Monday, May 10, 2010

The making of a preamp

Building the 66-001p Vacuum Tube Reference Preamp
(an overview)

Well, it's been a while since I've posted here. I've been working on a preamp that is bound for review. The picture above is the regulator section for the outboard PSU, which in this case is configured for 120VAC / 60Hz. The regulator board is the only circuit board used in the entire product. Circuit boards are convenient but can color the sound as they are a giant capacitor with not so good dielectric. All of my products use point to point wiring as seen in this photo of the RIAA section under construction.

The circuit resides on an aluminum sub-chassis that is isolated from the main casework via 8 nitrile rubber bushings. The entire sub-chassis is the ground plane, and there is copper buss wire connecting all grounded points in the circuit. This provides a low-impedance ground path and helps to shield against noise. The RIAA preamp uses a fet/triode cascode for the input stage for high gain and low capacitance.

All the wiring is high-temp Teflon insulated solid conductor for signal and stranded for power. Silver solder is used throughout along with close tolerance metal film and wire-wound power resistors, polypropylene capacitors, (hand matched polypropylene and polystyrene caps used in the RIAA filter), and high-quality electrolytic and polyester caps for power supply bypassing. Solid polymer electrolytics are used for the cathode bypass capacitors; these have very low ESR and a long life span. You can see them in the picture above zip-tied to the yellow poly bypass caps. Below shows each tube section has 110uF of bypass capacitance. High quality ceramic tube sockets are used for longevity.

Below you can see things starting to come together. Those are the output transformers, used here for gain reduction and impedance matching. These transformers have an output impedance of 8 ohms! They drive headphones with authority and since the output impedance is so low you can run very long interconnects without high frequency roll off. The signal cable you see is high-speed digital coax used for HDTV and VGA applications. It has very low capacitance and uses foamed Teflon for the dielectric, has a silver plated solid copper center conductor and dual shield (foil and braid).

The front panel is where you see the functional interfaces; volume control (Alps Blue), balance, mute and mono, input selector (silver contact) and headphone jack. The balance control works in shunt mode and is out of the direct signal path. Moreover, it's action is limited to about 10dB of variation. The balance, mono, volume and mute controls can be seen in this picture.
The casework is steel with a nice satin texture paint job which hides fingerprints and other maladies well. The steel does a good job blocking RF interference.
Below is the nearly completed preamp awaiting the top and bottom covers and faceplate. These units take about 24 hours of labor to put together as every component has to be placed, every wire cut to size and dressed, components have to be measured and matched. Then multiple tests and measurements are done and finally a listening evaluation. This is not to mention the amount of engineering, prototyping, testing, measuring, listening, re-evaluating and modifications that went into the design. But a labor of love, to be sure!
So, I'm not going to give away all my secrets~ But I can assure you that this preamp performs very well in the measurement world and even better in the listening room! For full specifications and performance data please visit my website: