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Designing a Low-Distortion 12-Watt Amplifier
August 1958 Radio-Electronics

August 1958 Radio-Electronics

August 1958 Radio-Electronics Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

Website visitor Daniel O. wrote to request that I post this "Designing a Low-Distortion 12-Watt Amplifier" article from the August 1958 issue of Radio-Electronics magazine, which of course I was glad to do. It is nice to know there is still some interest in building vacuum tube circuits, just to keep the knowledge alive. When reading through the vintage magazines in search of good articles, I typically do not include ones like this not because I don't think they are worthy of posting, but because they are narrowly focused and would not be of interest to a wide number of people. If you run across an article which appears in the table of contents of one of the hundreds of old magazines I have posted, let me know and I'll be glad to do the same for you.

Designing Low-Distortion 12-Watt Amplifier

12-watt vacuum tube amplifier chassis layout (top side) - RF Cafe

Space on the chassis is tight, so proper parts placement is important.

12-watt vacuum tube amplifier chassis layout (inside bottom) - RF Cafe - RF Cafe

As few components are used, there is plenty of room under the chassis.

by Robert M. Voss

Few and far between are 10- or 12-watt amplifiers which deliver clean undistorted power at the levels required for filling a small room. Current commercial design trends point toward units with power ratings in three-digit figures, on mammoth chassis. I am not taking sides in the apparently never-ending battle between the high-power and low-power factions among the audiophiles. Few people, however, will argue that 150 square feet of floor space need more than 10 or 12 watts (over the complete audio range) for adequate reproduction of music, the 7th Avenue Express coming into Times Square or whatever else the listener may care to subject himself to.

Recently, I was asked to design a power amplifier to be used with an 8-inch shelf type speaker system in a room measuring 10 x 15 feet. The unit had to satisfy the following requirements:

1. Inaudible distortion at all feasible sound levels (in the 10 x 15-foot room).

2. Low source impedance.

3. High efficiency.

4. Best possible stability characteristics.

5. Hum and noise below audibility (with input shorted or when driven by moderately low impedance, such as cathode follower).

In designing the amplifier, these considerations, except for the last item, were taken as a group. The first four all apply to the output stage and the application of feedback around it. Item 5 is of course, dependent upon the amount of feedback used, but I decided to aim for as low a hum and noise figure as possible without feedback. With inherently low extraneous noise, the minimum amount of feedback used can depend upon the ratio of distortion and internal resistance reduction necessary, rather than noise considerations (10 db of feedback will reduce distortion by a factor of 3.1; 20 db by a factor of 10). This often results in a more stable arrangement.

12-watt vacuum tube amplifier schematic - RF Cafe

The 12-watt amplifier uses only 4 tubes.

To satisfy requirement 1, a power output of 10 to 12 watts (undistorted) was considered adequate. Requirement -triode-connected tubes of the 6L6 variety, as used in the original Williamson circuit, or Ultra-Linear connection of the 6V6 family. The Ultra- Linear (also known as "tapped- screen" or "gilded lily") mode of operation first achieved popularity as a replacement for the triode output stage in the Williamson circuit. It combines the advantages of the high efficiency characteristic of tetrodes with the inherently low distortion of triodes (or triode -connected tetrodes). Today, it is probably the most commonly used type of audio output circuit. In addition, its low distortion characteristics make a minimum amount of feedback necessary, thus satisfying requirement 4. Because of the high efficiency (requirement 3) of Ultra-Linear operation, I used this type of output stage. I feel it is wiser to design an amplifier along conventional lines, aiming at simplicity, than to incorporate revolutionary circuitry, extremely critical components or very heavy feedback loops. The unit described here sounds clean, well damped and noiseless with the feedback loop removed.

