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Gainclone - The most popular amplifier in the world(?)

LM3886 from National Semiconductor. The heart of a Gainclone.

It took a while to understand what "GC" was, or Gainclone. What a name! A couple of IC's (LM3875LM3876 and LM3886) from National Semiconductor are very popular in the DIY community. The LM3886 is the most powerful and delivers around 68 watts and peak power of 120 watts. The IC is only 20 x 15 mm and contains a whole power amplifer with 10 A output capability. I think this IC is amazing, so much power in so small volume and few external parts.

Interesting features

Click on the pictures to get a larger view.

I have designed a Gainclone and it's universal in most respects? My intentions led to this:

DeLuxe version:

  • Inverting or non-inverting LM3886, can be chosen
  • Inverting or non-inverting input buffer, with or without gain, can be chosen
  • Feedback network which can satisfy everyone, full gain at DC or gain of 1 using a capacitor in the feedback
  • With or without DC-servo for both inverting or non-inverting LM3886, can be chosen
  • Output filter, L//R, small handwound inductor plus a resistor.
  • Inputfilter, lowpass and also highpass to block incoming DC-signals. You could also skip the filters if you want to. I recommend though to use a high frequency lowpass filter.
  • Mute, the LM3886 is "dead" for a short time at power up, can be chosen.
  • Bridge connection and the LM3886 can operate in inverting or non-inverting mode, can be chosen.
  • Pcb for 2 separate channels, stereo, or one channel in bridge connection. It's possible to change mode easily with help from a jumper.
  • Power supply be on the pcb with sufficient amount of smoothing capacitance.
  • Stabilized power supply for the buffers. +-12 V is chosen (can easily be changed) so the amazing AD8620 can be used.

Normal verison (not designed yet):

  • Inverting or non-inverting LM3886, can be chosen.
  • Feedback network which can satisfy everyone, full gain at DC or gain of 1 using a capacitor in the feedback.
  • Output filter, L//R, small handwound inductor plus a resistor
  • Inputfilter, lowpass and also highpass to block incoming DC-signals. You could also skip the filters if you want to. I recommend though to use a high frequency lowpass filter.
  • Mute, the LM3886 is "dead" for a short time at power up, can be chosen.
  • Pcb for one channel.
  • Power supply be on the pcb with a certain amount of smoothing capacitance.
Read More 

 

 

Where does the name come from?

See the excellent description at Wikipedia. It began with Sakura Systems which designed a high-end (or at least expensive) power amp based on LM3875 and a minimal power supply. The product is called GaincardPictures are floating around on the internet and the inside is amazing I think. Some (rather many actually) are a little bit bugged (Rod Elliott among others and I for instance) over Gaincard but others, mostly DIY'ers build their own Gaincard and then call them Gainclone. Some of the Gainclones are really beautiful, very good craftmanship.

Here are reviews of the original.

http://www.tnt-audio.com/ampli/47gaincard_e.html

http://www.6moons.com/audioreviews/47labs5/gaincard.html

 

 

 

 

The beautiful things about the LM3875 and LM3886 are that they are reliable, cheap, sounds good and are easy to get working, even if the are hardwired (no pcb).

 

 

Schematic description

Click on the picture to get a larger view

The schematics, page 1

Click on the picture to get a larger view. The picture shows the schematics of the amp. Of course you can't use it for anything except for an overview. Please download the pdf-file instead if you want to see the details.

This is the left channel including a phase shifter for bridge connection, bottom left.

The schematics, page 2

Click on the picture to get a larger view. The picture shows the schematics of the amp. Of course you can't use it for anything except for an overview. Please download the pdf-file instead if you want to see the details.

This is the right channel.

The schematics, page 3

Click on the picture to get a larger view. The picture shows the schematics of the amp. Of course you can't use it for anything except for an overview. Please download the pdf-file instead if you want to see the details.

This is the power supply for both channels.

Circuit description

Before you read the text below, download the AN-1192 and try to understand this very good application note. Much of this design is based on this application note. I have made some adjustments but the basic idea is very good.

