IntroductionGuitar amplifiers are always an interesting challenge. The tone controls, gain and overload characteristics are very individual, and the ideal combination varies from one guitarist to the next, and from one guitar to the next. There is no amp that satisfies everyone's requirements, and this offering is not expected to be an exception. The preamp is now at Revision-A, and although the complete schematic of the new version is not shown below, the fundamental characteristics are not changed - it still has the same tone control "stack" and other controls, but now has a second opamp to reduce output impedance and improve gain characteristics.
One major difference from any "store bought" amplifier is that if you build it yourself, you can modify things to suit your own needs. The ability to experiment is the key to this circuit, which is although presented in complete form, there is every expectation that builders will make modifications to suit themselves.
The amp is rated at 100W into a 4 Ohms load, as this is typical of a "combo" type amp with two 8 Ohm speakers in parallel. Alternatively, you can run the amp into a "quad" box (4 x 8 Ohm speakers in series parallel - see Figure 5 in Project 27b, the original article) and will get about 60 Watts. For the really adventurous, 2 quad boxes and the amp head will provide 100W, but will be much louder than the twin. This is a common combination for guitarists, but it does make it hard for the sound guy to bring everything else up to the same level.
Note: This is a fully revised version of the original 100W guitar amp, and although there are a great many similarities, there are some substantial differences - so much so that a new version was warranted. This is (in part) because PCBs are now available for both the power and preamps. The update was sufficiently substantial to warrant retaining the original version, which is still available as
The Pre-Amplifier
A photo of the Revision-A preamp is shown below. You'll see that there are two dual opamps, but the schematic only shows one. This is the main part of the Rev-A update - the output section now has gain (which is easily selected), and a better buffered low output impedance. The remainder of the circuit is unchanged. Full details of the new version are available on the secure site for those who purchase the PCBs.
Guitar Pre-Amplifier Board (Revision A)
The preamp circuit is shown in Figure 1, and has a few interesting characteristics that separate it from the "normal" - assuming that there is such a thing. This is simple but elegant design, that provides excellent tonal range. The gain structure is designed to provide a huge amount of gain, which is ideal for those guitarists who like to get that fully distorted "fat" sound.
However, with a couple of simple changes, the preamp can be tamed to suit just about any style of playing. Likewise, the tone controls as shown have sufficient range to cover almost anything from an electrified violin to a bass guitar - The response can be limited if you wish (by experimenting with the tone control capacitor values), but I suggest that you try it "as is" before making any changes.
Figure 1 - Guitar Pre-Amplifier
From Figure 1, you can see that the preamp uses a dual opamp as its only amplification. The lone transistor is an emitter follower, and maintains a low output impedance after the master volume control. As shown, with a typical guitar input, it is possible to get a very fat overdrive sound by winding up the volume, and then setting the master for a suitable level. The overall frequency response is deliberately limited to prevent extreme low-end waffle, and to cut the extreme highs to help reduce noise and to limit the response to the normal requirements for guitar. If you use the TL072 opamp as shown, you may find that noise is a problem - especially at high gain with lots of treble boost. I strongly suggest that you use an OPA2134 - a premium audio opamp from Texas Instruments (Burr-Brown division), you will then find this quite possibly the quietest guitar amp you have ever heard (or not heard :-). At any gain setting, there is more pickup noise from my guitar than circuit noise - and for the prototype I used carbon resistors!
| Notes: 1 - IC pinouts are industry standard for dual opamps - pin 4 is -ve supply, and pin 8 is +ve supply. 2 - Opamp supply pins must be bypassed to earth with 100nF caps (preferably ceramic) as close as possible to the opamp itself. 3 - Diodes are 1N4148, 1N914 or similar. 4 - Pots should be linear for tone controls, and log for volume and master. |
The power supply section (bottom left corner) connects directly to the main +/-35V power amp supply. Use 1 Watt zener diodes (D5 and D6), and make sure that the zener supply resistors (R18 and R19, 680 ohm 1 Watt) are kept away from other components, as they will get quite warm in operation. Again, the preamp PCB accommodates the supply on the board.
The pin connections shown (either large dots or "port" symbols) are the pins from the PCB. Normally, all pots would be PCB types, and mounted directly to the board. For a DIY project, that would limit the layout to that imposed by the board, so all connections use wiring. It may look a bit hard, but is quite simple and looks fine when the unit is completed. Cable ties keep the wiring neat, and only a single connection to the GND point should be used (several are provided, so choose one that suits your layout. VCC is +35V from the main supply, and VEE is the -35V supply.
