Leach Amplifier Construction

Pi-Amp Leach Amp V4.5 Completed Circuit Boards

The amplifier used in this project was designed by Dr. Marshall Leach. Design, construction and testing are covered in depth at his web site -

Leach Amp at Georgia Tech (the live site)
Leach Amp at Georgia Tech (mirror - in case of take down)

My sincere thanks to Dr. Leach for sharing this design and helping me trouble shoot the first build.

Items on this web page are intended to supplement material on the Leach web site -

Amplifier Printed Circuit Board (PCB)

The amplifier used in this project is the Marshall Leach, Low TIM, version 4.5. The PCB's were available directly from Dr. Leach, however he passed away November 20, 2010. His web page is currently active however there is no indication if PCB orders will be filled.

PCB's can be drawn and etched by hand. Making you own PCB's is beyond the scope of this manual, however kits are readily available.

A scaled PCB layout can be found here

Another alternative is using an on-line PCB prototype builder. The process is simple though higher in per unit cost. Some prototype builders provide Windows compatible CAD software or you can use the open source application PCB Designer and use it to generate Gerber (RS-274-X) files used by all PCB fabricators. For Linux users, PCB Designer is excellent, and you can download the PCB Designer layout file of the Leach V4.5 PCB here (right click, save link as ...) and generate your own Gerber files for use by any PCB prototype shop.

Amp Components

This table is up to date as of January 2010. The stock numbers reference Digi-Key, an electronics supplier with on-line sales. Allied Electronics and MCM are also good on-line alternatives. (DK = Digi-Key, RS = Radio Shack). "No." is total required for two amplifiers.