Overture_Design_Guide13.xls (and instructions for the file) is a very good tool when you want to estimate needed transformer and other power supply parts, heatsink etc.

I will describe the left channel in detail. The right is the same with exception of the phase shifter IC6, IC7.

The design consists of an input buffer, the power ICLM3886, DC-servo, a phase shifter and a power supply. The things which are missing is a transformer and a rectifier bridge.

The input buffer is IC1 or IC2. Many good audio opamps don't come in DIL08 package anymore so I have made an option for DIL08 or SMD SO08. A SO08 IC isn't so hard to solder and I can really recommend AD8620, because it's a really, really good opamp. The most impressing feature besides the sound quality is the extremely low offset voltage, around 70 µV! OPA2134 is also good. I recommend an opamp with low input bias currents because you can get better bass using small input coupling capacitors but any good audio opamp will fit as long as you take care of designing the passive parts.

Six different models

I have made six different models in order to satisfy the big crowd but my examples are just suggestions. I you feel like choosing other parts or other values of the parts, feel free to do so. But now I have to test my configurations so I'll know that least those work good.

1 Non-inverting buffer, Non-inverting LM3886, DC-servo

2 Non-inverting buffer, Inverting LM3886, DC-servo

3 Inverting buffer, Non-inverting LM3886, DC-servo

4 Inverting buffer, Inverting LM3886, DC-servo

5 No buffer, Non-inverting LM3886, No DC-servo

6 No buffer, Inverting LM3886, No DC-servo

Input buffer

Input filter for non-inverting buffer

R1(R23) is just a pulldown resistor. It can be of any value from 100 kohms to 2.2 MOhms. The purpose is to not letting C3(C27) float when the input is unconnected. The capacitor can pick up charge and you will/may get a transient when the amp get connected to a signal source and the amp is powered up. This resistor is just a precaution.

C3(C27) and R2(R24) is a highpass filter which may not be needed if your signal source is 100% DC offset free. The cutoff frequency is given by f = 1/(2*pi*R2*C3). 1 µF and 220 kohms gives 0.72 Hz.

R3(R25) and C5(C29) forms a lowpass filter. This is for RFI protection. It's wise to have very high frequencies cut off. The formula is the same as above. 1 kohms and 2.2 nF gives 72 kHz. If you feel that you don't want this filter at all, just short R3(R25) and omit C5(C29). I recommend this filter also for limiting high-speed signals into the LM3886. The filter should block frequencies above the power bandwidth of the LM3886 which is 100 kHz.

Gain for non-inverting buffer

The gain is set by R5(R27) and R4(R26). A = R5/R4 + 1 The gain should be 1-2 i normal cases. C6(C30) is setting the gain to 1 at low frequencies, hardly necessary if you use AD8620 orOPA2134. If you of some reason want to use it you must increase the R4 and R5 or have a very large C6. C7(C31) is for just in case, hardly necessary when R4 and R5 has resistor values under 10 kohms. The purpose of this is to compensate for the input capacitance of the opamp itself. If high resistor values are chosen this input capacitance can cause oscillations. C7(C31) is the medicine for it.

Input filter for inverting buffer

R1(R23) is just a pulldown resistor. It can be of any value from 100 kohms to 2.2 MOhms. The purpose is to not letting C3(C27) float when the input is unconnected. The capacitor can pick up charge and you will/may get a transient when the amp get connected to a signal source and the amp is powered up. This resistor is just a precaution.

C4(C28) and R4(R26) is a highpass filter which may not be needed if your signal source is 100% DC offset free. The cutoff frequency is given by f = 1/(2*pi*R4*C4). 1 µF and 47 kohms gives 3.4 Hz. I recommend lower values of the resistors if this is possible, 10 kohms is better but then you vill maybe increase the C4(C28). 1 µF and 10 kohms gives 16 Hz which may be a bit high in same situations.

C3(C27), C5(C29), is not used and R2(R24) and R3(R25) should have 0 ohms together or max some other value up to 1 kohms. Those two resistors or jumper wires forms a DC-patch for the inputs of the opamp. VERY IMPORTANT! The opamp won't work otherwise.

