A Compact 50W Integrated Amplifier with Meier Headphone Section.

by Tim Harrison

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My reason for constructing this project was to develop a design for a compact integrated stereo amplifier suitable for use by a poor (but sound quality conscious!) student living in a university or college dorm. The amplifier drives a pair of loudspeakers using two LM3876 integrated power amp ICs (50 watts per channel), or a pair of headphones via a Meier crossfeed filter and an OPA2134 dual opamp. It provides four switchable line level inputs, and an unbuffered line level output for recording purposes. The design uses readily available good quality components, and is based around four separate PCBs; one for each power amp channel, one for the power supply board, and one for the preamp/headphone driver.

THE CIRCUIT

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Figure 1

The block schematic for one channel of the design is shown above (figure 1). The preamp and the first stage of the headphone amp are separate in this application, ‘straddling’ the gain across the volume control. There is an initial gain of 2.5 before the control, followed by a further gain stage of x3 after it. This arrangement allows the power amp to be driven directly from the output of the volume control without further gain, and makes for lower noise operation of the headphones. The input selector switch is a 4-way, 3-gang type, so one gang isvused for each channel, and one gang is used to switch the input indicator LEDs.

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Figure 2

Above is the schematic for part of the preamp board (figure 2). The output of the selector switch is sent to pins J1 and J3. Looking at the left channel, C1 and R2 form a low pass filter with a -3dB point of 40kHz, which rejects any RF interference picked up on the interconnects. R2 also sets the input impedance of the unit, in this case 47k ohms. R1 ensures the opamp U1 is presented with an equal impedance at both its inputs, helping improve its distortion performance as outlined on the OPA2134 datasheet. The value of R1 (9k1) is the nearest commonly available value to the parallel combination of R3 and R4 (22k and 15k respectively). R3 and R4 set the gain of this stage, just under 2.5 in this case. This value allows ample headroom for a wide range of source signals, which could be as much as 3VRMS. In this case, the peak output voltage of 10.6V would be fine with the suggested ±15V power supply.

This initial gain brings the signal up to a level whereby the output from the volume control can drive the power amp circuits directly, with no further gain, and allows the headphone driver circuit to operate with a lower gain, giving lower noise performance. C7 forms a 100kHz low pass filter with R3, rolling off the gain to unity at very high frequencies, and helping promote stability of the opamp. It is not strictly necessary with the suggested OPA2134 device, but allows the drop-in substitution of a cheaper but more oscillation prone device, such as the NE5532, if budgets are tight. C19 AC couples the output from this stage to the volume control, and with a 50k potentiometer, sets the -3dB point of the headphone amp’s response at 1.4Hz (the power amp has further high pass filtering). This capacitor is very important, as all the other stages are DC coupled, and C19 prevents any DC offsets from source components being amplified and presented to the headphones or speakers.

The resistor R9 links the output of the input selector to a recording device, such as a tape deck or minidisc recorder. It helps prevent the source becoming too loaded down feeding both the input gain stage and the recording device, and protects the source should the output become shorted to ground for any reason. The outputs from J5 and J6 are fed into the volume control pot, which should be a good quality type. Finally, C3 to C6 provide local decoupling of both the power supply rails, C5 and C6 decoupling the high frequencies, with C3 and C4 decoupling the lower ones.

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Figure 3

The output of the pot feeds the power amp and the headphone driver, which is also mounted on the preamp board. Looking at the above schematic for the headphone driver (figure 3), we can see that the opamp U2 is used in a similar configuration to the input amp U1. In this case, R24 matches as closely as possible the parallel combination of R11 and R12, helping reduce distortion as before. Again, C21 allows compatibility with cheaper opamps. R11 and R12 set the gain of the stage at just over 3, bringing the signal up to a level sufficient to drive a pair of headphones. This stage also acts as a buffer, isolating the Meier crossfeed filter from the varying output impedance of the volume control. C8, R14, (with C10, R21, and R15) form a crossfeed filter, which in this case is permanently wired in circuit. A detailed description of the operation of this circuit can be found in Jan Meier’s article A DIY Headphone Amplifier with Natural Crossfeed.

