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. If you can't find the mentioned files, send me a message. I have them also.

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 IC, LM3886, 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 or OPA2134. 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!