Gain for inverting buffer

The gain is set by R4(R26) and R5(R27). A = R5/R4 R4 and R5 should be 10-47 kohms and R5 should be x1 to x1.5 of R4. The gain should be 1-1.5. C7(C31) is recommend to cut high frequencies. R5 = 47 kohms and C7 = 22 pF gives 153 kHz.

C5(C29) is just replaced with jumper wire in order to create a DC-path for the non-inverting inputs of the opamps. VERY IMPORTANT! The opamp won't work otherwise.

Phase shifter

When you want to bridge connect I have chosen to have a precision inverter, IC6 (SMD) or IC7. This inverter should be a bit faster than the input buffer so you will get an exact copy of the original signal but phase shift of 180 degrees. If you plan to use this feature you should match R19 and R20 so they will have the same value, 0.1% matching tolerance is good and quite easy to achieve with an ordinary DVM and a couple of 10 kohms resistors. IMPORTANT, you should also match the feedback resistors for the lM3886 as well. The matching should be between then channels. R8 and R30, R9 and R31 should have the same values if the LM3886 are used in non-inverting mode. For inverting mode it's, R7 and R29, R9 and R31.

The unused opamps in IC6 or IC7 must be tied to ground and have feedback. Most opamp can be set at gain of 1 which means that R22 isn't necessary but it's no harm to set the gain to something higher, this just for avoid any sign of unstability.

DC-servo

The DC-servo can be configured as non-inverting or inverting. For non-inverting LM3886 you must use a non-inverting DC-servo and for inverting LM3886 you must use an inverting DC-servo.

The DC-servo has one important design rule: The servo must not saturate at max output power and the lowest defined frequency, 20 Hz in my case. The reason for this is that the servo has +- 12 or 15 volts as supply voltage but the LM3886 has max +- 42 volts. This property sets the max speed of the servo.

The LM3886

Which mode?

The LM3886 can be used in three basic modes, stereo single IC, bridged mono or paralleled mono. All these modes can be in non-inverting or inverting mode.

Study the application note AN-1192 and the datasheet carefully. This is important because it's easier to succeed if you "know" the LM3886. I have read a lot on the internet about these LM3875 and LM3886 amps and many of the instructions are based on rumors, not facts. Because of that many people don't succeed as much as the ought to. Some of the component choices are totally wrong which leeds to mostly excessive output offset voltages and oscillations.

Some people claims that particular values "sounds" good and the penalty is bad performance in offset mainly.... which is the good thing about the IC if you do it right..... Anyway, my component choices are based on the application note AN-1192 because I think that they are good. It's possible that some values have to be changed when the prototype is built.

Non-inverting mode with DC-servo

R6(R26) and C8(C32) is lowpass-filter and protection at power down (only the R6). With 1 kohms and 220 pF you will get 723 kHz. The C6 value can be changed to anything because of the presence of R6. Frequencies down to 50 kHz is technically correct I think. f = 1/(2*pi*R6*C8)

C11(C35) is not used. See the datasheet of LM3886 for information.

R10(R30) and C13(C37) are for the mute function. The LM3886 is shut off at power on and smoothly powered up. This for avoiding dumps at the speaker output. If the pin 8 is left open the amp is muted, therefore you must always use this resistor and the current must be at least 0.5 mA. If you don't want this mute function, just omit the C13(C37).

C12(C36) is shorted if the DC-servo is used.

R18(R40), C14(C38) and L1(L2) forms an output filter which sometimes need to be used. The inductor is normally not very critical in value. 0.5-2 µH will do. Don't forget to use rather thick wire, 1-1.5 mm diam. Consult the Overture_Design_Guide13.xls for advice's. If you don't experience any stability problems, just replace the L1(L2) with a wire and omit R18(R40) and C14(C38).

The (non-inverting) DC-servo for the non-inverting LM3886 consists of R11(R31), C15(C39), R13(R33), C16(C40), R14(R34), C17(C41), R42(R40), IC4 or IC5, C18, C19.

