DIY Speaker Testing Microphone Preamp

Introduction

I have been thinking about designing a preamp for speaker testing, something that would take one of those Panasonic electret capsules and amplify its signal up to a line level. Well, I finally got around to doing just that. There are a couple of preamp schematics out there on the web, such as the El-Cheapo-Mic and the Mitey Mike II. The El-Cheapo seems to be wired wrong, and the MMII is rather pricy and uses surface mount components, which might present some difficulties for the average electronics hobbyist.

I don't want to beat up on the MMII too much, it really is a great exercise in low-power design, and I'm sure it works very well. (Strange, but I've looked high and low for the MAX402 op-amp used in it, but it doesn't seem to exist, even on MAXIM's web page.) See the MMII schematic at Dick Campbell's home page:

http://www.ultranet.com/~rhcamp

and the El Cheapo-Mic at:

http://project.ee.mtu.edu/stu_orgs/aes/cheapo-mic.html.

Quick quiz: can you spot the biasing flaw in the El Cheapo-Mic? (The last stage has DC bias for both the '-' and '+' inputs, guaranteed to rail the output. Also, the DC point of the '+' input is fixed at the negative supply rail, which is your basic no-no.)

I wasn't that concerned with power consumption when I designed this preamp, that is, it was not my number one priority. The speaker testing I do only lasts for a few hours at a time, and I really don't mind if the battery in my preamp dies after 50 hours or so. No, my main priorities are good frequency response, low noise, and ease of construction. If the thing could be left on for a year, great, but I didn't want to cut any corners in the important departments.

The graphics conversion program I was using to capture my AutoCad drawings, IrfanView32, crapped out on me (guess it doesn't like Win98) so I had to convert my programs using this rather awkward process: first "copy vectors" in Acad to a *.wmf on the clipboard, then screengrab the clipboard view back to the clipboard at which point it is a bitmap, then paste this into Paint and edit it. I can't believe Windows doesn't have an easier, more direct route to a bitmap from a vector *.wmf!

The Circuit

Parts List:

  IC1             TL074
  Q1              2N3904 or similar
  D1-5            1N914 or similar
  D6              Red LED
  R1,3,7,8,13,15  100k
  R2              220k
  R4              1k
  R5              3.3k
  R6              33k
  R9,11           3.3k
  R10             68k
  R12             100 ohm
  R14,16          47k
  R17             10k
  R18             470 ohm
  C1,8            100uF
  C2              100pF
  C3,6,9          10uF
  C4              47uF
  C5              220pF
  C7              1uF
  SW1             DPDT center-off mini toggle (275-1545)
  B1              9V battery
  J1,2            Gold plated RCA jacks (274-852)

  MISC            Enclosure (270-211)
                  PC prototyping board (276-150A)
                  14 pin IC socket


Note:  All resistors are 1/4W or 1/8W (your choice).
       All capacitors are ceramic (NPO) and electrolytic (10V or better).

Circuit Description

• R1 and D1, D2, and D3 form a voltage reference, which is increased and buffered by the first op-amp section. Normally, the reference voltage is itself derived from the output of the op-amp, but the TL074 is operating rather close to its minimum voltage, which makes this topology actually perform better.
• R4 and C1 form a low-pass filter that reduces the noise of the voltage reference section.
• R5 and the microphone capsule plugged into J1 form a voltage divider across which the audio signal is manifest. R5 provides the "phantom" voltage to the capsule.
• C2 filters out any RF that wanders in on the microphone cable.
• R6, R7, and R8 bias the op-amp section to 1/2 the supply voltage. C9 allows this bias to happen.
• R9 and R10 form a beta section for the second op-amp section, producing a gain of approximately 20. This gain is reduced to approximately 2 when R11 / C5 are switched in-circuit by SW1A. The ratio of these two gains is about 10, or 20dB. R9 and C4 form a high-pass filter with a cutoff frequency of 1Hz. R11 and C5 form a low-pass filter with a cutoff frequency of 200kHz.
• R12 supresses any oscillation the second op-amp section might otherwise experience driving the output cable. C6 and R13 restore the output DC level, and also form a high-pass filter with the load. For this reason, loads lower that 10k or so should be avoided unless some low frequency rolloff can be tolerated.
• R14, R15, and R16 form a resistive divider tree that provide voltage references to the window comparator formed by the third and fourth op-amp sections. If the voltage at point 'X' goes outside of these references, than either D4 or D5 conduct, charging the pulse-stretching capacitor C7, and turning on Q1, thus lighting D6. D6 is placed in the rather unconventional location of the emitter of Q1, vs the collector, because the opamp output can't swing far enough to ground to turn Q1 off otherwise. The LED forward voltage drop adds to the base-emitter voltage drop to make this threshold more than 2V above ground, which is sufficient to fully turn off Q1 and D6.
• Switch Section SW1B provides power switching, and C8 filters the battery power.

Construction

Here is how to layout the parts on the pc board (thanks to Ellen Tunstall for the component annotation!). This is a top view, the side with all of the components (as opposed to the bottom side, which has the copper pads). All semiconductors are shown in green, resistors in grape, and capacitors in violet.

