W. Marshall Leach, Jr., Professor
Georgia Institute of
Technology
School of Electrical and Computer Engineering
Atlanta, Georgia
30332-0250
Copyright 2000. All rights reserved.

Click here to
download an LTSpice file for the amplifier. It was developed by Valerio
Maglietta who generously shared it with me.

Warning to amplifier builders! Dated 08/14/06. One amplifier builder found
that the 2N5416 PNP transistors that he purchased were NPN. I have tested them
and verified this. Two of the transistors are shown in the linked photo. They
are identified by the letters ON surrounded by a circle, signifying they are
products of ON Semiconductor. You can check for a transistor type as follows:
Connect an ohmmeter from base to emitter. You should get a low resistance with
the positive lead to the base if it is NPN, otherwise it is PNP. With the
ohmmeter connected from base to collector, you should get the same result.
Repeat the tests with the negative lead connected to the base. The resistances
should be reversed. You should get an open circuit from collector to emitter
with both polarities of the ohmmeter. Defective transistors usually can be
determined with these tests. The transistors should be removed from the circuit
for the most reliable results. I have later found that these transistors could
be counterfit units made by another company and labeled with the ON
Semiconductor name.

PDF of
Overall Circuit Diagram

Photograph
of assembled circuit board.

Background

The Cascode
Diff Amp Input Stage

The Second
Stage

The Output
Stage

The Feedback
Network

The Protection
Circuit

The Power
Supply

Construction
Details

Parts List
and List of Suppliers

Clipping
Indicator - Several students who have built the amplifier wanted to add a
clipping indicator. I found this circuit in my filing cabinet. It was given to
me many years ago. I understand that it was described in an article by Chris
Russell in Issue 3 of The Audio Amateur in 1975. We changed some of the
resistor values and it seems to work quite well with the amplifier. The two 12
kohm resistors can be changed to vary its sensitivity.

Frequently
Asked Questions

Component Change: R37 through R40 - 680 ohm (changed from 470 on
4/4/2). This change makes the protection circuit less sensitive so that the
amplifier can drive more current into lower impedance loads. I made this change
after a correspondent told me that he built the amp to use with his home built
electrostatic speakers which have a very low impedance. He said the amp sounded
distorted. He removed the protection circuit and that solved the problem. He
said that he has had no problems with the amp with the protection circuit
removed. My original amp had no protection circuit, and I never had any problems
with it.

PDF
of Circuit board foil pattern and template for drilling heat sinks. The
circuit board measures 4 inches by 4.4 inches. There are layouts for both the
solder and the component sides of the board. The template for drilling the heat
sinks has the holes marked for the power transistors and the bias diodes. You
must verify that the layout and template print to the correct size! You do
this by verifying that the line at the bottom of the page measures six inches.
If it doesn't, you will have to change the print settings for the Adobe reader.
Do not use the board layout or heat sink drill template if it doesn't print to
the correct size! Thanks to Damir Arbula for cleaning up some printer artifacts
that were on my original layout and making the layout for the circuit side of
the board.

Layout of
parts on the circuit board with the copper traces beneath the board shown.

Layout of
parts on the circuit board with the copper traces beneath the board not
shown.

Color
layout of parts on the circuit board with the copper traces beneath the
board shown. Thanks to amplifier builder Todd Johnson for this!

I used a DOS program called Tango to lay out the amplifier circuit board. The
Tango layout and the Gerber files it created are in this
zipped folder. Click on the link and save the folder to your computer. I
intend to check to see if all of the files are in the directory and add any
missing ones, provided I can find them.

Here is a Powerpoint
Slide Show sent to me by Bradley Koeppel and Jeff Stout. Bradley is an EE
and Jeff is a ME. It shows the constuction of two 5 channel amplifiers, which
they built. These are some of the best looking amplifiers I have seen. Here
is a picture of the finished front panel of Bradley's amplifier.

Parts Sources
and Prices Prepared by Students I am sure many of these prices are out of
date. For example, MCM has almost tripled the price for their transistors.

View a picture of student Allen Robinson's amplifier here.

View a picture of student Dan Leverett's amplifier here.

John Mitchell received his amplifier circuit boards in September 2004. In
April 2005, he sent me pictures of his work in progress. He said that it was his
first electronics project and is a lot of work. He is building his amplifier on
aluminum plates and installing it inside a commercial chassis. You can see
pictures of his amplifier here.

View an amplifier concept designed by Carl-Fredrik Gustavsen here.

Derek
Beamer's Amplifier Page. Derek is in the Toronto area.

Klas Malman's Amplifier Page.
Klas lives in Sweden.

The Double Barreled
Amplifier Page

Greg's Download Page - A wealth of
free software for electronics projects.

I have circuit boards for the amplifier which are fabricated by a local PC
board manufacturer. They are pre-drilled for the correct wire sizes and have a
solder mask to prevent solder bridges between traces. These are Vers. 4.5 boards
which have the hole spacings between two resistors increased by 0.1 inch
compared to the Vers. 4.4 boards. If you wish information on these, I can be
contacted at

Please put "Low-TIM PCB" in the subject header of any email that you send to
me. I get so many spam emails and emails from our school that sometimes I just
don't see inquiries about amplifier circuit boards before my email gets deleted.
I apologize if any of your inquiries do not get answered on time.

I do not recommend that you make the circuit boards unless you have
experience in doing it. A source of materials for making your own printed
circuits can be found here. I have been
told that their "Press and Peel Blue" product (not the wet stuff they sell) can
be used to successfully make boards with traces as narrow as 0.01 inch. The
smallest traces on the amplifier layout are 0.03 inch wide. The PnP Blue product
is basically a transfer medium that allows you to transfer the toner image from
a laser printer directly onto bare copper clad board and then etch it with
FeCl3 (ferric chloride). This is nasty stuff and it should not be
disposed of in your drain.

After you etch the board, the copper should be cleaned with steel wool,
lightly coated with solder flux, and then "tinned" with a soldering iron and
rosin core solder. Do not use a commercial tinning solution that you dip the
board into. It is almost impossible to solder a board that is tinned with one of
these products because they corrode very quickly. When you drill the board, you
should use the correct size drill bit for the pads. The hole diameters on my
boards are: small pads - 0.032 inch, medium pads - 0.040 inch, large pads -
0.059 inch, mounting holes - 0.125 inch. If you do not use a sharp drill bit,
you can pull the pads off the board when you drill it.

LSR&D and
Electronics One amplifier circuit diagrams.

Home

This page is not a publication of the Georgia Institute of
Technology and the Georgia Institute of Technology has not edited or examined
the content. The author of this page is solely responsible for the
content.