Basically, the circuit performs a frequency selective mix of the two channels into each other, allowing recordings meant for speaker listening to sound natural on headphones. I had built projects with the filter made switchable in the past, but I never turned it off, so the switch was omitted here. Finally, the opamp U3 forms a simple noninverting buffer to drive the headphones. R17 forms a minimum load when the phones are disconnected, and helps prevent pops and clicks when they are connected with the unit powered up. While it is possible to substitute cheaper opamps in other parts of the circuit, the device used here needs to have a high output current capacity, and must remain stable when driving difficult loads. J10 and J12 are the output to the headphone socket, which should have its ground isolated from the chassis so as not to defeat the ground loop breaker circuit. Again, C11 to C18 provide local supply decoupling for the opamps.

You can find more information on the detailed operation of opamp based circuits, such as the preamp and headphone amp circuits presented here, in Chu Moy’s article Designing an Opamp Headphone Amplifier. Figure 4 is the power amp schematic for one channel (both channels are identical – and use one power amp board each).

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Figure 4

The circuit given here is similar to the one presented in the article, Single Chip 50 Watt / 8 Ohm Power Amplifier, on Rod Elliott’s site, ESP. The LM3876 is a good quality component capable of delivering 56W continuously into an 8 ohm load and 100W peak – enough for any dorm! It has a quoted distortion figure of 0.06% at 40W output, and offers good sound quality in a simple design. It has comprehensive output protection circuitry, preventing not only thermal runaway, but protecting the device from short circuits on the output, and voltage spikes from inductive loads.

Looking at the circuit, R3 and R1 set the gain of the power opamp at 23, and C1 limits the DC gain to unity. It also forms a low pass filter with a -3dB point of 7.2Hz. R2 draws roughly 1.5mA from pin 8, disabling the internal muting function of the LM3876, and C2 provides a large time constant for the action of the muting circuit. R4 should be a 1W resistor, and has 10 turns of 0.4mm enamelled wire wound round it, with its ends soldered to the resistor leads, giving a roughly 0.7uH inductor in series with the 10 ohm resistance. The inductor acts to promote stability of the power opamp, by ensuring a minimum 10 ohm load at higher frequencies. Likewise, the low pass zobel network formed by C7 and R5 (which should also be a 1W type), helps prevent oscillation should any RF appear on the output. C3 to C6 provide local supply decoupling for the power amp IC.

To enable the power amplifier to deliver its full rated power (56W/ch) continuously, and to cater for the potential 100W peaks, I decided to build a good quality power supply for the project, capable of supplying 200W. The main power supply for the speaker amps was built directly into the chassis, and is a fairly standard design. It supplies ±35V, and is capable of just over 3A continuous per rail for both the power amps. A ±15V supply for the preamp and headphone driver is provided from the main supply by the PSU board. Firstly, I will describe the main power supply, whose schematic is shown below (figure 5):

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Figure 5

The mains enters the chassis via a filtered IEC inlet, and the live line is fed through a 1A antisurge type fuse mounted in an insulated chassis fuse holder, before both the live and neutral lines are fed to a DPST rocker switch mounted on the front panel. The mains feed from the switch is connected to the primary of the power transformer, and a pair of transient suppressors are wired in parallel with it (only one is shown in the diagram). They should be rated for the mains voltage where you are, and should be mounted securely on the base of the chassis, I used two sections from an insulated terminal block.

The secondaries of the transformer are wired in series, and the wires from the toroidal types can be connected directly to a heavy duty chassis mounted bridge rectifier. The output of the bridge rectifier is sent to a pair of reservoir capacitors, C2 and C3, connected in parallel with C4 and C5, which provide high frequency decoupling. The only other point about the power supply that needs explaining is the ground loop breaker circuit. The 0V rail is connected to chassis ground and mains earth via R1, a 10 ohm wire wound resistor, in parallel with C1, a mains rated 100nF capacitor. The resistor prevents any currents flowing round the loop created by the mains earth and the ground in unbalanced phono interconnects. The 100nF capacitor shorts the resistor at high frequencies, allowing any RF to flow to ground in the normal way. I placed C1 and R1 on the underside of the stripboard I used to mount the reservoir and decoupling capacitors.

