Introduction

Theory of Operation

This stage amplifies the quadrature audio frequency difference products from the Mixer stage via R17 and R18. R19 and R20 make up a voltage divider that provides the 2.5 Vdc bias to the Op-Amps, configured as an inverting amplifier. The ratios of R19/R17 and R20/R18, respectively, determine the voltage gain of the output over the input for each Op-Amp. That voltage gain is theoretically 499:1, or about 54 dB. Each Op-Amp's output is capacitively coupled through a 100 ohm resistor to the "Ring" (Q) and "Tip" (I) Audio Out terminals for input to the PC's sound card

Schematic

This is a subset of the overall schematic. Note: red dot indicates resistor testpoints (hairpin, top, or left-hand lead)
05 Operational Amplifiers

Bill of Materials

DesignationValueColor/CodeOrientationCategoryNotes
U8LT6231CS8SMT
C310.1uFSMT 1206black marked strip
C320.1uFSMT 1206black marked strip
C280.1uFSMT 1206black marked strip
C250.1uFSMT 1206black marked strip
R1710W-E
R1810flat-h
R151kflat-h
R161kW-E
R194.99kE-W
R204.99kflat-h
R21100flat-v
R22100flat-h
C260.047uF473ceramic
C270.047uF473ceramic
C29220pF221ceramic
C30220pF221ceramic
C244.7uF475ceramic

Summary Build Notes

  • Install SMT IC U8 (bottom)
  • Install 4 x 0.1 µF SMT capacitors (bottom)
  • Install 8 resistors R15-R22 (top)
  • Install 5 ceramic capacitors C24 and C26-C30(top)
  • Test the Stage

Detailed Build Notes

Bottom of the Board

Install U8

DesignationValueColor/CodeOrientationCategoryNotes
U8LT6231CS8 SMT SOIC-8

Install SMT Caps

DesignationValueColor/CodeOrientationCategoryNotes
C310.1uFSMT 1206black marked strip
C320.1uFSMT 1206black marked strip
C280.1uFSMT 1206black marked strip
C250.1uFSMT 1206black marked strip

Top of the Board

Install Resistors

Install Ceramic Capacitors

DesignationValueColor/CodeOrientationCategoryNotes
R1710W-E
R1810flat-h
R151kflat-h
R161kW-E
R194.99kE-W
R204.99kflat-h
R21100flat-v
R22100flat-h
DesignationValueColor/CodeOrientationCategoryNotes
C260.047uF473ceramic
C270.047uF473ceramic
C29220pF221ceramic
C30220pF221ceramic
C244.7uF475ceramic

Completed Stage

Topside

Completed topside

Bottomside

Completed bottomside

Testing

Current Draw(DMM)

  • Current numbers here are for the CMOS version of the Si570.  You will need to adjust these up by about 14 mA for the LVDS version.
  • Power the board up
  • Measure the current draw and 5 V rail voltage with a 1K Ω limiting resistor
  • Measure the current draw without the limiting resistor.
TestpointNominal ValueAuthor'sYours
Current Limited mA6-10 mA7.5 mA______
Current limited 5V rail1-2 Vdc971 mV______
Non limited draw mA90-100 mA94.9 mA______
#voltage_divider_test

Voltage Divider R15/R16(DMM)

  • Measure the voltage at the R16 hairpin lead with respect to ground.
  • It should read approximately 2.5 Vdc (½ the 5 volt rail).
TestpointNominal ValueAuthor'sYours
R16 hairpin lead2.5 Vdc2.48 Vdc______

Pin Voltages (DMM - 5, 2.5, and 0 Vdc)

  • Measure the voltages at the pins of U8.  (see bottomside image above)
  • It is best to test for pin voltages at the actual pins (not the pads), thereby ensuring correct soldering of the pins to the pads.
TestpointNominal ValueAuthor'sYours
U8, Pins 1, 2, 3, 5, 6 & 72.5 Vdc2.48-2.51 Vdc______
U8, Pin 85 Vdc4.96 Vdc______
U8, Pin 40 Vdc0 Vdc______

OpAmp Test - DMM (No Scope)

Tony Parks suggested this next test,which requires only a DMM, a 10 k resistor, and some clip leads.

The test will test each of the two Op-Amps.  If the Op-Amp being tested is working, then the voltage measured at the output of the Op-Amp will increase to accomodate the effect of the changed bias on the input.   Passing these tests gives you more than enough confidence to move on to the Mixer stage.

Testpoints for No-Scope Test of OpAmps
  • Obtain a 10k resistor (you can use R11 from the next stage's BOM)
  • using the DMM, measure the dc voltage with respect to ground at the hairpin of R19.
    The result should be approximately 2.5 Vdc (½ the 5 Vdc rail).
  • keep the DMM lead on R19's hairpin
  • Using two clip leads, "bridge" the 10k resistor between the hairpin of R17 and ground. See the diagram to the left.
  • Observe the voltage reading at R19 hairpin. If OpAmp 1 is working, the voltage should have jumped to approximately 3.75 Vdc
  • Remove the resistor/clip lead from R17 and the voltage at R19 should go back to the 2.5 Vdc level.
  • Follow these same steps for OpAmp2, substituting:
    • R18 (right-hand lead) for R17 (hairpin) and
    • R20 (left-hand lead)  for R19 (hairpin).
TestpointNominal ValueAuthor'sYours
R19 hairpin - no bridge2.5 Vdc2.51 Vdc______
R19 hairpin - R17 bridged3.75 Vdc3.75 Vdc______
R20 left-hand lead - no bridge2.5 Vdc2.51 Vdc______
R20 left-hand lead - R18 bridged3.75 Vdc3.75 Vdc______
J1 - Main Bus Jack Mixer Stage Power Supply Stage