If you don't need all the gain that is available, simply increase the value of R6 (the first 4k7 resistor) - for even less noise and gain, increase R11 (the second 4k7) as well. For more gain, decrease R11 - I suggest a minimum of 2k2 here.
If the bright switch is too bright (too much treble), increase the 1k resistor (R5) to tame it down again. Reduce the value to get more bite. The tone control arrangement shown will give zero output if all controls are set to minimum - this is unlikely to be a common requirement in use, but be aware of it when testing.
The diode network at the output is designed to allow the preamp to generate a "soft" clipping characteristic when the volume is turned up. Because of the diode clipping, the power amp needs to have an input sensitivity of about 750mV for full output, otherwise it will not be possible to get full power even with the Master gain control at the maximum setting.
Make sure that the input connectors are isolated from the chassis. The earth isolation components in the power supply help to prevent hum (especially when the amp is connected to other mains powered equipment).
If problems are encountered with this circuit, then you have made a wiring mistake ... period. A golden rule here is to check the wiring, then keep on checking it until you find the error, since I can assure you that if it does not work properly there is at least one mistake, and probably more.
The input, effects and output connections are shown in Figure 1B.
- Input - these are quite the opposite of what you might think. The same basic idea is used on Fender amps, as well as nearly all others that have dual inputs for a channel. The Hi input is used for normal (relatively low output) guitar pickups, and is "Hi" gain. "Lo" in this design has about 14 dB less gain, and is intended for high output pickups so the first amplifier stage does not distort. The switching jack on the Hi input means that when a guitar is connected to the Lo input, it forms a voltage divider because the other input is shorted to earth.
- Effects - Preamp out and power amp in connections allow you to insert effects, such as compression (for really cool sustain, that keeps notes just hanging there), reverb, digital effects units, etc. The preamp out is wired so that the preamp signal can be extracted without disconnecting the power amp, so can be used as a direct feed to the mixer if desired. This is especially useful for bass. The preamp output can also be used to slave another power amplifier (as if you need even more - you do for bass, but not guitar).
- Output - A pair of output connectors is always handy, so that you can use two speaker boxes (don't go below 4 ohms though), or one can be used for a speaker level DI box. Because of the high impedance output stage, headphones cannot (and must not!) be connected to the speaker outputs. The 'phones will be damaged at the very least, but (and much, much worse) you could easily cause instant permanent hearing loss.
Figure 1B - Internal Wiring
The connections shown are very similar (ok, virtually identical :-) to those used in my prototype. Noise is extremely low, and probably could have been lower if I had made the amp a little bigger. All connectors must be fully insulated types, so there is no connection to chassis. This is very important !
You will see from the above diagram that I did not include the "loop breaker" circuit shown in the power supply diagram. For my needs, it is not required, for your needs, I shall let you decide. If you choose to use it, then the earth (chassis) connection marked * (next to the input connectors) must be left off.
A few important points ...
- The main zero volt point is the connection between the filter caps. This is the reference for all zero volt returns, including the 0.1 ohm speaker feedback resistor. Do not connect the feedback resistor directly to the amp's GND point, or you will create distortion and possible instability.
- The supply for the amp and preamp must be taken directly from the filter caps - the diagram above is literal - that means that you follow the path of the wiring as shown.
- Although mentioned above, you might well ask why the pots don't mount directly to the PCB to save wiring. Simple really. Had I done it that way, you would have to use the same type pots as I designed for, and the panel layout would have to be the same too, with exactly the same spacings. I figured that this would be too limiting, so wiring it is. The wiring actually doesn't take long and is quite simple to do, so is not a problem.
- I did not include the "Bright" switch in Figure 1B for clarity. I expect that it will cause few problems.
Bass Guitar, Electric PianoAs shown, the preamp is just as usable for bass or electric piano as for rhythm or lead guitar. A couple of changes that you may consider are ...
- Delete the clipping diodes (unless fuzz bass/piano is something you want, of course). If these are removed, then the output should be taken directly from the Master output pin (M-OUT in Figure 1), so leave out / change the following ...
- Delete R14, and D1-D4
- Delete Q1 and associated components (C14, C15, R15, R16, R17)
- Delete VR5
- Change R13 from 4.7k to 100 ohms
You may also want to experiment with the tone control caps - I shall leave it to the builder to decide what to change, based on listening tests. C3 and C8 may be increased to 4.7uF to provide an extended bass response. If the gain is too high, simply increase R11 (10k would be a good starting point and will halve the gain).
