Amplifier Hook Up



Amp Hookup Parts Description No. Supplier Stock No. USD ea. Total
S1 SWITCH TOGGLE 4PDT 6A SLDR 1 DK 360-1874-ND 10.12 10.12
S2 Toggle Switch, DPDT, On-On, 5A 1 DK 450-1533-ND 3.13 3.13
J1 JACK PHONE 1/4" 3POS OPEN 1 DK SC1094-ND 2.60 2.60
BP1 POST BINDING BANANA INSULATED BLK 4 DK J165-ND 3.51 14.04
BP2 POST BINDING BANANA INSULATED RED 4 DK J164-ND 3.51 14.04
P2 POT, 100K OHM, Log 1 RS RS-271-1732 3.50 3.50
R50 RESISTOR SILICONE 10 OHM 5W 4 DK ALSR5F-10-ND 1.71 6.84
C25 CAP .1UF 100V STACK METAL FILM 4 DK P4725-ND 0.34 1.36
Total NA NA NA NA NA 55.63


ESP Project 09-RevA

The schematic above shows the ESP 09-Revision A active crossover. The version used in Pi-Amp Bi-Amp is Revision B. The Revision B schematic cannot be shown due to copyright protection.

The table below shows the total cost of ESP 09-Revision B (PCB and on board components)

ESP P 05-RevB Description No. Supplier Stock No. USD ea. Total
R1 – R10 10KR, 1%, MTL 10 DK 10.0KXBK-ND 0.13 1.30
R11, R12 100R, 1%, MTL 5 DK 100XBK-ND 0.13 0.65
RF As Cackulated, 1%, MTL 30 DK As Calculated 0.13 3.90
VR1, VR2, VR3, VR4 20KR, 1/4W, 25 TURN 4 DK 490-2878-ND 1.36 5.44
C1, C2 1uF, 250V, MTL POLY 2 DK P10979-ND 0.68 1.36
CF As Calculated, 250V, MTL POLY 30 DK As Calculated 0.36 10.80
CB (3) 10 uF, 63V, R-ELEC 3 DK 493-1351-ND 0.25 0.75
CB (5) 0.1 uF, 250V, MTL POLY 5 DK P10967-ND 0.36 1.80
OP AMP (Through Hole) OP275GSZ-ND 7 DK OP275GPZ-ND 3.01 21.07
ESP Project 09-RevB PCB 1 ESP 09-RevB 24.00 24.00
Total NA NA NA NA NA 71.07

The leads on the line level signal side of the circuit (from the PC sound card up to the Leach amp PCP input) should be 22 or 20 AWG stranded copper hook up wire. The leads from the Leach amp PCB amplified signal output should be 18 or 16 AWG stranded copper hook up wire hook up wire.

Rear Panel
  1. The purpose of the Zobel Bridge is to shunt high frequency signal from amplifier output to ground. Build the circuit board using the angel bracket and a piece of 2" x 2-1/2" Plexiglas as shown below. Plexiglas makes excellent mounting board for small groups of large components requiring simple wiring. Buff the Plexiglas with sand paper, lay out the hole pattern and drill 1/16" holes. Note that each R50 / C25 pair share a hole ( three holes per R50 / C25 pair). Feed the component leads through the mounting holes, apply a small amount of flux to each lead, tin with solder, solder wire leads and and insulate with shrink wrap. When soldering, make the joint close to the Plexiglas surface and allow a small amount of solder to flow into the hole. The solder will lock each component securely to the board. Mount the Zobel Bridge to the chassis rear panel below the speaker binding posts.