Wires on the front side of the board are shown in red, while wires on the back side of the board are shown in blue. Often, you can use leads from the components to complete these backside wire connections, so don't cut those leads after soldering to the pad until you are sure you don't need them.

I tend to solder wires right to the top of the lead coming out of a resistor in order to make connections to off-board components such as switches, input, outupt, etc. If you want to do this then be careful as to how you mount the resistors so that the correct lead is accessible. I also like to put wire loops on the top of the board for external wiring purposes, and several of these can be seen above going to the LED and battery. Both of these types of connection points are shown above.

Note that the pc board pad layout is asymmetrical, that there are more of the "single" holes on the lower half of the board as shown here (three in a row on the bottom, two in a row on the top).

Here you can see the inside of the preamp. The pc board is fairly spare. Note also the knot in the battery leads located above the lower left case standoff. This forms a natural strain relief.

Here is a drilling guide for the end panel for the case. Dimensions (except for diameters) are shown in millimeters. All controls and I/O (J1, J2, SW1, and the clipping LED) fit on this panel. I mounted the jacks on either end, with the switch and LED between them, so that they are all in a row and 14 mm apart from each other. The 15/64" hole is for SW1, and the two 1/4" holes are for J1 and J2. The last hole (the one labeled "see text") is for the LED. Drill it whatever size is necessary for the LED holder you choose to use.

Here is a picture of the preamp with a short microphone plugged into the input, and a cable plugged into the output. A microphone this short really can't be used for speaker testing since reflections off of the preamp box will confound all of your measurements. I was just using it for a quick checkout of the preamp itself. (Don't try this at home kids!)

Performance Characterization

Frequency response data:

                       Gain Setting:

   Output dB           X2         X20
   ---------       ------      ------
     -3            1.25Hz      1.25Hz
     -2             1.6Hz       1.6Hz
     -1             2.5Hz       2.5Hz
      0
     -1            120kHz       55kHz
     -2            200kHz       88kHz
     -3            290kHz      115kHz

• The clipping indicator functions very well, lighting up before the onset of visible sine wave distortion. It also works well for signals such as narrow pulses, lighting up before the onset of clipping.

• The output swings 2V peak (4V p-p) at full output.

• Phantom voltage regulation is better than 2% for supply voltages between 7V and 9V.

• Current draw from the 9V battery is 8.25mA, good for 50 hours or so of intermittent use (so says Don Lancaster's CMOS Cookbook).

Which Microphone Cartridge Should Be Used With This Preamp?

Before proceeding, let me say that these results should be interpreted with several things in mind. For testing, I sent John that very short microphone shown connected to the preamp above. John had to use several extensions and adapters to get it to work, and this may have influenced the frequency response somewhat due to the bulky nature of the interface and reflections off of it and such. The following text is adapted from several of John's letters to me, and includes the images produced by IMP during the calibration procedure.

The testing was done with John's Mitey Mic which dates back to 1993, but it was calibrated by Joe D'Appolitio through the Old Colony Sound program. The calibration curve for it is shown above. John reports that in comparisons with Rudi's LAUD mic it appears to be 'holding' it's calibration pretty well. An RS ribbon tweeter was used as the sound source, which is why the calibration was only performed from 3kHz up.

Shown above, measured at about 1", the Mitey Mic with it's 18" wand produced an extremely clean impulse with no discernable reflections. The mic stand had to be moved forward a lot to get the test mic 1" from the RS ribbon tweeter. There was a double combination of adaptors on it to get it to mate to the BNC cables used. Lots and lots of reflections! In the end, it was necessary to embed the mic / adapter assembly in 4" thick foam in an attempt to quell the reflections.

The image above shows the response of the MMI and the test microphone cartridge. It is obvious that the short test mic assembly is a poor test subject.

The reflections could have influenced the response a bit - but perhaps not as much as one might fear. You can really distinguish the effects of the reflections by the amount of 'hash' in the unsmoothed response of the test compared with the MM. Note that the unsmoothed MM response is almost (but not quite) line, the test mic's response is very hashy by comparison.

Based upon reflections it seems that implementing a wand would be the best thing to do. In the past, John has used 1/4" diameter brass tubing and fitted the capsule to the end with adhesive. As the capsule is the same diameter as the tubing this should optimize it's reflection problems.

Here is the same data, but the test data has been smoothed. With 1/12 octave smoothing there is good comparability.

The image above shows the mathematical difference between the responses of the two microphones, which then is used to determine the calibration of the test microphone. Nothing changed between measurements except for moving the mic stand, though the mic gain input on the IMP required adjustment.

With massive fiddling and tricks to fool the IMP software John was able to produce a frequency response plot for the test mic above 3000Hz, and this is shown in the image above.

So, given all of this, which microphone element should you use? I would say that it is a toss up. The higher gain of the newer element certainly makes designing a preamp for it easier, and this was the element I used to set the gain values in my preamp circuit.

My many thanks to John Whittaker in this endeavor!

How To Use This Preamp

FAQ SECTION

06/12/99
here is an offer from Jason M. Neal regarding this project.  
He has been gracious enough to design a pc board and a kit 
of parts, and is offering it to anyone concerned.  
Click here for info / instructions.
The pcb looks really great, and Jason is not making any profit on this deal,
so be sure to give him a big DIY "attaboy" if you order a kit!