A LINE PREAMP WITH A TWIST

This is Somewhat Embarassing

I've spent the last ten years trying to get the line amp out of the system, because it has no technical purpose, other than source switching and volume control; which functions can be incorporated into the power amplifier.  The line amp has looked to me like a solution without a problem.

Except one.  Terry Cain, loudspeaker tycoon and all around Good Guy, had an amp that needed a linestage, and made me an offer I couldn't refuse.  One major revision and a good deal of developmental buffing later, I can offer what I think is an outstanding minimalist line amp which will improve the sound of almost any power amplifier with which it is teamed, as well as providing source switching and volume control.

The problem I've found with vacuum tube line amps is that due to the nature of the available small signal tubes, they have too much voltage gain and not enough current drive (unless cathode follower output is used, which is another whole problem in itself). This has been neatly addressed here by the use of a small power tube, the 6AQ5/6005, in a low Zout circuit, the SRPP.  As a result, voltage gain and output impedence are both about one third those of the usual tube circuit.  As you see, excess gain present in many line amps has been converted to driving power; this preamp will drive multiple power amplifiers, even plate amps, with ease.

Both noise and distortion are very difficult to measure, with my gear.  I do have a good lab millivoltmeter, so I could at least get some sort of a reading, though it would change if I moved my head or hand around the DUT.  I couldn't get a decent distortion trace on my scope when nulling out the signal waveform at one volt out, even at the most sensitive setting, just grass.  It's certainly down at a very small fraction of a percent.  The max output swing is taken at the onset of soft clipping; at an output of five volts AC, the distortion is not measurably higher than at one volt.

Since each piece is made on order for the individual, input/output configurations and parts choices are to your preference.  I do have a base model of which I try to keep an example either on hand or chassis-constructed.  This will have dual parallel outputs (in order to make available the driving power of the circuit to those with separate subwoofer amps), and two inputs selected by a center-off toggle.   Switches are SwitchCraft silver contact, volume control is now the superbly transparent DACT CT-2 discrete attenuator, tube sockets are ceramic/gold.  Power rectification is by shunted ultrafast switching diodes, power supply capacitors are Panasonic TSHA and Nichicon Muse, the power supply is heavily choke-filtered.  Signal capacitors are Mundorf Silver Supreme, signal resistors are Riken and Kiwame.  Signal wire is high purity solid core silver in teflon. The chassis rim is cabinet quality exotic hardwood, chassis plates are ⅛” aircraft aluminum, all fasteners are stainless steel. Construction is by a thirty-year guitarmaker.

If you have a dedicated power amp, that requires a line preamp for signal switching and volume control, Mœbius will bring out the absolute best it can be.  Price for the base configuration is $2000. Extra inputs on a rotary switch (Elma, same as the DACT uses) are really the only meaningful upgrade I can think of, the cost will be very moderate; $100 for three, $150 for four.  I am now also able to supply remote volume in a format that is entirely out of the signal path (in fact it's completely turned off except when adjusting the volume), and which also uses the DACT for volume control, for $400.

If you would like to build your own, I'll have the project up on the Projects side of the site as soon as possible.  For the immediate present, I'll put up the owners manual, which has the circuit schematics, parts list, dimensional drawings, all the hard data.  For those contemplating purchase, it will give you a good idea of exactly what's in the box, as well as the documentation you can expect with your unit.

Interested, hm?  Drop me a post; my mailto is at the bottom of the page.  Let's talk story about it.

Poinz

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Last Updated Sept. 15, 2006
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