  2. Install the speaker binding posts (BP1 / BP2) to the chassis rear panel as shown above. These binding posts are made of nylon and will strip if the retaining nut is over tightened.
  3. Attach the Zobel bridge leads to the binding posts in accordance with the wiring diagram. Each resistor/capacitor series forms a bridge between each red/black speaker binding post pair. The most expedient way to prepare the wire lead for the small binding post connection is to strip back about 1/2" of insulation, twist the strands together, form the bare wire in a loop around a nail, remove the nail and tin the loop with solder. The small wire lead retaining nuts should be put aside in a safe place until later.
Controls Panel
  1. Attach leads to the ESP active crossover PCB as follows -
    • Two signal inputs.
    • Two signal input grounds.
    • Four signal output.
    • Four signal output grounds.
    • One each, on the power connector, +15V, -15V, and ground, 18 AWG.
    • Fifteen total leads. Ensure that the three power leads have sufficient length to reach the PSDU.
  2. Attach the active crossover PCB to the chassis using the two #6 machine screws that attach the controls panel mounting bracket to the chassis base as shown above. The hole spacing on the PCB is the same as the hole spacing on the bracket (3 3/4"), however the PCB mounting holes must be opened to 5/32".

  3. Prepare the volume control potentiometer (P2) by soldering four twisted pairs (2 - S/G pairs, 2 - W/G pairs) to their respective lugs in accordance with the wiring diagram.

    Potentiometer lead configuration can be confusing but this summary should help.
    • The connections are termed "Signal", "Wiper" and "Ground" (one set for each channel).
    • S is input and W is the attenuated output.
    • The illustration below shows the schematic representation of P2 (RS-271-1732), lead configuration, and expected ohm meter measurements.

  4. Mount the volume control (P2) on the controls panel. This volume control has a small anti rotation pin next to the shaft that must be snapped off with pliers before mounting.
  5. Attach leads to the headphone selector switch (S1) and mount to the front controls panel.
    • The headphones will monitor the line level signal from the PC sound card and attenuated not by P2, but by the operating system volume control. Check the OS volume level before connecting headphones.
    • The center lugs of S1 should be long enough to reach the PC sound card
  6. Attach leads to the bi-amp selector switch (S2) and mount to the front controls panel.
    • This selector switch toggles Leach amp input between bi-amp mode and 4 channel stereo mode. This switch is not necessary if stereo is not required.
    • The center lugs of S2 should be long enough to reach the Leach amp PCB's.
  7. Attach leads to the headphone jack (J1) and mount to the front controls panel.
  8. Complete all of the wiring between these controls and the active crossover PCB.
  9. There are thirteen ground points that must be brought together and later connected to one signal ground on the sound card -
    • Two on the volume control (P2).
    • One on the headphone jack (J1)
    • Two signal in on the ESP active cross over.
    • Four signal out on the ESP active cross over.
    • Four signal in on the Leach amp PCB's (not yet installed).
    • Bring ground leads together in stages to make a harness finishing with one lead that will tie to the sound card pin out header. Be sure to include four leads long enough to reach the signal in side of the Leach amp PCB's.
    • Do not use central ground or the amp will hum.
  10. Connect the ESP active cross over to the regulated power supply as follows -
    • Route and connect the +15V and -15V leads to the +15V and -15V connections on the voltage regulator PCB.
    • Route and connect the ground lead directly to the ground connection on the voltage regulator PCB.
  11. After wiring the controls and ESP active cross over, the following leads remain to be connected -
    • Two signal leads on the center lugs of the headphone selector switch (S1) that will connect to the pin out header on the sound card.
    • One signal ground lead, from the "harness" built in step 9, that will be twisted together with the two signal leads from S1 and that will connect to the pin out header on the sound card.
    • Four signal leads on the center lugs of the bi-amp selector switch (S2) that will connect to signal in on the four Leach amp PCB's.
Rear Panel
  1. Route four leads from the red binding posts (BP1) on the rear panel back to the amp fins. To keep the binding posts and Leach amp PCB's symmetrical , the leads from the top two amp PCB's connect to the top left and right (viewed from the front) red binding posts. These amps drive the tweeters (or mid + tweeter for a three way speaker). The leads from the bottom two amp PCB's connect to the bottom red binding posts and drive the woofers. The illustration below shows the leads carefully routed and wire tied.

