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Old 30th August 2003, 03:37 PM   #1
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Default Benefit of adding transformer to electronically balanced mic preamp?

Here is a real dumb newbie question.

There are a lot of good mic preamps on the market that use electronic balancing rather than input transformers, as good input transformers are expensive, and cheap input transformers can degrade the signal. The electronically balanced good mic preamps, popular for their excellent cost/performance, are never sonically quite as good as equally well-designed preamps using quality input transformers.

So, here is my dumb newbie question:

If the owner of an electronically balanced mic preamp also happens to have a first-rate 1:1 input transformer of the correct impedance, will the insertion of that transformer between the mic and the preamp tend to IMPROVE the sound, WORSEN it, or make no noticeable differene?

I'm not talking about modifying the electronic circuitry of the preamp in any way, exept to insert this transformer into the signal path between the mic and the preamp.

THANKS!

-- Chris Witmer
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Old 30th August 2003, 05:15 PM   #2
haldor is offline haldor  United States
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Advantages of transformers:
- Greatly improved common mode noise rejection (prevents hum)
- Galvanic isolation (prevents ground loops, also prevents hum).
- RFI protection is much easier to implement without negatively effecting the sound quality (avoid picking up the local AM radio station on your mic preamps).

Disadvantages of transformers:
- Physical size. May not fit inside compact gear.
- Cost. Good transformers are expensive and cheap ones aren't worth bothering with since they will saturate easily on low frequencies.
- Will block phantom power from getting to your condensor mic (only when added after the fact, products designed with transformers can supply phantom power).

Transformers are really good in an adverse environment (long distance cable runs, RFI, hum and or ground loop problems).

Bill Whitlock has a good writeup on balanced inputs and transformers. http://www.jensen-transformers.com/an/an002.pdf

Phil
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Old 30th August 2003, 10:10 PM   #3
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Quote:
Originally posted by haldor
Will block phantom power from getting to your condensor mic (only when added after the fact, products designed with transformers can supply phantom power).

Thanks for the information! Duh! I had forgotten that simple insertion of a transformer would indeed block the phantom 48V supply to the microphones. So much for that idea . . .

-- Chris
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Old 31st August 2003, 12:51 PM   #4
haldor is offline haldor  United States
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Hi Chris,

If one of the windings on your transformer has a centertap then you can feed phantom power into that (that's how preamps designed with transformers do it). You will need a low noise 48V DC supply, but the current requirements are very slight (50 mA is way more than enough). This is probably more work than it is worth however.

Phil
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Old 31st August 2003, 03:52 PM   #5
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Default transformers

This is a huge topic among professional audio engineers. The questions really is whether you want to HEAR transformer coloration or you are using them for their intrinsic electronic advantages. For the former you should look for transformers that introduce considerable amounts of harmonic distortions and other artifacts in the signal path. Certain Jensens tend to be too clean for this. Check out Buzz Audio's website for a discussion on how different transformers affect the signal. Also, a transformer can act as a noisegate for certain signals (takes a good signal to pass) and can delay high frequencies over low frequencies hence giving you a perception of more "bottom" in your microphone signal. Usually the circuit that follows the tranny is just as important for obvious reasons. If you insist on transformer coloration try out a MIC SPLITTER box with one input and two outputs (one being transformer coupled) and compare the two. You can send phantom through the direct output.
Cheers

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Old 31st August 2003, 08:50 PM   #6
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I really appreciate all the replies, and I like the idea of doing a comparison with the use of a mic splitter to hear the difference. Thanks!

-- Chris Witmer
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