Amplifier Hookup

Pi-Amp Leach Amplifier Installed in Chassis

Pi-Amp Amplifier Electrical Hookup Schematic
Amp Hookup Parts Description No. Supplier Stock No. USD ea. Total
S2 Toggle Switch, DPDT, On-On, 5A 2 DK 450-1533-ND 3.13 6.26
J2 JACK PHONE 1/4" 3POS OPEN 1 DK SC1094-ND 2.60 2.60
R-HP1 RES 560 OHM 1W 5% METAL FILM AX 2 DK 560WCT-ND 0.20 0.40
R-HP2 RES 22 OHM 1W 5% METAL FILM AX 2 DK 22WCT-ND 0.20 0.40
R-HP3 RES 120 OHM 1W 5% METAL FILM AX 2 DK 120WCT-ND 0.20 0.40
P2 POT, 100K OHM, Log 1 RS RS-271-1732 3.50 3.50
C25 CAP .1UF 100V STACK METAL FILM 2 DK P4725-ND 0.34 0.68
Total NA NA NA NA NA 52.98
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.

    Pi-Amp Zoebel Bridge
  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.
  4. Mount the RCA input jacks (J1) to a piece of Plexiglas. The Plexiglas isolates input signal ground from the chassis (i.e. central ground). If the RCA jacks are mounted directly to the rear panel a hum will be heard through the speakers. At no time should the input signal share ground with the chassis. Mount the Plexiglas / RCA jack unit to the rear panel using two #6 machine screws. Check to ensure that the top of the Plexiglas is 1" below the top of the rear panel. In the illustration below, signal is red and ground is black.

    Pi-Amp Rear Panel Signal Jacks
Controls Panel
Pi-Amp Front Panel Wiring
  1. Build the headphone attenuator circuit board using the same method as used for the Zobel bridge. Cut a piece of Plexiglas 1" x 3", layout and drill eight 1/16" component holes and one 5/32" mount hole as shown in the drawing below. The component holes should place the resistors close as practical to the end of the board to leave a space for the headphone jack protrusion. Attach the six wire leads to the resistors and insulate with shrink wrap. Attach the circuit board to the machine screw that holds the controls panel mount bracket to the chassis base (lower right corner of the illustration above). Mount the head phone jack (J2) on the controls panel and connect the three wire leads from the attenuator circuit board.

    Pi-Amp Head Phone Mechanical Schematic
  2. Run the ground from the headphone jack to central ground as shown below.

    Pi-Amp Central Ground
  3. Prepare the speaker A/B and headphone selector toggle switches (two - S2) by soldering wire leads to the toggle switches. The headphone switch routes the output signal from the amp PCB's to either the headphone attenuator or the center lugs on the speaker A/B selector switch. The speaker A/B selector then routes the signal to either the A or B channel binding posts.

    Ensure that each lead is long enough to reach its destination!

    Four leads from the speaker A/B selector are routed to the rear panel binding posts and the other two connect to the headphone selector. The headphone selector requires two leads long enough to reach the left and right channel amp PCB's. The other two leads are brought in from the headphone attenuator. Mount both switches on the controls panel.

    Twisting logical pairs of leads together keeps the wiring organized and shielded from EMI. Use a hand drill with a small eye-hooked in the chuck to make twisted pairs.
  4. 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.

    Pi-Amp Alps Pot Internal Wiring and Resistance
  5. 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.
Controls Panel to Rear Panel Wiring
  1. Route the four wire leads from the speaker A/B selector switch to the red binding posts. The leads for left and right channel are each paired and twisted together. Before connecting the leads to the binding posts, determine A and B channel with a multi-meter. The illustration below shows the leads carefully routed and wire tied.

    Pi-Amp Rear Panel Speaker Lead Wiring
  2. Make the four black binding posts common with one wire lead and route that lead to central ground.
  3. Route the four leads from the volume control (P2) signal and ground lugs to the rear panel RCA jacks (J1). The leads should be twisted together as S/G pairs.
Installing and Wiring Amplifier PCB's
  1. Install the eight brass stand offs (1451EK-ND) to the amplifier fins using #6 x 1/2" machine screws.
  2. Install the power transistor heat sinks (345-1050-ND) to the amplifier fins using four #6 x 3/4" machine screws. In order to ensure that the heat sinks are 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 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.

    Pi-Amp Leach Amp PCB Mounted on Front Panel
  4. Route the power leads as follows-
    • Both PCB grounds to central ground.
    • PCB "+" and "-" power from terminal strip to PCB power rails.

    Cut the leads to their final length.
  5. Remove the PCB's from the chassis. Solder the leads to the PCB and install the PCB's to the chassis.
  6. 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.
  7. Route the control leads as follows-
    • Volume control W/G twisted pairs to PCB signal input.
    • Headphone selector center lugs to PCB speaker output.

    Cut the leads to their final length.
  8. Remove the PCB's from the chassis. Solder the leads to the PCB and install the PCB's to the chassis.
  9. Sound test the amplifier to ensure that speaker A/B channels, headphone and volume work as intended. An MP3 player, connected to the RCA jacks (J1) on the rear panel, using a stereo 3.5mm mini phone plug male to RCA male adapter, is a good signal source. This adapter will be used a the permanent connection between the PC line level audio and J1.
Pi-Amp Rear Panel Sound Card to Amplifier Cable