    Note: The picture below is from the "Pi-Amp" portion of this web site, however the binding post configuration is identical because "Pi-Amp" is stereo A or B channel and "Pi-Amp Bi-Amp" is bi-amp or 4 channel stereo.

  2. Make the four black binding posts (BP2) common with one wire lead and route that lead to central ground lug on the PSDU.
Leach Amplifier PCB's
  1. Install sixteen brass stand offs (1451EK-ND) to the amplifier fins using #6 x 1/2" counter sink machine screws.
  2. Install the power transistor heat sinks (345-1050-ND), with power transistors and bias diodes installed, to the amplifier fins using four #6 x 3/4" machine screws.
    • To ensure that the heat sinks are grounded, file off the anodized coating around one mounting hole on each heat sink. If a power transistor collector shorts to the heat sink, the heat sink will be grounded. If the heat sink is not grounded and a collector shorts, then the heat sink is a potential shock point.
  3. Mount the PCB's to the standoffs temporarily. Route the leads from the power transistors and bias diodes between the bottom standoffs. Determine each leads termination point on the PCB and cut the leads to final length. Keep track of the leads with masking tape labels.

  4. Route Leach amp ground and power supply leads as follows-
    • Four PCB output grounds to central ground.
    • Four PCB signal input grounds to signal ground (discussed in the Front Panel section). There is an additional ground connection on the signal input portion of the amp PCB that Leach specifies as connected to central ground. Do not make this connection to central ground or the amp will hum. I have worked several ground configurations and have determined that the all signal grounding, up through the Leach amp signal input, must be to signal ground (i.e. grounded to the PC sound card pin out).
    • Four "+58V" and four "-58V" power leads from the PSDU terminal strips to the PCB power rails.
    • Cut the leads to their final length.
  5. Route the four signal leads from the center lugs of S2 to the Leach amp PCB signal inputs and cut to final length.
  6. Remove the PCB's from the chassis. Solder the transistor, bias diode, power, signal and ground leads to the PCB and install the PCB's to the chassis.
  7. Perform the power up test described under "Initial Tests on the Completed Amplifier" and "Setting the Bias Current" located at the Construction portion of the Leach web site.
Calibrate the ESP Active Cross Over

The following steps compare active cross over input signal to output signal. The signal amplitude is adjusted using VR1, VR2, VR3 and VR4 on the active cross over. The LC and RC high pass amplitudes should be equal as well as LC and RC low pass amplitudes for a given fixed voltage input. The difference between high and low pass amplitude, for either channel, depends on the speaker enclosure.

  1. Set the volume level to zero (counter clockwise).
  2. Set the bi-amp selector switch (S2) to the 4-channel stereo position (this isolates the active cross over output signal from the Leach amp PCB's.
  3. Connect the signal generator to the left channel (LC) wiper lug on the volume control (P2) and connect the signal generator ground to the signal ground "harness".
  4. Connect channel one of the oscilloscope to the signal generator output and ground the probe to the signal ground.
  5. Connect channel two of the oscilloscope to the left channel (LC) high pass on bi-amp selector switch (S2) and ground the probe to signal ground.
  6. Adjust the signal generator to read 1.5V peak at 10 KHz.
  7. Turn on the "Pi-Amp Bi-Amp" master power switch.
  8. Measure the cross over output voltage on scope channel two and compare to channel one.
  9. Connect channel two of the oscilloscope to the left channel (LC) low pass on bi-amp selector switch (S2) and ground the probe to signal ground.
  10. Adjust the signal generator to read 1.5V peak at 100 Hz.
  11. Measure the cross over output voltage on scope channel two and compare to channel one.
  12. Adjust the signal generator frequency from 100 Hz to 10 KHz and note the attenuation on scope channel 2 as the frequency approaches design cross over frequency.
  13. Play around with variable resistors (VR1 - VR4) until the cross over is balanced and attenuated as needed. This takes some time.