Low Power Amplifiers

Why They Make Sense

Current Project
6SN7 as output tube

This project is to see if a four-watt-per-channel amplifier using 6SN7s as output tubes is practical. At four watts, operation should be within the 6SN7's maximum ratings. So far, I have the transformers. Next, a chassis will be fabricated to start an experimental amplifier. The picture shows progress as of 5/8/2025. There has been a delay waiting for some parts to finish drilling and punching chassis holes.

All the holes are in the chassis. Burrs from the holes need to be removed, then the chassis will be lightly sanded with fine sandpaper. I'll get a progress picture up after I have my worktable tidied up.

last project update 06/03/2025

This second edition of the book has much of the original book with the addition of a more detailed example of designing and building a vacuum tube amplifier. There are 110 concise reference pages covering technical information that pertains to vacuum tube circuits. To see a complete list of the technical reference pages, look at the book sample PDF Index. The amplifier design example takes you through the steps of fabricating a traditional chassis layout and designing a Class A stereo amplifier (pictured on the book cover).

Book Reviews
read them here

The book on CD
available here

Why PDF?

PDF documents are readable on just about any device. Standard eBook reflowable formats scatter content, causing illustrations to be moved away from associated text. PDF content remains fixed, presenting content like a printed book. For technical documents, maintaining a fixed layout is best.

Project Information

Half-watt 6SN7 headphone amplifier

A definitive report based on technical tests concerning the best type of capacitors with good stability and low distortion.

Selecting capacitors for lowest distortion

From a 1947 Wireless World magazine issue, a detailed article on balancing push-pull output circuits.

Push-pull balance - W T Cocking

Wide-band response and low distortion circuits for cathode follower output using either a 6SN7 or a 12BH7.

Cathode follower output

Low Power Amplifiers

Why they make sense

For the audiophile working on a home system, referring to sound pressure levels for loudness can save you money and save your hearing.

Consider building a lower-power system. Components such as transformers and output tubes used in lower-power amplifiers are significantly less expensive. Besides costing less, lower-power transformers are physically smaller, requiring less chassis space. Tube amplifiers with lower-power output draw less current and have lower electrical utility costs.

Amplifier power output does not affect sound. An amplifier with 10 watts of output power will sound the same as a 100-watt amplifier. The difference is loudness. Rather than using amplifier output watts as a gauge for loudness, use sound pressure levels (SPL).

When considering sound pressure levels, take into consideration how your ears respond to them. Using very low SPLs of around 40 to 50 dB as reference, to double the loudness you need to increase amplifier power four times. As you increase amplifier power, your ears will start to compress the sound in response to the increased SPL. At higher sound pressure levels, it will require eight times the amplifier power to double the apparent loudness. In the average living room, depending on the efficiency of the speakers, a comfortable background listening level only requires around a half-watt of amplifier power.

The following is a comparison of everyday sound pressure levels based on average published values.

Sound pressure levels above 85 dB are high enough to cause some hearing loss, depending on exposure time. The higher the SPL, the greater the chance of hearing loss. At an SPL of 100 dB, you really should be wearing ear protection. At 120 dB without ear protection, the chance of hearing damage is high, even for short periods of exposure. At 140 dB, ear damage will most certainly happen without ear protection. If you would wear ear protection out in the environment, then why would you want such a high SPL in your listening room? Unless, of course, you plan to wear ear protection.

The sensitivity of speakers is usually specified as "so many dB @ 1W/1m" (1 watt at 1 meter) from the speaker. For example, 93 dB @ 1W/1m. Higher sensitivity speakers are more efficient and require fewer amplifier watts for a specific sound pressure level. A pair of speakers with a sensitivity of 96 dB @ 1W/1m requires less amplifier power than speakers with a sensitivity of 93 dB @ 1W/1m. Assembling an audio system using 85 dB as a reference for maximum SPL will provide substantial loudness. If paired with the right speakers, 12 amplifier watts can provide an SPL of 85 dB 20 feet from the speakers.

On crownaudio.com, you can use the 'Amplifier Power Required' calculator to determine amplifier power output. The primary factors are the desired SPL level at listening distance from the speakers, and speaker sensitivity. Enter the listener's distance from the source (speaker) in meters. If 20 feet, then enter 6.1 meters. Enter the desired SPL level in dB at the listener distance, for example, 85. Enter the sensitivity of the speakers, for example, 93 (93 dB @ 1W/1m). For amplifier headroom, enter 3 dB to accommodate audio peaks. Entering those parameters, you should find that it only requires 12 watts of amplifier power.

For music, a normal SPL listening level is around 70 dB; 80 dB is loud. Sound pressure levels above 85 dB can cause hearing loss, especially with long-term exposure. A home audio system designed to produce an SPL of 80 dB to 85 dB at ten feet (or twenty feet in larger rooms) from the speakers should be sufficiently loud. Over time, you will save money if you pay a little more for more efficient speakers and drive them with a lower-wattage amplifier. Besides the savings in amplifier costs, lower-power amplifiers draw less current, which means lower electric power costs. Vacuum tubes used in lower-power amplifiers are less expensive, a savings each time you replace tubes.

Loudspeaker distortion increases at higher volume levels. At lower listening levels, the average speaker system has one to two percent distortion. Besides placing more pressure on your ears, higher amplifier levels will push speakers higher into distortion. From an audiophile point of view, keeping the SPL level within reason equates to a better, cleaner listening experience.

More information:
Noise-Induced Hearing Loss
Hearing loss and music
Loud Noise Can Cause Hearing Loss