lm3886 chipamp guide

8/10/2019 LM3886 ChipAmp Guide

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LM3886 Chip Amp Guide

First of all, this guide is an explanation of the methods and decisions taken by me in building

my own amp. This was successful and to my ears sounds good, however I dont imagine it is the

perfect solution. Hence some of the methods may not be the last word in quality, but they do at

least work.

This amp is based on the dual mono kit using the National Semiconductor LM3886 which is available

from chipamp.com. I recommend you download their guide and also the official LM3886 datasheet

from National Semi.

Amplifier Schematic

Note: VR1, R9 and L1 are not part of the kit.

Power Supply Schematic

Fig 1

Fig 2


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Note: The fuse, switch and transformer are not part of the kit.

D7, D8, R17 & C3 are part of the kit, through not shown in the schematic in the guide from

Chipamp.com. They are optional anyway.

The PCBs have a good silk screen but below are some good quality photos so that you can

see what you are getting before ordering the kit:

PSU

Amplifier

Fig 4

Fig 3


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Transformer choice & general parameter calculations

Toroidal transformers are the most common choice for DIY audio amps. This is due to their

relatively small size and cost. Also they have low magnetic field leakage reducing interference with

other components. I bought mine from CPC Farnell but Airlinktransformers.com also sell them

cheaply.

Size depends on the power of your amp, a stereo (ie two LM3886s) seem to require at least 120VA,

with 300VA being recommended for their better regulation (see below). Beyond 300VA youll

probably need a soft-start circuit for the amp to avoid large in-rush current at start up so are best

avoided unless it is needed.

Specifications:

Power: Giving in volt amps [VA]. Power = Input Vac RMS* Input Iac RMSInput Vac Input voltage, AC, RMS

Output Vac Output voltage, AC, RMS. This may be given as 25/25 or 25+25. This means it hastwo outputs and, as we will be using it, can produce +25V, 0V and -25V

Regulation: The percentage by which the output voltage will rise above the specified output,

when there is no load. I.e., 25V with 6% regulation25*1.06 = 26.5V

This is important as the amp will spend most of its time near no load, and the

maximum input voltage must not be exceeded.

Here comes the maths, we need to determine the correct voltage and power ratings

required from our transformer. First though, I will make clear where the circuit we expect each value

and the components whose values are used in the equations that follow.

VMAINS

VO AC

VO DC

VIN VO DC

RL

RF

RI

Fig 5

Fig 6


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The following equations are best used in an excel document but by presenting them here we

aid understanding and you can build your own excel document from these.

The RMS AC output voltage from the transformer is given as:

Where VO AC = no-load transformer RMS output voltage, as AC

VO AC RATED = rated full-load transformer RMS output voltage, as AC

This is converted to peak AC output voltage by: After smoothing, the DC output voltage from our PSU should be:

And now we have enough to determine the type of transformer required and ensure thatthe amplifiers maximum ratings are not exceeded. Below ABS Max is the absolute maximum for

that parameter, as stated on the datasheet. These must not be exceeded.

The amps power supply voltage is givenby: ABS Max: 84 [V]

Output voltage (to speaker):

ABS Max: 36 [V]

Power dissipated (peak):

ABS Max: 125 [W]

Power output (RMS):

ABS Max: 68 [W]

Current output (RMS):

ABS Max: 11.5* [A]

*Internally limited to typically 11.5A

Power input (RMS): ABS Max: Transformer dependant

From this page you should be able to determine several important parameters of theamplifier. The minimum VA rating of the transformer is found at the end so again, if it is presented in


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a spreadsheet you could vary the specifications of the transformer and see the influence it has on

the rest of the design.

Mains voltage: Mains voltage within the UK is officially 230VRMSbut this is rarely the case, it

is typically closer to 240V, and can stray as high as 243V. You should design the amp to operate at

this maximum mains voltage, even if it means sacrificing some power when the mains is runningslightly lower. The value is the MOST important, the other values will only be reachedwhen the amp is at full load (i.e really very loud), but this maximum voltage could be exceeded even

without music playing.

Gain

While we are at the mathematics well calculatethe gain and gain bandwidth product

(GBWP). The resistors that come with the kit will give you suitable values so this section is only if you

wish to change the input and feedback resistors.

Minimum: 20 Minimum: 2 [MHz]

(Note: For a full (frequency) range amplifier, a typical bandwith is 100kHz).

See pages 20&21 of the datasheet to find out why the GBWP is important.

Connecting the transformer:

The transformer will come with 2 or 4 input connections, and 4 output connections as in Fig

7. The colours may be different but for now I will refer to the colours in the photo.

First we establish what each wire is connected to. We eventually want the transformer connected as

in Fig 6 so remember the coil of a transformer is a low resistance length of copper so you can

determine which wires terminate each coil by measuring the resistance between two wires.

Fig 7

Input terminals

2x

Output terminals

4x


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Hopefully the transformer will come with a diagram to indicate at least which terminals are input

and output. Do NOT get them confused, otherwise the mains voltage will get stepped up, roughly

10x, and the result will be very dangerous.

