Plans to Build a Single-Ended Cathode Follower Amp
♦ Pure Class A Single-Ended
♦ Cool Running
♦ Solid Reliability
♦ Wide Bandwidth
♦ 3 watts per Channel
OTL technology affords the finest in sonic reproduction. There is no question, no doubt. No other amplifier configuration can match the qualities that OTL amplification can bring to music. However, not everyone wants an OTL amplifier. They may have low impedance speakers, they may be uncomfortable with multiple output tubes, or they just want something more traditional. We recognize this and have developed a low cost transformer coupled amplifier that rivals OTL performance.
So how does it sound?
It has a very mellow, smooth sound. Music is very clear and accurate. It is not overly warm or heavy sounding. Treble is fully extended with crisp transients. It’s not fair to compare a transformer coupled amplifier to an OTL in regards to clarity, quickness and openness of the sound, but in direct comparisons with the T16, the cathode follower amp has a more weighty sound with more body with a slight reduction in detail and openness. Some people actually prefer this kind of sound. Only an OTL can project music into the room with the true sense a live performance, but this comes close. We feel that this design approaches the limits of what it is possible to accomplish with a transformer coupled output. Don't let the low power rating fool you. This is a very dynamic sounding amplifier. It will work well with most speakers that have an efficiency of 92 dB at one watt or higher.
What is a Cathode Follower Output?
With traditional single-ended amps, one lead of the output transformer is connected to the power supply and the other is connected to the plate of the tube, and the tube's cathode is grounded. The output tube in this configuration provides both voltage and current amplification. In this amplifier, one lead of the transformer is connected to the cathode of the tube and the other lead is grounded. The output tube provides only current amplification. This configuration dramatically improves performance because the cathode follower reduces the impedance of the output stage to less than 40 ohms while a conventional configuration has an output impedance of 700 ohms or more depending on the tube used.
If the cathode follower is so great, why doesn't everyone else use it?
Quite simply, because it is much more difficult to design. Through some clever engineering, we have been able to develop a simple, low cost circuit that incorporates the superiority of the cathode follower output.
So what specifically are the benefits of this amplifier?
1. Powerful bass. The output impedance is very low at 0.4 ohm giving clean, accurate bass transients. High current pulses are available to drive the woofer. The bottom end is down 1 dB at 30 Hz and 3 dB down at 20 Hz. Conventional single-ended amplifiers have little bass drive below 100 Hz.
2. Extended high-frequency response. The low impedance drive of the output stage easily drives transformer capacitance extending high frequency power bandwidth. The top end is down 1 dB at 50 kHz giving the amp OTL like quality.
3. Very linear, low distortion. The cathode follower is inherently a very low distortion configuration. Residual THD is less than 1% just before clipping. Distortion drops to just a few tenths of one percent at lower power levels.
4. Super quiet. It wasn’t easy to do, but we were able to get the noise down to well below 1 mV. The amp is as quiet as a low noise T16, which are commonly used to drive high-efficiency horn speakers. It is much more difficult to eliminate noise from single-ended tube amps than push-pull amps like the T16. Output transformers make the problem even more difficult.
5. Loves 4 ohm speakers. Four ohm speakers can be easily driven with no loss of performance. The transformer has output taps at 4, 8 and 16 ohms allowing it to match to a wide variety of speakers.
6. Low heat. This amp is green! Total power consumption is about 100 watts. The amp can be comfortably used in small rooms and in the summer months. Glorious tube sound can be enjoyed without consuming very much energy.
Use it for Home Theater!
Connect it your home theater system and really enjoy those movies. Eliminate that harsh brittle digital sound. Smooth out voices and bring those characters to life.