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User Guide for the Non-Inverting

LM3875 Kit 


Table of Contents

Safety Precautions

  1. Introduction
  1. History
  2. Terminology Used In This Document
  3. Tools Required To Assemble the Kit
  1. How to assemble this kit
  1. Kit Contents
  2. Assembling the Amplifier PCB
  3. Assembling the Rectifier PCB
  1. How to Turn the Assembled Kit into a Working Amplifier
  1. Choosing a transformer
  2. Wiring the power supply and installing into the chassis
  3. Chassis Considerations
  4. Volume Control
  1. Miscellaneous Information
  1. The Zobel Network
  2. The Bridged LM3875
  3. An Input Buffer
  1. FrequentlyAsked Questions (FAQ)
  1. What started this group order?
  2. Where did you first find out about the gainclone?
  3. I just ordered the PCB set, what premium components should I use with it?
  4. I would rather start with basic components, what works with the board?
  5. I have a suggestion for material to be added to this manual
  6. Soldering Tips


Safety Precautions

It is assumed the builder of this kit is familiar with the dangers of handling exposed electrical wiring. The assembly of this kit will require the practice of safety around household voltage, be it 120 or 240 VAC. It is recommended that you work under the guidance of someone knowledgeable of the safety concerns if that person is not yourself.

  1. Introduction

Thank you for purchasing the Non-Inverted LM3875 kit. The goal of this kit is to get easy to build, good sounding DIY amplifiers into the hands of as many people as possible. If you follow the instructions provided in this manual, you will have built a high quality power amplifier in a reasonable amount of time. It should be noted that the currently available kit uses a Revision B board. Additions to this board have been taken from feedback from those who purchased the Rev A boards, as well as the design guidance from members of the diyAudio.com community. Major changes to the Rev B board include the addition of a second rectifier board, allowing the construction of true monoblock amplifiers without purchasing additional boards, the ability to mount the feedback resistor on the PCB directly and an area for a Zobel network to be integrated directly on the board.

  1. History

The term gainclone is based on the 47 Laboratory 4706 Gaincard. After rave reviews of this amplifier in Japan, it was determined that the main component of this amplifier was a readily available $5 chip, made by National Semiconductor. This chip, the LM3875, when properly implemented, creates a high quality amplifier. This started a new trend in DIY amplifiers, as this is the first time that such a high quality amplifier could be made so easily and for such little money.

  1. Terminology Used In This Document

Non-Inverting – Operational Amplifier (opamp) topology used in this kit. An opamp has 2 input terminals. If the input goes into the +input, then it is generally non-inverting, meaning that the output is in phase with the input.

Negative Feedback (NFB) – A technique used in most power amplifiers to set the voltage gain. The output is fed back into the negative input on the amplifier. The effect of negative feedback is to cancel out distortion and negate the effect from non-ideal characteristics. The NFB loop should be as short as possible. This leads to the NFB resistor not being placed on the PCB, but soldered directly to the device pins. On the Rev B board, the option of including the NFB resistor is included, but not required and the builder is encouraged to still solder the NFB directly as described in the following instructions.

LM3875 – Amplifier IC used in this kit. It is available in 2 different packages, insulated LM3875TF, and un-insulated LM3875T. The insulated package is usually preferred, due to the ease of use. Some people say that the insulated package also sounds better. The actual Gaincard uses the LM3875TF.

