i am looking for suitable replacement for SSM2166 (mic preamp)...pls suggest the best solution

     Hello,

     I don't see an ADI drop-in replacement for the SSM2166, however I've experienced mic preamp RFI susceptibility & have found it best to use RF filtering ahead of the chip.  Series 47 uH inductors in each input line with shunt 0.01 uF caps to ground work well.  This combination resonates at about 230 KHz, sufficiently below the AM broadcast band.  Be careful not to allow your filter to resonate within broadcast frequencies or else you may end up with a radio receiver instead of a mic preamp.

     All differential amplifiers have sensitive input transistors with junctions that can rectify / detect RF energy.  Keeping RF away from them definitely helps.

     Best regards,

     Bob

Hi,

Thank you, Bob! The suggestion of an LC on the front end is excellent.

I would add that the SSM2166 can be treated as a differential input. Is your source differential? Are you using a twisted pair wire with a grounded shield, to connect your source with the SSM2166? Even a single-ended source can be improved by treating it as much like a differential source as possible. Can you share some more information about your system?

Best regards,

Coleman

hi KJ Bob & Coleman,

Sorry for the delay. It was long time i had opened engineer forum. My system is differential and today i made one filter circuit with series 47uH and shunt 0.01uF. Two circuits ( each one for Mic-HI line and Mic_Lo line) and i had made and connected the filter circuit before the microphone  signal reaches the SSM 2166.

i will let u know the results of RFI susceptibility with this circuit.

Thanks & Regards,

Naveen

Dear Mr. Bob,

I had made the circuit...47uH in series and 0.01uF in (for each line of Mic_HI and Mic_LO) shunt to ground. And tested for the RF susceptibility. I didnt got any improvement. When ever near by VHF system operating at 118MHz to 134MHz goes into Tx mode, i am getting continuos whistle type of noise/audio. If i remove SSM2166 in my circuit, then system is OK. But i have no chance i have to use SSM2166 in my system. And I increased order of the flter to further cut off the RF energy into the audio system. But no improvement.

Please suggest the suitable circuit or how to overcome this problem.....

Thanks & Regards,

Naveen

Dear Mr. Bob,

I had made the circuit...47uH in series and 0.01uF in (for each line of Mic_HI and Mic_LO) shunt to ground. And tested for the RF susceptibility. I didnt got any improvement. When ever near by VHF system operating at 118MHz to 134MHz goes into Tx mode, i am getting continuos whistle type of noise/audio. If i remove SSM2166 in my circuit, then system is OK. But i have no chance i have to use SSM2166 in my system. And I increased order of the flter to further cut off the RF energy into the audio system. But no improvement.

Please suggest the suitable circuit or how to overcome this problem.....

Thanks & Regards,

Naveen

    Hello Naveen,

     Sorry that this difficulty remains.  I had confused the SSM2166 with the differential-input pre-amp ICs often used where I work, but actually the SSM2166's data sheet shows it has a single-ended input.  Thus the MIC-Low input is the same as ground, and only one LC filter is needed as shown below.  This schematic is simply that of the chip's evaluation board, to which I added the LC filter.  I show it as a pseudo-differential input in case you wish to use a three-pin mic with the usual "balanced" (2 conductors plus shield) cable -- however in this case you would be better off with a true differential preamp (or an input transformer, but those get expensive)..

     I'm wondering how strong, and how close, is the FM transmitter involved.  Also whether the preamp circuit is in a shield box.  I also show in red a 220 pF cap you could try adding directly across the op-amp inputs.

     Best regards,

     Bob

Dear Mr.Bob,

Mr.Bob, thanks for ur reply and showing interest in this problem.

Here in our system the MIC-HI & MIC-LO lines are given to a transformer which is a centre tapped one. And the secondary of the transformer is connected to Pin.no.7 (IN+) of SSM2166. And other end of secondary of the transformer is connected to pin no.6 (IN-) through Resistor and capacitor.

So, i will try to connect 220pF in b/w pin no. 6 & 7. It is an VHF AM transceiver antenna is near by to the audio system. And our audio system is in a sheilded box only. Pls suggest any material comes which can be pasted inside the audio system box to eliminate the RF entering into audio system.

Or any other differential amplifier will u suggest beofre the micrphone signals enters into the Audio system. In our audio system SSM2166 is the front end stage.

Thanks & Regards,

Naveen S

Could you share the schematic for the MIC-HI and MIC-LO inputs through to the SSM?  Are these two inputs going to opposite sides of the primary, are they using normalling jacks, etc?  How is the ground scheme implemented?

     Hi Jon,

     I'm glad to have you jump in here with your expertise.  In fact I was going to "@ mention" you but thought that a bit presumptuous.  I'm sure you can help Naveen get the RF out of his preamp as you helped get the noise out of the mixer I've been working with.

     Best regards,

     Bob

Hello Jon,

Thanks for sharing my problem.

Yes, these mic-hi & mic-lo lines are twisted shielded pair and are going to opposite ends of the centre tapped primary transformer. And the centre tap is connected to ground. Basically there is an antenna which works in VHF band near by the audio system. The power output from the VHF transceiver when in Tx mode is around 40W. So, at times when the transceiver in transmission mode, we will listen whistling/whining background noise and resulting unable to transmit through the transceiver. The audio system is housed in a metallic box and well grounded.

Some how the RF esp.VHF is entering into audio system and creating oscillations in the total system.

Pls suggest me how to aviod this RF entering into the audio system.

The secondary of the transformer terminals are connected to IN+ and IN- of SSM 2166 IC.

Thanks & Regards,

Naveen S