Power AmplifierThe power amp board has remained unchanged since it was first published in 2002. It certainly isn't broken, so there's no reason to fix it. The photo below shows a fully assembled board (available as shown as M27). Using TIP35/36C transistors, the output stage is deliberately massive overkill. This ensures reliability under the most arduous stage conditions. No amplifier can be made immune from everything, but this does come close.
Guitar Power Amplifier Board
The power amp (like the previous version) is loosely based on the 60 Watt amp previously published (Project 03), but it has increased gain to match the preamp. Other modifications include the short circuit protection - the two little groups of components next to the bias diodes (D2 and D3). This new version is not massively different from the original, but has adjustable bias, and is designed to provide a "constant current" (i.e. high impedance) output to the speakers - this is achieved using R23 and R26. Note that with this arrangement, the gain will change depending on the load impedance, with lower impedances giving lower power amp gain. This is not a problem, so may safely be ignored.
Should the output be shorted, the constant current output characteristic will provide an initial level of protection, but is not completely foolproof. The short circuit protection will limit the output current to a relatively safe level, but a sustained short will cause the output transistors to fail if the amp is driven hard. The protection is designed not to operate under normal conditions, but will limit the peak output current to about 8.5 Amps. Under these conditions, the internal fuses (or the output transistors) will probably blow if the short is not detected in time.
Figure 2 - Power Amplifier
Figure 2 shows the power amp PCB components - except for R26 which does not mount on the board. See Figure 1B to see where this should be physically mounted. The bias current is adjustable, and should be set for about 25mA quiescent current (more on this later). The recommendation for power transistors has been changed to higher power devices. This will give improved reliability under sustained heavy usage.
| As shown, the power transistors will have an easy time driving any load down to 4 ohms. If you don't use the PCB (or are happy to mount power transistors off the board), you can use TO3 transistors for the output stage. MJ15003/4 transistors are very high power, and will run cooler because of the TO-3 casing (lower thermal resistance). Beware of counterfeits though! There are many other high power transistors that can be used, and the amp is quite tolerant of substitutes (as long as their ratings are at least equal to the devices shown). The PCB can accommodate Toshiba or Motorola 150W flat-pack power transistors with relative ease - if you wanted to go that way. TIP3055/2966 or MJE3055/2955 can also be used for light or ordinary duty. |
At the input end (as shown in Figure 1B), there is provision for an auxiliary output, and an input. The latter is switched by the jack, so you can use the "Out" and "In" connections for an external effects unit. Alternatively, the input jack can be used to connect an external preamp to the power amp, disconnecting the preamp.
The speaker connections allow up to two 8 Ohm speaker cabinets (giving 4 Ohms). Do not use less than 4 ohm loads on this amplifier - it is not designed for it, and will not give reliable service!
All the low value (i.e. 0.1 and 0.22 ohm) resistors must be rated at 5W. The two 0.22 ohm resistors will get quite warm, so mount them away from other components. Needless to say, I recommend using the PCB, as this has been designed for optimum performance, and the amp gives a very good account of itself. So good in fact, that it can also be used as a hi-fi amp, and it sounds excellent. If you were to use the amp for hi-fi, the bias current should be increased to 50mA. Ideally, you would use better (faster / more linear) output transistors as well, but even with those specified the amp performs very well indeed. This is largely because they are run at relatively low power, and the severe non-linearity effects one would expect with only two transistors do not occur because of the parallel output stage.
Make sure that the bias transistor is attached to one of the drivers (the PCB is laid out to make this easy to do). A small quantity of heatsink compound and a cable tie will do the job well. The diodes are there to protect the amp from catastrophic failure should the bias servo be incorrectly wired (or set for maximum current). All diodes should be 1N4001 (or 1N400? - anything in the 1N400x range is fine). A heatsink is not needed for any of the driver transistors.
The life of a guitar amp is a hard one, and I suggest that you use the largest heatsink you can afford, since it is very common to have elevated temperatures on stage (mainly due to all the lighting), and this reduces the safety margin that normally applies for domestic equipment. The heatsink should be rated at 0.5° C/Watt to allow for worst case long term operation at up to 40°C (this is not uncommon on stage).
Make sure that the speaker connectors are isolated from the chassis, to keep the integrity of the earth isolation components in the power supply, and to ensure that the high impedance output is maintained.
Power SupplyWARNING - Do not attempt construction of the power supply if you do not know how to wire mains equipment.