Amplifier Parts Description No. Supplier Stock No. USD ea. Total
Q1, Q2, Q5, Q7, Q9, Q10 TRANS NPN GP SS 80V TO92 12 DK MPS8099GOS-ND 0.47 5.64
Q3, Q4, Q6, Q8, Q11 TRANS PNP GP SS 80V TO92 10 DK MPS8599RLRAGOSCT-ND 0.44 4.40
Q13, Q14 TRANS NPN 450V 1A TO39 4 DK 497-2576-5-ND 1.21 4.84
Q12, Q15 TRANS PNP 200V 1A TO39 4 DK 497-2593-5-ND 1.23 4.92
Q16 TRANS PWR NPN 8A 150V TO220 2 DK MJE15030GOS-ND 1.53 3.06
Q17 TRANS PWR PNP 8A 150V TO220 2 DK MJE15031GOS-ND 1.53 3.06
Q18, Q20 TRANS PWR NPN 20A 140V TO3 4 DK MJ15003GOS-ND 3.92 15.68
Q19, Q21 TRANS PWR PNP 20A 140V TO3 4 DK MJ15004GOS-ND 3.92 15.68
Heat Sink for Q18-Q21, HEATSINK, WAKEFIELD, 423K 2 DK 345-1050-ND 29.60 59.20
Heat Sink for Q16, Q17 HEATSINK TO-220 CLIP-ON BLK 4 DK HS111-ND 0.37 1.48
Heat Sink for Q13, Q14 HEATSINK PRESS ON .25"H BLK TO5 4 DK HS218-ND 2.90 11.60
Q18-21 Socket SOCKET FOR .090" TO-3 CASE 8 DK 4601K-ND 2.38 19.04
Q18-21 Wafer THERMAL PAD TO-3 8 DK BER100-ND 0.52 4.16
D1, D2, D3, D4, D11, D12 RECTIFIER 400V 1A DO41 12 DK 1N4004DICT-ND 0.30 3.60
D5, D6, D7, D8, D9, D10 DIODE SS FAST 100V 200MA DO35 12 DK 1N4148TACT-ND 0.14 1.68
D13, D14, D15, D16 DIODE ZENER 20V 500MW DO35 8 DK 1N5250B-ND 0.27 2.16
C1 CAP 390PF 500V MICA RADIAL 2 DK 338-1057-ND 2.84 5.68
C2, C3, C15, C16, C23, C24 CAP .1UF 100V STACK METAL FILM 12 DK P4725-ND 0.34 4.08
C7, C12, C17, C18 CAP .1UF 50V STACK METAL FILM 8 DK P4525-ND 0.29 2.32
C4, C5, C13, C14, C21, C22 CAP 100UF 63V ELECT FC RADIAL 12 DK P10343-ND 0.54 6.48
C6 CAP ELECT 220UF 16V SU BI-POLAR 2 DK P1168-ND 0.73 1.46
C6 CAP 330UF 16V ELECT FC RADIAL 2 DK P10246-ND 0.48 0.96
C8 CAP 180PF 500V MICA RADIAL 2 DK 338-1082-ND 1.86 3.72
C9 CAP 47PF 300V MICA RADIAL 2 DK 338-1084-ND 1.86 3.72
C10, C11 CAP 10PF 500V MICA RADIAL 4 DK 338-1068-ND 1.37 5.48
C19, C20 CAP .01UF 50V STACK METAL FILM 4 DK P4513-ND 0.29 1.16
P1 POT 2.0K OHM 3/8" SQ CERM SL ST 2 DK 3386P-202LF-ND 1.29 2.58
P2 POT, 100K OHM, Log 1 RS RS-271-1732 3.50 3.50
R1 RES METAL FILM 20.0K OHM 1/4W 1% 10 DK 20.0KXBK-ND 0.15 1.50
R2 RES 2.00K OHM 1/4W 1% METAL FILM 10 DK 2.00KXBK-ND 0.15 1.50
R3-R10 RES 301 OHM 1/4W 1% METAL FILM 20 DK 301XBK-ND 0.15 3.00
R11, R12, R27 RES 1.21K OHM 1/4W 1% METAL FILM 10 DK 1.21KXBK-ND 0.15 1.50
R13, R14 RES 2.21K OHM 1/4W 1% METAL FILM 10 DK 2.21KXBK-ND 0.15 1.50
R15, R16 RES 12.1K OHM 1/4W 1% METAL FILM 10 DK 12.1KXBK-ND 0.15 1.50
R17, R18 RES 11.0K OHM 1/4W 1% METAL FILM 10 DK 11.0KXBK-ND 0.15 1.50
R19 RES 1.10K OHM 1/4W 1% METAL FILM 10 DK 1.10KXBK-ND 0.15 1.50
R20 RES 22.1K OHM 1/4W 1% METAL FILM 10 DK 22.1KXBK-ND 0.15 1.50
R21, R22 RES 29.4 OHM 1/4W 1% METAL FILM 10 DK 29.4XBK-ND 0.15 1.50
R23, R24 RES 365 OHM 1/4W 1% METAL FILM 10 DK 365XBK-ND 0.15 1.50
R25, R26 RES 1.00K OHM 1/4W 1% METAL FILM 10 DK 1.00KXBK-ND 0.15 1.50
R28, R29 RES 274 OHM 1/4W 1% METAL FILM 10 DK 274XBK-ND 0.15 1.50
R30, R31 RES 3.92K OHM METAL FILM .50W 1% 10 DK PPC3.92KXCT-ND 0.25 2.50
R32, R33, R51 RES 82.5 OHM 1/4W 1% METAL FILM 10 DK 82.5XBK-ND 0.15 1.50
R34, R35 RES 332 OHM 1/4W 1% METAL FILM 10 DK 332XTR-ND 0.15 1.50
R36 RES 221 OHM METAL FILM .50W 1% 10 DK PPC221XCT-ND 0.25 2.50
R37-R40 RES 681 OHM 1/4W 1% METAL FILM 10 DK 681XBK-ND 0.15 1.50
R41-R44 RES 10.0 OHM METAL FILM .50W 1% 10 DK PPC10.0XCT-ND 0.25 2.50
R45-R48 RES .33 OHM 5W 5% WIREWOUND 10 DK 0.33W-5-ND 0.35 3.50
R49 RESISTOR SILICONE 10 OHM 5W 6 DK ALSR5F-10-ND 1.71 10.26
R-Test RES 100 OHM 1/4W 1% METAL FILM 20 DK 100XBK-ND 0.15 3.00
Standoff STANDOFF HEX 6-32THR .750"L 8 DK 1451EK-ND 0.88 7.04
PCB Marshal Leach V4.5 2 ML GaTech 25.00 50.00
Total NA NA NA NA NA 314.98
Component Testing

The four amplifier components (resistors, diodes, capacitors and transistors) must be checked with a multi-meter before soldering to the circuit board. The objective of the multi-meter test is identification of a faulty component before it is mounted in the circuit board. A faulty component mounted in a circuit board is difficult to identify. The tests described below are conducted using the resistance function contained in a simple digital V-I-R multi-meter. More advanced multi-meters have diode, transistor and capacitor testing functions.

Pi-Amp Transitor Pin or Lead Identification
Pi-Amp Transitor PNP or NPN Identification by Resistance

The following comments parallel the Construction portion of the Leach Amp web site.

