Leach Amplifier Construction
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 -
- Parts list with Digi-Key part numbers and Jan 2011 pricing.
- Comments to Construction portion of the Leach Amp web site specific to this chassis configuration.
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.
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|
|R50||RESISTOR SILICONE 10 OHM 5W||4||DK||ALSR5F-10-ND||1.71||6.84|
|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|
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.
- Resistors - Reject resistors that fall outside of their tolerance. For example, a 100 KR (one hundred thousand ohm), 1% resistor should measure 999 KR - 101 KR.
- Capacitors - Reject capacitors that measure non-infinite resistance. Use the lowest resistance range setting (200R - 400R typical). The lowest range applies the highest DC voltage (3 VDC typical) and will charge the capacitor quicker. The highest resistance range setting (2 - 50 MR typical) applies a low DC voltage (100 mV typical) and will charge the capacitor slowly and usually result in a large, but not infinite, final resistance reading. A low resistance indicates a short, or current leakage, between the capacitor "plates". Small capacitors charge quickly and infinite resistance is measured instantly. Large electrolytic capacitors charge slowly and may show a large, but not infinite, final resistance depending on the resistance range setting. Large capacitors should be discharged with a resistor before testing. Honor polar capacitor polarity markings with the meter probes.
- Diodes - Reject diodes that measure non-infinite reverse bias resistance. A reverse bias is applied to the resistor when, in the resistance function, the positive meter probe is placed on the diode lead closest to the polarity band (cathode) and negative meter probe placed on the anode. The reverse bias resistance should measure infinite. The forward bias resistance should measure 20KR - 30KR.
- Transistors - Confirm the transistor gender (NPN or PNP) and junction integrity by measuring the resistance with the leads connected as shown in the table below. Determine the base, emitter and collector (B, E, C) using the pin out diagram in the next section. Notice the collector to emitter test; low resistance would indicate a damaged transistor.
The following comments parallel the Construction portion of the Leach Amp web site.
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.
Q1 hfe as measured with a 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
- 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.
- Fasten the TO-3 / Diode template to the heat sink securely with masking tape.
- Strike the hole centers with a scratch awl. Double check that all holes have been marked before removing the template.
- Drill pilot holes, using the same bit size as required for the diode holes, through each mark made with the awl.
- Place a piece of masking tape over the diode holes for safety.
- Open up the TO-3 mount holes with a 7/32" bit.
- 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.
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).
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
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.
Initial Tests on the Completed Amplifier
- P1 is at maximum resistance when turned fully counter clock wise.
- 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 -
Is There a Hum in the Loudspeaker?
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Hints if Problems are Encountered
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