WhitakerAudio
 
New Project – 60 W Stereo Power Amplifier

This page documents the design and construction of a 60 W stereo power amplifier. The log below traces development of the amplifier, shown on the right in finished form, from planning through construction, testing, and optimization.

60 W stereo amplifier

Background

The concept here is a 60 W stereo power amplifier, built on a platform similar to the 40 W stereo amplifier described elsewhere on this site. The new amplifier is essentially identical to the 40 W amplifier except for the output tubes (7027A instead of 7868) and the power supply. The 6U8A input/phase splitter has sufficient headroom to drive the new output tubes to 30 W per channel. The power supply for the 40 W amplifier has been modified to provide slightly higher voltage output. This has been accomplished by reducing the value of the series resistor between the cathode of the 5U4 rectifier tube and the first bank of filter capacitors. In addition, provisions to power a separate preamplifier have been eliminated. Taken together, these provide the necessary B+ operating voltages.

The output transformers for the 60 W stereo version are identical to the 40 W stereo version, having been conservatively specified for the 20 W per channel product. (The transformers are rated for 40 W per channel.) The same power transformer and choke are used in the 60 W and 40 W units.

Update 1

Construction began today with drilling/punching of the chassis. As this amplifier is very similar in design and layout to the 40 W stereo amplifier, no surprises are expected. Chassis work is time consuming and detail-oriented. As I begin each project, I try and find a better way to do the chassis. I would like to find a vendor that will take my Adobe Illustrator drawings and produce a finished, painted chassis. I have contacted more than a few vendors in an effort to accomplish this over the past two years, but with no luck.

So, with no better alternative, I continue to produce the needed chassis with a drill press and hole punches. The end result is actually quite good, but it takes time.

Bare chassis

The finished chassis is shown on the left. Note that the drilling task is rather simple, with only a few different sized holes required. All work is done against a drill pattern overlay, which makes for an efficient project. A black hammered finish is applied after all of the holes are cut.

The painted chassis and transformers are shown on the right. I have tried various finishes on the transformers and arrived at black satin as my favorite. It provides a subtle contrast to the hammered finish chassis.

Chassis and transformers

I should mention that all of the chassis used in the projects described on this site are from Hammond Manufacturing. I am using their heavy-duty steel product line, which comes finished in gray or black. A number of sizes are available, in addition to optional base plates and walnut side panels. Aluminum chassis are also available, but given the weight of the transformers for this and other projects, I have settled on using a steel chassis. In addition, I have standardized on a particular size (17-in by 14-inch by 3-inch) for nearly all projects. This simplifies inventory management and provides a uniform look across the different projects.

Update 2

An important part of the preliminary work on this project (and other projects documented on this site) is organization of the parts. Having an accurate bill of materials (BOM) is a critical—and time consuming—process. After the parts are received, they are divided into packages, which include: 1) right channel amplifier block, 2) left channel amplifier block, 3) power supply, and 4) chassis. Each part is labeled and checked off the BOM list. This process identifies any missing parts and serves to double-check the BOM. It also tends to divide the project into more manageable stages.

Although building the four "packages" described here takes a fair amount of time, it speeds the construction of the unit by separating the parts management step from the construction step. This is a valuable process for a project of any substantial size.

Update 3

Progress continues to be made on construction of the amplifier. As of today, the power supply and left/right channel amplifier boards had been completed.

Amplifier PWB

The 60 W stereo amplifier is built around two identical 30 amplifier blocks. The component side is shown on the left; the foil/socket side is shown on the right.

Construction techniques closely follow the 40 W stereo amplifier, the major exception being different tube sockets for the push-pull output devices. The 40 W stereo amplifier uses 7868 tubes, which utilize a 9-pin novar base. The 60 W stereo amplifier uses a pair of 7027A devices, which use a standard 8-pin octal socket.

Amplifier PWB, foiil side
Power supply PWB

In order to achieve 30 W output from the amplifier blocks, plate voltage in excess of 450 V is required. The power supply for the 40 W amplifier can supply the necessary operating voltages with some modifications.

In the original design, a 50 ohm series resistor is placed between the cathode of the 5U4 rectifier tube and the input capacitor filter bank. For the 60 W amplifier, that resistor has been reduced in value to 10 ohms. In addition, provisions to power a separate preamplifier have been eliminated. These steps, combined with generous filtering, provide the needed power for the output stages.

Power supply PWB, foil side

Update 4

Work continues on the amplifier, with most assembly tasks completed as of today. Owing to the similarity of this amplifier with the 40 W stereo amplifier already built, the detailed documentation developed for the 40 W unit has proved quite valuable in construction of the 60 W amplifier.

