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RE: Tube Buffers ?

Posted by FlaCharlie (A) on July 19, 2021 at 06:55:52

In Reply to: Tube Buffers ? posted by mark.korda@myfairpoint.net on July 17, 2021 at 12:32:06:

As viridian has pointed out, the FX and the many similar units that are so popular, are not buffers. They are preamps with gain. So nothing that Nelson Pass has to say about the characteristics of buffers applies to the FX.

The biggest problem with the implementation of the FX is that it operates the tubes at very low voltages.

A poster on the massive FX thread over on AK, linked earlier, measured voltages for me a while back. I was in the process of breadboarding a preamp using a variety of tubes and the same simple (anode follower) circuit. I was curious so I tried the FX tube at stock voltages and then at more "normal" voltages suggested by the tube data sheet.

Not surprisingly, it sounded significantly better at normal voltages but the sound at lower voltage was better than I expected. If you take the triode plate curves (the FX uses a pentode wired in triode) and draw a load line it's truly horrible. Yet it doesn't sound as bad as it looks.

I just breadboarded the FX out of curiosity because I knew it had much more gain than I needed. Most modern systems don't really need any gain at all, since amps can normally be driven to full power by the output of a CD player or DAC alone. Some "tube flavor" is nice, though, and an active stage can often improve dynamics compared to a "passive preamp", aka a volume control.

While many owners like the sound, the gain is probably the biggest problem. There are a lot of complaints about how touchy the volume control is - the useful range is quite limited. So it's difficult to make minor level adjustments, it's often either too quiet or too loud.

I breadboarded several different tube types using the same circuit. Here's a link to what I built, if anyone's interested.

• 1626 Preamp Build   (Open in New Window)

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