I recommend the use of DC-servo because if you use it the offset voltage at the output will be in the range of 1 mV or less. If you use the extremely good AD8620 you will get output offset voltages less than 100 µV, probably less then 70µV! The two suggestions for opamps in this place are also very good audio opamps with extremely low distortion so the interference at low frequencies will be very low. If you do decide not to use it just omit all parts which are mentioned and use only C12.

Inverting mode with DC-servo

The advantage with inverting mode is lower distortion. In same cases you will ten times lower distortion than in non-inverting mode. The disadvantage is very low input impedance.

C8 is replaced with a jumper. The non-inverting input must be grounded. R6 should be omitted

C11 is not used. See the datasheet of LM3886 for information.

R10 and C13 are for the mute function. The LM3886 is shut off at power on and smoothly powered up. This for avoiding dumps at the speaker output. If the pin 8 is left open the amp is muted, therefore you must always you this resistor and the current must be at least 0.5 mA. If you don't want this mute function, just omit the C13.

R7 and R9 determines the gain. A = R9/R7 If you of some reason want to use the LM3886 without an input buffer you may want to have input coupling capacitors, C9 and maybe C10. C10 is an option if you want to have very low cut off frequency and a low value of R7. f = 1/(2*pi*R7*(C9+C10))

R18, C14 and L1 forms an output filter which sometimes need to be used. The inductor is normally not very critical in value. 0.5-2 µH will do. Don't forget to use rather thick wire, 1-1.5 mm diam. Consult the Overture_Design_Guide13.xls for advice's. If you don't experience any stability problems, just replace the L1 with a wire and omit R18 and C14.

The (inverting) DC-servo for the inverting LM3886 consists of R12, D3, D4, C16, R14, C17, R15, R41, IC4 or IC5, C18, C19. D1 replaced with a jumper.

Inverting mode without DC-servo

If you of some reason don't want to use DC-servo, use only the following parts: C8 replaced with a jumper, C9, C10, R7, R9, R10, C13, R18, C14, L1.

Power supply

You need only 2 x 15-30 VAC fullwave rectified as power voltage. As rectifier bridge you can useRFB02 or any other suitable bridge. I think 10 A, 200 V (more than 100 V) or more is sufficient for normal use and 15-25 A for heavy use. The pcb is equipped with room for four big electrolytic caps with 10 mm between the pins. I have chosen ELNA LP3J but any other suitable type will do. 20000 µF is much for this amp but you can chose to have less too. 4700 µF is the lower limit I think. Some people claim it's even less, 1000 µF according to some people. This is not yet verified by me. The reason for this is simply "it sounds better". A possible explanation could be smaller current spikes from the rectifier diodes.

The chosen fuses are 4 A slow but they can be changed for the application. Choose those so you can play music at full volume, not a sinus test tone.

The opamps are fed from ordinary 7812/7912 regulators but those can be changed to any other pin compatible 3-pin regulator. R37 and R38 take down the incoming voltage a bit. The max voltage of the regulators is 35 volts according to datasheets but in real life the limit is higher. If you are sure the you have max 35 volts in you can just replace these resistors with jumper wires.

Bridge connection mono 1 x 120 W

If you want 1 x 120 watts (or so), place a jumper at J1, J3. Bridge connection will work in all modes but I suspect that it is an advantage if you have DC-servo because you will get lower losses due to offset voltages.

Two channel connection stereo 2 x 68 W

If you want 2 x 68 watts (or so), place a jumper only at J2. You can also solder this jumper if you are sure you don't want the bridge connection feature.

Download special schematics first

For the following section, please download the special schematics for these models. When you build, check the schematics very closely so you will choose the right parts and place them in the right positions. Later I will also make component placement pictures for each model but now there is only the component print. This is more demanding so please check very carefully where you put the parts.

Which part first?

As a general rule, start with low and small parts first, for example, resistors, then plastic capacitors, then IC's but not the LM3886 (wait), then high parts like the big caps, then connectors and maybe the inductor. Test the pcb without the LM3886, then mount the LM3886. For unexperienced builders I recommend though that you build some parts and then test them. It's easier to succeed if you are cautious. See instructions below.

