JFET Phono Preamp Kit


Minimalist audiophile phono stage using 2sk170 JFETs. Coupling and EQ caps are all Russian paper in oil. High performance. Low budget. Available as a kit including the PCB and all the parts to complete it.

Available for $89 at boozhoundlabs.com


Boozhound Laboratories JFET Phono Preamp
Assembly manual (rev.2)

The Boozhoundlabs Philosophy

The plan here is to offer kits that let the curious audiophile experience designs that they would otherwise have to build from scratch. The parts used in this kit are for the most part considered obsolete and are probably no longer being manufactured. I source this stuff from overseas via eBay.

I think simplicity is a huge part of why classic equipment sounds so good, and modern stuff can sound so bad. In the days when capacitors and transformers were expensive, designers minimized the parts count in any design, and this approach is audible even when designing with modern devices. And for those of us who not only want to build stuff, but to understand how it works, simple designs are much more comprehensible, with no "black boxes" that we only understand through the abstraction of a spec sheet.

Why not have fun building stuff instead of just pouring dollars into your system on the quest for ultimate-ness? Part of the fun for me is the ability to try something new without having to shell out the big bucks.


The JFET Phono Preamp

This is just about the simplest circuit possible that will accomplish what we need - reverse-RIAA equalization with gain. This is 2 JFET gain stages with a passive (no feedback) RIAA equalization network sandwiched between them.

For more info, and to see how other builders have done something similar, you can search for a circuit called “Le Pacific” which is the same topology, but with a slightly different EQ section and parts values.


Pretty nifty, huh? Only 22 parts per channel including the power supply filter stuff. The JFET gain stages are straight from any electronics textbook. The RIAA filter section made up of R5, R8, C1, C2, and C3 are calculated using standard formulas. Nothing special here – except that almost nobody does it like this.

Gain is approximately 40dB (about 30dB per gain stage, minus about 20dB for the RIAA EQ).

Inventory


Start by verifying that you have all of the parts you need. I endeavor to make sure I send only complete kits, but it is always possible I missed something. If I screwed up and left something out, please email me immediately at jsn@boozhoundlabs.com and I will make it right.

Here is what is included with each kit, with checkboxes to make it easy to verify that you have all of this stuff:

( ) 1 Printed Circuit Board
( ) 4 2sk170 transistors
( ) 4 220uF electrolytic capacitor Nichicon Muse
( ) 4 0.1uF PIO capacitor Russian K40-Y
( ) 2 0.033uF PIO capacitor Russian K40-Y
( ) 2 0.001uF PIO capacitor Russian K40-Y
( ) 2 1uF PIO capacitor Russian K42-Y
( ) 4 10 ohm resistor Brown, black, black, gold, brown
( ) 4 49.9 ohm resistor Yellow, white, white, gold, brown
( ) 4 100 ohm resistor Brown, black, black, black, brown
( ) 6 3.16k resistor Orange, brown, blue, brown, brown
( ) 2 28k resistor Red, gray, black, red, brown
( ) 2 47k resistor Yellow, violet, black, red, brown
( ) 2 100k resistor Brown, black, black, orange, brown
( ) 2 1M resistor Brown, black, black, yellow, brown


Assembly

This is almost self-explanatory, but I will offer a few tips, and a few photos.

It is generally a good idea to install the little stuff first and the big stuff afterwards, so that you aren’t melting the big stuff trying to get to the little stuff. Start with the resistors and the JFETs.

There will be 2 matched pairs of JFETs. Use each pair in the same position in both channels – one pair will be Q1 in both channels, the other Q2. This will maintain the same amount of gain on both channels. It doesn’t matter which pair is Q1 and which is Q2.


I like to solder from the bottom of the board because it is easier to get to things, and the odds of overheating a part are lower because you are that much further from the part itself. Be sure to heat the pad and the leads sufficiently to let the solder flow all the way to the top of the board though. These boards have through-plated holes, so it will be easy.


These boards have traces only on one side, leaving the bottom side to be nothing but a huge ground plane. This will reduce grounding problems and make this a very quiet design.

I also made the gap between the ground plane and the pads really huge so that it would be hard to accidentally ground out a component. I tend to use a soldering iron tip until it is far from pointy, so the spacing helps me keep everything tidy.

The points where the components do attach to the ground plane will take a bit more heat to solder properly since the ground plane will act as a heat sink.


Next install the capacitors. Or instead of “next” I should say “last” because you are done!

The only thing left to do is visually inspect the solder joints to make sure everything looks good and there are no solder bridges or obvious cold solder joints.

