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This is the home page for
EJ Jurich
author of the book
Vacuum Tube Amplifier Basics
This project is to see if a four-watt-per-channel amplifier using 6SN7s as output tubes is practical.
6SN7 Amplifier Project
This current version of the book includes technical information from previous versions plus a more detailed example of amplifier design and assembly. Using a concise format, 110 reference pages cover technical information that pertains to vacuum tube circuits. Design examples take you through the steps of fabricating a traditional chassis layout and designing a Class A stereo amplifier (pictured on the book cover).
NOTE: The file download version of the book Vacuum Tube Amplifier Basics will have a price reduction of 50% for the month of July, 2025.
Headphone stereo amplifier project from 2014 using a 6SN7 as the output tube provides 1/2 watt of audio for each channel. Speaker connector posts are included for background listening.
Half-watt 6SN7 headphone amplifier
Russell O Hamm paper on vacuum tube amplifier performance versus solid state.
A definitive article on selecting capacitors for the best stability and lowest distortion.
Selecting capacitors for the best performance
Amplifier damping factor and what a realistic damping figure is in relation to amplifier performance.
Wide-band response and low distortion circuits for cathode follower output using either a 6SN7 or a 12BH7.
From a 1947 Wireless World magazine issue, a detailed article on balancing push-pull output circuits.
Push-pull balance - W T Cocking
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.
SPL COMPARISON
dB — SOURCE
40 — Quiet Library
50 — Average Home
60 — Speech @ 3 feet
70 — Vacuum Cleaner @ 3 feet
80 — Side of Busy Street
90 — Diesel Truck @ 30 feet
100 — Jack Hammer @ 3 feet
110 — Chainsaw @ 3 feet
120 — Pneumatic Drill @ operator
140 — Train Horn @ 20 feet
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 Hearing Loss
Speaker Distortion
Danger High Voltage
be careful where you put your hands
© 2014 -
All Rights Reserved.
EJ Jurich
PO Box 3416
Des Moines, IA 50316
last update 06/22/2025
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