1 Non-inverting buffer, Non-inverting LM3886, DC-servo

+12 V and -12 V

The non-inverting input buffers

The non-inverting LM3886

Mounting the LM3886

The amp is now very much likely functioning! Don't forget to report to me about your impressions!

2 Non-inverting buffer, Inverting LM3886, DC-servo

+12 V and -12 V

The non-inverting input buffers

The inverting LM3886

Mounting the LM3886

The amp is now very much likely functioning! Don't forget to report to me about your impressions!

3 Inverting buffer, Non-inverting LM3886, DC-servo

+12 V and -12 V

The inverting input buffers

The non-inverting LM3886

Mounting the LM3886

The amp is now very much likely functioning! Don't forget to report to me about your impressions!

4 Inverting buffer, Inverting LM3886, DC-servo

+12 V and -12 V

The inverting input buffers

The inverting LM3886

Mounting the LM3886

The amp is now very much likely functioning! Don't forget to report to me about your impressions!

5 No buffer, Non-inverting LM3886, No DC-servo

The smoothing caps

Connectors X1 or A1-A4, C46 - C49. Notice how the electrolytic caps should be turned. Note the stripes for indicating negative pole on the capacitors and see also the corresponding marking in the pcb. I have also + signs in the pcb but those can sometimes be rather unclear.

Connectors X1 or A1-A4, X2, X3 or A5, X4, X5 or A6.

The LM3886

R1-R3, IC2_ wire between pin 1 and 3. Wire in R6. R7, R8, R18. C3, C5, C12, C13 if you want the mute feature, C14, C20-C23.

R23-R25, IC2_ wire between pin 5 and 7. Wire in R28. R30, R31, R32. C27, C29, C36, C37, if you want the mute feature, C38, C42-C45. Wire in J2

L1, L2. L1 and L2 should be around 1 µH in parallel with a 10 ohms resistor, 0.6 watts to start with. Maybe you will need 1-2 W but I don't think so. This filter may not be necessary at all. See comment here.

Mounting the LM3886

The amp is now very much likely functioning! Don't forget to report to me about your impressions!

6 No buffer, Inverting LM3886, No DC-servo

The smoothing caps

Connectors X1 or A1-A4, C46 - C49. Notice how the electrolytic caps should be turned. Note the stripes for indicating negative pole on the capacitors and see also the corresponding marking in the pcb. I have also + signs in the pcb but those can sometimes be rather unclear.

Connectors X1 or A1-A4, X2, X3 or A5, X4, X5 or A6.

The LM3886

R1, wire in R3, IC2_ wire between pin 1 and 3, R7, R9, R10, R16, R19, wire in C3, C8-C10, C13 if you want the mute feature, C14, C20-C23

R23, wire in R25, IC2_ wire between pin 5 and 7, R29, R31, R32, R38, R40, wire in C27, C32-C34, C37 if you want the mute feature, C38, C42-C45. Wire in J2.

L1, L2. L1 and L2 should be around 1 µH in parallel with a 10 ohms resistor, 0.6 watts to start with. Maybe you will need 1-2 W but I don't think so. This filter may not be necessary at all. See comment here.

Please note that suggested component values causes the amp to oscillate the the input is unconnected. The values are chosen to optimize speed and noise. If the amp may be unconnected you may also change component values.

Why does it oscillate? The gain must be more than 10. When the amp is unconnected the gain will be R9/(R7+R1). This means also that the signal source must have an impedance of less than 2 kohms. You can't use a potentiometer with a resistance more than 2.2 kohms.

Mounting the LM3886

The amp is now very much likely functioning! Don't forget to report to me about your impressions!

 

 

Matching parts for the bridge connection feature

Matching parts for the bridge connection feature. Click on the pictures to get a larger view.

If you are really sure that you want the bridge connection feature add, R19-R22, C24-C26 and IC6 or IC7, J1-J3. You can skip this and solder those parts when you really need bridge connection. It's very easy to do this afterwards but you should match some resistors first. See below.

If you are planning to use the amp in bridge mode, read this, important:

In order to get maximum performance you should match the resistors so they are alike across the channels. Strive for 0.05-0.2% tolerance. The goal is to get identical gain, the whole idea of bridge connection.