Integration


Connecting this to the various inputs and outputs is also super easy. The in and out pads are obvious. There is one more pad I probably should have labeled “gnd” that is for a wire to connect to the chassis, ideally at a ground lug where you connect the ground wire from your turntable.

The pwr pads need to be connected to a source of roughly 24 volts filtered DC. There is a bit of filtering (more like decoupling) on the board, but not enough to filter AC. I use a mid-grade switch mode wall wart power supply for this because they are cheap, easy, and very quiet. An upgraded regulated power supply would be a nice upgrade I’m sure.

There is no onboard fuse. The wall wart power supplies I imagine almost everyone will be using for this are internally current limited. If you use another power supply, a fuse might be a good idea. Current draw is less than 50 mA.

I include the standard 47k load resistor, but feel free to change that to something that is better matched to your cartridge or stepup transformer or whatever.

The mounting holes at the corners are designed for 4-40 thread screws. The boards measure 5.3” by 3.5” by 1.25” assembled

Now go play a couple of those Steely Dan albums you buy every time you see them in the thrift store!

-jsn


FAQ:

power supply options

This circuit should be happy with any supply voltage between 12V and 24V, so feel free to experiment.

I have sold many of these with a simple but high quality switch-mode AC adapter, but the one I used to sell has been discontinued. I have found that some switch mode AC adapters will produce a "motorboating" effect, but they are cheap so if you get this, try another one.

AC adapters are cheap, easy, and usually quiet - but there are certainly better sounding options. One of the best fits for this circuit is to use ~24V worth of batteries in series. Batteries tend to sound really great, are the quietest option by a mile.

The H-PS-1 power supply from Glass Ware (TubeCad Journal) would be really amazingly good I bet (though I haven't built one). The 18V version would be ideal.

component matching

I match the JFETs to within 1mA Idss. Matched pairs should be used in the same position for right and left channels. Matching only ensures that the gain of the circuit be equal in each channel. The capacitors in the RIAA eq section are matched to 10%. Since these are setting the breakpoint of a low 6dB/octave slope, a 10% change is minor. The resistors in the RIAA circuit are standard values, with 1% tolerance.

capacitor upgrade options

The Surplus Russian paper in oil caps were selected because I personally like the sound of these caps a lot, I find they compliment the JFET sound, and because the price/performance ratio is unapproachable by modern new parts. That said, capacitor sound if often a matter of personal taste, and system synergy, so feel free to experiment. The values that matter are the RIAA EQ section (C1, C2, C3) so fee free to replace the coupling caps with larger or smaller values. The output cap is relatively small at 1uF since the relatively high output impedance of the circuit requires an easy load.

options for getting more gain

I have had several kit builders ask about the options for getting more gain out of the BHL Phono. There are LOTS of options. Gain is determined (approximately) by the ratio of the Drain Resistor (R4 and R11) to the Source Resistor (R3 and R12). R4 if off limits though, because it sets the output impedance of that stage, and changing it would effect the RIAA equalization (which can always be recalculated). For a modest increase in gain, you can reduce the size of R3/R12 or increase the size of R11. Start by roughly doubling/halving them. If you want more gain, you can remove the Source resistors completely, or bypass them with a large (typically electrolytic) capacitor. If you want to get a bit more exotic, you can replace the drain resistor with current sources, or perhaps a current limiting diode (which is simply a preset current source).

Another option to consider is the addition of a stepup transformer in front of the stage, which can improve the impedance matching with many low output cartridges as well. Conveniently, low output cartridges tend to prefer lower impedance loads, so if you need more gain definitely look into stepup transformers since you might be able to get more gain with the added benefit of better impedance matching.

grounding

The circuit board is designed so that the entire bottom copper layer is a ground plane. This should make for the quietest possible circuit with no possibility of ground loops on the board itself. To install this into a chassis, you should use insulated RCA jacks and make sure the board connects to the chassis at only one point, using a wire soldered from the ground pad to a ground lug, which is grounded to the chassis, and is where you connect the ground wire from your turntable. Exotic cartridge/arm grounding schemes may benefit from other ways to do it, but if you have something like that you probably already have equally exotic grounding plans for the phono stage :)

operating points - what to expect

The JFETs used can potentially vary in Idss between 6-12mA. In the circuit, expect somewhere around 10-15mA of current. The batches of JFETs I have received so far have been in the 8-9mA Idss range, but it's possible that future batches will be towards one of the extremes, so don't worry about this too much as long ans it sounds right.

more specs

Output impedance = R11 = 3.16k ohms
Dimensions of assembled kit = 5.3” by 3.5” by 1.25”