A HIGH PRECISION TUBE PHONO PREAMPLIFIER
by Fred Nachbaur, Dogstar Music ©1998, 2001

4: CONSTRUCTION HINTS


The first prototype of the phono preamp was built into a small metal box chassis, using PCB construction. I subsequently re-used the PCB module for use as one of the preamps in my RA-100 integrated amplifier.

If you wish to duplicate the layout of the PCB-based prototype, download the file sapart.zip, (be sure to read "artwork.txt" included in the zipfile) which contains the images necessary to create your PC board. Write me if you want the original Gerber files, should you have access to a "one of" PCB manufacturer. Note that the preamplifier module is designed for double-sided boards. This can be tricky, and is recommended only for experienced PC prototypers.

Note also: There is a polarized component shown on either side of the centre tube (12AT7) which is not used in normal applications; this was left as an option should someone wish to use the preamp in a "cathodyne" configuration. This, and its associated output connection pads are ignored in this design.



The second prototype (pictured below) used the same type and size (210 mm x 155 mm x 50 mm) steel box chassis. However, for the purpose of this article (and to assure myself that the design does indeed give good results using conventional wiring) this version was built using point-to-point wiring techniques, employing the tube socket pins and rows of terminal strips to provide the required connection points.

Testing showed no measurable (or audible) difference in the two design approaches, so use whichever technique you're more comfortable with.



This second prototype was constructed with the bulk of the preamp circuitry on the top case-half, and the power supply, mode switching, input/output jacks, and feedback networks in the bottom case-half. If I had it all to do over, I'd leave myself more room in the top section for the many resistors and capacitors, and also more room in the bottom for the feedback networks. However, since it's unlikely that I'll build yet another of these in the foreseeable future, I leave it to you to come up with a superior mechanical layout. ;-)


The Inner Workings of the second prototype.
Note the shielded cables on all audio lines, and twisted pairs on power wires.



The top case-half, showing most of the preamp wiring.
You'll probably want to leave yourself more room by increasing the spacing
between tube sockets, and mounting more terminal strips.



The power supply section (lower case-half). High-voltage supply
on the left, DC filament supply on the right. Feedback network components
wired near the mode switch on the left side of the front panel.


Wiring tips

• Be extremely careful. This cannot be overemphasised. The voltages and currents in this project can hurt you. Be constantly aware of this fact, and the worst you can do is make a bunch of bad smoke. Burned out components are replaceable; you're not.



• Start by doing all the mechanical work. Lay out, drill and cut your enclosure for the power transformers, input and output jacks, power cord, switches, and standoffs for the PC board (if used) or tube sockets and terminal strips (if using the point-to-point approach). In other words, do as I say, don't do as I do. When prototyping, the design and construction aspects often go hand in hand, leaving plenty of opportunity for a slip of the drill to send you crashing through some delicate part of the circuit. You, as the builder, have the luxury of being able to methodically start with the heavy stuff, and progress to the more delicate circuitry once all the grunt-work has been done.



• Your enclosure should be metal (steel or aluminum). Plastic or other non-conductive enclosures are simply not appropriate for this design, mainly because they are a lot more prone to hum. The prototype shown in the photographs was built in an 8" x 6" x 2" steel chassis box (formerly available from Radio Shack). However, this particular box is a rather tight fit; you should go for the compact look only if you're relatively experienced. A somewhat larger box will give you more elbow room, and allow you to place the transformers further away from the more sensitive circuitry.



• Make provision to connect the ground wire from your turntable to the chassis of the preamp. A bolt, lockwasher, and a couple nuts is all it takes for this. Or, make arrangements for a wing-nut, thumb-screw or wire clip arrangement if you prefer.



• Next wire up the power supply. This is best done using the point-to-point approach. Carefully follow the schematic diagram, being very very careful to observe polarity on diodes and capacitors. This is not just an idle warning; reverse polarity at these voltages can cause electrolytic capacitors to explode enthusiastically, making a heck of a mess at best and possibly injuring someone (e.g. you) at worst.
• Run all power wiring as twisted pairs. This includes AC input, filament supplies, and B+ lines.



• An important part of building the power supply is making an insulated "bleeder resistor." This consists of a 10K, 1 watt resistor soldered to insulated wire, with shrink-wrap or tape completely covering the connections. At the ends of the wires install insulated 'gator clips. This is to discharge the capacitors after testing or otherwise working on the power supply. Connect the clips between the positive end of C7, and "Pwr Ground" for at least ten seconds before touching anything in the power supply area. Getting sloppy with this can result in nasty (and potentially dangerous) surprises. However, don't forget to unclip the bleeder resistor before powering up; the resistor will heat up in a hurry if you do, letting you know in a startling (and smelly) way.



• Ground returns should be grounded at a central grounding point only. (This is also called the "star-grounding" method.) Resist the temptation to use the chassis as a ground return. The only components that may be safely chassis-grounded are the Ceramic inputs and the output jacks on the rear apron. Failing to pay careful attention to proper grounding can result in hard-to-remove AC hum and other artifacts.