The power supply is again nice and simple, and does not even use traditional regulators for the preamp (details are on the preamp schematic in Figure 1). The power transformer should be a toroidal for best performance, but a convention tranny will do just fine if you cannot get the toroidal.
| Do not use a higher voltage than shown - the amplifier is designed for a maximum loaded supply voltage of +/-35V, and this must not be exceeded. Normal tolerance for mains variations is +/-10%, and this is allowed for. The transformer must be rated for a nominal 25-0-25 volt output, and no more. Less is Ok if the full 100W is not needed. |
Figure 3 - Power Supply
The transformer rating should be 150VA (3A) minimum - there is no maximum, but the larger sizes start to get seriously expensive. Anything over 250VA is overkill, and will provide no benefit. The slow-blow fuse is needed if a toroidal transformer is used, because these have a much higher "inrush" current at power-on than a conventional transformer. Note that the 2 Amp rating is for operation from 220 to 240 Volt mains and as shown is suitable for a 200VA transformer - you will need an 4 or 5 Amp fuse here for operation at 115 Volts. Smaller transformers can use a smaller fuse - I am using a 2A slow blow fuse in my prototype (160VA transformer at 240V mains input), which seems to be fine - it allows for a maximum load of 480VA which will never be achieved except under fault conditions.
Use good quality electrolytics (50V rating, preferably 105°C types), since they will also be subjected to the higher than normal temperatures of stage work. The bridge rectifier should be a 35 Amp chassis mount type (mounted on the chassis with thermal compound).
The earth isolation components are designed to prevent hum from interconnected equipment, and provide safety for the guitarist (did I just hear 3,000 drummers asking "Why ??"). The 10 Ohm resistor stops any earth loop problems (the major cause of hum), and the 100nF capacitor bypasses radio frequencies. The bridge rectifier should be rated at least 5A, and is designed to conduct fault currents. Should a major fault occur (such as the transformer breaking down between primary and secondary), the internal diodes will become short circuited (due to the overload). This type of fault is extremely rare, but it is better to be prepared than not.
Another alternative is to use a pair of high current diodes in parallel (but facing in opposite directions). This will work well, but will probably cost as much (or even more) than the bridge.
All fuses should be as specified - do not be tempted to use a higher rating (e.g. aluminium foil, a nail, or anything else that is not a fuse). Don't laugh, I have seen all of the above used in desperation. The result is that far more damage is done to the equipment than should have been the case, and there is always the added risk of electrocution, fire, or both.
Electrical Safety
Once mains wiring is completed, use heatshrink tubing to ensure that all connections are insulated. Exposed mains wiring is hazardous to your health, and can reduce life expectancy to a matter of a few seconds !
Also, make sure that the mains lead is securely fastened, in a manner acceptable to local regulations. Ensure that the earth lead is longer than the active and neutral, and has some slack. This guarantees that it will be the last lead to break should the mains lead become detached from its restraint. Better still, use an IEC mains connector and a standard IEC mains lead. These are available with integral filters, and in some cases a fuse as well. A detachable mains lead is always more convenient than a fixed type (until your "roadie" loses the lead, of course. You will never do such a thing yourself :-)
The mains earth connection should use a separate bolt (do not use a component mounting bolt or screw), and must be very secure. Use washers, a lock washer and two nuts (the second is a locknut) to stop vibration from loosening the connection.
TestingIf you do not have a dual output bench power supply
Before power is first applied, temporarily install 22 Ohm 5W wirewound "safety" resistors in place of the fuses. Do not connect the load at this time! When power is applied, check that the DC voltage at the output is less than 1V, and measure each supply rail. They may be slightly different, but both should be no less than about 20V. If widely different from the above, check all transistors for heating - if any device is hot, turn off the power immediately, then correct the mistake.
If you do have a suitable bench supply
This is much easier! Do not connect a load at this time. Slowly advance the voltage until you have about +/-20V, watching the supply current. If current suddenly starts to climb rapidly, and voltage stops increasing then something is wrong, otherwise continue with testing. (Note: as the supply voltage is increased, the output voltage will fluctuate initially, then drop to near 0V at a supply voltage of about +/-15V or so. This is normal.)
Once all is well, connect a speaker load and signal source (still with the safety resistors installed), and check that suitable noises (such as music or tone) issue forth - keep the volume low, or the amp will distort badly with the resistors still there if you try to get too much power out of it.
If the amp has passed these tests, remove the safety resistors and re-install the fuses. Disconnect the speaker load, and turn the amp back on. Verify that the DC voltage at the speaker terminal does not exceed 100mV, and perform another "heat test" on all transistors and resistors.