PCB – Abbreviation for Printed Circuit Board

Gain – Ratio of output power  input power

Gainclone – Amplifier using a chip amplifier, such as the LM3875 used in this amplifier

  1. Tools Required To Assemble the Kit


  1. How to assemble this kit

  1. Kit Contents

Parts for the Amplifier Board

Description/Component Designation on PCB

Mono Quantity

Stereo Quantity

Dual Mono Quantity

Amplifier rev. 3  PCB

Single Channel board for the LM3875

1

2

2

LM3875TF

National Semiconductor Chip Amplifier

1

2

2

1k Ohm Resistor

R1

1

2

2

22.1k Ohm Resistor

R2,Rf

2

4

4

680 Ohm Resistor

R3

1

1

2

2.7 Ohm Resistor

Rz

1

1

2

1500uF Panasonic FC Capacitor

C1,C2

2

4

4

0.1 uF Capacitor

Cz

1

2

2

Parts for the Power Supply Board

Description/Component Designation

Mono Quantity

Stereo Quantity

Dual Mono Quantity

Power Supply rev. 3  PCB

PCB for Power Supply Components

1

1

2

1 Ohm Resistor

R1,R2

2

2

4

2.2k Ohm Resistor

R3,R4

2

2

4

10k Ohm Resistor

R5

1

1

2

MUR860G

D1,D2,D3,....D8

8

8

16

47 uF

C6,C7

2

2

4

LED

LED

  1. Assembling the Amplifier PCB

It’s not absolutely necessary to install the components in this order, but doing so should help you avoid soldering in tight spaces. Start with the components closest to the center of the board, the resistors.

  1. Bend the leads of the 1k ohm resistor (R1).

IMG_1284.JPG

  1. Thread the leads through the holes of the PCB where R1 is labelled.

  1. Bend the leads down on the back of the PCB to secure the component while you solder.

  1. Apply the soldering iron to the resistor lead and pad while feeding solder until the connection is formed.

  1. Trim the leads of the resistor.

  1. Repeat these steps for the rest of the resistors.

  1. Next, solder the 1500uF capacitors, C1 and C2.  Make sure the longer lead goes into the + hole. Note that there are three holes instead of two. This is for those who wish to use capacitors not sold with the kit.

  1. If you wish to utilize the zobel network, add the Rz and Cz components.

  1. Solder the LM3875 IC onto the board. Insert the LM3875 IC into the PCB, applying pressure on pin 9. Pin 9 will be a tight fit, but will help position the chip properly.

  1. Here is a picture of the bottom side, after leads have been trimmed.

  1. Assembling the Rectifier PCB

  1. Solder the LED onto the board. The small arrow indicates where the longer lead should be inserted.

  1. Solder the 10k ohm resistor, R5, next to the LED.

  1. Gather the MUR860 diodes, and solder them to the board. The white line on the PCB board indicates which way the diode should be facing. As shown in the picture, the metallic plate should be facing the the white line.

  1. One row down, one to go. Remember those white lines are for the other row. If you look at the diodes toward the right of the image you can see the corresponding line.

  1. This is what the board should look like when all diodes have been soldered.

  1. Now solder the .1 uF capacitors, C3, and C4.

  1. Follow up with C7 and C8, the 47 uF capacitors. Observe polarity, the longer lead goes into the positive (+) hole.

  1. Now you can solder the rest of the components, R1, R2, R3, R4, C1, and C2. Here are a couple pictures of a completed power supply.

Now that you have all the boards finished, you are ready to assemble them into a working amplifier. At this point you can clean up the PCB with rubbing alcohol and a plastic brush. This will remove the flux from the solder, and the board will look better.


  1. How to Turn the Assembled Kit into a Working Amplifier

  1. Choosing a transformer

There are many different options for producing a power supply, but using the supplied diode bridge gives three main options when choosing a transformer.

The original standard application of this kit uses a transformer with dual secondaries. In this application, the primaries are attached to the mains AC voltage, while V+ and 0+ are attached to V+ and PG+ and V- and 0- are attached to V- and PG-. Note that for a stereo amp, one diode bridge can be used for both channels. For monoblock applications, both bridges and two separate transformers can be used.

For the application of a true center tapped transformer, one with only three wires, V+, V- and 0, the following alternate arrangement can be used.

Connect such that one transformer connects it V to V+ and 0 to PG- and the second transformer V to V- and 0 to PG+.