• Check, double-check, and re-check your wiring before proceeding. Then do a preliminary test by turning the supply on very briefly (2 seconds or so). There should be no smoke. Use your VOM in 500 to 2000 volt range, and check the voltage at your B+ outputs; they should read several hundred volts. If ok so far, turn the unit on while monitoring the B+ voltage. After a few seconds it should stabilize at 450-470 volts. Power down, and don't forget to bleed the capacitors!


Similarly check the filament supply. The filament supplies should ride around +/- 8 volts DC (16 volts end-to-end) with no load.


• Optionally, you can check the supply under load by temporarily soldering 100K, 2 watt resistors between each B+ line and "power ground," and a 27 ohm, 5 watt resistor across the filament lines. Your B+ voltage should be around 390 volts DC, (no less than about 360), and filament voltage should be right about 12 volts AC (6 volts either side of "heater ground".) Don't leave the supply on for too long; the resistors will get quite hot after a while.



• Run all audio lines using good-quality shielded cable. The shield should be grounded at one end only (I suggest using the "source" end for the ground, making it easier to trace signal direction later should it become necessary.) Cut away the shield at the other end, and shrink-wrap or tape the cut to prevent the little "fuzzies" from shorting against anything. The exception is the magnetic phono inputs (see next item).



• The phono and mic inputs require special handling. The input jacks should be isolated from ground using insulated mounting hardware. This is the only case where the shield of the cable is connected at both ends. Connect to the jack's ground return at the jack end, and to a common tie point with the grounds going to the mode switch (if implementing the Ceramic phono option). The output of your switching The line from the common switch terminal should be connected to the input of the preamp using shielded cable, again with the switch end grounded to the common tie point, and the preamp end grounded to the ground pad nearest the input terminal (PCB construction) or to the terminal strip connection to which R101, C102 and R105 are connected (or the corresponding right-channel parts).



• Keep all lines as short as is practical. Try to keep input lines away from AC and filament supply lines, and from the output lines.



• The lines to the input and output jacks on the rear apron may optionally be two-conductor shielded cable, one for each channel. This is what I used on the prototype. However, if you want to maintain the highest possible channel separation, run separate single-core cables instead.



• Don't be careless with any lines that carry supply voltages. Make sure that all connections are well-insulated. Don't let any uninsulated high-voltage point come any closer to any other point (including, of course, the chassis) by less than about 5 mm worst-case.

Other tips

• Don't skimp on capacitor voltage ratings. If you can find capacitors rated at 500 volts continuous, these would be preferable; however, most sources only supply up to 450 volt units, which have held up fine in my prototypes; the slight over-voltage that can occur before the tubes heat up and start drawing current doesn't seem to cause adverse effects.



• Because of the unit's low power consumption, no special heat-sinking techniques need be observed. However, if you enclose the entire unit (including the tubes) inside a closed case, allow at least a few ventilation holes.



• Keep this project well out of reach of small children and pets at all times. I keep coming back to the safety aspect, not because I'm paranoid, but because there is real danger in carelessness.

Adjustment and testing

Assuming that everything goes well, there is not much to adjust. Turn the unit on and let it warm up for at least a minute. Measure the voltages at the following points (relative to chassis ground). A deviation of about 10% is nothing to worry about, but larger differences should be investigated. Compare the readings for both channels; they should agree quite well:

Plate V1A (bottom of R106/R206)	190-210 V
Plate V1B (bottom of R107/R207)	270-290 V
Cathodes V1 (top of R104/R204)	26-32 V
Grid supply V1A (top of C102/C202)	1.5 V less than cathodes
Grid supply V1B (ACG)	2V less than cathodes
Grid V2 (R115/R215)	same as Plate V1A
Cathode V2 (top of R108/R208)	1.5V higher than Grid V2
Plate V2 (OUT)	290-310 V
+IN (top of R101/R201), Outputs	0V
Filaments	+6.3 V (pins 4), =6.3V (pins 5)

OK so far? Congratulations! Your DC servo loops are working just great. If not, don't fret. Remember that any problem in the DC loop will probably cause all voltages to be wonky, so it may not be easy to troubleshoot. Take your time, go over all your connections, verify parts, etc. Still having trouble with one channel? Swap 12AX7A's. If the problem follows the tube, you've got a bad one. If the problem stays with the same channel, there's a problem on the board circuitry. There, you got it? Well done!

Ideally, final testing should be done using a sine-wave signal generator and oscilloscope, to verify that your performance is similar to the curves shown in the "How It Works" section. If this kind of test gear is not available to you, you'll have to use the "plug it in and see if it works" approach. Do be careful, when connecting to your amplifier, to bring the amp's volume control up from zero very gradually, in case you have a problem such as excessive hum or oscillation.

Using the preamp is pretty straightforward; connect your phono (Ceramic or Magnetic cartridge) to the appropriate input jacks, and connect the output jacks to your amplifier. And away you go!

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