Pre-amp for interface of a Balanced/MC mic into 2-wire electret mic input

Hi, Tim,

We are working on this for you... Please stay tuned...

-d2

My sample TLV1018 and TLV 1015 arrived.I was over-optimistic that we could proptotype with these even with the lab's smallest solder tip. Is there a leaded variant or sample available mounted into a test board?

Hi, Tim,

I don't support this product directly, but I checked our internal doc servers, and there is no EVM available for this device, nor other package options...

I assume you looked at the INA parts, those are in bigger packages.

-d2

Tim, Don,

I don't believe that our INA products would be suited to this application. A tricky op amp circuit might work.

Are you able to make additional connections to the circuitry? A normal computer type electret mic interface only has two connections--ground and the supply/signal pin (through the resistor). Must the circuit reside on the two-wire side of these connections?

Regards, Bruce.

No Bruce, this product necessitates an interface to the two wire circuit intended for interface to an electret without modification, or access to, the host system. The LVT1012 parts, et al, present something of a challenge. Pity that a SOT23  or similar isn't available but I can quite understand why not

Tim

Tim,

Here is a possible approach using an op amp. The op amp is phantom powered from audio output line. I've used this approach to power op amps on a pro-audio balanced phantom powered microphone:

http://www.edn.com/article/486291-Microphone_uses_phantom_power_.php

I've never tried it on a computer mic interface where the voltage and current available is much lower. It seem to work in simulation but I have not tried it. I'm not sure if a gain of 10 is enough. It requires a micropower op amp and GBW is modest so you can't get much gain.

Regards, Bruce.

Thanks Bruce,

Regarding power availability, my nominal 2v (not a pc) supply is via 1k8 source AND it can drop further so I am aiming to have my circuit functional at 1.7v, nevertheless the OPA330’s 1.8v & 35uA/ch ratings look promising. Not least it’s SOT23 package!

Our dynamic microphone input source is balanced but I may treat the mic as a single-ended device as I have no long cable runs. Gain-wise, I am looking at between x2–x8.

 Tim

Tim,

As you can see, I simulated with 2V and 1k8 on the receiving end. It gets messy to create a differential input. so I made it single-ended.

A note on the choice of op amp. The OPA330 is a chopper op amp with very low offset voltage. This is, of course, not needed in this application but the combination of bandwidth, quiescent current and operating voltage is a good fit. The idea of a chopper op amp in an audio application sounds scary but I think you would find it to be no problem. Chopping noise is very high frequency and well below the broadband noise level of the device. Your max gain of 8 may open other possibilities for op amp choices. Also, if your frequency response requirements are not hi-fidelity it opens more choices. Is this just for voice applications?

Regards, Bruce.

Yes, low bandwidth, it's only for voice. That said, I don't want to profile its response excessively in that purpose. I missed the chopper spec, I don’t have a need for super-low drift/offset. Would you suggest any other contenders in this low v/I application? I have other HF noise in the vicinity anyway so I am keen to get in the lab with the OPA330.

Tim