DIY Speaker Testing Microphone Preamp - Take 2

Introduction

I've been meaning to update my preamp circuit to reflect the changes it underwent for Jason Neil's kit offering. Now that Jason's page is down (Jason where are you?) it seemed that it was high time to do it. The design presented here uses a low power opamp and a precision voltage reference. For those of you interested in the orignal design, it is located here: http://mysite.verizon.net/tammie_eric/audio/preamp/preamp.html. You might want to consult it for construction tips and the like.

Here is a handy Word document of the schematic, parts placement, and PWB: preamp2.doc.

The Circuit

Parts List:

  IC1             LM6134AIN
  REF1            LM4040 2.5V precision reference
  Q1              2N3904 or similar
  D1-2            1N914 or similar
  LED1            Red LED
  R1              15k
  R2              22k
  R3              33k
  R4              1k
  R5              3.3k
  R6              33k
  R7,8,15,13      100k
  R9,11           3.3k
  R10             68k
  R12             100 ohm
  R14,16          47k
  R17             10k
  R18             470 ohm
  C1,8            100uF electrolytic
  C2              100pF NPO
  C3,6            10uF electrolytic
  C4              47uF electrolytic
  C5              330pF NPO
  C7              1uF electrolytic
  C9              10uF non-polar electrolytic
  C10             5pF NPO
  SW1             DPDT center-off mini toggle (275-1545)
  B1              9V battery
  J1,2            Gold plated RCA jacks (274-852)

  MISC            Enclosure (270-211)
                  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

• REF1 is a temperature compensated voltage reference biased via R1, the output of which is amplified and buffered by the first op-amp section configured in a non-inverting topology. Note that the supply for this reference is itself derrived from the actual regulated output of the op-amp, which increases the regulation effect. That really isn't necessary for the high-quality reference used, but hey, its essentially free. The voltage at the output of this opamp section should be 4.17V +/- whatever the precision of the resistors used for R2 and R3 is.
• 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 line powering to the capsule.
• C2 filters out any RF that may wander 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 to ground, 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 conducts, which charges the pulse-stretching capacitor C7, and turns on Q1, thus lighting LED1.
• Switch Section SW1B provides power switching, and C8 filters the battery power.

Physical Design

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And here is how it could be assembled on an RS perfboard, similar to Preamp I (thanks to Ellen Tunstall for the component annotations!):

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Performance Characterization

  -1dB @ 100kHz
  -2dB @ 145kHz
  -3dB @ 275kHz

• Confirmed the -3dB lower cutoff frequency (both gain settings) at ~1Hz.

• Current draw @ 9V with mic element connected: 4mA

• The clipping indicator works fine, as before, with the threshold at +/-2.25V @ 9v supply.

• The voltage regulation is rock steady at 4.174V open circuit with the supply varied from 4.25V to 9V. Very insensitive to temperature.

Here are a couple of graphs Jason created which show the phase and magnitude response of both gain settings:

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Microphone Elements

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Here is a picture of the WM-61A element disassembled from the mic wand:

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If you are serious at all about speaker measurement, you should either buy a calibrated mic or get the mic you are using calibrated. I haven't personally used his services, but Jason reports good and affordable results from Kim Girardin; contact info below:

Kim Girardin
Suite 2
1400 Homer Road
Winona, MN 55987
Voice/Fax 507-454-8844
kmgrdn@luminet.net

FAQ SECTION

For those having trouble printing out my web pages:

IE version 5.5 has a print preview which works pretty well, and might keep you
from wasting paper on things that won't print right.  If you are using IE, go 
to the menu item View | Text Size | and pick "Smallest".  Now go to File | Page Setup 
and click on the Landscape radio button, then click the OK button.  This looks like 
it will print right on my computer anyway via the print preview:  
File | Print Preview... is how you see this.

For more flexibility you can paste the web page contents into Word.  In IE 
select the menu item Edit | Select All and then do a Ctrl-C.  Open Word and do a Ctrl-V 
on a new blank document.  If you have the 2000 version all of the info should paste 
in and you can resize the pictures as you like.  If you have Word 97 you may have 
to paste the images in separately (right click on the image in IE and then click on 
"Copy"; then click in Word where you want the picture to go and do a Ctrl-V to 
perform the paste).  

Next question?



Some additional info on the transistor and the components external to the PWB:

The middle leg of the 2N3904 on the PCB is inserted in the hole directly underneath 
the body of the transistor, as the pins are all in a row.  Here is a list of possibilities 
for the transistor package (top view of transistor, like in the PCB wiring diagram, with 
the flat or tab on the body indicated by dashes):

Plastic body with flat, all three pins in a row (I don't know what idiot made it different 
from the following two):

  ----- 
 (C B E)
 


Plastic body with a flat, center pin staggered out:
  -----
 (E   C)
    B



Round metal body with a tab, center pin staggered out:

 (C   E)
    B  -  tab



Since the layout shown is for the in-line pin plastic body transistor (first one above) 
you may have to make adjustments if you use a transistor with different packaging.  For 
example, if you are using a plastic body transistor with staggered pins, you will want to 
rotate the transistor 180 degrees so that it is oriented thus:

    B
 (C   E)
  -----

Then you can bend the base lead (B) so that it is roughly in line with the collector (C) 
and emitter (E) leads, then insert it in the PCB.

As for the off-board components and the switch:

 switch

  A B
  C D
  E F

C2 (100pF) is soldered between the center conductor and the ground tab of the input jack.  
The input jack ground tab must be somehow wired to the negative terminal of the battery connector. 

C5 (220pF) and R11 (3.3k) are wired in parallel and then one end of this parallel assembly 
is connected to the B connection of the switch, and the other end is connected to the exposed 
wire of R9 on top of the PCB.  The D connection of the switch is connected to the exposed wire of R10 on top of the PCB.

Connect A and E together, then connect them to the "+" terminal on the PWB.
Connect C to the red or positive wire from the 9V battery connector.
Terminal F on the switch is a no-connect.

Connect the "GND" terminal on the PWB to the black wire from the 9V battery connector. 

Connect the center conductor of the output jack to the exposed wire of R13 on top of the PCB.

If you want, you could mount C5 / R11 on the PCB, there is certainly plenty of room to do this left.  
I just wired them in the air between the switch and the PCB.