Ultra-Linear operation of 6V6's requires a tap at 25% of the primary impedance on each side of the output transformer's center tap. In addition, this component must be able to handle 10 watts over the complete audio range and have excellent frequency response at all levels. To satisfy these requirements, an Acrosound TO-310 is used. It is an outstanding performer, delivering 10 watts from 20 to 30,000 cycles and 20 watts from 30 to 20,000 cycles per second. It has enough primary inductance to provide adequate low-frequency response.

Amplifier Circuit

For the voltage -amplifier and phase - splitter stages, I decided to use a direct-coupled split-load phase inverter. This arrangement has many subtle, frequently overlooked advantages. The DC coupling means one less capacitor -coupled stage, resulting in greater feedback stability. Putting the phase inverter directly before the output stage (6V6's do not require much grid drive) eliminates the hum frequently associated with the large potential difference between the heater and unbypassed cathode. The plate load of the voltage amplifier also functions as the grid 34 resistor of the phase splitter; hence the voltage amplifier works into an extremely high impedance, and high gain and low- distortion signal voltage at high levels are available from the first stage. The phase splitter has excellent distortion characteristics because of the large amount of degeneration inherent in the circuit. The output at either side of the phase splitter is approximately equal to the input.

Great pains were taken to keep hum and noise to a minimum. An isolated ground system is used, connected to the chassis only at the input jack. A deposited-carbon resistor is used for the plate load of the first stage. The power supply has more than the normal amount of filtering. A 5Z4 is used as the rectifier because its long warm-up time protects the filter capacitors and it has a lower internal voltage drop than the more common 5Y3, allowing for more filtering. The center tap of the heater winding is connected to the output tube cathodes. This puts a bias of about 24 volts on the heaters, which prevents any hum from being generated at the cathode of the voltage amplifier.

When building the amplifier, match R5 and R6 within 1% to assure equal voltages from each side of the phase splitter. C2 and C3 should, if equipment is available, be matched and it may also be desirable to match R7 and R8. Standard 10% tolerances are used elsewhere, with the exception of R1, R3, R9, R10 and C1, which should have no more than 5% tolerance. The power supply is conventional, except that - as mentioned before - more than the usual amount of filtering is used before the push-pull output stage. With this arrangement, the strongest noise component should be the hiss originating in the voltage amplifier.

With the input open, noise produced by a very efficient speaker is inaudible more than 1 foot from the cone. Shorting the input brings noise down to the point at which the listener must press his ear against the cone to hear it.

No balancing circuit was used in the output stage, but you can easily add one in the cathode circuit of the 6V6's.

Actually, from a fresh carton of six 6V6's, the two farthest apart differed less than 5% in the amount of plate current they drew. Gm matched tubes may further reduce distortion. An ECC83 or 5751 may be used in place of the 12AX7, and EL84's may be used as power amplifiers (replacing R11 with a 130 -ohm unit) . The output transformer should, in any case, be a TO310; feedback components are based on the characteristics of this unit. (R1 -C1's time constant determines ultrasonic rolloff and transient response.)

The amplifier was built on a 5 x 9% x 1% -inch chassis, which is just about as small as is possible since the transformers are rather on the large side. If the layout shown is used, the 6V6 farthest from the 12AX7 should be driven by the cathode of the phase inverter. This tends to compensate for the difference in output impedances of the cathode -follower and plate -loaded outputs. Connect the leads from the output transformer with the tracers (gr-wh, blu-wh) to the output tube which is connected to the plate of the phase splitter. (Blu and blu-wh to plates, gr and gr-wh to screens.) If leads are reversed, feedback will be in a positive phase. The amplifier will kick off, possibly damaging the output transformer and tubes.

The finished amplifier fulfills all the requirements. Sound is clean and not colored by the amplifier. The improvement in damping characteristics is very noticeable when compared with a typical commercial 10 or 12 watter. The simplicity of design and the straightforward circuitry lead some to believe that the amplifier will continue to deliver clean audio for years without servicing.




Posted May 21, 2021

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RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while tying up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived...

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