Preliminary Information

The 6SN7 is a classic tube known for its quality sound. This design shines as a headphone amplifier delivering a sound comparable to expensive high-end amplifiers. Using an isolation L-pad, the headphone output jack provides output for 30-ohm to 600-ohm headphones. It was found that with two 6SN7 output tubes, all four triode sections connected in parallel, a power output of 4 watts into 8 ohms is available. In a dual channel stereo configuration, this would be 4 watts per channel. This is an ideal power level for a home audio system where speakers are in proximity to the listener and high power output levels are not required. Rather than power output, sound pressure levels (SPL) are used for relative loudness based on speaker SPL one-watt sensitivity.
Amplifier power = 1-watt
Spk Sesitivity - SPL at 6.5 feet
86dB/1w/1m – SPL = 78dB
88dB/1w/1m – SPL = 80dB
90dB/1w/1m – SPL = 82dB
92dB/1w/1m – SPL = 84dB
94dB/1w/1m – SPL = 86dB

From a cost perspective, a pair of 6SN7 tubes, when compared to the cost of a triode power tube of similar power output, can be competitive. The amplifier output is transformer-coupled from the output tube plates. This provides both a headphone output and an 8-ohm speaker output. Speakers are connected by a switch directly to the output transformers 8-ohm secondary. The 8-ohm secondary is connected to headphones through an isolation L-pad. The L-pad provides a lower noise floor and raises the output transformer primary impedance. When speakers are removed from the output load, at 1 kHz reference, the L-pad load transforms the output transformer’s 3,500-ohm primary to an impedance of around 100K ohms with 38-ohm headphones and about 200K ohms with 300-ohm headphones. The higher primary impedance reduces significant plate loading on the output tubes, reducing lower frequency waveform distortion.

This is a controlled constant current design. During lower volume levels, current through the output tubes is rolled back. This allows the output tubes to run cooler. As output tube current flow is reduced, overall amplifier power consumption is reduced. Current through the output tubes is increased by 57% at higher volume levels. Regardless of the power level, the output is true Class A single-ended.

Several tests were run at 4 watts output to determine how well the tubes performed under full output and full output driven into overload saturation. At full output, the output tubes do get hot similar to traditional output tubes. When pushed into plate overload saturation, the output tube’s cathode bias starts backing off current through the tubes, reducing plate dissipation. Listening tests were run for several weeks at various volume levels under low and high output power levels.

During testing of the amplifier, single-tone measurements indicated higher levels of distortion at lower frequencies, especially during 8-ohm load tests. However, it was noted that even at higher volume levels, the lower frequencies were not noticeably distorted. There is criticism voiced by some that triode single-ended outputs suffer from low-end distortion due to output transformer plate loading. Using an online 20 Hz signal, an oscilloscope was calibrated to display a two-cycle waveform. Then four complete cycles would be 40 Hz, and so on. Several hours of monitoring low frequency waveforms of a large selection of music resulted in little or no content below 50 Hz. Bass seems to fall mainly in the above 50Hz region. Even music with heavy bass displayed sinusoidal waveforms. Those may well have been waveforms with high distortion, but yet the sound did not indicate so.
The end conclusion was that single-tone measurements are not necessarily the final word. Consider low frequency distortion from another viewpoint. As long as the bass frequency waveforms are sinusoidal in nature, distortion may not be as objectionable to the ear as higher frequency distortion. Low frequency sound waves have longer wavelengths and carry more physical energy than higher frequencies. Even at moderate volume levels, bass frequencies tend to be felt in addition to being heard, creating a physical sensation. Beyond tone measurements, this would be more of a psychoacoustics issue.

The following performance measurements were made while testing the final version of the amplifier circuit. Frequency response and distortion readings were made using a classic HP 331A distortion analyzer and a Heath IG-18 generator. Internal distortion of the IG-18 = 0.2%.

HEADPHONE
Freq, Response, Distortion
40Hz,    −1.2dB,    2.7%
50Hz,    −0.6dB,    1.6%
100HZ,    0 dB,      0.57%
200HZ,    0 dB,      0.3%
300HZ,    0 dB,      0.28%
400HZ,    0 dB,      0.24%
500HZ,    0 dB,      0.24%
1KHZ,     0 dB,      0.24% (ref)
2KHZ,     0 dB,      0.21%
3KHZ,     0 dB,      0.2%
4KHZ,     0 dB,      0.2%
5KHZ,   −0.2 dB,   0.2%
6KHZ,   −0.2 dB,   0.2%
7KHZ,   −0.2 dB,   0.2%
8KHZ,   −0.2 dB,   0.2%
9KHZ,   −0.3 dB,   0.2%
10KHZ, −0.5 dB,   0.2%
11KHZ, −0.5 dB,   0.2%
12KHZ, −0.5 dB,   0.2%
13KHZ, −0.6 dB,   0.2%
14KHZ, −0.6 dB,   0.2%
15KHZ, −0.6 dB,   0.2%
16KHZ, −0.7 dB,   0.2%
17KHZ, −0.7 dB,   0.2%
18KHZ, −0.8 dB,   0.2%
19KHZ, −0.8 dB,   0.2%
20KHZ, −1.0 dB,   0.2%
30KHZ, −1.9 dB,   0.2%
40KHZ, −2.8 dB,   0.2%


SPEAKERS (8-Ohm)
Power = 1-Watt
Freq,  Response,  Distortion
40HZ,    −1.0 dB,   8.8%
50HZ,    −0.3 dB,   6.6%
100HZ,  −0.1 dB,   3.6%
200HZ,    0 dB,      2.8%
300HZ,    0 dB,      1.7%
400HZ,    0 dB,      1.5%
500HZ,    0 dB,      1.4%
1KHZ,      0 dB,      1.4% (ref)
2KHZ,      0 dB,      1.3%
3KHZ,      0 dB,      1.3%
4KHZ,      0 dB,      1.2%
5KHZ,      0 dB,      1.2%
6KHZ,      0 dB,      1.2%
7KHZ,      0 dB,      1.2%
8KHZ,    −0.1 dB,   1.2%
9KHZ,    −0.1 dB,   1.2%
10KHZ,  −0.2 dB,   1.2%
11KHZ,  −0.3 dB,   1.2%
12KHZ,  −0.4 dB,   1.2%
13KHZ,  −0.4 dB,   1.2%
14KHZ,  −0.5 dB,   1.1%
15KHZ,  −0.6 dB,   1.0%
16KHZ,  −0.8 dB,   1.0%
17KHZ,  −0.8 dB,   1.0%
18KHZ,  −1.0 dB,   1.0%
19KHZ,  −1.0 dB,   1.0%
20KHZ,  −1.2 dB,   0.9%
30KHZ,  −2.4 dB,   0.7%
40KHZ,  −3.8 dB,   0.6%
Distortion at 0.5-watt is approximately 50% lower.

Speaker distortion figures do not include distortion produced by a typical 2-way speaker system, usually around 3% at moderate listening levels.

Up to now, the design has been a single-channel experimental circuit. A finished working two-channel amplifier has been initiated starting 09/14/2026 with a CAD drawing for an engraved chassis plate. When a completed tested amplifier is finished, a technical publication will be available. It will contain the necessary information for the DIY amplifier builder to replicate the design. The design includes a bias control circuit that will require a PCB or hand wired on a breadboard type circuit board. All things considered, it could be early 2027 when it is finished.