The output from the main PSU is fed to the power amp boards via a front panel DPST switch, allowing the speaker amps to be switched off for headphone only listening, and also (unswitched) to the preamp PSU board. Below is the schematic for the preamp PSU board (figure 6):

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Figure 6

The ±35V rails must be reduced to around ±20-22V before they can be fed to standard three pin regulators. I simply used a potential divider comprising R3 and R6 for the positive rail, and R4 and R7 for the negative rail. Simply placing a reverse biased 12-15V zener diode in series with the supply, i.e. in place of R3 and R4 (and omitting R6 and R7), would be an alternative option, and probably simpler – this option didn’t occur to me until after I built the prototype! C1 to C4 decouple the output of the regulator, and R1, R2, and R5 set the current flowing though the LED indicators, around 15mA in this case. The stabilised ±15V supply is presented on pins J1-J3, and the remainder of the pins provide supplies for a pair of power on indicator LEDs (mounted next to the mains rocker switch), and the input selector LEDs. These are mounted above the input selector switch, and light to show which input has been selected. They are controlled by the remaining gang of the three gang rotary switch.

CONSTRUCTION

I have provided my PCB artwork for you to use to make your own PCBs if you are interested in building all or just part of this project. Below are links to the artwork files, and the relevant placement guides (all GIF format):

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Download PC Board Artwork and Placement Guides 1
Download PC Board Artwork and Placement Guides 2
Download PC Board Artwork and Placement Guides 3
Download PC Board Artwork and Placement Guides 4
Download PC Board Artwork and Placement Guides 5
Download PC Board Artwork and Placement Guides 6

The artwork will print the correct size if you set your graphics software to output 600dpi to your printer. The placement guides should be printed at 300dpi. If you have trouble getting them the right size, the power amp boards should be 41mm wide, the PSU board should be 113mm wide, and the preamp board 132mm. I made the artwork for the lead pitch and size of the components I could source, so I suggest you print out the placement guides real-size (300dpi), and compare the sizes and lead pitches of the components you can source, selecting the ones that best fit the board.

As a guide to component selection, I used 0.6W metal film resistors throughout, except for R3, R4, R6, and R7 in the PSU, which should be 5W wire wound radial lead types, and R4 and R5 in the power amp, which should be 1W wire wound types. For the decoupling capacitors use ceramic disc types, and for capacitors in the signal path (C19, C20, C8 and C10 in the preamp), I used the Wima MKS4 250V series, although any metal film type will do (but may not fit).

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The line level signal from the sources is received via an array of gold plated phono plugs on the rear of the unit. The plugs I used had red and black identification bands on them to indicate which channel should be connected to them. This was important, as I was not planning to print any lettering onto the case, so the connections and controls had to be fairly self-explanatory. The phono plugs should be mounted using an insulating bush, as the design uses a ground loop breaker circuit, and the signal and earth (chassis) grounds are separated.

The source signals are routed via screened cable to a rotary selector switch mounted on the front panel which is used to select the source to be listened to (and recorded from). The switch should be a good quality part, as a positive tactile response from it enhances the feel of the finished project. The part I used was a 3-gang 4-way type, allowing 4 stereo inputs to be accommodated, leaving one gang free to switch the source indicator LEDs mounted above the control. I used a cheap part by a company called Alpha, their SR2611 series. This switch works fine and only cost a few pounds (roughly $5 US).

For a volume control, I used a 50k ALPS pot, but a cheaper type of any value between 10-100k could be used. A conductive plastic track type is preferable to a carbon track, and should be logarithmic law (also called audio taper). The ALPS RK27 series pots (the blue ones), while pricey, come highly recommended, as they have a very nice tactile feel to them, and exhibit good tracking between the gangs.

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For the preamp and headphone section opamps, I recommend the OPA2134 by Burr Brown, and DIL sockets are a good idea to help prevent heat/static damage during soldering. Note, the LM3876T power opamp in figure 4 must be used with my PCBs, the T suffix denotes the package type. The power opamps share a large 2 degrees C per watt heatsink mounted on the rear panel in the prototype and are mounted using greaseless silicone insulators and insulating bushes. Make sure the metal tab of both the power amp ICs is isolated from the chassis – this is very important.

Power supply

The value of the mains fuse in figure 4 varies depending on what type of transformer you use, and the supply voltage in your country. Since I live in the UK where the mains supply is 230V, and I am using a 225VA rated toroidal transformer, a 1A antisurge fuse was used. Take care to get this value right, as if it is too low, you will suffer nuisance blowing, and if it is too high, you will not get proper protection in the event of a fault. The fuse rating can be calculated in the normal way using I = P / V. A double pole type switch is preferable to a single pole type, as it allows the unit to be completely isolated from the mains when it is switched off. The mains rocker switch used should be rated to handle the in-rush current of the transformer, anything over 4-5A should be fine in this case.