Transistor Matching

Use a multi-meter to measure hfe (hfe = IC / IB, the ratio of the Collector to Base current). Pair transistors with closest hfe values. An acceptable difference is within 5%.

For a given transistor, the value of hfe is dependent on VCE (Collector to Emitter Voltage) and temperature. VCE is set by the multi-meter.

Pi-Amp Transitor Current Gain vs. Collector Current

Q1 hfe as measured with a common multi-meter -

Pi-Amp Common Multi Meter
Assembly of Circuit Boards

In order to mount the Leach PCB's using the brass standoffs (1451EK-ND) and #6 machine screws, open up the four mounting holes with a 5/32" bit

Trouble can be avoided by organizing components and double checking the value and orientation before soldering to the PCB.

Testing the Circuit Boards

An mp3 player can be used in place of the signal generator. Audacity or any number of freeware signal generators can be used to make single frequency mp3 files. An mp3 player produces about 1.5V RMS at no load.

Drilling the Heat Sinks
  1. Determine the diode hole size by drilling a scrap and test fitting with one of the bias diodes (1N4004). The hole size will vary by diode manufacturer.
  2. Fasten the TO-3 / Diode template to the heat sink securely with masking tape.
  3. Strike the hole centers with a scratch awl. Double check that all holes have been marked before removing the template.
  4. Drill pilot holes, using the same bit size as required for the diode holes, through each mark made with the awl.
  5. Place a piece of masking tape over the diode holes for safety.
  6. Open up the TO-3 mount holes with a 7/32" bit.
  7. Open up the TO-3 base-emitter holes with a 1/4" bit.
Installing the Bias Diodes

Because the heat sinks are exposed on the front of the unit, each row of power transistors should be covered with a strip of Plexiglas to prevent contact with wandering fingers or metal objects. This requires slight modification to the #1 to #2 and #3 to #4 bias diode connection. The leads are folded flat before soldering together.

Pi-Amp Power Transistors and Heat Sink

A clean way to install the diodes is to solder two pairs together using a jig, install the pairs in the heat sink, and then solder the connection between D2 and D3, as shown below. Glue the diodes using one small drop of standard Loctite Super Glue applied to each after the diodes are in the heat sink. Standard Loctite will flow easily into the space between diode and heat sink (do not use the gel).

Pi-Amp Bias Diode Configuration

Immediately after installing a set of four diodes in one heat sink, solder 12" long 20 gauge wire leads to D1 and D4, insulate with shrink wrap, and label D1 lead "A" and D4 lead "B" with masking tape. If you don't label you will surely forget the location of D1 and D4!

Use the soldering tip for attaching wire leads to small components under Amp PSU.

Installing the Power Transistors
Pi-Amp TO-3 Transistor Sil Pad and Socket

The slightest burr from the transistor socket mounting holes will penetrate the insulating pad and short the power transistor collector to chassis ground resulting in blown transistor. This point cannot be over emphasized! Even if you check for continuity between the collector and heat sink after initial installation, when the amp is powered up, the heat sinks get hot, the pads get soft and then the burr contacts the collector.

Transistor/Diode sequence across the heat sink is as follows: Q18-Q20-Diodes-Q19-Q21.

Before the sockets and transistors are installed in the heat sinks, solder 12" long, 18 gauge wire leads to the transistor sockets and insulate with shrink wrap (24 total leads).

Label each socket lead with masking tape as follows:

Q18B, Q18E, Q18C
Q20B, Q20E, Q20C
Q19B, Q19E, Q19C
Q21B, Q21E, Q21C

Each socket requires two #6 x 3/4" round head sheet metal screws.

Test to ensure no continuity between all collectors and heat sink with the multi-meter before installing in the chassis. If the amp boards have been installed in the chassis, disconnect the boards from central ground before checking continuity otherwise a false short will be measured.

Pi-Amp TO-3 Power Transistor Installation Close View

Preparing and Assembling the Rear Panel
Wiring the Chassis
Optional Input Level Control
Initial Tests on the Completed Amplifier
  1. P1 is at maximum resistance when turned fully counter clock wise.
  2. Do not power the amplifier circuit with the amplifier power supply without installing the 100R resistors in place of F2/F3. Do one channel at a time.
Setting the Bias Current

Multi-meter installed in series across F1 measuring current in mA -

Pi-Amp Setting Leach Amp Bias Current
Is There a Hum in the Loudspeaker?

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Hints if Problems are Encountered

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