Using the first example, one can choose a variety of VA ratings and rail voltages. Keep in mind that after rectification, the rail voltages are somewhat higher than the non-rectified AC secondary rating of the transformer. The secondary voltage averages to 1.4 * the AC voltage, minus diode losses. The transformer regulation is also a factor, dependent on the size and regulation characteristic of the transformer in question. Suffice it to say that a commonly used 18V transformer results in approximately 25V rectified, while a common 22V supply produces around 34V rectified.

Referencing the National datasheet LM3875.pdf, you can find the practical maximum rail voltage rating for the average impedance of your speaker. The Output Power vs. Supply Voltage chart on page 9 is a good indication of the maximum rail voltages for a given speaker that you will design to. When looking over the curves a speaker with a nominal impedance of 4 ohms tend toward 25V rails with a reasonable margin of safety, while voltages above 35V are still well within the range for 8 ohm speakers. This shows that transformers with 18-22V secondaries are well within reason for many common commercial and DIY speakers. A transformer with 25V transformer secondaries can also be successfully with less of a safety factor.

The VA ratings on transformers is also a consideration. Within reason, a larger transformer has more constant regulation under load, but this chip operates very successfully without extremely large transformers. Many have successfully used 160VA transformers, while the 220VA range seems to be adequate for almost all stereo implementations, not straining the transformer. The price point between 220VA and 330VA, however might lead one to purchase the larger of the two. Anything above this could be considered frivolous for a stereo pair, unless one happens to be on the shelf or in a surplus vendors stock. Don’t be tempted to buy an extremely large transformer such as used for Class-A applications as it is simply not required. The fuse required will be dictated by the size of the transformer, due to the inrush current when power is first applied. A 2 amp slo-blo typically works fine for transformers around 220VA or less, while a 3 amp slo-blo fuse might be required if using a larger transformer.

Using an EI transformer or a toroidal transformer is up to the end user. Both have their advantages, but the common availability of toroids and their successful use in many designs makes them a common choice. If you have access to a good quality EI, don’t be afraid to use it.


  1. Wiring the power supply and installing into the chassis

  1. Wire the secondary windings from the transformer into the rectifier PCB.

The primaries of the transformer should be wired as shown below with a fuse and power switch. If constructing monoblocks, this procedure will be duplicated using one transformer and rectifier board per monoblock.

  1. Once you have verified that the voltages from your rectifier board are correct, by measuring from V+ to +PGND and V- to –PGND (should be the same), solder wires to the rectifier board for running the power supply voltages to each channel. It is helpful to choose a color scheme for your wiring, so that things are not improperly wired. Reversing the voltage supply rails on the amplifier can ruin the LM3875 IC.

  1. Next, route the power supply wires in your chassis. You will also need to run a ground wire from your chassis ground to each channel:

The chassis ground essentially goes to what could be referred to as the star ground. There will be 3 wires tied here, one each channel, and AC ground connection.

  1. Next, solder the power supply wires, along with the input and output wires into the PCB for each channel. Be sure to wire the polarity of the input and output correctly. Wiring the input backwards might cause the input to be shorted to ground.

        

  1. Now it is time to mount the boards to the chassis. For the standoffs on the PCB, I used regular 1” standoffs with #4 screws. Heatsink compound is required for mounting the LM3875 to the chassis. If you are using the LM3875T IC, make sure to use the provided Aluminum Oxide insulator and the screw flange. #4-40 screws is recommended for mounting the boards.

        

  1. Here is a picture of my chassis with the boards mounted: Assuming that you took care of tying the input and output to the board, it should now be a functional amplifier.

  1. Chassis Considerations

There are many options on chassis design. Much of this us up to the builder’s taste, tools, budget and time. One fundamental issue which is the same, regardless of final design, is safety. The chassis keeps dangerous voltages from harming anyone or anything that is in the area of the amplifier.