When you are satisfied that all is well, set the bias current. Connect a multimeter between the collectors of Q10 and Q11 - you are measuring the voltage drop across the two 0.22 ohm resistors (R20 and R21). The desired quiescent current is 25mA, so the voltage you measure across the resistors should be set to 11mV +/-2mV. The setting is not overly critical, but at lower currents, there is less dissipation in the output transistors. Current is approximately 2.2mA / mV, so 10mV (for example) will be 22mA.
After the current is set, allow the amp to warm up, and readjust the bias when the temperature stabilises. This may need to be re-checked a couple of times, as the temperature and quiescent current are slightly interdependent. When you are happy with the bias setting, you may seal the trimpot with a dab of nail polish.
15 Watt Amplifier
Description:
A 15 watt amplifier made using discrete components. Sergio designed this circuit for his Electronics Level II course.
Notes:
This amplifier uses a dual 20 Volt power supply and delivers 15 watts RMS into an 8 ohm load. Q1 operates in common emitter, the input signal being passed to the bias chain consisting of Q8, Q9, D6, D13 and D14. Q8 and Q9 provide a constant current through the bias chain to minimize distortion, the output stage formed by a discrete darlington pair (Q2,Q4) and (Q7,Q11). The last two transistors are power Transitors, specifically the 2N3055 and MJ2955. The 7.02K resistor, R16 was made using a series combination of a 4.7K, 680 Ohms, and two 820 Ohms. The 1.1K resistor, R3 was made using a 100 Ohms and a 1K resistor. You can use this circuit with any walkman or CD player since it is designed to take a standard 500mv RMS signal.
16 watt amp:
click here to enlarge schematic
parts:
U1 U2 | LM383 8 watt audio amplifier ic |
R1, R3 | 220 ohm resistor |
R2, R4 | 2.2 ohm resistor |
R5 | 1 megohm resistor |
R6 | 100k audio taper potentiometer |
C1, C7 | 10uf electrolytic capacitor |
C2, C5 | 470uf electrolytic capacitor |
C3, C4, C6 | 0.2uf ceramic capacitor |
SPKR1 | 4 to 8 ohm speaker (up to 8 inches diameter) |
all resistors are 5 or 10 percent tolerance, 1/4-watt
all capacitors are 10 percent tolerance,
rated 35 volts or higher
2 Watt Amplifier
Description:
A 2 Watt audio amplifier made from discrete components.
Notes:
This was one of the earliest circuits that I ever designed and built, in Spring 1982. At that time I had only an analogue meter and a calculator to work with. Although not perfect, this amplifier does have a wide frequency response, low harmonic distortion about 3%, and is capable of driving an 8 ohm speaker to output levels of around 5 watts with slightly higher distortion. Any power supply in the range 12 to 18 Volts DC may be used.
Circuit Description
The amplifier operates in Class AB mode; the single 470R preset resistor, PR1 controls the quiescent current flowing through the BD139/140 complimentary output transistors. Adjustment here, is a trade-off between low distortion and low quiescent current. Typically, under quiescent conditions, current is about 15 mA rising to 150 mA with a 50 mV input signal. The frequency response is shown below and is flat from 20Hz to 100kHz:
Bode Blot:
The circuit is DC biased so that the emitters of the BD139 and BD140 are at approximately half supply voltage, to allow for a maximum output voltage swing. R9 and R10 provide a degree of temperature stabilization which works as follows. If the output transistors are warm, the emitter currents will increase. This causes a greater voltage drop across R9 and R10 reducing the available bias current. All four transistors are direct coupled which ensures:-
(i) A good low frequency response
(ii) Temperature and bias change stability.
The following voltage checks were made on my prototype. All voltage are made with respect to (wrt) 0 Volt and shown in the table below.DC Voltages Measured wrt 0V |
Q1 Emitter | 9.12V |
Q1 Base | 9.77V |
Q1 Collector | 14.22V |
Q2 Base | 14.22V |
Q3 Base | 7.65V |
Q4 Base | 5.98V |
Junction R9/10 | 6.82V |
Capacitor C3 | 14.95V |
The BC109C and 2N3906 operate in common emitter. This alone will provide a very high open loop gain. The output BD139/140 pair operate in emitter follower, allowing the amplifier to drive low impedance speakers. The signal to noise ration is shown below:
Signal to Noise Ratio:
This amplifier has a S/n ratio of 115dB at 1kHz. Overall gain is provided by the ratio of the 22k and 1k resistor. A heat sink on the BD139/140 pair is recommended but not essential, though the transistors will run "hot" to the touch.
Fourier Analysis
A quick measure of the distortion of this amplifier was performed. Operating on a 15V DC power supply with an input sinusoidal waveform of 100mV peak to peak at 1KHz produced the following results in.