R19, R20

Non-inverting LM3886

R8, R30

R9, R31

R17, R39

R14, R36

Inverting LM3886

R7, R29

R9, R31

This matching procedure is only necessary if you are planning to use the bridge connection feature. It's possible that it's not necessary to match but experience will tell. I will report later.

 

 

Building +12 V and -12 V power supply

+12 V and -12 V

Start with the voltage regulators for the opamps.

Connectors X1 or A1-A4, F1, F2, C46 - C57, R41, R42, IC9, IC10. Notice how the electrolytic caps should be turned. Note the stripes for indicating negative pole on the capacitors and see also the corresponding marking in the pcb. I have also + signs in the pcb but those can sometimes be rather unclear.

Connectors X2, X3 or A5, X4 (please note that I have forgotten the spade connectors for incoming ground), X5 or A6.

Apply voltage, +20-42 V DC and -20 to 42 V DC. Check that you have +12 V +-5% at pin 8, IC2 and -12 V +-5% at pin 4, IC2 or other suitable point.

If you don't get 12 V you may load the regulators a bit. Load them with 1-10 kohms. Certain brands of regulators demands a minimum load i order to regulate.

Disconnect the voltage and wait until you will have less than 1 V at the big caps. If you can't wait and also have big 10000 uF caps, discharge with a 3.3 kohms /0.6 W resistor or a 100 ohms, 4 W. USE NOT A WIRE. YOU CAN DAMAGE THE CAPS IF THEY ARE DISCHARGED TOO BRUTALLY.

 

 

Building inverting input buffers

The inverting input buffers

R1, R4, R5, C1, C2, C4, wire in C5, C7, IC1 or IC2. Use IC sockets only if you want to change the opamps easy for experimenting purposes but if you are pretty sure of the opamp choice, skip the sockets. If you do want to use these use high quality sockets with "tulip" contacts and goldplated (round holes). Avoid sockets with flat contact springs for permanent use. They have less contact pressure and may cause you trouble especially if you have very small currents which the opamp inputs have. Don't save any money on sockets. Use the best, but best is to avoid them completely.

R23, R26, R27, C28, wire in C29, C31.

Apply voltage, +20-42 V DC and -20 to 42 V DC. Check the output offset voltage. Less than 2 mV is acceptable. Measure with a voltmeter at pin 1 and pin 7 at IC1 or IC2.

If the offset voltages are OK, apply some sinus signal or music signal. Use an oscilloscope or headphones with 100-560 ohms in series (just to protect the headphones) and observe superb sound quality. If all work so far (good!) then start with the main IC, the LM3886.

 

 

 

Building non-inverting input buffers

The non-inverting input buffers

R1-R5, C1, C2, C3, C5. Wire in C6. IC1 or IC2. Use IC sockets only if you want to change the opamps easy for experimenting purposes but if you are pretty sure of the opamp choice, skip the sockets. If you do want to use these use high quality sockets with "tulip" contacts and goldplated (round holes). Avoid sockets with flat contact springs for permanent use. They have less contact pressure and may cause you trouble especially if you have very small currents which the opamp inputs have. Don't save any money on sockets. Use the best, but best is to avoid them completely.

R23-R27, C27, C27, C29, a wire at C30.

Apply voltage, +20-42 V DC and -20 to 42 V DC. Check the output offset voltage. Less than 2 mV is acceptable. Measure with a voltmeter at pin 1 and pin 7 at IC1 or IC2.

If the offset voltages are OK, apply some sinus signal or music signal. Use an oscilloscope or headphones with 100-560 ohms in series (just to protect the headphones) and observe superb sound quality. If all work so far (good!) then start with the main IC, the LM3886.

 

 

Building inverting LM3886

The inverting LM3886

R7, R9, R10, R12, R14, R15-R18, R29, R31, R32, R34, R36-R38, R40, a wire at C10, C13 if you want the mute feature, C14, C16-C23, C32, a wire at C34, C37 if you want the mute feature, C38-C45, wire jumper at D2, D3, D4, wire jumper at D6, D7, D8, wait with IC4 or IC5, L1, L2. L1 and L2 should be around 1 µH in parallel with a 10 ohms resistor, 0.6 watts to start with. Maybe you will need 1-2 W but I don't think so. This filter may not be necessary at all. See commenthere.