The chassis must be properly grounded to the mains ground to prevent possibly lethal voltages from being seen on the chassis in the event of failure. Ground loops, which commonly are a source of hum in situations may still be alleviated by proper chassis layout, shielding and the use of a star ground. Stopping the source of hum is the only option, instead of disconnecting the chassis ground. At times, when connected to a complex system, hum can be caused by interfacing to coax cable used for video. Try disconnecting this cable from the system entirely and if this solves the problem, consider an isolation transformer for cable systems. Chassis layout also can affect hum. Keeping signal level wiring away from the transformer is a good place to start.

Just about anything which can safely isolate the user from the amplifier could be a viable chassis. Some designs focus on aesthetics, while others offer simplicity and others focus on optimising various materials to provide a stable platform for the amp to subjectively sound its best.

The last consideration for chassis construction is heatsinking. This amplifier does not require huge heatsinks. Successful implementations have used computer heatsinks without fans, solid pieces of bar stock or aluminum angle and others rely on standard large heatsinks. An aluminum plate around 3” x 3” x ½” should be plenty for most applications if allowed to circulate in free air.

When attaching a toroidal transformer to the chassis, care must be taken to not create a shorted turn. This occurs when a ferrous (metal) object creates a loop through the center of the transformer. In the following example, eliminating the top metal bracket and using only the washer supplied with the transformer would eliminate the shorted turn.

  1. Volume Control

Volume control can be achieved by use of a receiver or pre-amp or via signal attenuation. An attenuator can be made using a potentiometer with a shunt resistor or a multi-position switch with various combinations of resistors to attenuate the signal to appropriate levels. If a switch is used, a make before break switch should be used to to eliminate the possibility of popping when switching from one setting to another.

If a receiver or pre-amp is used and has sufficient gain, the amplifier could be overpowered and clipping could occur. If the volume control on the pre-amp gets extremely loud with small variation, lowering the gain of the amplifier is in order. Lower the gain of the amplifier by varying R3 per the equation Gain = 1+ Rf/R3. The standard supplied gain set by R3 is 23.1dB. Gain should remain above 10dB to reduce the chance of the amplifier going into oscillation. To lower the gain, increase the value of R3.

The following diagram shows the wiring scheme for a typical three-leg potentiometer:

Here is an example of a Noble stereo potentiometer:

  1. Miscellaneous Information

  1. The Zobel Network

A brief description of the implementation of a Zobel network and why this would be done follows.

Please note that this is per the recommendation of some experienced designers and is included in the National datasheet.

What does the Zobel do? It helps prevent high frequency oscillation, which may occur with difficult loads/capacitive cables. Basically, it creates a low pass filter, with the .1 uF cap and 2.7 ohm resistor creating a pole at 1/(2*pi*r*c), which can solve issues with cables, as well as reducing RF interference. According to some people, the Zobel can have a negative effect on the sonic performance. Builders who are concerned about the presence of oscillations induced by cables should indeed experiment with this addition and are encouraged to post their results!

  1. The Bridged LM3875

The following is a brief explanation of options for bridging the LM3875 chip. There are many different options, including buffering, which would result in a different topology than the one alluded to below.

The first thing you need to determine is the load you want to drive. Keep in mind the effect of bridging and paralleling of amplifiers:

Now, to figure out the power, simply figure out how much load each amplifier is driving. If you are driving 8-ohm speakers and bridging, the power will more than double. If you are driving a 4-ohm speaker with a paralleled set-up, the power will increase, but not quite double.

The ultimate configuration for power would be a bridged-parallel configuration with 4 amplifiers per channel. Paralleling is easy, with the addition of the 0.22 ohm resistors in series with the output. Bridging will require a circuit that will invert the phase of one channel, such as a simple circuit using the DRV134.

        

Feel free to search for more on your favourite website for more background on bridging this amp or look into the LM4870 chip discussions.

  1. An Input Buffer

Nick Whetstone at Decibel Dungeon has compiled some information from other sources regarding the addition of an input buffer. It is a good place to start to determine if this addition is something you would like to try. The following diagram is posted with Nick’s permission and is one way to achieve a simple buffer for the circuit.