Fourier Coefficients:
The number of samples was set to 4096 and Fourier coefficients up to the 16th harmonic were calculated. The sum of the all harmonics up to 16KHz amounted to just under 2.9% total harmonic distortion, the results are plotted below.
Harmonic Distortion:
The second and third harmonic are the biggest contribution to overall distortion. Choosing a different amplifier design, a different viasing scheme or more evenly match components can reduce distortion accordingly. At the time this amplifier was made, I only had an analogue multimeter, so all things considered, it was not too bad an effort.
Picture of My Prototype
Finally an image of the original which has stood the test of time. The BD139,140 power transistors can be seen on the left hand side, the preset near top centre, the BC109C centre right and 2N3906 is buried under a miniature screened audio cable, centre bottom.
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20 Watt / Channel Stereo Power Amplifier
Circuit DescriptionThere are many instances where a simple and reliable power amplifier is needed - rear and centre channel speakers for surround-sound, beefing up the PC speakers, low powered tweeter amplifier, etc. For those who want to build their own 'Gainclone' amplifier, this will certainly do the job :-)
This project (unlike most of the others, but in a similar vein to Project 19) is based almost directly on the typical application circuit in the National Semiconductor specification sheet. You can also use the TDA2050 (from SGS-Thompson), which has almost identical performance and (remarkably) the same pinouts! As it turns out, the amp in the NS application circuit is pretty good, as is the (very similar) one from SGS. The amp is also remarkably simple to build - if you have a PCB! These ICs are a cow to wire on Veroboard - it is possible, but results are unpredictable.
Figure 1 shows the schematic - this is almost the same as in the application note (redrawn), and with added RF protection at the input. Note that the speaker must return to the central 'star' earth (ground) point. If connected to the amplifier's earth bus, you will get oscillation and/or poor distortion performance.
Figure 1 - LM1875 / TDA2050 Power Amplifier Circuit Diagram (One Channel)
Voltage gain is 27dB as shown, but this can be changed by using a different value resistor for the feedback path (R4, currently 22k, between pins 2 and 4). The amplifier must not be operated at any gain less than 10 (20dB) as set by R4 and R5, as it will oscillate. In some cases, an inductor may be needed in series with the output to prevent instability with capacitive loads (10 turns of 0.5mm wire wound around a 10 Ohm 1W resistor). The most common capacitive load is the speaker cable itself, and 'audiophile' leads are often worse than standard grade cables in this respect.
The 1 Ohm resistor (R6) should be a 1W or 0.5W type, and all others should be 1/4W 1% metal film (as I always recommend). All electrolytic capacitors should be rated at 50V if at all possible, and the 100nF (0.1uF) caps for the supplies should be as close as possible to the IC to prevent oscillation. C1 should be a bipolar (non-polarised) electrolytic, or may be plastic film if you prefer.
The supply voltage should be about ±25 Volts at full load, which will let this little guy provide a maximum of 25 Watts (rated minimum output at 25°C). To enable maximum power, it is important to get the lowest possible case to heatsink thermal resistance. This will be achieved by mounting with no insulating mica washer, but be warned that the heatsink will be at the -ve supply voltage and will have to be insulated from the chassis. For more info on reducing thermal resistance, read the article on the design of heatsinks - the same principles can be applied to ICs - even running in parallel. I haven't tried it with this unit, but it is possible by using a low resistance in series with the outputs to balance the load.
Note that the supply voltage must not exceed ±30V at any time - this is the absolute maximum voltage rating for the LM1875. Note that the TDA2050 is rated for a maximum of ±25V.
Figure 2 - IC PinoutsFigure 2 shows the pinouts for the LM1875, and it should be noted that the pins on this device are staggered to allow adequate sized PCB tracks to be run to the IC pins.
The PCB for this amp is for a stereo amplifier, is single sided, and supply fuses are located on the PCB. The entire stereo board including four fuses is 115mm x 40mm (i.e. really small). The heatsink needs to be bigger than you might expect, largely because of the relatively high thermal resistance of the TO-220 case. National recommend that the heatsink should be no smaller than 1.2°C / Watt (it is actually suggested that the heatsink be 0.6°C / W, but this is a very large heatsink indeed, and is not necessary for normal audio into reasonably well behaved loads.
Never operate these ICs with no heatsink, even without any load connected. The quiescent dissipation will cause them to overheat very quickly, and may damage the internal circuitry.