If you of some reason don't want to use DC-servo, omit the following parts: R12, R14, R15, R16, R34, R36, R37, R38, C16-C19, C40, C41, IC4, IC5.

A wire at R16, R38, capacitor in C12, C36 instead of wire jumper.

 

Mounting the LM3886

Mounting the LM3886

Now it's time to solder in the LM3886. First, drill the holes in the heatsink. Make sure that the holes have no sharp edges. Mount the IC's without having them soldered but you must have them mounted at the pcb. This is for avoiding mechanical tensions. If you use LM3886TF which is insulated, you only need thermal grease, compound. Don't mess with it! Use a thin layer equally distributed. Bolt the IC's and than solder them also. If you use the LM3886T which is NOT insulated (but have higher power dissipation) you must use a washer of mica, Kapton (orange thin plastic film) plus a bushing insulating the screw. Apply a thin and smooth layer of thermal grease on both sides of the washer and tighten the screw. You can also use silicone rubber with or without the grease.

Last a warning: The grease is messy. Don't use more than you need and you need very little! The grease will also creep away from the LM3886 and the result is dust collecting.

Change fuses to 500 mA fast, or 1 A fast, just as a precaution.

Check once more that the LM3886 has got both it's inputs connected somewhere, if not the fuses will go. Check especially J2.

Apply voltage and check the current consumption which should be max 150 mA (guessing right now). Measure the voltage at the output. Should be less than 1 V (guessing). Disconnect the supply voltage.

If you have chosen to have DC servo, solder in the IC's for the DC-servo, IC4 or IC5.

Apply supply voltage, check the output DC voltage. It should very low. With AD8620 max 2 mV, probably much less, like 70-100 µV! With OPA2134 I'll expect a slightly higher value, maybe less than 5 mV.

 

Building non-inverting LM3886

The non-inverting LM3886

R6, R8, R9, R10, R11, R13, R14, R17, R18, R28, R30, R31, R32, R33, R35, R36, R39, R40, C8, a wire at C12, C13 if you want the mute feature, C14 - C23, C32, a wire at C36, C37 if you want the mute feature, C38-C45, wait with IC4 or IC5, L1, L2. L1 and L2 should be around 1 µH in parallel with a 10 ohms resistor, 0.6 watts to start with. Maybe you will need 1-2 W but I don't think so. This filter may not be necessary at all. See comment here.

If you of some reason don't want to use DC-servo, omit the following parts: R11, R13, R14, R17, R33, R35, R36, R39, C15-C19, IC4, IC5.

Capacitor in C12, C36 instead of wire jumper.

 

Technical data

For complete data, please read the datasheet for the IC's in mind.

Output power: 68 W at 4 ohms, see this for more info
Frequency response: 0 Hz - 400 kHz, -3 dB
Frequency response with DC-servo: 0,4 Hz - 400 Hz, -3 dB
Power bandwidth at 20 Vrms, 8 ohms: 0,4 Hz - 100 kHz, -3 dB
Equivalent input noise: 2 µV typical
Signal to noise ratio: 92.5 dB at 1 W out
Signal to noise ratio: 117 dB at 1 V in
Dynamic headroom: 119 dB
Distortion: 0,002 %
Slew rate: 19 V/us
Step response: TBD
Gain: 19 (25.6 dB), non-inverting mode
Input impedance: 100 kohms, with non-inverting input buffer
Output current: 11.5 A
Gain bandwidth product: 8 MHz
Dimensions: 152 x 120 mm, 6" x 4.7"

 

 

 

 

 

 

 

 

 

 

 

 

LM3886

AN-1192

Overture_Design_Guide13.xls

AD8620

OPA2134

 

 

To do

Glömt flatstift för inkommande jord

Glömt ta bort regulatorer när opampar saknas.

Tjockare hot releaf bars för kraftjordsplinten, kanske, kolla detta.

Svårt att löda AD8620 när allt annat sitter omkring.

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