If this addition interests you, be sure to check out Nick’s website http://www.decdun.fsnet.co.uk, as well as those of his references and the typical DIY sites for additional information.

  1. FrequentlyAsked Questions (FAQ)

Here is a list that I compiled of frequently asked questions:

  1. What started this group order?

I was teaching my friend how to use some PCB layout software for his projects, and I gave him the assignment to make a PCB for the gainclone, as it is the simplest circuit that I know of, yet still functional. I had been playing around with the gainclone for the last year, trying different versions, but never actually taking it seriously. The circuit was based off of Peter Daniel’s simplified version that he posted on the forum. My friend returned to me a board layout that was twice the size of the current board, and had long unholy traces running everywhere. I decided that I would make my own layout and show him what I thought would be the best implementation. I spent a night working on it, and posted the result the next day to DIYaudio to see if others had comments on it. I received several e-mails requesting boards, and a recommendation to offer a kit, as I had chosen a specific set of components for the design.

Anyway, I worked on the design more, and started a new thread a couple of days later to see how much interest a group order of pcbs for this would generate. There was quite a substantial interest, so I created a rectifier PCB also, and decided to put 2 channels + rectifier on a single PCB. I worked with Peter Daniel to refine my PCB layout, and get a design that would be work best, for chassis mounting, and optimal signal paths. I researched components and availability and put up an order page.

From here, I generated orders for well over 100 kits.

  1. Where did you first find out about the gainclone?

There was a discussion on DIYaudio over a year ago about the term Frugal-phile™, which Dave (planet10) started by saying:

“It is relatively easy to spend big-bucks and get a reasonable sounding hifi (but not as easy as you might think, given the number of mega-buck audio-pile systems that are not very listenable). The goal here is to spend as little as possible and get a hifi that is musically enjoyable.”

From this, I got curious about what kind of amplifiers would be considered Frugal-phile™. I asked about it, and the term GainKlone came up. I found a reference to the Chip Amp Forum. From here, I saw Thorsten’s design, and decided to try it. I made one, and never took it seriously, until I saw Peter Daniel’s latest amplifiers. At this point, I was hooked on using the gainclone.

  1. I just ordered the PCB set, what premium components should I use with it?

If you want to use premium components, like I used with the kits, here are a list of vendors that carry these parts:

Riken resistors: Michael Percy: www.percyaudio.com, Angela: www.angela.com

Caddock resistors: Michael Percy: www.percyaudio.com, pcX: www.partsconnexion.com Blackgate caps: Michael Percy: www.percyaudio.com, pcX: www.partsconnexion.com, Reference Audio Mods: www.referenceaudiomods.com 

Rest of components: Digikey: www.digikey.com

  1. I would rather start with basic components, what works with the board?

If you just want to use standard components, the board will work fine, and you can always upgrade later on. Here is a parts list for Digikey:

#

Part Name/Description

Digikey Part Number

2

LM3875TF

LM3875TF-ND

4

1500uF 50v Panasonic FC Capacitor

P10334-ND

4

22.1k ohm resistor (min quantity = 5)

PPC22.1KXCT-ND

2

680 ohm resistor (min quantity = 10)

P680BBCT-ND

2

220 ohm resistor (min quantity = 5)

PPC20.0KXCT-ND

8

On Semiconductor MUR860 fast diodes

MUR860OS-ND

2

4.7uF 50v Panasonic FC Capacitor

P10315-ND

If you are just buying these components for yourself from Digikey, in small quantities, expect to pay about $35 for all these parts, plus shipping from Digikey. The budget kit will cost around $35, including all of the parts above and a PCB set.

  1. I have a suggestion for material to be added to this manual

Feel free to drop an e-mail to: manual@chipamp.com and I will try to look over this account once and a while to make revisions to this manual. Thanks to Sandy for all of his help in preparing this manual.