Output power is rated at 20W per channel, but with music signals you will probably be able to get a peak power of about 25W into an 8 ohm load. Refer to the data sheet (see link below) for the full specification on the IC. Note that the TDA spec sheet claims 50W, but this is overly optimistic and cannot be achieved in practice.
Photo of Completed Amp (On Heatsink)
How Does It Sound?
The sound quality is very good - as I said at the beginning, I would not call it audiophile hi-fi (but then again - I might, with caveats), and provided the amp is never allowed to clip it sounds excellent. Because of the overload protection (which I have never liked in any form) this amp provides somewhat nastier artefacts as it clips than most discrete amplifiers.For those who think an incredibly short feedback path length is actually important, a surface mount resistor can be used for R5, either soldered directly to the leads (pins 2 and 4) or the pads on the copper side of the board. This will provide a feedback path of less than 20mm in total, and could be made less than 10mm (at the risk of damaging the IC with excess heat).
This amp is ideal for Hi-Fi PC speakers, and could also be used as a midrange and/or tweeter amp in a tri-amped system - there are a lot of possibilities, so I will leave it to you to come up with more.
Power Supply
A suitable power supply diagram is shown below. This is adequate for as many amplifiers as needed, simply by increasing the size of the transformer. 18-0-18 volt transformers are available (they are commonly used for 12V lead-acid battery chargers), and this provides the required +/-25V. | WARNING: Mains wiring must be performed by a qualified electrician - Do not attempt the power supply unless suitably qualified. Faulty or inadequate mains wiring may result in death or serious injury. |
Figure 3 - Power Supply
Although 10,000uF capacitors are shown, the amplifier will operate quite happily with less - I do not recommend anything less than 4,700uF for a pair of amps. The transformer rating is up to you. It should not be less than 150VA, and more than 300VA is unwarranted - the regulation improves with greater VA ratings, but the law of diminishing returns comes into play quite quickly.
Signal earth (the triangle) and mains earth should be tied together at a common point, which will become the 'star' earthing point for the whole amplifier. This should be as close as possible to the common of the filter capacitors. The main earth must connect to the chassis to prevent electric shock in case of a transformer "meltdown".
20 Watt Class-A Power Amplifier
single-ended Class-A amplifier is essentially one where there is only one active driven output device. The passive "load" may be a resistor, an inductor (or transformer) or - as in this amplifier - a current sink. Of the three basic options, the current sink offers the highest linearity for the lowest cost, so is the ideal choice.
Some esoteric (some might say idiosyncratic) designs use inductors or 1:1 transformers, but these are bulky and very expensive. Unless made to the utmost standards of construction, they will invariably have a negative effect on the sound quality, since the losses are frequency dependent and non-linear.
This amp uses the basic circuitry of the 60W power amp , but modified for true Class-A operation - it should be pretty nice! This amp has been built by several readers, and the reports I have received have been very positive.
With simulations, everything appears to be as expected, but although I have yet to actually build it and test it out thoroughly, no-one has had any problems so far. Using +/-20 Volt supplies - either conventional, regulated or using a capacitance multiplier, it should actually be capable of about 22 W before clipping, but expect to use a big heatsink - this amp will run hot.
Quiescent current has been reduced from my earlier attempts and simulations from a bit over 3A down to 2.6A - but it will still dissipate nearly 110W per amplifier!
There are a few things which must be considered - In my original article, I suggested a suitable current sink. Although this would certainly work, the dissipation actually exceeds the maximum for the MJE2955 devices. Running at 55 W each, and considering that they will be at an elevated temperature (probably around 70°C), the maximum safe power is only a little over 45W, so clearly two devices must be used. With two, the dissipation of each transistor is "only" 27.5W, and this also allows a lower thermal resistance from case to heatsink.
I strongly suggest that you use either TO-3 transistors, or large (high dissipation) plastic case devices. Heat transfer from transistors to heatsink will be the biggest problem you will face with this amplifier.
Figure 1 - Power Derating For The MJE2955An alternative is to use bigger transistors (even reverting to the TO-3 style), but in the long run using two paralleled transistors is still a cheaper option, and provides an adequate safety margin for the MJE2955 devices. Note that TIP2955 transistors may also be used, since they are more or less direct equivalents for this design. If you want to use more robust devices, I suggest TIP36 (A, B or C).
The modifications from the original 60W amp are as follows:
- Biasing diodes and the 47 Ohm resistor removed
- Lower transistor array removed, and replaced with a current sink.
- Power supply voltage reduced to +/-25V Maximum (+/-22V recommended)
- The "tail" of the long-tailed pair has been simplified to a simple resistor
- The DC offset can be set using the trimpot - this balances the LTP
- However big the heatsink you were thinking of, use a bigger one !
