Yet Another OCL Tube Buffer

The concept of a unity gain (or near unity gain) OCL tube buffer has been around for a while. The original idea of a naked tube buffer was proposed in the early 1990s. The idea behind the buffer is obvious - that there are some situations where an audio system needs an impedance transformation (from high Zi to low Zo) but doesn't need gain. Sometimes the user just wants an attenuator in the signal path, but no gain. For example, some audiophiles use passive preamps which are, essentially, jacks with pots in between them. Passive preamps (or passive attenuators) are always damaging to an audio system because they insert resistance into the signal path. A unity gain buffer is a better choice for this situation because it shows the source a very high impedance (good for the source) while driving cables with a very low impedance. Of course, once you introduce an active component into the signal path, then you have all the issues associated with that device's distortion characteristics. There is no free lunch.

John Broskie did a treatment of this idea a decade ago in the Tubecad Journal. Another buffer design was created in a series of threads at the DIY Audio Forum. These three threads are:

• Heretical Unity Gain Line Stage 1.
• Heretical Unity Gain Line Stage 2.
• Heretical Unity Gain Line Stage 3.

After all of this super work by these brilliant individuals, it is more than fair to ask why we need yet another version of this concept. And the only honest answer is that we don't, but some of us just can't help ourselves anyway. So, if you're interested in a slightly different way to do this, read on. The goal will be to design the perfect OCL tube buffer.