Chassis, components installed

The top chassis view of the amplifier is shown on the left. The output transformers can be seen on each side of the chassis. The choke is mounted center-front. The power transformer is mounted center-rear. Note that the power transformer is oriented 90 degrees relative to the choke.

The back panel (shown on the right) is clean and uncluttered. Audio inputs are located at the center of the back panel, with a chassis ground terminal mounted just below the RCA jacks. The barrier strip output terminals are placed on separate sides of the back panel. The ac power connector and circuit breaker can be seen at the far right side of the chassis.

Rear panel

Update 5

Assembly was completed today on the 60 W stereo amplifier. Initial tests confirmed proper construction. Prior to powering up a new project, I always take resistance measurements between chassis ground and each tube socket pin. This process does a good job of identifying any major assembly errors. Following successful completion of the ohmmeter tests, power-on measurements are taken with the rectifier tube installed but all other tubes removed. This step is set for tomorrow.

Chassis view, bottom

The bottom-view of the chassis is shown on the left. As stated previously, the amplifier consists of one power supply PWB, two amplifier PWBs (one for each channel), and two speaker terminal PWBs (one for each channel).

Nearly all connections are made on the circuit boards. The interface point for the power transformer leads is the power supply PWB, shown on the right. By eliminating discreet connections, assembly is simplified.

Although the connections to the power supply PWB are somewhat dense, the rest of the chassis is roomy.

Close up of power supply board

Update 6

Functional tests were completed on the amplifier today, with good results. All voltage measurements agreed with the predicted values, and the amplifier sounded great with program audio.

Amplifier on bench for testing

The amplifier is shown on the left on the bench undergoing testing. A close-up view of the unit is shown on the right.

With the big 7027A tubes, this is an impressive amplifier. I will begin performance tests on it tomorrow. I expect good numbers. The big question is power output. I’m hoping I can make 30 W per channel. This may be a bit of a stretch, given the power supply, which will be running wide open to hit 30 W per channel. We’ll see how it goes. Regardless, this as big an amplifier as I plan on building (at least for now).

60 W amplifier under test

Update 7

Initial performance tests were completed today on the amplifier. The starting point was a default bias voltage on the output tubes of –40 V. This value will provide for minimal idling current and consequently long tube life. Measurements were in line with expectations, notably sub-3% distortion at 25 W and decreasing to less than 0.5 % at normal listening levels of 5 W or so. Frequency response was within 1 dB from less than 20 Hz to more than 20 kHz. Noise was quite good at –90 dB, referenced to full power output. The amplifier was able to make just over 30 W per channel at 1 kHz with 120 V ac line input. Square wave performance was exceptional at full power with 1 kHz, and very respectable at 100 Hz and 10 kHz.

This design requires matched output tubes, which are readily available. As a test, one channel used matched tubes and the tubes in the other channel were not matched. As expected, the matched pair out performed the unmatched pair by about 1% THD at maximum power output. A greater difference was observed in IMD. At 50% of power and below, the difference in performance had disappeared. In a bit of a surprise, the unmatched pair outperformed the matched pair in terms of THD and IMD by a small amount at low power levels.

Remaining tests include optimizing the bias to find a good compromise between low idling current and low distortion. Also, I plan to take a close look at the feedback circuit values. Above 20 kHz, high frequency response is down about 1 dB relative to 1 kHz, increasing with higher frequencies. Coincidentally, distortion at high frequencies is exceptionally low compared with sub-100 Hz performance. It may be possible to flatten the high frequency response at the expense of some distortion at the high end. This should also improve the square wave response at 10 kHz.

Update 8

Final square wave measurements were taken on the amplifier today. The traces were very similar to those observed for the 40 W amplifier. Performance was exceptional at 1 kHz, even at just below the clipping point of the amplifier. Some tilt was observed at 100 Hz, and rounding of the leading edge at 10 kHz. These issues were not significant, and not unexpected.

The optimum bias point was also explored. A starting point of –40 V was assumed, based on guidance from the RCA Receiving Tube Manual. As it turns out, –40 V was precisely the optimal setting for bias on the 7027A tubes for this circuit. THD measurements at just below clipping (30 W) came in below 3%. IMD at just below clipping was 9%. These numbers are not anything to write home about. Still, for an amplifier at the edge of clipping, performance was not bad. The assumption here is that the amplifier will typically be run at power levels considerably below the peak of 30 W. All THD and IMD measurements below 15 W were in the sub-1% range.

Next on the agenda is to see if improvement in distortion can be achieved by better balancing the phase-splitter/driver stage.

