t much you can do about that .... the problem happens when you plug a High impedance source into a low impedance input ......

Just think most guitar amps have a very high impedance input , because most guitars are high impedance , But if your guitar has active electronics then it has an extremely low output impedance but there is no problem plugging it into a High impedance guitar amp and will actually be quieter in most cases than a Passive high impedance Guitar ......


Cheers
 
Now it gets interesting, because you know about electricity, but not about recording equipment. I am a perfectly good hack who knows how to hook up recording equipment, but my electrical theory sucks. I expect very soon, one of the many people on this board that know both will come along and give us both a perfectly unintelligible (to me) answer that will explain why what I said will work. I just know it will, because I've been plugging mics into things for a long time. Is there an audio engineer in the house?-Richie
 
Richard , I"m not saying that what you are saying won"t work because it will all I"m saying is that it probably isn"t necessary ......
 
OK Minion, I'm all ears. You think it's OK to plug low impedence condenser mics into high-z inputs? It may not be, in that sense *necessary*- it will pass a signal, but experience tells me it will generate a weak signal and an unacceptable noise floor. There is a reason why we don't plug low impedence mics into guitar amps, and why for about 6 years, we all had to use transformers like that to plug our new fangled low-z mics into our aging high impedence PA's.-Richie
 
Hi richard , I think you have it mixed up a bit , You loose signal when you plug a High impedance source into a Low impedance Amp .....

A High impedance Amp Needs very little current to drive it , Alternately a Low impedance source supplies much more current/Voltage than a high impedance source , So when you plug a low impedance source into a high impedance amp there is plenty of current to drive it ....
The problem is when you put a High impedance source into a Low impedance amp , The source gets loaded down badly and you loose signal ....

here is a snippet from a site ...
-----------------------------------------------------------------------

In order to preserve signal level and frequency response, it's important to drive equipment with a source signal that is lower in impedance than the destination equipment's input impedance. If the input impedance of a device is not significantly higher than the source impedance, the signal will be reduced or "loaded down" and its signal to noise ratio and frequency response will suffer.

Think of this as having a nozzle at the end of a garden hose. The garden hose is a low impedance source (there is little resistance to the flow of water) and the nozzle is the higher impedance of the input being fed by the hose.

When the nozzle valve is closed (open circuit):


Input impedance is VERY high
Pressure (voltage) is at maximum
Flow (current) is zero


Now open the nozzle just a little:


Input impedance reduces but remains high
Pressure reduces but remains high
Flow is small


As you continue to open up the nozzle:


Input impedance reduces further
Pressure reduces
Flow increases


With the nozzle open all the way:


Input impedance is very low
Pressure falls dramatically
Flow is greatest

-----------------------------------------------------------------------

You don"t plug mic into a guitar amp because a guitar amp doesn"t have enough gain to amplify a mic level signal .......

You would use a Transformer if you had a Low impedance source with a old High impedance PA to properly Unballance the signal and at the PA side (as old high impedance PA"s were usually unballanced inputs) there keeping the signal ballanced till just before the pa ....

I don"t just make this stuff up
 
OK Minion. First, understand that I am *not* arguing with you. So- let's assume that everything you say is true (I don't argue with facts). If a short XLR to TS cable is used from the phantom power unit to the TEAC 1/4" mic input, it should work fine, as long as the gain can be turned down enough to not clip the preamps in the TEAC. Because the main cable run (from the mic to the phantom power unit) is balanced, there should only be a nominal loss of signal by unbalancing the signal, which should be more than compensated for by the impedence mismatch. I think my confusion arises by misunderstanding that a high-z instrument input and a high-z mic input are two different animals. What I don't understand is- if you can unbalance the signal of a low-z mic, and plug it into an unbalanced 1/4" high-z mic input without any problem, why did they bother to build those transformers in the first place? It seems like all they needed was an XLR to TS cable.
 
I believe they used transformers to unballance because if the Mic has a transformer in it (SM 57/58 and many other Transformer Ballanced Dynamics) and you disconnect The negitive lead and just use Posistive and the Ground the Mic won"t work , with a transformer ballanced Mic both the negitive and the positive need to be used or it doesn"t make a complete curcuit .....

Pluss if you disconnect the Negitive on the mic of any Mic you instantly make it unballanced so you pick up noise through the cable , if you put the transformer at the PA end of the cable the mic stays ballanced through the cable and only is unballanced from the short distance from the transformer to the PA therefore you don"t get as much noise and still enjoy most of the benefits of being ballanced , It"s probably not much of a noise problem for jotting ideas down or jamming with the PA system but I wouldn"t want to use unballanced mic in a studio situation , so yes a Transformer would be good if you are useing the the High impedance Preamp in a studio situation ....


Cheers
 
Yo jviss! After all that, I think he just said you should spend the $12. I told you I was a hack, but I'm a perfectly good hack.-Richie
 
Richard Monroe said:
Yo jviss! After all that, I think he just said you should spend the $12. I told you I was a hack, but I'm a perfectly good hack.-Richie
Click to expand...

Yea, I felt bad after while asking the question, and then, I didn't want to interrupt.

What I get out of this is that impedance bridging is the standard practice, i.e., low source impedance to high load impedance; the transformer is necessary to keep the cable balanced; most convenient solution would be a phantom power supply/mic pre that has an unbalanced output, and use a short unbalanced cable from power supply to mic input, like the Rolls MP13; but it's probably cheaper and more flexible to just get a power supply and the transformer.

By the way, the transformer is just $10 via amazon and others.

Still not certain:
  • transformer has the right turns ratio for my application;
  • still not sure how to interpret the mic input spec from the Teac manual.

Thanks very much to both of you for your interest in this.
 
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