Simple Microphone ECM buffer questions

[oneoldude]

This is an unbalanced mic buffer for measurement mic use. It will be built into the mic wand and allow easy swapping of ECM capsules for loudspeaker testing and capsule testing and provide for longer cables than the ECM capsule alone.

The output will go to an unbalanced mic pre with switchable gain and from there to unbalanced sound card line in. All inputs and outputs in the chain are unbalanced.

I posted my first try in another forum. See post #37 here: http://www.diyaudio.com/forums/solid...quested-4.html

After a lot of reading and outlining the discussion of jan.didden and Amblifier in the thread above, I think I have come up with an improved circuit below.

Here are my calculations. Pardon the non-conventional presentation.

Ic = .0045 A = 4.5 mA (Should swing a bit less than 4.5V P-P into 1k. Input will probably never see more than 1V P-P [but you never know!] and load should be well above 1K)

Re = 9V/.0045 A = 2000 Ohm = 2k

2N5210 hfe = 250

Ib = .0045/250 = .000018 A = .018 mA

Bias string I = (10)(.000018 A) = .00018 A = .18 mA

Bias string Rb = 9V/.00018 A = 50000 Ohm = 50k

Rb2 = (4.5V+.65V)/.000018 A = 28611 Ohm = 28.7k

Rb1 = 50000 Ohm - 28611 Ohm = 21389 Ohm or 21.5k or Rb1a = 5k and Rb1b = 16.5k

MIC bias resistance 9k or Rm1a = 3.01k and Rm1b = 6.04k

RL and CL are loads for the sim. PS is either 9V battery or 9V regulated supply located at the mic pre about 40 feet from the buffer circuit.

Can C4 and or C5 be left out since C3 is very close to the rest of the circuit?

Any help or advice on improving this unbalanced buffer will be appreciated. The idea is to be able to get it to work well and to get it into a mic wand or connector.

Here is the circuit and its sim from LTS.

Thanks

[pebe]

Are the two green traces the phase responses?
What is the significance of the bottom trace being 25dB down on the top one?

[oneoldude]

Are the two green traces the phase responses?

No.

What is the significance of the bottom trace being 25dB down on the top one?

None.

Upper red trace is input to the buffer in dB
Upper green trace is output from buffer in dB
Scale for upper traces is on the left in dB

Lower red trace is phase of input to buffer in degrees
Lower green trace is phase of output from buffer in degrees
Scale for lower traces is on the right in degrees

Frequency scale is across the bottom

This is a buffer. Therefore no gain. So, input and output across the circuit's bandwidth are the same at 0 dB.

Is this the right forum for my kind of questions?

[pebe]

The link of your original post did not work, so I could only go on your present circuit. And as the circuit first displayed on my screen had its RH side (past about 200MHz) cut off I could not see the phase legend on the side - hence my question.

RL and CL are loads for the sim. PS is either 9V battery or 9V regulated supply located at the mic pre about 40 feet from the buffer circuit.

Can C4 and or C5 be left out since C3 is very close to the rest of the circuit?

C3 across the 9V rail is probably enough. But try removing C4 and C5 and add a resistance equivalent to the supply cable between the 9V supply and C3. Then see how it looks on the sim.

Otherwise the response looks flat enough. Personally, I would have preferred a 600Ω terminated balanced screened cable for a 40ft audio run to ensure no noise pickup, but if you are doing subjective tests it probably won't matter.

[oneoldude]

My image does not have the right side cut off as shown on this forum. My guess is the forum software cropped the image. Sorry bout that.

My purpose here is for the mic to provide for accurate objective information not subjective information.

The pro audio equipment I have on hand will not allow a bypass of its tone controls and its use is more complex than necessary. The result is that one must engage in subjective adjustment of the system and objectivity is lost. What pro audio stuff does is take an unbalanced signal, put in a lot of circuitry and then convert it back to unbalanced for use with a sound card. I want to avoid all that with high quality objective results, if at all possible.

Many extremely high quality industrial objective measurement systems employ unbalanced mic capsules and are single ended throughout. They use hundreds of meters of unbalanced cable with no noise problems. They use a constant current design so as to avoid signal loss over long distances. Those systems are more robust than I need and are over my head technically.

Since the typical mic capsule is unbalanced and since the input of decent quality sound cards is greater than 10k and is also unbalanced I suspect a very accurate, simple to use, low noise, objective measurement system can be created with an unbalanced design. The system may not be able to handle hundreds of meters of cable when measuring next to an antenna farm. But 40 feet in a quiet (EMI, RFI) location should be doable. To do it one must overcome pro audio bias and give it a go.

That is what I am trying to do.

Thanks

[pebe]

I did say that balanced cable was just my personal preference – it is by no means mandatory. I doubt that unbalanced cables would ever be used in a recording studio, though. In fact, I know of an unbalanced installation in a small studio that had to be ripped out and replaced because of noise ingress and the hum induced by ground loops.

I should be very interested to know more details of industrial installations that use hundreds of meters of unbalanced cable with no noise problems, and I don't know what you mean by 'constant current design'. The upper limit recommended by many sources is about 10ft but, as you say, 40ft will no doubt be doable.

[oneoldude]

I did say that balanced cable was just my personal preference – it is by no means mandatory. I doubt that unbalanced cables would ever be used in a recording studio, though. In fact, I know of an unbalanced installation in a small studio that had to be ripped out and replaced because of noise ingress and the hum induced by ground loops.

I have read the very same thing about balanced systems. In fact, any system that is badly engineered and badly installed will need to be ripped out and replaced. There are some advantages re: noise in very difficult environments for balanced systems. But I think they are really not necessary for a home user if an unbalanced system is properly designed and installed in a reasonably quiet (EMI, RFI) environment.

I should be very interested to know more details of industrial installations that use hundreds of meters of unbalanced cable with no noise problems, and I don't know what you mean by 'constant current design'. The upper limit recommended by many sources is about 10ft but, as you say, 40ft will no doubt be doable.

I have routinely used 40ft of unbalanced mic cable and never had a noise problem whatsoever. The problem I had was a bit of a loss in HF resolution due to the capacitance of the cables. That can be resolved by having a mic pre at or in the mic or having a buffer at or in the mic. Hence my post.

The super hi qual industrial measurement mic companies have a bunch of different names for their designs. But I think they are all constant current loop designs. They just don't tell you much about their proprietary circuits. The engineers sometimes call the mics sensors or transmitters (which is confusing vs a variety of piezo sensors, stress gauges and other sensors). B&K is a big industrial mic company that uses constant current loop design but does not call it that. There are many more. http://www.bksv.com/Products/transducers/acoustic/microphones/microphone-cartridges.aspx

Here is a nice article on a DIY constant current loop mic and a source for some information. http://sound.westhost.com/project134.htm

I hope you find it interesting.

Goodby.

[pebe]

Here is a nice article on a DIY constant current loop mic and a source for some information. http://sound.westhost.com/project134.htm

A very low output buffer working into a very high load. Nice one!