Update 9

Major progress was made today on reducing distortion at high operating levels. As designed, the phase splitter stage does not provide the means to adjust for small differences in the characteristics of the 7027A output tubes. Instead, balance is dependent on the matching of the tubes. Faced with the high distortion at 30 W output documented in Update #8, I did some experiments with changing the value of the cathode resistor in the phase splitter. This seemed the simplest way of adjusting the drive to one tube, relative to the other. The improvements in THD and IMD were significant. As documented above, THD at 30 W output per channel was 3% and IMD was 9%. After changing the value of the cathode-to-ground resistor in the phase splitter circuit, THD had dropped to 1.5% and IMD was 1.8% at full power. Given the improvements achieved, I will add a potentiometer in the cathode circuit to permit adjustment of drive over a small range.

The other issue identified in preliminary tests was a slight drop-off in frequency response above 10 kHz. This issue was corrected with some small changes to the input circuit. Following the modifications, frequency response was within 1 dB from 20 Hz to 20 kHz. The 3dB point on the high end was 35 kHz.

Update 10

The drive balance potentiometers were installed in the amplifier today (one for each channel) and performance tests were repeated. The distortion at 30 W is quite good (THD = 1.5%, IMD = 1.8%), and falls off rapidly as power output is reduced. The major challenge with regard to THD is the low end. At frequencies above about 100 Hz, distortion is well below 1 percent, falling to less than 0.5% at mid-band for most measurements.

Assembly is nearly complete on the automatic protection board described elsewhere on this site. Installation of the auto-off option will complete the project.

Update 11

With all assembly and testing completed, the amplifier was subjected to critical listening tests today, using the 50 W amplifier as the benchmark. Of all the amplifiers I have built, the 50 seemed to have the most pleasing sound. In any event, the 60 W amplifier performed very well.

Amplifier top view, completed

The completed 60 W stereo amplifier is show in the photos on the left and right. I am quite pleased with the results.

The amplifier has now been placed into service. Some long-form listening tests with a variety of music sources and types are planned.

The unit is heavy—about the same as the 40 W stereo amplifier. The handles on the sides of the chassis began as a cosmetic trim idea. As it turns out, they are very useful for moving the amplifiers around. Most of the weight is in the transformers, of course. The handles are off-set from center in recognition of the placement of the transformers on the chassis.

Amplifier rear view, completed

Update 12

The cost of building any project is an important consideration for the audio enthusiast and hobbyist. The total bill of materials (BOM), including shipping of components for the 60 W stereo amplifier was $2,268. The major cost centers included:
• Electronic components = $467
• Transformers = $530
• Printed wiring boards (PWB) = $650
• Plexiglas cover = $125
• Tubes = $112
• Decals = $120
• Front panel = $105

Builders could eliminate the PWB cost by using hand-wired terminal strip construction. Note that three PWB designs are used in this amplifier—one for each channel, one for the power supply, and one for each speaker output circuit, for a total of five boards. Other potential areas of cost reductions include the back and top-side decals, and the Plexiglas cover.

The total time required to build the 60 W stereo amplifier—from ordering parts to completing performance measurements—was approximately 26 hours.

The BOM cost for the optional auto-protect board was $686. The cost breakdown was roughly 50% for parts and 50% for the PWB. Construction time was about 2 hours.


Update 13 (January 2017)

Detailed step-by-step documentation is provided in a 154 page User and Assembly Manual that is available for download. If you are familiar with the Heathkit assembly manuals of the past, the approach taken with this new manual should seem comfortable. For those who want a printed version, it is available for purchase on Lulu—a web site for specialized, print-on-demand documents. (There is charge of $14.95 for the printed manual; see "60 W Stereo Amplifier: User and Assembly Manual." on the Lulu web site.)

In addition, a ZIP file is available for download that will assist in building this amplifier. The ZIP file contains the following individual files:

• Schematic diagrams of the amplifier and auto-protect circuits as ".pdf" (Acrobat) files
• Bill of materials for the amplifier and auto-protect circuits as an Excel file
• Printed wiring board layout files for the amplifier, speaker terminal, and auto-protect boards as ".pcb" ( ExpressPCB) files
• Front panel layout as a ".fdp" ( Front Panel Express) file
• Chassis layout, bottom view, as an Acrobat file
• Chassis layout, top view, as an Acrobat file
• Drill pattern for the chassis as an Acrobat file
• Layout for the acrylic trim piece as an Acrobat file

Note that the files above are provided as-is. Every effort has been made to make sure they are complete and accurate, but no warranties are expressed or implied. Builders are encouraged to double-check the information contained in the above files prior to proceeding. For the front panel layout, users can customize the text as desired; e.g., "Built by John for Mary, January 2015."