Tube Buffer Preamp?

[Geezer]

This probably falls in the "can't leave well enough alone category"...I know that most folks simply place a volume control (and input selector if needed) at the inputs of an ST35 for a completely passive preamp stage and it works well. I've currently got an ST35 w/ tube rectifier, EFB mod and a 100K stepped ladder attenuator hooked up to my CD player and it does the job. Still, several people have commented that the ST35 benefits from a little help at the front end and I tend to agree (I've also used mine with an active preamp and liked it, though some objectionable coloration creeped in, mostly from a pair of solid state opamps).

I recently splurged $12.50 on a tube buffer preamp PCB from a Hong Kong company, http://www.analogmetric.com/gallery.php?id=88 I already had the loose parts for completion on hand so my only additional cost will be $20 for a transformer and 5 bucks for a tube. Though they call it a "Unity Gain Buffer" and it can be configured so, by jumpering a couple of resistors it can increase the input gain by 1.5.

I've been tossing around ideas for an "integrated" diy ST35 including a preamp stage. I know that passive generally means "less noise" and this is more of a learning experiment than anything else. While impedance matching isn't a major issue with the ST35, I'd like to be able to switch between different input devices, interconnects and speakers and present a constant load to the amp. I've noticed that small things such as interconnect capacitance are audible with direct input, and RFI can be an issue (I carelessly turned on a different tube amp with interconnects that weren't attached to an input device and picked up a local AM radio station loud and clear, without a tuner).

The only drawback to a passive pre is that if input impedance from the signal source is high then output impedance from the volume control will also be high, possibly presenting a difficult load. The inverse is also true--if an amp prefers low source impedance then the signal source might have to look at low impedance at the volume control.

A buffer before the volume control makes the control's low impedance look like high impedance; with the buffer placed after the volume control the output impedance will be substantially lower.

I don't know where the ST35 falls or where to place the buffer to achieve what I'm looking for...I want the buffer itself to be as neutral as possible (and possibly my Chinese gizmo isn't the ideal device for the job) but for now I'm just tinkering around. I also don't know whether to configure the buffer for unity gain or perhaps increase the gain by 50-100 percent. Dave Gillespie's EFB circuit corrects gain issues in the ST35, where the fixed cathode bias resistor decreases the output of one channel in relation to the other depending on relative loads, and this adds another unknown (though it wouldn't seem to be relevant to input configuration).

I'm a paint-by-the-numbers diy'er and I'll probably be experiment with different configurations and simply see what I get, but I wondered if someone more familiar with load presentations might chime in and give me a leg up on where to start. I learn best by getting my hands dirty and this is new territory for me.

Any comments or observations are welcome.

[Shannon Parks]

I would suggest a 12B4 Budgie line stage. It would work perfect in this application. Right now I have mine with a plate follower mod which allows adjustable output gain.

Shannon