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I don’t normally include transient suppressors in the power supplies of audio projects I build, as I run the mains supply to my home audio system through a filter which includes them. However, as this integrated amp was designed to be used away from home, they are included here. You can use a pair together in parallel as suggested to increase their dissipation capacity.

The toroidal power transformer was made by Nuvotem and sourced from RS components in the UK, at a cost of about 23UKP (or about $35 US). In general, the power transformer itself should be a good quality type, and I recommend a 225VA toroidal part for this project. If the budget is tight, a lower value toroidal (say, 160VA) could be used, or even a conventional EI laminate type. Although the standard EI transformers are cheaper than toroidal types, and exhibit a lower inrush current, they are less than ideal for a compact unit. They tend to be quite bulky, and emit strong electromagnetic fields, leading to hum pickup in adjacent circuitry. A toroidal transformer is both compact, and emits a far less strong field.

Although the rated current of the power supply is only 3A, the charging current of the reservoir capacitors will be much higher than this at times. I recommend using a 35A type bridge rectifier, such as the KBPC3506. A pair of heavy duty insulated terminal blocks should be mounted nearby, and the centre tap of the secondaries connected to this. The terminal block will now form a star grounding point, and should be the place all the 0V rails in the unit are connected together. This method of grounding ensures hum free operation. Save yourself a lot of grief and use this method the first time – hum free results are almost guaranteed.

If the budget is tight, 4,700uF capacitors can be substituted for the 10,000uF ones specified in figure 4, especially if a 160VA transformer is used. I had trouble fitting 10,000uF caps into the chassis, so I used two 4,700uF caps in parallel per rail. I couldn’t get any capacitor mounting brackets, so I simply soldered C2 – C5 onto a small piece of stripboard. You could use either method, but be sure to take your DC output from the capacitors and not the rectifier.

For the 5W resistors in figure 5, I used vertically mounting ceramic, wire wound resistors, but you could use standard axial types, with one leg bent down the side, if you find the radial types hard to get. C5 to C8 decouple and stabilise the output of the potential divider (or zener diode), before it is fed into a pair of standard voltage regulators. These should be mounted with a pair of small flag type (clip-on) heat sinks with a thermal resistance of around 20-25 deg. C per watt. I used Redpoint Theramalloy PF752.

Chassis

I mounted the project in a compact instrument case (300mm W x 150mm D x 100mm H), which has a removable internal chassis. The case was supplied painted grey, but once I had drilled it, I decided to repaint the chassis blue to make the project look more individual. I prepared the chassis by sanding it down thoroughly, making sure that all surfaces would provide a good key the paint could adhere to. I then cleaned all the surfaces with white spirit, and applied three thin coats of standard car spray paint. I used diffused blue 3mm LEDs, black rocker switches with blue markings, and black aluminium knobs to complete the effect.

You can see the layout I used pretty clearly from the pics inside the unit, all the boards were mounted on the base of the chassis, except for the two power amp boards which were mounted on the rear panel. The bridge rectifier is bolted to the bottom of the metal chassis. I used 4mm binding posts for the speaker terminals, two black ones for the ground connection, a green one for the left channel, and red for the right. All the signal wiring should be done using shielded cable, with the screen grounded at one end only. Ribbon cable can be used for LED wiring, 32/0.2mm hookup wire should be used for power amp supply and speaker connections, and 7/0.2mm hookup wire can be used for other low power connections.

If there is hum on the output of the completed project, the problem is almost certainly to do with the ground scheme used. Make sure that there is a 10 ohm resistance between the chassis and signal ground (i.e. that you have not defeated the ground loop breaker), and make sure you have not accidentally grounded a point by two paths simultaneously. The star grounding scheme as outlined earlier is highly recommended. The path to ground on the volume control pot is particularly critical, in the prototype the unit refused to stop humming until the far end of the wiper had its own separate connection to the star ground point. It should be possible to set the volume control to zero and, with the unit on, put your ear to the speaker and hear nothing but a faint hiss.

RESULTS

My impression of the project overall is very good, it sounds good, and is very compact. The performance from the IC power opamp is impressive, and I think my prototype looks nice, too! Listen to your favourite cans through it late into the night, or let it provide some serious slam through speakers for a small room or dorm.

c. 2002 Tim Harrison.

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