If you have 4 input terminals, connect the middle two together and apply 230V across the

combination (Fig 8), otherwise the configuration for EU/US use is shown in Fig9.

If you have 2 input terminals, apply 230V and 0V across them.

Connect the 0V terminals together on the output. This will give +V andV on the other terminals.

Ensure the transformer is producing the expected voltages before connecting it to the PSU board.

Referring to Fig 3, connect the terminals to the PSU PCB as per Fig10 below:

30V AC10V 0V

-30V AC2

Grounding

There are many theories regarding grounding but they all relate back to the creation of ground

loops, which cause a continuous hum amongst other problems. My amp is very close to silent and

this was achieved for two reasons:

Use of a ground start Use of very thick wires

Ground Loops:

If one component is connected to the ground of the next and so on, with that cable eventually

reaching ground the resistance of the cables (or pcb track) can become enough to present a

different ground potential at each component. As in Fig11, the two resistors would act as a

potential divider, presenting a different ground potential to each LED.

Fig 8 Fig 9

Fig 10

Fig 11


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Ground stars get around this by using separate wires to a central point, from which a single wire

connects back to the PSU PCB. In Fig12 each component would be presented with the same ground.

The star is the circular join,obviously many other areas of the circuit join this point as well.

Some people insist on separate ground stars for power and signal grounds. A power ground takes

the power connections from the components just discussed, the signal grounds would be those from

the audio input ground. (An RCA lead has positive audio and ground connections, this ground needs

to be connected to a ground of some description in the amp). My has two stars, one for the leftchannel and one for the right channel, I did not bother with power/signal stars and the hum is still

barely audible.

Thick cable:

Since ground loops are caused by the resistance of the wiring, I used thick, low resistance wire

throughout. I used 14 AWG speaker cable for everything which was a hassle to solder because of the

thickness but again, hum was not a problem so maybe it was worth doing.

Input coupling & Output offset voltage

Large DC currents can ruin speakers very quickly, and since they have low impendence, asmall DC potential at the output of the amp is very undesirable. Before plugging your amp into your

speakers for the first time, measure the potential difference between the output and ground. It

should be no more than 100mV.

The output offset can be reduced by ensuring the resistors used have exactly the right value. I had a

box of several so picked out two which measured exactly 22k.

To prevent a DC input to your amp becoming a large DC output a coupling capacitor can be added to

the input (Fig1, C10). Capacitors cannot pass DC so are perfect for this, however some say sound

quality is compromised by this step. I havent compared but went for the safe option of including

them. Polyester capacitors are supposed to sound the best as well, with electrolytic caps sounding

the worst. At least 1uF is required, but a larger value 3.3uF or 4.7uF will allow better bass

performance.

Fig 12


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Feedback inductance

The inductance of the feedback loop (Fig1, R10) should be as low as possible to allow very

fast feedback. Although there are pads for it on the PCB, a common trick is to solder it directly to the

LM3886, keeping the wire lengths to an absolute minimum. This is quite easy to do and should look

like Fig 13.

This is between pins 3 & 9. The pin arrangement is the same as Fig14, though the LM3886 has 11

pins and not 15.

Heat-sink choice

The LM3886 is packaged in the Multiwatt11 package (exact dimensions are given in the

datasheet), though you will struggle to find a heat-sink specifically for this package.

I bought a generic 200x100x27mm sink from Fischer Elektronik (Fig 1) and drilled a 3mm hole

between two of the fins to secure the amp to the sink with a nut and bolt. Drilling between two fins

is fiddly but a tight bolt provides good contact with the metal.

Note, if using bare metal remember the metal tag on the top of the LM3886 is connected to -V, so

should not be pressed directly against the heat-sink. It is common practice to use a mica insulator

on the back side of the IC and a shoulder washer on the front side to isolate the exposed tab from

the heat-sink.[1]

Choosing the correct size:s

The NS LM3886 datasheet gives the following details:

Fig 15

Fig 13

Fig 14


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= thermal conductivity

JC = 1 C/W Junction (die) to case

CS = 0.2 C/W Case to sink (with thermal grease)

SA = - Sink to ambient air

JA = JC+ CS+ SA

PDMAX = (TJmaxTAmb)/JA

Which rearranges to:

SA = [ (TJmaxTAmb)PDMAX (JC+ CS) ] / PDMAX

SA will be quoted on the heat sink specifications. My heat-sink was rated SA = 2 C/W but is much

larger than is really needed.

Volume control

Volume is controlled by a potentiometer on the input line (see Fig 1, VR1). Look for a dual

gang logarithmic potentiometer with maximum resistance of about 10k. It will look like Fig 2

Safety

I am not qualified to offer advice on the safety aspects of building an amplifier and take noresponsibility for the consequences of a homemade electrical item. If in doubt, contact someone

who is qualified to assist you.

Completed Photos

These photos may help when trying to visualise how the amp will come together.

PSU

Fig 16


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Before adding the terminal blocks consider how the PCB will be mounted, otherwise they may get inthe way. Also, solder in the resistors under the PSU before adding the capacitors.

Completed amplifier


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[1] raudio1969Diyaudio.com

lm3886 chipamp guide

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