Figure 2- The New 20W Class-A AmplifierThe current sink shown should have very high linearity, since it is based on the same concept as the output stage devices. The 0.25 Ohm resistor should cause little grief (4 x 1 Ohm 1W resistors in parallel), but some experimentation may be needed here, since the base-emitter voltage of the BC549 determines the current. This circuit works by using the BC549 to steal any excess base current from the compound pair. As soon as the voltage across the 0.25 Ohm resistor exceeds 0.65V, the transistor turns on and achieves balance virtually instantly.
The 1k trimpot in the collector of the first LTP transistor allows the DC offset to be adjusted. The nominal value is around 400 ohms, but making it variable allows you to set the output DC offset to within a few mV of zero.
Determining The Optimum CurrentThe ideal operating current for a Class-A amp will be about 110% of the peak speaker current. If the loudspeaker system has a nominal impedance of 8 Ohms (the design impedance for this amp), then with a +/- 22V supply the maximum (theoretical) speaker current is ...
I = V / R = 22 / 8 = 2.75A
In my original calculations, I decided on a quiescent current of 2.6A - this is really Ok, because the above calculation does not consider the losses in the output stage. In practice, it is likely that up to 3 Volts will be lost in the output circuit, based on the losses in the output devices, emitter resistors and driver transistors.
This now gives a maximum voltage of 19V peak (2.375A @ 8 Ohms). Applying the 110% fudge factor gives an operating current of 2.6125A, or 2.6A close enough. If these peaks are met in practice, this gives an output power of 22.5W into 8 Ohms.
Note that the current in the -ve supply rail remains constant, but that in the +ve supply rail will vary from the normal steady state current (same as the -ve supply). At signal extremes, the current will double (upper transistors turned on), or will drop to almost zero for negative peaks. This is common for single-ended Class-A amplifiers, although you will not see it stated in the text for most designs. This can complicate the design of the power supply.
Adjusting The Quiescent CurrentIf the current sense resistor is made a higher value than optimal (say 0.33 Ohm 5W), you can use a trimpot across the resistor with the wiper going to the base of the BC549. This will allow you to set the current more accurately. Note that the sense transistor must be kept away from heat sources (such as heatsinks and power resistors) or the current will fall as the amp gets hotter. Be very careful if you use a trimpot, because if the wiper is wound down to the -35V supply line, the current sink will attempt to sink infinite current - this is likely to cause damage (to put it mildly). Start with the wiper at the most positive end (i.e. the collectors of the output devices), and carefully increase the current until the desired setting is reached. Use of a multiturn pot is highly recommended (almost mandatory, actually).
Figure 3 - Variable Current SourceFigure 3 shows a suggested way to make the current sink variable. The 1k fixed resistor ensures that even if the pot becomes open circuit (whichdoes happen, although rarely), the stage will not try to sink an infinite current. Remember to allow time for the temperature to stabilise - this may take 10 minutes or more, depending on the size of the heatsink. Larger heatsinks have a greater thermal mass, and take longer to reach the final operating temperature.
The heatsink is a critical part of a Class-A design, and for this amp a sink with a thermal rating of less than 0.5°C / Watt is mandatory. With a dissipation of about 110W quiescent, an 0.5°C/W heatsink will give a temperature rise (above the ambient) of 55°, so for the "British Standard" 25°C ambient temperature the transistors will operate at 80°C. This is hot. If possible, 0.25°C/W thermal rating is preferred, which will keep the temperature down to a more moderate 55°C or so - this is still hot but tolerable.
I suggest that any intending builder reads the article on heatsinks, to gain a better understanding of the difficulties involved in obtaining a good thermal transfer from transistor to heatsink. The use of TO-3 power transistors (MJ2955) will also help considerably in this respect.
200 Watt Amplifier
22 Watt Audio Amplifier
rt | Total Qty. | Description | Substitutions |
R1 | 1 | 39K 1/4 Watt Resistor | |
C1,C2 | 2 | 10uf 25V Electrolytic Capacitor | |
C3 | 1 | 100uf 25V Electrolytic Capacitor | |
C4 | 1 | 47uf 25V Electrolytic Capacitor | |
C5 | 1 | 0.1uf 25V Ceramic Capacitor | |
C6 | 1 | 2200uf 25V Electrolytic Capacitor | |
U1 | 1 | TDA1554 Two Channel Audio Amp Chip | |
MISC | 1 | Heatsink For U1, Binding Posts (For Output), RCA Jacks (For Input), Wire, Board
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