Electrical – How to connect microphone to computer? Pre-amp with galvanic isolation


I have audio card with Hi-Fi input, but it is strictly stereo TRS (not balanced).

I have condenser microphone with balanced TRS output.

I found that mic output is very low, and I receive too much noize from its shield.

I need to connect microphone with galvanic isolation to drop all noize, so it should have absolutly clean zero level. I know it is capable of at least 50 dB of SNR, but now only 15 dB.

I opened mic, everything looks good, it is supplied with small (passive) powering scheme inside, involving few small transistors.

Ok, I have done some research and came to conclusion, I just used condenser mic without any power supply (hehe).

After some considerations I came up with this scheme.
Am I right to not connect shield from everything to GROUND/MIC_IN of sound card?
I will probably use batteries to achieve perfect noize-free environment.
Any recommendations on using capacitors before amplifier?

enter image description here

Best Answer

You haven't given any datasheet link for the microphone so this is all a guess.

  • The mic is low-Z - probably 600 Ω output.
  • Your amp is hi-Z - probably > 10 kΩ input.

The simplest solution is a microphone transformer.


simulate this circuit – Schematic created using CircuitLab

Figure 1. Microphone "unbalancer" and impedance matching.

A really good transformer is expensive and some active circuits may give better frequency response for the same money. The big advantage with the transformer solution is that no power is required.

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Figure 2. A typical microphone impedance matching transformer. No messing!

It now appears that the microphone is a 48 V electret. This usually means that it is phantom powered.


simulate this circuit

Figure 3. Rough schema for a balanced line microphone to unbalanced hi-Z input.

The usual scheme for a balanced line, phantom-powered microphone is as shown in Figure 3.

  • Power is fed through the centre-tap of the unbalancing transformer at the amplifier end. Current splits both ways through the transformer (cancelling out each other's magnetising so the core doesn't saturate) and travels along both signal lines to the mic. The screen is the 48 V return.
  • At the mic the reverse is done and the 48 V current is extracted from the centre-tap. This is filtered and regulated (neither shown) to power the signal amplifier and provide the electret bias (not shown).
  • The electret capsule will have a high output impedance. The amplifier will buffer this and drive the hi-Z input to the transformer - typically 10 kΩ.
  • The transformer steps down the signal and decreases the impedance to 200 - 600 Ω.
  • At the receiving end the signal is unbalanced, the signal stepped up and, as a result, the impedance raised to 10 kΩ again.
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