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Author Topic: Balanced dynamic mic pre-amp  (Read 3904 times)
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VE3ELQ
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« on: February 15, 2019, 01:10:52 PM »

This project started out as a 10 band EQ based on an inexpensive imported EQ board. A balanced input/output pre-amp with regulated power supply was designed and constructed to amplify a EV RE-27 ND dynamic mic input to about .3V rms to feed the EQ board and then buffer the EQ output to drive a fully isolated output transformer to the transmitter.  The intent was to tailor the lows, mids, and highs for best quality and roll off the high stuff above about 3.5 khz to tame the bandwidth.  The results were less than spectacular to say the least so the project was scrapped. But all was not lost because the little pre-amp part worked beautifully and when re-packaged into a metal project box it served very well to drive my Behringer 1124DSP parametric EQ that I had not used in years.  Following much bench testing and fiddling with an audio generator and scope the 12 Band parametric was programmed to achieve the goals producing superb audio while keeping the AM bandwidth under control. So far all on air audio reports have been great.

Attached are some pics and a schematic. For those who would like to try making a SMT PCB this would be a useful and easy to build first project.  The PCB pattern can be provided ready to laser print onto glossy photo paper and ironed onto some FR4 or G10 board ready to etch in a Peroxide/Muriatic acid mix. Guys its really not that hard.

FWIW, have fun, and if you build it please post your results.

73s  Nigel


* 20190210_160109.jpg (3616.25 KB, 3264x2448 - viewed 306 times.)

* 20190215_102337.jpg (2366.31 KB, 3264x2448 - viewed 329 times.)
* MicPreamp.pdf (3064.12 KB - downloaded 164 times.)
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« Reply #1 on: February 16, 2019, 05:55:13 PM »

Very Nice Nigel. 
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Patrick J. / KD5OEI
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« Reply #2 on: February 16, 2019, 08:33:05 PM »

Yeah real nice work there.
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« Reply #3 on: February 18, 2019, 08:29:21 PM »

With 300 uV in, what is the highest p-p voltage seen at the top of the output pot?

Am also considering a balanced mike input for some equipment.


Thanks


Phil
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VE3ELQ
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« Reply #4 on: February 19, 2019, 08:22:44 AM »

With 300 uV in, what is the highest p-p voltage seen at the top of the output pot?

Am also considering a balanced mike input for some equipment.


Thanks


Phil


In my current setup the OP pot is about 70% giving just the right level to drive the 1124DSP EQ to just below clip with a RE-27 mic, so about 500MV at the top of the pot.  Several ways to increase the gain if needed.  Increase the two 47K Rs to say 68K or more. Increase the mic gain trim pot to say 50K or more.  Modify the OP buffer op amp by adding a R from pin 9 to Gnd and a feedback R from 8 to 9 and play with the ratio.  Best to assemble it on an experimenters plug board (without the IP RF filters) and play with it. The low cost OP transformer does roll off the lows about 3DB per octave starting at 200HZ but I needed about 7db attenuation down there anyway so it is not a problem.  It does work very well as presented for my particular application.

Have also been playing with different EQ profiles using all 12 filters and have achieved close to a brick wall low pass response starting at 3.7khz dropping like a stone above that.  Sure keeps the BW respectable in a PW mod TX.

73s  Nigel
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« Reply #5 on: February 20, 2019, 11:53:49 AM »

Thankss Nigel.

I guess I misread your OP.

What is the p-p or rms output voltage of your mike with a normal speaking speaking voice of say 1" from the mic into your differential load of 2k?

Many low Z mics have a low output voltage in the millivolts range for speech at ~ 60 db SPL.


Thanks

Phil
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VE3ELQ
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« Reply #6 on: February 20, 2019, 04:25:31 PM »

Phil,
Sorry I misunderstood your question.  The RE27 spec quotes output per Pascal of sound pressure.  OK I guess if you know the sound pressure which I dont.  So I put the scope on it and measured 5 to 7 millivolts RMS in a normal voice with no load R.  Impedance is 150 ohms so it takes a bunch of gain to get to 500 mv.  This little pre-amp does it well with very low noise.

73s  Nigel
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« Reply #7 on: February 20, 2019, 07:32:21 PM »

That is similar to one of my 200 ohm Shure mics which develops about 3.5 mVrms across a 1k load with a normal speaking voice at about 1" from the grill.

So your pre-amp has a voltage gain of at least 50 or 34 dbV.

Good deal.


Phil


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MikeKE0ZUinkcmo
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« Reply #8 on: February 22, 2019, 12:04:52 AM »

Also similar to my station Tx audio buffer/amp.  This has 40 dB gain, and I drive it with a D-104 element.



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VE3ELQ
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« Reply #9 on: February 23, 2019, 07:24:52 AM »

Looks good Mike.  Have worked Ron WB2RON a few times who is also running a D104 into a High Z FET pre-amp which sounds really good just like a studio mic.  A good solution for that mic.

73s  Nigel
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« Reply #10 on: February 23, 2019, 11:44:17 AM »

Don Chester has a tube version of the differential Mic amp in ER #306.

It has a differential mic input and a differential output for driving a Push Pull modulator tube set.

Phil

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